# Diameter for Threading.



## kcmillin (Jun 20, 2010)

I am having difficulty trying to find the correct diameter to make stock to get it ready for threading.

Is there a rule of thumb here, like a certain percent less that the major diameter?

I always have to make parts smaller than I think to get the die to start.

Also, can this be applied to making threads on the lathe?

Example, If I wanted to make a 1/2 13 thread in an aluminum bar, would I start with a 1/2 inch diameter, or something smaller?

Kel


I guess my question is, Is there an Index for threading like there is Tapping?

Kel (again)


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## websterz (Jun 20, 2010)

The major diameter of the thread is the size of your material. a 3/4-10 requires 3/4" material, a 1/2-13 requires 1/2" and so forth. This is for male threads of course, if you are tapping holes get a chart.


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## Blogwitch (Jun 20, 2010)

Kel,

Webs is correct on that.

If you are cutting with a die, you should preferably use a split one. This allows you to leave the two outer screws in the diestock loose, and tighten the centre one into the split. This effectively makes the die cut larger, and so should start cutting a lot easier as well. If you look at either side of the die thread, you should find one side has a fairly large lead in taper, this goes towards the job, again to help start the thread off. 
If needed, you could put a small lead in taper on the workpiece, which I do as a matter of course, that way you will end up with a thread that starts much easier into the hole than it would do if it was square ended. After you have cut the first run on the thread, you gently unscrew the centre screw and tighten up the two outers, then take another cut. What you are doing is bringing the thread size down gently until it is a perfect fit into the internal threaded hole, you might have to take it down in a few stages until you reach the good fit size.

This tapered lead in does have one disadvantage. It leaves the threads at the end of the cut tapered as well, so they might not screw all the way into the threaded hole. The way around that is to cut a recess into the workpiece to the thread core dimension, this should be the same width as the tapered bit left after cutting the thread, so allowing the thread to go all the way into the hole. Another way is to turn the die around in the diestock, and use the non tapered side to cut the tapered end at the end of thread cut, so you end up with a parallel thread all the way along.

Confusing ain't it

Bogs


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## steamer (Jun 20, 2010)

If you go through this thread

http://www.homemodelenginemachinist.com/index.php?topic=4025.msg41557;topicseen#msg41557

There's a chart with tap drill sizes....handy for the shop... as an attachment to one of the posts....that may help with tap drill sizes.

Dave


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## Tin Falcon (Jun 20, 2010)

Kel:
 the littlemachineshop.com threading chart also gives major diameters as well as minor diameters as well as tap drill and clearance drill sizes. http://littlemachineshop.com/Reference/tapdrill.php
any machineries handbook will have more in depth charts. If you do not have a copy you can download a copy from archive.org
http://www.archive.org/details/cu31924003638024 
Tin


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## mklotz (Jun 20, 2010)

The formula for the size of the tap drill, which is also the size of the hole to bore when single-pointing internal threads, is:

TD = MD - 0.013*DOT/TPI

where:

TD = tap drill diameter
MD = major diameter
DOT = desired depth of thread expressed as a percentage
TPI = threads per inch of the desired thread


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## Twmaster (Jun 20, 2010)

It sounds like Kel is looking for info on male threads. 

I read somewhere, and use this in my shop, go 95% of the diameter. So if you wanted to cut 3/4" threads with a die you'd turn the part to .713 than thread.


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## kcmillin (Jun 20, 2010)

Thank you all for your information.

What my real problem is in making male threads. I have all the charts for tapping. But whenever I try to make threads using the lathe or with a die, it never works how I want it to. When I am using a die, it wont start unless I go .010 smaller than the diameter of a screw with the same threads. Anything larger and it just ruins the threads. 

I can see my problem is more than likely poor quality dies. I should invest in better dies.

However, when I try screwcutting on the lathe, the threads are always the wrong size, and dont fit the nut. 

When I measure an off the shelf 1/2 13 bolt, the outer diameter is .490.???

Sometimes when I start with say .5 inches, when I am done it measures larger than what I started with, like .515.???

I have put this off long enough, I see many people make threads on the lathe with no problem, I should be able to do it too.

So, I think I need some information on threading. 

Is one half the difference between the major diameter and minor diameter, the depth of cut?

sorry for such a weird question. It seems I am the only one with trouble making male threads work. I am most certainly doing something wrong.

