Taps and drilling

[Bentwings]

Question about ts drill sizes I know you can look any thing up on the internet , but suppose you come to a tap you just don’t have the real size you need. Small metric come to mind. As a start can you just measure the flite thickness on day a two flute tap and drill close to that size then if it doesn’t start easily go up day one drill size as clos as possible I have an imperial number drill set but not a full metric set. One of m normal tap drills is bent so I really have no choice untilI get new tap drills. They are ordered but it will be a week at least m3 tap is what I’m looking at for the issue I don’t see well and measured the small lead in but I must have not done it right as I eventually had to go one size larger tap which led to over sizing the mounting holes. Chain of events ok y’all can stop laughing parts fit together just do not have correct screws or through holes

 

[Lloyd-ss]

General rule of thumb for tap drill is nominal screw diameter minus one thread pitch.

So, for a #10-32, it would be .190 minus (1/32) = .190 - .031 = .159 tap drill That gives about 75% full thread.

 

[Lloyd-ss]

I would just like to add, that the larger the diameter, and the finer the thread pitch, the more critical the diameters become. I make a lot of high pressure air tubes (3,000psi+) out of different materials, with threaded end plugs, usually odd ball diameters. I use this calculator
http://theoreticalmachinist.com/Threads_UnifiedImperial.aspx
(they have metric, too) and the larger the tube diameter the coarser I like to make the thread. Anything over about 1.25", I try not to go finer than 24 pitch. The reason is that with my manual machines, and doing single point threading on the lathe, a thou or 2 might be acceptable on a 24 pitch at that diameter, but with a 32 pitch, the strength can really drop especially if it is thin wall where some distortion might occur. And any more threads than about 7 (or less) should never be counted for strength calculations. Thread failure can many different forms, and no one likes surprises. I have tested many tube threads to failure and not been surprised.... lately.
Lloyd

 

[Bentwings]

That’s an interesting way for getting quick tap drill sides I’ll give it a try . I have a bunch of metric sides to drill . I always seem to have trouble with these especially imperial wire sizes

 

[timo_gross]

That’s an interesting way for getting quick tap drill sides I’ll give it a try . I have a bunch of metric sides to drill . I always seem to have trouble with these especially imperial wire sizes

Can confirm Lloyd-ss rule is also valid for metric e.g. M8x1.25 goes with a 6.8 tap drill (they round to next full 0.1 mm ) M6x1 drill is 5 mm.
I got a free "drillchart" from the tool shop. I pinned it to the wall, always a good confirmation for the standard pitch.

 

[larryg]

https://www.guhring.com/Support/Technical/Tap-Drill-Sizes
Above is one of the best tap charts that I have found. If you spend some time wandering around the technical section you can find a lot of good information.

lg
no neat sig line

 

[SmithDoor]

Question about ts drill sizes I know you can look any thing up on the internet , but suppose you come to a tap you just don’t have the real size you need. Small metric come to mind. As a start can you just measure the flite thickness on day a two flute tap and drill close to that size then if it doesn’t start easily go up day one drill size as clos as possible I have an imperial number drill set but not a full metric set. One of m normal tap drills is bent so I really have no choice untilI get new tap drills. They are ordered but it will be a week at least m3 tap is what I’m looking at for the issue I don’t see well and measured the small lead in but I must have not done it right as I eventually had to go one size larger tap which led to over sizing the mounting holes. Chain of events ok y’all can stop laughing parts fit together just do not have correct screws or through holes

Most do not know about the percentage of threads in section of tap drill.

Hard steel then %60 to %65 thread
Steel %70 thread
Aluminum %80 thread.

This helps in tapping not break the tap in hole

Dave

 

[L98fiero]

Most do not know about the percentage of threads in section of tap drill.

Hard steel then %60 to %65 thread
Steel %70 thread
Aluminum %80 thread.

