Measuring thread depth without wires - your views, please

[BaronJ]

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]

With great difficulty !



Brave man ! Not something that I would like to attempt



Stay safe.

'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.

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.

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]

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...

...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

 

[Ken I]

Just to flog this dead horse a few more steps.

I was cutting a thread last night - M12 x 1.5 :-

Holding the three wires is a job requiring an extra pair of hands - so I use a piece of modelling clay to hold the wires.

Since I was replicating a thread I had to hand, I simply took three pieces of Ø1.5 piano wire and measured the existing for calibration purposes - no calculations whatsoever.

Regards, Ken I

 

[Richard Hed]

Just to flog this dead horse a few more steps.

I was cutting a thread last night - M12 x 1.5 :-
View attachment 147527
Holding the three wires is a job requiring an extra pair of hands - so I use a piece of modelling clay to hold the wires.

Since I was replicating a thread I had to hand, I simply took three pieces of Ø1.5 piano wire and measured the existing for calibration purposes - no calculations whatsoever.

Regards, Ken I

I do a lot of outside threads, odd sizes, never if I have a die for the size, but I have never used the three wire system--don't intend to either. Will eventually get one of those micrometers for threads but in the mean time, I use a mating part and go VERY carefully near the end. Occasionally I do a "screw up" and have to make a second one, but that has very little to do with measuring the threads usually. However, for those of you who use that 3 wire system--good on ya.

 

[kf2qd]

Your method would work, but you have to take into account any variation in the OD of the thread. Thread wires use just the thread as a reference, your method uses one side of the thread, and on the other side the reference is the OD, so the OD measurement must also be compared to come up with a number that gives you the thread Dimension.

You might try cutting a nut in thirds and using it as a comparison tool.a larger nut with the sme thread pitch would give you a tool to compare your thread to a known good thread.

 

[stanstocker]

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:

Sewing needles are quite inexpensive at least in the US. The goal is to have three "wires" that fit well in the thread and are of a known single diameter. Commercial thread wires are not the only way to do good three wire measurement. Another option is a cheap digital microscope with measuring software.

Music wire is often available in 36 inch lengths at hardware stores, and clock repair parts vendors sell nice quality wire for replacing clock pivots in 0.1mm increments.

Perhaps none of these are possible options in South Africa, but maybe they will give you some ideas.

Cheers,
Stan

 

[colby15642]

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:
View attachment 117086

This is a close up of the 60º cone and wedge anvils:
View attachment 117087

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).
View attachment 117088

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.

View attachment 117089

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.

View attachment 117090

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

This is how this clumsy machinist in Pennsylvania holds the three thread wires:

https://www.amazon.com/FLEXBAR-1670...dc-9fdd-7001def7c3a4&pd_rd_i=B00DHN0BPG&psc=1

 

[Mike Ginn]

Colby
Please check your amazon link - seems to be off topic!

I find the 3 wire approach, although accurate, a pain to use and as a result I never use the wires be they needles or other wire. If I am cutting single point external thread then I always make a nut (internal thread) first using a tap. OK I might not have the correct tap (No 1inch BSP tap in my drawer!) but most of the time it works for me.

Mike

 

[ignator]

Colby
Please check your amazon link - seems to be off topic!

I find the 3 wire approach, although accurate, a pain to use and as a result I never use the wires be they needles or other wire. If I am cutting single point external thread then I always make a nut (internal thread) first using a tap. OK I might not have the correct tap (No 1inch BSP tap in my drawer!) but most of the time it works for me.

Mike

On my computer it brings up this;

 

[TSutrina]

I have seem a micrometer my dad own, he worked for a screw manufacture. The tips were ground into a 60 degree triangle that wasn't very tall one side had one and another two. Works for a single thread per inch measurement.
That suggest that getting three 3mm triangular files and use them as you would wire. That works for up to about 8 thread per inch.

 

[colby15642]

On my computer it brings up this;

Yes. These soft plastic bits hold your three thread wires on the micrometer spindle and anvil. Just like having extra fingers that never become tired! These sets are sold by most USA machinist supply houses and also by Amazon and probably others.

 

[Mike Ginn]

They are still a pain to use even with the plastic holders.

 

[mcostello]

I use a strip of sponge from the dollar store.

 

[krypto]

Since the topic has veered off into holding the wires, here's a shot of the thread wires in use.

The two bottom wires are attached to the micrometer with the Flexbar holder. You set them on the threads first, then use your free hand to hold and set the remaining wire while closing the micrometer. It's very easy to do assuming you have all your fingers and can work a micrometer with one hand. It's certainly much easier than grease, sponge, clay etc.

Thread wires are very inexpensive and probably the cheapest way to measure threads. Following the currency equivalent of the OP, in the USA a set of wires is two big Macs? I haven't been in that place in years...

You can also use thread measuring triangles with a micrometer. These come with their own rubbery holders, but are much more expensive and seemed harder to use on fine threads, although I haven't used them that much.

 

[michael.kilpatrick]

https://www.applegate.co.uk/suppliers/o-vee-spring-gauges-ltd-1080713These are much more better, i bought some on ebay years ago