# Measuring thread depth without wires - your views, please



## HennieL (Jun 12, 2020)

Hello everyone,

Here in South Africa where I live it is very difficult, and extremely expensive, to obtain the correct sizes and accuracy wires to use in the 3-wire method for determining thread pitch diameters, (and thus confirming that your single point threading is cut to the correct depth). So, until recently, I have been doing what most machinists do - screwing a nut onto the threaded bolt, and calling it OK if the nut goes on easily, but tightly...

Some time ago I purchased a new micrometer that has a 0.001mm resolution "sight unseen" through the internet, and although I'm not very happy with the "feel" quality, it is giving me consistently the same (rounded) results as my trusty old Mitotoyo 0.01mm resolution micrometer - hence it is at least accurate to 2 decimals of a mm. Anyway, to make a long story short, this micrometer came boxed with a set of exchangeable "anvils", one set being 60º wedges, another a set of 60º cones, and a third set are wide, flat disks. Last weekend I opening the box to remove the micrometer in order to take some readings on a rod that I was turning down to make a special bolt, and the 60º wedge shaped anvils caught my eye just as I was thinking about the critically sized threading that I had to do... light-bulb moment... _Why not use a flat disk anvil on the one jaw, and either the wedge or the cone in the other jaw of the micrometer - then I can calculate the thread depth by measuring the thickness of the (outside) major diameter of the "bolt" normally, and the diameter of the "bolt" with the wedge fitting into the thread on the one side, and subtracting the one measurement from the other should give me the thread depth..._

Bit difficult to explain - let me show you some photos to make this clear...

The micrometer fitted with the flat disk and wedge anvils into it's two jaws:






This is a close up of the 60º cone and wedge anvils:





And here is a photo showing the flat disk straddling the outside of the threads on the one side of the bolt, and the 60º wedge fitting in between two threads, measuring to the minor diameter of the bolt (M16x2).





So, is this an accurate way to measure screw threads???

I wanted to confirm if this idea is actually worth something, or just a "flash in the pan", so I raided my supply of bolts and machine screws, and measured these bolts varying from 20mm to 4mm diameter.






Keep in mind that all the bolts were Metric, and all were bought from shops (and one could thus hope that they were manufactured to prescribed tolerances).

Results are rather encouraging, I think - I measured 38 bolts, and found that three of them had major diameters outside of the normal 6g tolerance, and these were excluded from the final assessment. As can be seen in the spreadsheet summary below, the average measured thread depth of all the "in spec" bolts came to 0.72mm, whilst the calculated thread depth was 0.71mm when using the formula: _thread depth = dmax - d1max (also known as 5H/8)_, and 0.80mm when using the formula: _thread depth = 0.6134P_ (which is the most used formula). For interest sake, I also looked up the recommended total thread depth for two carbide inserts manufactured by Sandvic and Mitsubishi. Lastly, I calculated the % deviation of each individual measurement, expressed as a % deviation from the two calculated formulae, and highlighted the calculated depth closest to the actual measured depth.






I do accept that the wedge has a very narrow blunt edge - perhaps 0.25mm, and that there would be a smallest limiting size beyond which one would not get accurate readings - the above figures show that the % deviation increased substantially with the 4mm diameter machine screw readings, and this probably exceeds the minimum readable size. Also, there is no allowance for the changes in the helix angle resulting from the different pitch sizes, but I suspect this will be negligibly small.

I really would value any feedback on this: - Are any of my assumptions wrong? What am I missing? Why are the larger bolts closer to the 0.6134P values, and the smaller ones closer to the 5H/8 values? Why did I do this, do I need to see my shrink


----------



## SmithDoor (Jun 12, 2020)

I use rubber bands to hold the wires in place. I only use this when a test  thread for internal threading ( lathe chucks backs).

Most machine shops have board with every know thread and just feel using the nut. If not on board they ask for nut to fit the thread.

Update 6/13/2020
 Kvom have a good point, use what you are thread too the may be a different class fit.


----------



## kvom (Jun 13, 2020)

I've found that nuts are looser than tapped holes; hence I use a tapped hole when testing thread depth.


----------



## HennieL (Jun 13, 2020)

Thanks for the replies so far, but I'm afraid you're missing the point a bit  (with respect...)