This helps in tapping not break the tap in hole

Dave

It does depend on what the fastener is threading into and how many threads engagement. I worked for a company that made roll forming dies from D2 at Rc62 and the segments were bolted together with 3/8-16 socket head cap screws. They were having issues with breaking taps as they were drilling 5/16" that produces a 76% thread, I convinced them to drill 21/64 for a 57% thread and there was never an issue, at more than two diameters deep, you could break the bolt without stripping the threads, we checked to prove it. I couldn't do the math but I recall a conversation where someone figured out that anything more than 7 threads deep makes no difference in comparable materials.

 

[stevehuckss396]

They were having issues with breaking taps as they were drilling 5/16" that produces a 76% thread, I convinced them to drill 21/64 for a 57% thread and there was never an issue, at more than two diameters deep, you could break the bolt without stripping

Thanks for saying this. I have mentioned this where I used to work for years and they look at you like you just flew in from Mars. Finally a young open minded foreman ordered a 1/4-20 spiral flute tap and a #5 drill bit. Set for me and a set for himself. Had the pair until I retired and did well over 400 holes with it. In steel 75% threads make no sense cost wise. This is a construction setting where drilling and tapping is done by hand with battery drills and alignment is "eye balled". Tap life was increased 100X.

 

[Lloyd-ss]

Most do not know about the percentage of threads in section of tap drill.

Hard steel then %60 to %65 thread
Steel %70 thread
Aluminum %80 thread.

This helps in tapping not break the tap in hole

Dave

........................... I couldn't do the math but I recall a conversation where someone figured out that anything more than 7 threads deep makes no difference in comparable materials.

The percent of full thread, and, also knowing that any more than 7 threads doesn't add any additional strength are handy bits of info to know. I have heard the 7 thread rule from a very smart guy I knew.

I am going to be a bit bold here and say something that is meant only to be well-intentioned and complimentary, not conceited. And that is: all of the people on this forum (99%, anyway) are logic based and probably smarter than the average home do-it-your-selfer. Just look at what we build! But many of the calculations and charts we use are designed for the "average" situation. Make sure you use your critical thinking skills when using the chart info, and adjust it for your particular application. No one is going to burst into flames for using the tap drill or feed or speed that doesn't match the chart, but that DOES match the unique situation. Take an educated risk. You are smart enough. It will work.

 

[Lloyd-ss]

Strength and safety factor calculations

Threads are used in all of our projects, and they are usually plenty safe, but it is good to know just how safe.

Here is a picture of a tube burst failure and a thread failure from a test that I did. The tube is A513 Type5 ERW DOM steel tubing that I used as a high pressure air tank. The tube is .875 O.D. x .065 wall. The thread is 13/16-28. Design pressure for the tube was 3,000 psi. Calculated burst pressure was 13,800 psi, and the actual burst pressure was 15,000psi (using hydraulic oil). The failure pressure was about 8% better than the mechanical spec, which is what I was looking for, so that was good.

But here is the surprise, and it shows that multiple calculations needed to be done. If you look closely at the black powder coated surface at the end of the tube, you can see where the tube started to stretch at the thread root. Its yield strength was exceeded and it was approaching the point where the end of the tube would have ripped off from the tube and taken off across the test site. I didn't notice this until after the test while examining the tube. What it showed me was that I had neglected a critical calculation, the yield and tensile at the root of the internal thread. I have since added that calculation into my tube safety calculator spreadsheet. Had I done the calculation prior to the test, the yield at the thread root is about 12,000 psi of fill pressure, and the tensile is about 15,000psi. Given the stress concentration at the thread root (even with a mil-spec J-type thread with controlled root radius), it looks like what failed first: the tube burst, or the end popping off, was a bit of a crap shoot.

Tests like this should only be to prove that your calculations were correct, not to get a scary surprise, which I almost did.

 

[L98fiero]

Strength and safety factor calculations

Threads are used in all of our projects, and they are usually plenty safe, but it is good to know just how safe.

Here is a picture of a tube burst failure and a thread failure from a test that I did. The tube is A513 Type5 ERW DOM steel tubing that I used as a high pressure air tank. The tube is .875 O.D. x .065 wall. The thread is 13/16-28. Design pressure for the tube was 3,000 psi. Calculated burst pressure was 13,800 psi, and the actual burst pressure was 15,000psi (using hydraulic oil). The failure pressure was about 8% better than the mechanical spec, which is what I was looking for, so that was good.

