Calculating Hole Size For Odd Thread Sizes

[lkrestorer]

I have been searching the internet for a simple formula to calculate a hole size to use for internal threading of odd sizes with not much luck. As an example: if I were to have a threaded tube that I wanted to make a matching cap for and the tube O.D. was .800", how would I calculate the inside diameter of the cap I was making before I threaded it?

My guess is that there is one formula that would work with any diameter. I can't find it. Will someone help?

 

[danallen]

Subtract the thread pitch from the nominal diameter.

 

[lkrestorer]

Forgive my density but using a .800" tube with 32 TPI would need what diameter hole?

 

[Scott_M]

Here is the formula for obtaining tap drill size, right off of my CTD chart.

So for .800 and a 32tpi with 100 % thread ( use more like 75-85% )
.01299x 100=1.299/32=.0405 ( this is double thread depth )
So .800"-.040" =.760" tap drill size.


Scott

 

[danallen]

.800 - .031 = .769
OD minus thread pitch in thousandths

 

[lkrestorer]

Ok, using a known standard of a 1/4-20 bolt and using a 75% thread: 0.01299 x 75 = 0.97425 / 20 = 0.049 then 0.250 - 0.049 = 0.201 which is a #7 drill.

Next, 0.800-32 with a 75% thread: 0.01299 x 75 = 0.9475 / 32 = 0.0296 then 0.800 - 0.0296 = 0.7704

This appears to work out but, danallen, you came up with 0.769 and that's pretty darn close but .... "OD minus thread pitch in thousandths" (?) where did the ".031" come from or is it a typo for .032? I realize it is just a matter of 1 or 2 thousandths difference in hole size but I want to clarify it.

Thank you very much for the help. I do appreciate it.

 

[danallen]

No typo. The decimal equivalent of 1/32, the thread pitch in question, is .0312. I dropped the .0002

 

[lkrestorer]

OK, I think I've got it.

Back to the 1/4-20 as a standard: You are taking the 20 TPI as 1/20 so 0.250 - 0.05 = 0.200 and the closest size drill is still #7.

The other formula does pretty much the same thing with a lot more math. This one fits in the brain with some room to spare.

Thanks for the clarification.

 

[GrahamJTaylor49]

You could try looking in the "Machineries Screw Thread Book" It has never failed me.

 

[shorrog]

Hi guys, just checking but does the thread depth not depend upon the thread profile? So for threads with an angle of 60 deg eg Metric , Cycle, UNT etc it works but with threads such as BA, BSF where the angle is not 60 deg its not so accurate.
Graham

 

[danallen]

Hi guys, just checking but does the thread depth not depend upon the thread profile?
Graham

Correct. Subtracting the pitch from the nominal OD only works for 60 degree threads either metric or American.

 

[nealeb]

For a given thread pitch and thread form (e.g. UN series, Whitworth form) the thread depth is always the same, whatever the diameter. So, a 1/2x32TPI thread has exactly the same thread depth as a 1/4x32TPI. If you can't find the tapping size drill for a given hole (because it's non-standard, for example) then look in the table for a same-TPI thread of the same family (UN, Whit...) and find the difference between overall diameter and tapping drill size. Then use that to find the tapping size drill for your non-standard thread - take away the "difference" just found from the OD of the thread and that's your tapping size drill. Minimal arithmetic!

 

[SmithDoor]

I like picking a even number from the lathe.
If lathe has a 8tpi lead screw them pick 8,16,24,32 and 40tpi I do need to use a thread dial.
If lathe has a 6tpi then pick 6,12,18,24,30 and 36.
This trick also works for metric too.

Basically multiple the leadscrew by 1x,2x,3x,4x,5x and 6x and do not need to uses a thread dial

Dave

I have been searching the internet for a simple formula to calculate a hole size to use for internal threading of odd sizes with not much luck. As an example: if I were to have a threaded tube that I wanted to make a matching cap for and the tube O.D. was .800", how would I calculate the inside diameter of the cap I was making before I threaded it?

My guess is that there is one formula that would work with any diameter. I can't find it. Will someone help?

 

[Gene Pizzoli]

I have been searching the internet for a simple formula to calculate a hole size to use for internal threading of odd sizes with not much luck. As an example: if I were to have a threaded tube that I wanted to make a matching cap for and the tube O.D. was .800", how would I calculate the inside diameter of the cap I was making before I threaded it?

My guess is that there is one formula that would work with any diameter. I can't find it. Will someone help?

 

[Gene Pizzoli]

The ID of the matching part would be based on the number of threads per inch assuming the standard 60 degree thread. The pitch diameter is the standard method of measuring a thread. This would be the match up for an internal thread plus allowing for class of fit. Considering the limited measuring devises for the average hobby guy you will find when you get close the what you what, it will be cut and try. Hope this helps, Gene

 

[Richard Hed]

The thing with the 60deg forms is that all three sides of the form, form an equilateral triangle. That is, the three sides are exactly the same. what three sides? well the two side of a cut are two and the third is peak to peak (or vally to valley.) If you form a right triangle to any one of the sides to a peak opposite from it, you can use Pythagorean Theorem to calculate the depth of the cut. Actually you have two o0ptions one is to actually cut a bit deeper by a few thou or to make you OD a bit smaller in the first place than what is nominal. You can also use trigonometry (my dad taught me triggernometry which is what I thot I was going to lern in hiskool), but unless you dig trigonometry, I won't bore you with it.