When the professionals (and many of us amateurs) have a high-tolerance threading requirement, the standard "go to" method of confirming if you are within tolerance is to measure the pitch diameter using the 3-wire system - but as stated in my original post, they are very scarse and VERY expensive where I live. Also, in my experience, one could calculate the depth of cut to 10 decimal places, and then still need to make two or three "cleaning cuts" to achieve the correct thread depth due to tolerances and flex in our machines (at least, in mine... my threads always come out shallower than what I calculated).

Anyway, if you really want/need to know if your threads are within tolerance you either use the 3-wire method, Go-No Go gauges, or a special thread micrometer. The only difference between the method described above and the professional thread micrometer is that I don't have a V-slot anvil for my micrometer, and substitute this by using a wide flat disk anvil that straddles a few threads. Truth be told, I did not even know how a thread micrometer looked until after I thought about using the 60º taper wedge and flat disk, and completed the tests shown above - only Googling for "thread micrometer" early this morning, and discovered that "my" method is quite similar.

Still anxiously awaiting some in-depth feedback / comments to the following:


> Are any of my assumptions wrong? What am I missing? Why are the larger bolts closer to the 0.6134P values, and the smaller ones closer to the 5H/8 values?


----------



## SmithDoor (Jun 13, 2020)

Here a little trick for thread.
If making a thread where both ID & OD use same thread per inch as lead screw you do need the thread dial.

If lead screw is 8 TPI you use and thread 2X, 3X 4X, or basically any even multiple of lead screw that is finer.
Like 16tpi, 24tpi 32 tpi

If lathe has 12 tpi the same goes 24tpi, 36tpi or 48tpi

This works for your own project when you pick the thread pitch.

Dave



HennieL said:


> Thanks for the replies so far, but I'm afraid you're missing the point a bit  (with respect...)
> 
> When the professionals (and many of us amateurs) have a high-tolerance threading requirement, the standard "go to" method of confirming if you are within tolerance is to measure the pitch diameter using the 3-wire system - but as stated in my original post, they are very scarse and VERY expensive where I live. Also, in my experience, one could calculate the depth of cut to 10 decimal places, and then still need to make two or three "cleaning cuts" to achieve the correct thread depth due to tolerances and flex in our machines (at least, in mine... my threads always come out shallower than what I calculated).
> 
> ...


----------



## Ken I (Jun 13, 2020)

I have 3 sets of decimal drills available to me (at different locations) from 0.3 to 1.6 in 0.05mm steps and 1 to 6mm in 0.1mm steps - so I choose the nearest appropriate shank size - do the calculations and off you go.
You don't even need three you can use say two 0.8's and a 0.75 (illustrated below) and calculate accordingly (or draw it up in a CAD program to save you the trig.) - measure the actual shank diameters at the point you intend to use as they are slightly tapered with your typical twist drill shank being 0.05 to 0.1mm smaller than the cutting size  -diminishing with size.
I have two Microbox sets 0.3 to 1.6mm diameter in 0.05 steps which is small enough for most thread sizes.
I normally have two sets over my lathe so 2+1 is my go to method and CAD 'cause I'm lazy.
You obviously use a wire diameter that is going to register close to the effective thread diameter - but anywhere close is good enough if you do the calculations / CAD vis:- M8x1.25 nominal for illustrative purposes:-




Regards,  Ken


----------



## kvom (Jun 14, 2020)

With respect, how often do model engines require threads more precise than can be obtained by simpler methods than wires?  The micrometer setup you have looks like a good solution for checking once you get close while single-pointing.

FWIW, McMaster has a set of 48 wires for $36.30.  I'd be willing to bet that a member here would be willing to order a set and ship them to you at cost.









						McMaster-Carr
					

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.




					www.mcmaster.com


----------



## BIGTREV (Jun 14, 2020)

HennieL said:


> Thanks for the replies so far, but I'm afraid you're missing the point a bit  (with respect...)
> 
> Still anxiously awaiting some in-depth feedback / comments to the following:


I'm following you, it seems quite logical me, but I'm only a novice.
Cheers


----------



## Ken I (Jun 14, 2020)

Hennie,
             Yarwellnofine (that makes sense if you are South African) - I get the question - yes the method you are suggesting works but the crest radius / flat (and that varies greatly) is going to interfere with your measurements.

If you are single pointing the crest is going to be sharp - I often screw cut to the sharp point at full diameter as my "gauge" point.