But here is the surprise, and it shows that multiple calculations needed to be done. If you look closely at the black powder coated surface at the end of the tube, you can see where the tube started to stretch at the thread root. Its yield strength was exceeded and it was approaching the point where the end of the tube would have ripped off from the tube and taken off across the test site. I didn't notice this until after the test while examining the tube. What it showed me was that I had neglected a critical calculation, the yield and tensile at the root of the internal thread. I have since added that calculation into my tube safety calculator spreadsheet. Had I done the calculation prior to the test, the yield at the thread root is about 12,000 psi of fill pressure, and the tensile is about 15,000psi. Given the stress concentration at the thread root (even with a mil-spec J-type thread with controlled root radius), it looks like what failed first: the tube burst, or the end popping off, was a bit of a crap shoot.

Tests like this should only be to prove that your calculations were correct, not to get a scary surprise, which I almost did.

I was going to say that's interesting but who deals with pressures like that, but you can buy a 10,00 psi hydraulic pump for about $100 so there definitely is relevance to what we do.

 

[Nerd1000]

For some extra fun, consider what happens if you want to use a form tap that makes threads by cold forming the metal rather than cutting it! You need a much larger hole because some material will effectively be pushed up to make the threads... I think you really need a chart in such cases, though generally the correct size seems to be halfway between the tap drill for a normal tap and the nominal thread diameter (e.g. M6x1 tap drill is 5mm, but for a form tap you'd use 5.5mm).

I really like form taps, there's no swarf to deal with and the threads are actually stronger. But you would need a big drill set to cover all the common sizes, and you can't use them on brittle material like cast iron.

 

[Richard Hed]

The percent of full thread, and, also knowing that any more than 7 threads doesn't add any additional strength are handy bits of info to know. I have heard the 7 thread rule from a very smart guy I knew.

I am going to be a bit bold here and say something that is meant only to be well-intentioned and complimentary, not conceited. And that is: all of the people on this forum (99%, anyway) are logic based and probably smarter than the average home do-it-your-selfer. Just look at what we build! But many of the calculations and charts we use are designed for the "average" situation. Make sure you use your critical thinking skills when using the chart info, and adjust it for your particular application. No one is going to burst into flames for using the tap drill or feed or speed that doesn't match the chart, but that DOES match the unique situation. Take an educated risk. You are smart enough. It will work.

As said elsewhere, the average IQ of machinists (not really all that meaningful if yhou know what it reallhy is), is about 125--close to genius. Just thimpfk of all those things we have to know and remember and test and experiment with.

 

[L98fiero]

As said elsewhere, the average IQ of machinists (not really all that meaningful if yhou know what it reallhy is), is about 125--close to genius. Just thimpfk of all those things we have to know and remember and test and experiment with.

Along those lines, there's an online calculator from Guhring at Tap Drill Calculator - GUHRING

The formulae are;
inch sizes
tap drill size= nominal OD - (.0068 x %/ TPI)

Metric
tap drill size= nominal OD - (% x mm pitch/147.06)
Even with the formula, it depends on the material being tapped so they are estimates at best, typically 55 - 75% thread is recommended.

 

[awake]

Strength and safety factor calculations

Threads are used in all of our projects, and they are usually plenty safe, but it is good to know just how safe.

Here is a picture of a tube burst failure and a thread failure from a test that I did. The tube is A513 Type5 ERW DOM steel tubing that I used as a high pressure air tank. The tube is .875 O.D. x .065 wall. The thread is 13/16-28. Design pressure for the tube was 3,000 psi. Calculated burst pressure was 13,800 psi, and the actual burst pressure was 15,000psi (using hydraulic oil). The failure pressure was about 8% better than the mechanical spec, which is what I was looking for, so that was good.