Kel



			
				Twmaster  said:
			
		

> It sounds like Kel is looking for info on male threads.
> 
> I read somewhere, and use this in my shop, go 95% of the diameter. So if you wanted to cut 3/4" threads with a die you'd turn the part to .713 than thread.



Thats it. Thanks Twmaster.


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## stevehuckss396 (Jun 20, 2010)

See if this helps. Also chamfer the end of the stock to help get the die started. Buy a good quality die. The cheapo's will make more damage than thread.


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## zeeprogrammer (Jun 20, 2010)

kcmillin  said:
			
		

> It seems I am the only one with trouble making male threads work.



I seriously doubt that.

I find it interesting that some lathes are set up to do thread cutting and others are not. That, for me, is a clue.

One of the first things I asked about when I joined the forum is about thread cutting. And while there were some very helpful responses..I have yet to do any serious thread cutting other than with a die.

Keep at it. I'll be interested in finding out what you learn. I could use it.


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## mu38&Bg# (Jun 20, 2010)

For external threads the Maximum for a tight fitting Class 3A thread is usually the major diameter. The ranges for a 1/4-20 are as follows.

Class 1A
Major diameter .2367-.2489

Class 2A
Major diameter .2408-.2489

Class 3A
Major diameter .2419-.2500

For most work you'll be using 2A fits. The nominal is .245. That works out to 2%, but this figure is variable being a larger percentage for smaller threads(3% for 2-56), and smaller percentage for larger (1.4% for 1/2-13)threads.

If you turn the stock to the major diameter you'll end up with sharp threads that may not finish well, they tend to roll over, and will be tight in a tapped hole.

I shoot for the middle of the given range for the specification on the thread. If it needs to be tight I make the tapped part first and sneak up on a nice fit on the external thread.

The Machinery's Handbook is handy for this kind of info. When I worked in aerospace every thread and fastener had a class callout.

Greg


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## websterz (Jun 20, 2010)

kcmillin  said:
			
		

> ...When I measure an off the shelf 1/2 13 bolt, the outer diameter is .490.???...



That is only a difference of .005" per side. The material may vary that much (or more). I was taught to always use the full major diameter. If you are using a die to cut the threads you should be using a split die opened up to cut over-sized, then gradually tighten the die until it is on size. A hex die is generally used for chasing existing threads and will most often result in a ragged, undersized thread if used on full sized material.


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## rake60 (Jun 20, 2010)

I really enjoy single point threading on the lathe so I rarely use dies.

As Greg has noted, material that is the nominal size will result in a Class 3 
thread. That is at or very close to 100%. I have done a few jobs at work where
that was specified, but not many. 

A Class 1 fit is perfectly acceptable for almost any thread in hobby machining.
A 75% to 80% thread is usually sufficient. 
If your finished threads have sharp points at the crests, your OD was too big.

Rick


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## kcmillin (Jun 20, 2010)

rake60  said:
			
		

> I really enjoy single point threading on the lathe so I rarely use dies.
> 
> Rick



That's where I want to be.  

Thanks Rick. Yes I have been seeing sharp points on the crests, I will most definitely be using the method you described. 

A side note to this. I have been trying (recently) threading using a spindle hand crank. I feel I have more control this way, but the chips (swarf) is most irregular. Everything is sharp and setup properly, but the aluminum is lumpy looking, like regular swarf but with a nasty case of acne. Almost like scooping ice cream. The cut still looks clean though. I am using a pre sharpened bit that is flat on top. Should I grind this bit that is supposedly made for the job and put a rake on it? Is it better to cut under power? 

Kel


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## Twmaster (Jun 20, 2010)

Kel...

Are you using a lubricant? Is the aluminum perhaps a gummy nasty to cut variety?


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## shred (Jun 20, 2010)

This thread has the table I use (based on one of Marv's formulas, backed up by data in Machinery's handbook-- basically it pre-cuts the flats at the top of the thread, which makes threading with a die much easier since you don't have to plow off that metal during threading). It works well.

http://www.homemodelenginemachinist.com/index.php?topic=6487.0

Direct link to table PDF: http://www.homemodelenginemachinist.com/index.php?action=dlattach;topic=3554.0;attach=1602 Turn stock to "ext thread dia" and go to town


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## kcmillin (Jun 20, 2010)

I love this place.
 ;D ;D


Thanks Shred

Kel


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## Maryak (Jun 21, 2010)

kcmillin  said:
			
		

> I always have to make parts smaller than I think to get the die to start.