Using your micrometer set up is going to work just fine if you work to a truncated (flat) tip profile like ±7.8 diameter in the case of an M8.

Problem is how do you calibrate the nominal starting point ? Sure if you know the precise tip radius of your anvil you can work it out and calibrate against a known thickness  (slip block ?) or do you have a calibrated "V" - again this "V" can be calibrated using a wire.

A simple calibration piece would be a thread with a precisely finished O.D. and therefore a truncated thread (flat) profile - you could use any thread that you have measured - via the three wire method -  and documented. However you would still have to adjust for the truncation of the thread you are turning versus the thread you calibrated against.

If you work in a production environment and can get your hands on a selection of No-Go thread plugs these would make excellent calibration pieces.

I think your micrometer is just fine for production methods but for one offs it is probably more trouble to calibrate than it is to use the three wire method.

And all the errors of angle and symmetry etc. etc. are still there to haunt you regardless of method although the 3 wire method kind-of evens these errors out a bit.

In practice I normally screw cut to the theoretical sharp corner and test for fit against a nut or bolt / mating part as required.

That micrometer is a great piece of kit - did you make the wedge / point inserts ? Or did it come like that ?

Regards - Ken

P.S. All those commercial bolts were rolled threads and therefore the resultant finished effective & major diameters were all influenced by the diameter of the rod prior to rolling.
An undersized blank will result in excessive crest truncation and undersized major diameter but the effective diameter should still be O.K..
An on size blank should produce perfect results.
An oversize blank will look perfect but be oversize on effective and major diameters.
Trust me - years of thread rolling experience on millions of parts - I could go on.......


----------



## SmithDoor (Jun 14, 2020)

Being a novice is just having how to. Back I started there was no internet. You had only books and just did not say the finer details.

When first started did not of  know of thread wires, I use used a nut. 
Later I did try use drill bits for thread wires but it was hard to use and was pain. 
Once try using rubber bands witch made a lot better using thread wires. 
I was still highschool working RC planes. 

Dave



BIGTREV said:


> I'm following you, it seems quite logical me, but I'm only a novice.
> Cheers


----------



## HennieL (Jun 14, 2020)

Thanks guys - some good points, and you made me think some more...



> Why are the larger bolts closer to the 0.6134P values, and the smaller ones closer to the 5H/8 values?



I had a closer look at the wedge seating in the threads, and I think I discovered my own answer 

After checking with a strong magnifying glass, it's clear that the flat on the bottom of the wedge "bottomed out" against the side of the thread on the smaller pitched sizes (1.0mm and smaller), and not on the bottom of the thread - hence they did not measure the full thread depth as I originally thought. The larger thread sizes allowed the flat bottom of the wedge to touch the bottom of the thread, and thus agreed with the correct method of calculating the thread depth (0.6134P)... Oh well, another "light bulb moment" gone to waste 

No matter, this was yet another step in my continuous learning process...



SmithDoor said:


> If lead screw is 8 TPI you use and thread 2X, 3X 4X, or basically any even multiple of lead screw that is finer.
> Like 16tpi, 24tpi 32 tpi
> 
> If lathe has 12 tpi the same goes 24tpi, 36tpi or 48tpi



Thanks Dave, but having been metricated for more than 50 years (and living in a republic  ) I try to avoid anything imperial if I can - so it's strictly metric threads for me... and my leadscrew is 3mm, for what it's worth - but I do hear what you're saying.



Ken I said:


> Yarwellnofine (that makes sense if you are South African)



Shapshoot my Bra  (and that's another indigenous South African saying... apologies for this, but we're nearly as bad as the Auzzies)



> Problem is how do you calibrate the nominal starting point ? Sure if you know the precise tip radius of your anvil you can work it out and calibrate against a known thickness  (slip block ?) or do you have a calibrated "V" - again this "V" can be calibrated using a wire.



Ken, thanks for your input - you made me re-consider, and do some more research. I think I will continue to use the "wedge micrometer" on threads of 1.5mm pitch and larger, and will use the "one-wire" method to check if I'm in the ball park on the smaller diameters.



kvom said:


> With respect, how often do model engines require threads more precise than can be obtained by simpler methods than wires?  The micrometer setup you have looks like a good solution for checking once you get close while single-pointing.