But here is the surprise, and it shows that multiple calculations needed to be done. If you look closely at the black powder coated surface at the end of the tube, you can see where the tube started to stretch at the thread root. Its yield strength was exceeded and it was approaching the point where the end of the tube would have ripped off from the tube and taken off across the test site. I didn't notice this until after the test while examining the tube. What it showed me was that I had neglected a critical calculation, the yield and tensile at the root of the internal thread. I have since added that calculation into my tube safety calculator spreadsheet. Had I done the calculation prior to the test, the yield at the thread root is about 12,000 psi of fill pressure, and the tensile is about 15,000psi. Given the stress concentration at the thread root (even with a mil-spec J-type thread with controlled root radius), it looks like what failed first: the tube burst, or the end popping off, was a bit of a crap shoot.

Tests like this should only be to prove that your calculations were correct, not to get a scary surprise, which I almost did.


View attachment 144855

Lloyd, that is very interesting. Thanks for this "real-world" analysis!

A question about the picture; was the issue thread strength per se, or was it that the tube was thin enough at the root of the threads that it was actually stretching there? Either way a failure, of course!

 

[Lloyd-ss]

Lloyd, that is very interesting. Thanks for this "real-world" analysis!

A question about the picture; was the issue thread strength per se, or was it that the tube was thin enough at the root of the threads that it was actually stretching there? Either way a failure, of course!

Hi Andy,
You are correct, the threads themselves were fine, but the wall thickness at the thread root was thin. The tube O.D. was .875, and the max major dia of the internal thread was a nominal .8125, so the wall thickness could have been as thin as .031. The tube material has a yield of 72ksi and a tensile of 87ksi. You can see that the o-rings for the tube plug are in-board of the threads, so the threaded section sees basically no hoop stress. The axial load on that plug, when yield starts is 5,100 pounds force, and max tensile force was with 6,200 pounds force. At the rated max fill pressure of 3000psi, the force on the end plug is only 1,300 pounds force, so, plenty of safety factor. But pressurized to 15,000psi when the burst failure occurred, the axial force on the plug was 6,500 pounds force, so, the end of the tube could have popped off instead of the tube wall ripping open.

But here is the good news about the test, which gave many airgun owners peace of mind when I posted the video of this years ago. Currently, the max fill pressure that can be achieved with readily available small air compressors is 4,500 psi. So even if someone was careless and over-filled their 3,000 psi tank to 4,500psi, failure of the tube and end plug would not occur. But also, a proper design for such a tube is to design into the tube a source of benign failure that happens before a catastrophic failure occurs. In such air tubes, designing the failure into the o-rings is the usual method of choice. With excess wall clearance between the plug O.D. (where the o-ring is) and the tube wall, the o-ring will extrude thru the gap with a loud pop and all the air will be gone in an instant. Scary, but it will put the fear and respect back into you, LOL!!!

This test was done with hydraulic oil because doing it with air would have basically created a bomb because of all the potential energy in the compressed air. Hydraulic oil leaks past o-rings differently than air, and I think I just used 90 duro o-rings instead of 70 duro o-rings to make them more resistant to the high pressure of the test.

My apologies if I am hijacking the thread, but I hope these safety tid-bits raise a few eyebrows and make us think about what we are doing in our shops every day.

Just a little more, but very important. The other end of the tube is sealed by a valve cartridge with o-rings that is secured with three 10-32 cross screws. But CO2 versions (nominal 800psi) have only a pair of 6-32 screws. A guy who didn't really understand what he was doing managed, with great effort, to put high pressure air into his CO2 gun and blew the valve (a chunk of .75dia x 2" long aluminum) out the end of the tube and into his thigh. Luckily he survived ok. There is a graphic youtube video of this. Another big safety reminder. !

Again, my apologies for the long post. I lost a little finger to a table saw at age 15, but I still have the other 9. And it happened because I did something that I had no idea was wrong and dangerous.
Lloyd

 

[awake]

I can't speak for anyone else, but I am finding this very interesting. One important point emerges (actually, more than one, but this one stands out for me): Never forget that there may be a different point or kind of failure than what you thought you were designing for. I can imagine thinking, "This tube is rated for ### psi, so it will be fine" ... without factoring in the loss of strength induced by threading it.