 

[awake]

Here is the program that I keep on an old TI-8X calculator; it asks for the diameter and the thread pitch, and it returns the diameter for OD and ID, along with compound movement needed to achieve different thread engagement. Note some assumptions: this assumes the compound is set at 30° and that the tool tip is properly sized for the thread pitch. Note also that even though it is written with the assumption of imperial units, you can use it for metric so long as you convert the measurements - which you can actually enter as the input, e.g., 10/25.4 for a 10mm diameter, 25.4/1.5 for a 1.5mm thread.

TI-82 Program: THREADS
Fix 4
Degree
ClrHome
Input "DIAMETER: ",D
Input "TPI: ",P
(cos 30)/8/P->X
ClrHome
Disp "DIAMETER"
Disp " INSIDE:"
Disp " OUTSIDE:"
D-9*X->I
D-X->O
Output(2,11,I)
Output(3,11,O)
Disp “”
Disp "COMPOUND"
Disp " .875:"
Disp " .750:"
Output(8,2," .625:")
Output(6,11,.875/P)
Output(7,11,.750/P)
Output(8,11,.625/P)

 

[lkrestorer]

I was making an arbor for machining features on the body of an engine project that I've started and I wanted to test this. I wanted to thread the one end of the shaft and make a nut for it – but wanted to do it the right way. I chose the simplest method posted here (shaft diameter - 1/threads per inch). In my case I had a shaft (the arbor) that was 1.249” and I decided on 20 TPI (1.249 - 1/20 = 1.249 - 0.05 = 1.199). So a 1.199” hole size – and it worked out very nicely! The threads fit and there is no “wobble” in the assembly (actually my hole was more like 1.2”). The body of the arbor HAD to be exactly 1.249 to fit the bore of the engine so I started with that and wondered how to size the nut to lock it all together. I could have turned the end of the arbor down to 1.00" and used the standard drill size for that - but that wouldn't be any fun and wouldn't teach me anything new. I did use the shaft (that I threaded first) as a no-go gauge and snuck up on the finished thread testing after each cut.

I did not have to be concerned with thread form or class. Standard 60 degree threads were all I needed. For all "normal" holes it is a pretty straight forward drill n' tap adventure. My situation is something I have run into a couple of times and had a miserable time figuring out - such as making a threaded cap for a small tube.

Next I'll have to try the formula that Scott_M posted and see how I like that. More play time!

This was fun. Thank you all.

 

[L98fiero]

I was making an arbor for machining features on the body of an engine project that I've started and I wanted to test this. I wanted to thread the one end of the shaft and make a nut for it – but wanted to do it the right way. I chose the simplest method posted here (shaft diameter - 1/threads per inch). In my case I had a shaft (the arbor) that was 1.249” and I decided on 20 TPI (1.249 - 1/20 = 1.249 - 0.05 = 1.199). So a 1.199” hole size – and it worked out very nicely! The threads fit and there is no “wobble” in the assembly (actually my hole was more like 1.2”). The body of the arbor HAD to be exactly 1.249 to fit the bore of the engine so I started with that and wondered how to size the nut to lock it all together. I could have turned the end of the arbor down to 1.00" and used the standard drill size for that - but that wouldn't be any fun and wouldn't teach me anything new. I did use the shaft (that I threaded first) as a no-go gauge and snuck up on the finished thread testing after each cut.

I did not have to be concerned with thread form or class. Standard 60 degree threads were all I needed. For all "normal" holes it is a pretty straight forward drill n' tap adventure. My situation is something I have run into a couple of times and had a miserable time figuring out - such as making a threaded cap for a small tube.

Next I'll have to try the formula that Scott_M posted and see how I like that. More play time!

This was fun. Thank you all.

FWIW, you can have a 20% thread with a nice fit and no wobble, the percentage of thread is what determines the pull out strength/quality of the thread. That said, according to the machinery's handbook, tests have shown that more than 60% thread engagement provides no significant increase in strength. For thread engagements more than 1.5 diameters deep, 50% is usually sufficient assuming comparable materials in fastener and tapped material. Most tap drills are sized to cut a 70-75% thread. The 'H' number on the tap designates the oversize in 0.0005" increments to allow for shrinkage or plating. Tapmatic has a simple chart to figure out what percentage of thread to use.

And another for tapping torque vs the strength of the resulting thread.

 

[lkrestorer]

L98fiero,
I have glanced at all the Machinery Handbook information on taps and threading until it fogs my glasses. This is good basic information but my concern was for unusually odd sizes such as making a nut (with any random TPI) for this 1.249" shaft. When I used the simple formula that danallen posted gave me a hole size of 1.199" and the more precise formula from Scott_M gave me 1.200 with using a 75% factor. These formulas aren't needed for "normal" threads - just look at one of the tapping charts that everyone has. I'm not dismissing your post as it is good "school learnin' " and needs to be understood.

One other question that came to mind as soon as I read your "L98fiero": Do you really have a Fiero with a Tuned Port engine stuck in it? Wow!