Thanks for the offer kvom, I might just do that. Problem is, though, with our "junk status" currency that's at least 40 Bic Mac burgers, and I would still like to figure out another (cheaper) way, if possible. I do have a few sets of drills, though, and I'm going to try the "one wire" method using measured drills for a while and see how that compares to my micrometer (and my not-so-trusted nuts LOL). 

As for your question regarding why I need the precision - well, that's just how I am, I suppose... seriously, though, I have been doing other "hobby" work with the lathe that do require accurate threading from time to time for some years now - but truth be told, being new to this hobby and to this Forum, I have not even made a single engine yet, so I cannot argue with you on this one... but wait, it's going to happen soon


----------



## peter smith (Jun 14, 2020)

I think this is a good way to measure threads if you use the actual o/d of your work piece, and grind a small flat on the wedge so it will contact the effective diameter and not the radius in the root of the thread. I will be trying this method next time i cut a thread.


----------



## HennieL (Jun 15, 2020)

peter smith said:


> I think this is a good way to measure threads if you use the actual o/d of your work piece, and grind a small flat on the wedge so it will contact the effective diameter and not the radius in the root of the thread. I will be trying this method next time i cut a thread.


Thanks Peter.

After giving this some more thought, I agree, IF you can make a whole set of wedges, one for each pitch - with the flat being approximately equal to the pitch diameter (same as the "best wire" diameter used when measuring with three wires). I'm more inclined to grinding a very small radius on the end to the same diameter as the insert that I use (0.3mm, if I remember correctly), and still try to measure the root. Having made custom knives for many years I have the tools (and grinding belts and papers down to 3000 grit), so putting a small accurate radius onto the wedge should not be too difficult. Hmmm, perhaps I should make some new anvils instead, and try both methods...


----------



## ignator (Jun 15, 2020)

HennieL: I recently ran into this same issue with wanting to know that I made a correct thread without guessing that I  followed the procedure of feeding the compound slide the correct amount, as every description I read, I felt confusing. 
If I start with a sharp 60 degree cutting bit, and feed the compound that is set to 30 degrees off axis (~29.5), and I start with the round OD right at the described hardware diameter, I should feed the compound in the exact distance of the pitch of thread, as this is one side of the equilateral triangle. Ignoring the crest and trough radii. This should be a perfect sized thread (I understand a nut with the trough radii per the specification, will not fit on this cut thread). I've not seen a logical treatment of this. As it appears that any of the internet sites I find, talk about feeding in the cosine of 30 degrees times the pitch of the thread, which makes sense if you feeding in the cross slide, not the compound. So I'm still not understanding this single point process, and in the end, cut until a nut fits on my threads.
My solution was to buy a screw thread micrometer, in the 0-1inch size. This came with 5 sets of  anvils.



pitchAnvil Pair #1Anvil Pair#2Anvil Pair #3Anvil Pair #4Anvil Pair #5Threads Per Inch64-4844-2824-14 13-9 8-5 Metric 0.4 - 0.5mm0.6 - 0.9mm1.0 - 1.75mm 2.0 - 3.0mm 3.50 - 5.0mm

You have to zero the micrometer when you install the anvil for the size of interest. You zero the thimble reading, then push the bottom anvil into the mate, and lock it in with it's lever, as it is on a sliding foot.
But if you have problems getting the set of wires, you probably can't get this micrometer either.


----------



## peter smith (Jun 15, 2020)

When I was machining (on a lathe) in one of my jobs we used thread parallels, these are very similar to the wedge idea of yours, but you use a single wedge on top and a double wedge on the bottom these are very good to use and accurate, they are in sets. You put the pair together you are going to use measure them add this to the effective dia. of the thread you are going to cut and that is your size. if you google thread parallels you will find them. There is some on ebay but only BA ones.


----------



## peter smith (Jun 15, 2020)

I still think your single wedge is good and i an going to try it, but i will put a clearance flat on the point not a radius because you need to measure the effective diameter, if it sits on the radius you will get a false reading.


----------



## L98fiero (Jun 16, 2020)

Instead of all the alternatives that require a lot of calculations and/or purchasing more parts, why not make the 'bottom' anvil for a thread mic. you have a lathe and a mill so it's probably worth a try. Here's a picture of what they look like and given what you say prices are, you could spend quite a bit of time making some of these up, hardened drill rod or high carbon steel would work. They would probably even work unhardened as they have to be zeroed each time they are put in the mic. getting the vee anvil centered would be important.