Kel,

Most of the cheaper thread dies from our Asian cousins have no tapered lead in, hence the need to reduce the diameter to get them started and to allow for the "roll up" when the are deforming, (rolling), the thread rather than cutting it. Good quality dies have this lead in and cut more easily and straighter. In any event it always pays to make a chamfer on the leading edge to make starting a die easier.

Hope this helps

Best Regards
Bob


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## rake60 (Jun 21, 2010)

Bob makes another valid point.

I have a VERY CHEAP, carbon steel tap and die set that I bought at a local surplus
store for $8 USD. It was Pacific Rim manufactured.






Cheap as it was if you look closely at the dies you can see the difference between the
starting side and the back side. The starting side has a lead in chamfer on all of the lands.






On the back side the lands come to sharp points.






If you attempt to start any die from the back side the result will be frustration.

Rick


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## mklotz (Jun 21, 2010)

Some time ago I drew a detailed diagram of a 60 deg thread form and worked out the formulae for virtually every dimension of interest associated with this thread form. I've attached this to this post.

I encourage all of you to copy this diagram and put it in your shop notebook. It will answer many of the dimensional questions you may have about threads.

If you look at it, you'll see that the the small flat on the top of the thread, which accounts for the fact that the measured diameter is less than the nominal, is made by cutting off a small triangular section whose height is H/8 where H is the total height of the thread.

Near the bottom of the diagram you'll see a formula for 'U' labeled the "diametrical undercut from nominal diameter". 

U = [sqrt(3)/8]/n = 0.2165/n
where:
n = pitch expressed as tpi

The nominal diameter of the stock should be reduced by this amount before threading.* I believe that what Shred has done in his table is calculate the resulting diameter for common bolt sizes by subtracting U from the nominal diameters. (Shred, please correct me if I'm off base here.)

For some reason (perhaps an innate aversion to math), machinists seem to like tables better than formulae. I'm just the opposite. I can't (and don't want to) memorize a whole table but it's easy to memorize a formula. Knowing the formula allows me to figure out what I need even if the table is not to hand but, more importantly, it makes it possible to deal with oddball threads that aren't covered in the table.

--

* While this is technically correct, I have to admit that I seldom do it. I've threaded literally hundreds of things by simply applying good split dies to nominal diameter stock. I believe K's problem is due to some other problem - perhaps bad dies.


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## Lakc (Jun 21, 2010)

I actually have a nice set of dies from the mid 1980's, MATCO variety. They are not sized to cut class 3 threads, even starting the taper side. If I don't have a major diameter around the bottom of class 2 or top of class 1, the die has too much to cut. This basically extrudes the metal between the threads and you have a much bigger diameter after threading. If your threading with dies you just need to experiment to see what your dies like best. 

 For those who like threading on the lathe, I would wager you probably don't want to make a class 3 thread if you only have a class 1 tap. :


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## arnoldb (Jun 21, 2010)

Kel,


> Everything is sharp and setup properly, but the aluminum is lumpy looking, like regular swarf but with a nasty case of acne. Almost like scooping ice cream. The cut still looks clean though. I am using a pre sharpened bit that is flat on top. Should I grind this bit that is supposedly made for the job and put a rake on it? Is it better to cut under power?


If you're talking about the swarf, that would be pretty normal from my experience for single-pointing, especially on coarser threads if you're using direct infeed off the crosslide. If you use your topslide set to 29 1/2 degrees (I presume you are doing 60 degree thread form - UNF/UNC/Metric) this will be slightly less so, but single pointing on coarser threads can still be a pretty "brutal" process as you are removing a chip from a relatively large "irregular" contact area resulting in the ice cream scooping. If you mean the thread on the workpiece itself is looking like that, then I'd recommend you take another pass at the same infeed setting. For me this works very well, especially if I'm close to final dimensions on the thread; I sometimes do up to four passes on the same infeed setting for the last couple of thou; this smooths out the thread a lot.

I don't recommend that you put a rake on a pre-sharpened bit, as it actually changes the effective angle of the thread. With some care, it is possible to hand-grind a bit with rake for thread cutting, but the side-angles on the tip has to be adjusted a bit. OK, having said that - it is minimal, and I'm guilty of having added some rake without bothering to adjust the side angles; for a lot of the work we do as modelers, no-one will ever know! (Except that everyone now knows I do it :big For coarser threads, or where you have to match existing threads, this has more of an impact - for example, even though I cheat, I've been meticulous on matching the 12TPI 55 degree Whitworth thread of my lathe's spindle nose when I made my collet chuck, as well as the bit used for cutting the ACME thread form for the rotary table worm and gear cutter.