----------



## HennieL (Jun 16, 2020)

L98fiero said:


> Instead of all the alternatives that require a lot of calculations and/or purchasing more parts, why not make the 'bottom' anvil for a thread mic. you have a lathe and a mill so it's probably worth a try.



Thanks for the replies everyone - lots of good suggestions.

L98fiero, I plan to make some more "top" wedges with different edge radii to accommodate the various pitches - and I will probably make them from M45 tool steel, so I might as well harden them too. Making the "V" anvils to the required accuracy would be much more difficult, I think - unless one makes some properly shaped "inverted cutters" first and then cut the "V"s... Probably more work than it's worth. Will give it some thought, though...

By the way,your pic does not show...


----------



## L98fiero (Jun 16, 2020)

HennieL said:


> By the way,your pic does not show...


There are no dimensions where I got the picture(Google search) but the anvils are for a range of threads so knowing the range one could work out the pitch of the crests on the anvil. Just a guess without checking my set would suggest the pitch to be for the finest pitch of the range and they could be made by tilting a rotary table 30° and doing 2 passes with a hand ground fly cutter.


----------



## BaronJ (Jun 16, 2020)

Hi Guys,

An easy way to make some anvils would be to drill a suitably sized hole in some round material and thread it, and then mill just over half off the thread.  This will leave you with as many thread forms as you would need and the correct radius for that thread size.  When done they would look like those in the picture above.


----------



## goldstar31 (Jun 16, 2020)

BaronJ said:


> Hi Guys,
> 
> An easy way to make some anvils would be to drill a suitably sized hole in some round material and thread it, and then mill just over half off the thread.  This will leave you with as many thread forms as you would need and the correct radius for that thread size.  When done they would look like those in the picture above.



I agree with Baron and would go on to relate a bit in my past when I recall something similar but halving the test sample - with a hacksaw and filing it up and using a comparator.


Thanks John


----------



## holmes_ca (Jun 16, 2020)

I have just come into this conversation and I'm not sure if this can be of help, I recently received an email from Albany County Fasteners they have a new item in stock that may be worthwhile for some






						Albany County Fasteners: Product Search
					






					www.albanycountyfasteners.com
				




Edmund...........Alberta


----------



## HennieL (Jun 16, 2020)

ignator said:


> HennieL: I recently ran into this same issue with wanting to know that I made a correct thread without guessing that I  followed the procedure of feeding the compound slide the correct amount, as every description I read, I felt confusing.
> If I start with a sharp 60 degree cutting bit, and feed the compound that is set to 30 degrees off axis (~29.5), and I start with the round OD right at the described hardware diameter, I should feed the compound in the exact distance of the pitch of thread, as this is one side of the equilateral triangle. Ignoring the crest and trough radii. This should be a perfect sized thread (I understand a nut with the trough radii per the specification, will not fit on this cut thread)


Yes, this bugged me as well when I started threading some years ago, but I have come to the conclusion that flex in the machine, and unknown tolerances, are to blame for the constant need to make at least two shallow (0.05mm) cuts in addition to the calculated number and depth of cuts. Two things to keep in mind, that has helped me get closer to the required thread depth than just using the 0.6134P figure:

Don't thread on a bar that's at nominal diameter - look up or calculate the maximum and minimum values for the major diameter, and turn your bar to a diameter half way between these values - that puts you in the middle of the tolerance, and helps to achieve a good "in spec" fit.
Because I mainly use carbide insert cutters with radiused tips, and the radiused tip touches the bar when you zero the cross slide, I add the "thickness" of the tip radius and half of the fundamental deviation (_es_) to the calculated or tabulated thread depth - that usually helps to get me to within two or three "cleaning" passes of 0.05mm increments.
Hope this helps you to get closer as well.



BaronJ said:


> An easy way to make some anvils would be to drill a suitably sized hole in some round material and thread it, and then mill just over half off the thread.  This will leave you with as many thread forms as you would need and the correct radius for that thread size.



Thanks BaronJ - will have to give this some thought....


----------



## Mark Duquette (Jun 16, 2020)

You can use the shank of small drills to measure threads. If you know the size of the drill shanks it can be inputted into the formula to get the correct measurement.  I have used this method in the past with good results.


----------



## SmithDoor (Jun 17, 2020)

I started with drill bits
The down side is calculating the size.
When you buy a set thread wires it comes with chart. 

I have Excel work sheet for odd thread not cover by chart.

Most have a problem holding the wires
I use two rubber bands to hold the wires so put the mic the thread. Tape does not work and then need to clean the air after trying.

Dave



Mark Duquette said:


> You can use the shank of small drills to measure threads. If you know the size of the drill shanks it can be inputted into the formula to get the correct measurement.  I have used this method in the past with good results.


----------



## Richard Hed (Jun 17, 2020)

BaronJ said:


> Hi Guys,
> 
> An easy way to make some anvils would be to drill a suitably sized hole in some round material and thread it, and then mill just over half off the thread.  This will leave you with as many thread forms as you would need and the correct radius for that thread size.  When done they would look like those in the picture above.


Oh ho ho, you mean I've been doing this wrong all the time?  I was using "square stock" for threads.  Does you have a photo of what you are saying?


----------



## awake (Jun 17, 2020)

One trick for holding the wires is a dab of grease.


----------



## BaronJ (Jun 17, 2020)

Richard Hed said:


> Oh ho ho, you mean I've been doing this wrong all the time?  I was using "square stock" for threads.  Does you have a photo of what you are saying?



Hi Richard,
Have a look at post 17.


----------



## HennieL (Jun 18, 2020)

Richard Hed said:


> Oh ho ho, you mean I've been doing this wrong all the time?  I was using "square stock" for threads.  Does you have a photo of what you are saying?


Square stock, with a round shank turned on the one end to fit into the micrometre will probably be easier than trying to drill the round hole on a round bar...

Baron, Richard, Ken (and anyone else...), how would you then go about calculating the actual lathe cut thread depth of the bolt you are cutting, using this threaded half-round anvil? One cannot measure the actual half-round thread depth with wires or with a "V" wedge, and I would think the only way to confirm "within tolerance" (but still not actual thread depth value) would be to use a go-no go gauge to check the threaded hole before cutting it open... or perhaps with a 60° cone turned to an absolutely sharp point - but then one might as well just use that cone and the flat disk, as per my original post... or one can cut the "round hole" with a new set of taps and assume it is half-way within the specified tolerances... hmmm, I will have to think some more on this


----------



## Ken I (Jun 18, 2020)

Hennie, Typically I screw cut using a sharp point, go in at 30° and work to a truncated thread crest vis (R.H. Image) :-




I drew this M8 x 1.25 without reference to any tables so consider it illustrative.
I can do the trig but AutoCad is just so much less effort.
Once the sharp tip scores the O.D. you have your reference and can screw to depth - I normally keep a close eye on the diminishing crest flat as an indicator as I go.
Using carbide tips with radii (L.H. Image), the method is still the same but the trig gets a lot harder and therefore using AutoCad makes your life easier.
Also using Cad vs tables means you can adjust for anything not standard (i.e. what you have done {or want to do} wrong) such as tip radius, major diameter etc.
Regards,  Ken


----------



## goldstar31 (Jun 18, 2020)

Hennie


Does 'touching the bottom' as you describe it actually matter?
I always thought that it was the fit on the flanks that is inportant.

I recall the writings of K.C. Hart( as Martin Cleeve) not concerned with the  rounded crests. No it isn't in his book but in Model Engineer about his days  as a freelance nut and bolt maker after his redundancy. He wrote to a customer on this--- and received no reply!

For myself, and I insist being a 'bean counter' and certainly NOT and engineer or anything remotely connected, making a Quorn tool and cutter grinder. Apparently, it sorts the sheep from gloats(?) and noted that Professor Dennis Chaddock(  who incidentally made bits for atomic bombs), truncated the measuring thread on the leading bed bar.  Not that I wish to cause 'trouble


----------



## BaronJ (Jun 18, 2020)

goldstar31 said:


> Hennie
> Does 'touching the bottom' as you describe it actually matter?
> I always thought that it was the fit on the flanks that is important.



That was what I was taught and have always believed !  As long as the thread pitches and sizes match then the exact diameter isn't vital.


----------



## goldstar31 (Jun 18, 2020)

Hi John B
                  Thanks for the comments.
Thinking about the bottom of things, I have replaced the spindle Vee  belt on the 'new' Myford with a link belt which certainly doesn't reach the bottoms of the pulleys.