I don't know what lathe you have, but hand-cranking is just dandy if you're comfortable with it. It's a great way to start single-pointing, as you get to feel how much force it needs to turn the chuck and in turn you can start compensating for the depth of the cut; the deeper you go on a thread, the smaller your infeed should become. 

There are many differing opinions on the methods for single point thread cutting; you have to find the way that suits your habits through some trial and error.
From my own learning experience I initially found it best to start with small infeeds and many passes - it takes time but produces less heartache ; later on I started getting a feel for how much my lathe and toolbits could stand to take heavier cuts and still get good results. I prefer HSS toolbits for threading; I have a couple of tungsten tipped bits lying in my collection with their sharp tips broken off :

Cutting fluid is IMHO very important though - even for threading with a die. The only metals I cut dry is brass and cast iron, and I even found I get improved finishes on single-pointing brass if I use some of my favorite cutting fluid for the final passes. 

One thing's for sure though - even after cutting quite a couple of threads, it is always a treat to see that thread slowly emerging from a workpiece! - At least for myself and quite a couple of other HMEM members ;D

Regards, Arnold


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## shred (Jun 21, 2010)

mklotz  said:
			
		

> Some time ago I drew a detailed diagram of a 60 deg thread form and worked out the formulae for virtually every dimension of interest associated with this thread form. I've attached this to this post
> ....
> The nominal diameter of the stock should be reduced by this amount before threading.* I believe that what Shred has done in his table is calculate the resulting diameter for common bolt sizes by subtracting U from the nominal diameters. (Shred, please correct me if I'm off base here.)


That's exactly what I did. I like tables because I can stick them up in the shop near where I use them (this one is right over the lathe headstock) and refer to them instantly... "hmm... 7/13-24... that needs to be .456..." and away I go (rather, I usually take anything close on the larger side unless I'm being picky).

Lately I've been writing the formulas down on the table in case I encounter an item not on the table. I've got a shop calculator handy, but it's still a few moments of punching keys and then it turns itself off before I want to refer back to it


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## owenh (Jun 21, 2010)

thank you Shred very useful table.


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## Lew_Merrick_PE (Jun 22, 2010)

kcmillin  said:
			
		

> I am having difficulty trying to find the correct diameter to make stock to get it ready for threading.
> 
> Is there a rule of thumb here, like a certain percent less that the major diameter?
> 
> ...



Kel,

The designated major diameter is (almost always -- there are a couple of very strange exceptions, but they are extremely uncommon) the *maximum* major diameter of the screw thread. The *minimum* major diameter depends on the type of thread and its fit class. Acme threads have a fairly straightforward calculation set (see: _Acme Thread Data Sheets_ posted at http://www.scribd.com/Lew Merrick for the equations). 60°-V threads get a bit more complicated. There is a tolerance based on the maximum major diameter of the thread and an allowance calculated on both the circumference and tension area of the thread that create the minimum major diameter. This is why an attempt to plot the values from the tables in _Machinery's Handbook_ or _FED-STD-H28_ generally fail.

*I* live and work in that part of the universe where compliance with standards is unrelenting. The odds are very great that *your work* will never be held to such standards. The _general rule of thumb_ reduces to using a *minimum major diameter* of *95% of the maximum major diameter* for major diameters greater than #10 (.190). Increase that to *97% of the maximum major diameter* for major diameters #10 and smaller.

Additionally: (1) I highly recommend making a tool holder that allows you to use modern insert threading tools as they have really improved over the past few decades; and (2) that you get a set of thread measuring wires and use them to check the *pitch diameters* of your threading work. These will save you a world of heartache.


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## Brownstowmin (May 22, 2022)

I rarely use dies because I love single point threading on the lathe.

Material that is the nominal size will result in a Class 3 thread, as Greg mentioned. I am going to share this thread https://www.homemodelenginemachinist.com/threads/diameter-florida lottery-for-threading.9016/ to my kith and kin who want to know about that. That is 100 percent or pretty close to it. I've worked on a few jobs where such was required, but not many.

In hobby machining, a Class 1 fit is entirely acceptable for practically any thread.
A thread of 75 to 80 percent is usually sufficient.
If the crests of your final threads have sharp tips, your OD was too large.

Rick


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