Absolute so and so to keep removing the links as the thing is stretching.  I 'invented' a little jig to twist the links and then found that I had a pair of curved, locking pliers- probably suture ones from my late wife- which does the job beautifully.   Hell, how do you patent a notch in a bit of sheet metal

So back to 'screw cutting', well yesterday I was removing metal using the gear box and the power cross feed to cut--- A SCROLL

It's all Pythagoras really and so old, hardly worth re-inventing.

Keep well

N


----------



## BaronJ (Jun 18, 2020)

goldstar31 said:


> Hi John B
> Thanks for the comments.
> Thinking about the bottom of things, I have replaced the spindle Vee  belt on the 'new' Myford with a link belt which certainly doesn't reach the bottoms of the pulleys.
> 
> Absolute so and so to keep removing the links as the thing is stretching.  I 'invented' a little jig to twist the links and then found that I had a pair of curved, locking pliers- probably suture ones from my late wife- which does the job beautifully.   Hell, how do you patent a notch in a bit of sheet metal



With great difficulty !



> So back to 'screw cutting', well yesterday I was removing metal using the gear box and the power cross feed to cut--- A SCROLL



Brave man !  Not something that I would like to attempt



> It's all Pythagoras really and so old, hardly worth re-inventing.
> 
> Keep well
> 
> N



Stay safe.


----------



## HennieL (Jun 18, 2020)

goldstar31 said:


> Does 'touching the bottom' as you describe it actually matter?
> I always thought that it was the fit on the flanks that is inportant.



No, not actually, but... 

The fit on the flank is important as the force is transmitted through the flanks, and not through the crest and trough "bottoms".  The only reason why "touching the bottom" really matters to me is that it is an easy way that one can directly measure the cut depth whilst cutting the thread. Most manufacturers of threading inserts publish lists of cutting depths recommended for their inserts (but they all differ from each other, as the dimensions and tolerances on their inserts vary from each other as well), and using a direct depth measurement would be the easiest way to use this data to confirm if you are deep enough. Measuring with wires is tricky and expensive if you want to use the "proper" wire thicknesses and tight tolerances. Using small drill bits are a lot cheaper, but require some calculations, one need to have access to the published thread pitch diameters, and using the three drills is still tricky.

My original intention with this post was to just share an idea that could make the measurement of thread depth easier and/or cheaper, in order to confirm if the thread that you are cutting is within the specified tolerances and depth before removing the threaded bolt from the lathe - at least more accurately than using a nut... This discussion has brought up some very interesting points and ideas, and I appreciate everyone's comments - hopefully this can also help other people to understand the science/technology of cutting screw threads on a lathe.



BaronJ said:


> An easy way to make some anvils would be to drill a suitably sized hole in some round material and thread it, and then mill just over half off the thread.  This will leave you with as many thread forms as you would need and the correct radius for that thread size.  When done they would look like those in the picture above.



Baron, I made a "quick and nasty" 1.5mm pitch anvil this afternoon as per your suggestion, as a "proof of concept" of your idea, and made the following observations:

One would have to make such an anvil for each diameter and for each pitch in each diameter. I initially thought that one would only need one (largest) diameter "half-a-hole" for each pitch, but as soon as I measured a smaller diameter bolt with the same pitch in the larger hole, it rotated away from the cone point on the other side of the micrometer and gave false readings.
One would have to figure out a way to ensure that the wedge or cone on the other side of the micrometer actually aligns with a thread trough, and not with a crest (or hit the side of the thread) - any suggestions on this, other than making it only two threads wide??
O, here is a photo of the anvil (with a 1.5mm pitch in a MF14mm "half-a-hole") - will need to make it MUCH smaller when I make the real ones...


----------



## L98fiero (Jun 18, 2020)

HennieL said:


> O, here is a photo of the anvil (with a 1.5mm pitch in a MF14mm "half-a-hole") - will need to make it MUCH smaller when I make the real ones...


Instead of using a tapped hole, why not use a commercial bolt, mill/ grind away enough to produce a flat surface and leave the crest of only two threads and you'd have basically the same as the commercial thread mics, albeit somewhat cheaper and again, you have to set the mic at zero for each thread. Each anvil would have a range of useful pitches just like the commercial ones.


----------



## Tim1974 (Jun 19, 2020)

Just cut the thread job dun lol omg


----------



## BaronJ (Jun 19, 2020)

Hi Hennie, Guys,

To be honest, after thinking about it, a set of good quality thread gauges would probably do the same job.  Realistically you only need to ensure that the thread on one half fits the other half properly.

Making anvils as I suggested is really no different to a thread gauge.

Using your method of a mic with a 60 degree wedge on one side and a flat plate on the other would work just as well !


----------



## Woodster (Jun 19, 2020)

I use Mitutoyo Thread Mic's daily. You will need different anvils for various thread ranges. They have a 60deg Vee on the bottom and a 60deg cone on the top. Measurement is simple, as you just measure the pitch or effective diameter. The Vee should be free to rotate and you adjust the height with the bottom thimble to zero them.


----------



## HennieL (Jun 19, 2020)

Tim1974 said:


> Just cut the thread job dun lol omg


Thank you for your exceedingly helpful and positive contribution Tim1974 - I trust it boosted your little ego somewhat 

To all the rest of the people who actually did contribute towards making this an interesting discussion: I made a second half-a-hole" anvil with a M6-1.0mm pitch thread, cut by die (as was the first one), and it came out very nice - except that yet again the cone on the other side of the micrometer did not line up with the bottom of the thread, thus making it impossible to zero. I'm thus marking this one up as "not a practical solution", and will use the wedge and flat disk on all threads of 1.5mm pitch and larger, and an appropriately sized small drill in the "one wire" method for smaller pitched threads, as stated earlier in this discussion.

So, let's put this thread to rest - I certainly learned a lot more about screw threading that I learned during the 5+ years that I have been cutting threads with a lathe the way Tim suggested... 

Hopefully someone else can also pick up something useful from this discussion.


----------



## BaronJ (Jun 19, 2020)

Hi Hennie,

The issue with the anvil threads not lining up is because both sides of the anvils are fixed by the bore on the tail of the mic.  If the anvil on one side (the spindle side) is made to have a flat face that is in a recess a little greater than the thread pitch,  it will float and you will be able to get a zero.

Unfortunately I don't possess a mic with an inter changeable or removable anvil.  However it should work with a pair of floating anvils particularly with a digital mic that can be zeroed at any point.

Digressing slightly, I too have learnt from these posts.


----------



## goldstar31 (Jun 19, 2020)

BaronJ said:


> With great difficulty !
> 
> 
> 
> ...



'Faint heart never won fair lady!'
Meanwhile, it was a doddle . My first sort of decent-ish lathe was a Pools Major of which few will know.
It had a lead screw which ent to another one at the rear and had a pair of taper clamps. and gears which actually both faced and----hey hey cut tapers.
Unfortunately, it was a line shaft job but was worn out having been on 'War Work'. The bed would only turn  bananas.


BaronJ said:


> Hi Hennie,
> 
> The issue with the anvil threads not lining up is because both sides of the anvils are fixed by the bore on the tail of the mic.  If the anvil on one side (the spindle side) is made to have a flat face that is in a recess a little greater than the thread pitch,  it will float and you will be able to get a zero.
> 
> ...



John
         Instead of a conventional  hand held  'mike', why not a dial one and  make  the numerous anvils to change from the thingy will a ball? One of my clock gauges has an elephant's foot.

????

Norman


----------



## HennieL (Jun 19, 2020)

BaronJ said:


> The issue with the anvil threads not lining up is because both sides of the anvils are fixed by the bore on the tail of the mic.  If the anvil on one side (the spindle side) is made to have a flat face that is in a recess a little greater than the thread pitch,  it will float and you will be able to get a zero.


 Thanks for the thought, Baron, and yes, that might work. I've also considered the suggestion made by L98fiero in post 36 to grind a piece of bolt down to only two threads wide (with one trough between two ridges). I initially thought that was not a very good idea as the dimensions/tolerances on nuts are different from that on bolts, but after some further consideration think that it might work to make a "thread insert" that one can either epoxy into a recess in the anvil, or make various inserts of different pitches and lock them in a recess with a small grub screw - then one can manually line up the thread with the fixed cone on the other side. One could obviously also just grind a nut instead of a bolt, or drill a hole and tap, as per your original suggestion, and then grind it down to an insert size...



goldstar31 said:


> ...it was a line shaft job but was worn out having been on 'War Work'. The bed would only turn  bananas.



Norman, you made my day with that statement - I really laughed out loud when I read it. Thank you, sir


----------

