drill size for tapping threads...

who all would use a bigger drill to give 75% engagement in brass or ally...and 50% in steel or iron...is there a real need to tap close to thread to thread sizes...

John.

John,

I suggest you buy this book, should answer your questions and many more besides.

**LINK**

David

John

If I understand your question.

using "oversize" drills when tapping.

the effect is to reduce the thread depth cut in the side of the hole . This makes tapping easier but weakens the joint

similarly making the "bolt" under size ===easier on die but weaker

this can be offset to a degree by using longer engagement..

depending on the material, using less engagement can be a tradeoff..tougher material.=same strength for the reduced engagement

I aim for an engagement of around 60-65% in all metals. You simply do not need the extra thread depth in internal threads. Some years ago I did some experiments using 6082 aluminium alloy, as I needed to use it with highly loaded high tensile (grade 12.9) steel M4 SHCS. I drilled holes at 3.4, 3.5 and 3.6mm and then machine tapped them with a spiral flute tap. The 3.6mm hole represents less than 60% engagement. The SHCS broke before stripping the thread in the 3.6mm hole; I didn't bother testing the other two holes sizes.

Regards,

Andrew

Andrew

What length was the tapped hole that you used and do you think that ratio of bolt diameter to loaded thread length would hold roughly true across a range of thread diameters? I suppose the critical point is where the tapped hole fails before the screw/bolt fails, any feeling for this? I appreciate different steels will fail at varying stresses but a ball park guess could be useful.

Cheers

Martin

Edited By Martin W on 16/10/2013 00:20:51

Hi John,

As suggested a 65% engagement in steel should be sufficient for most purposes. Have you checked Harold Hall's website about tapping drill sizes? The next page will give taping drill sizes and depth.

Regards

Thor

Hi David,I have a very similar one to that book, Jason,as we are all Model engineers, I based the % of thread on the scaled down models, I think Andrew is closer to my way of thinking,,after all in full size ,a thread tolerance would be given on the drawing...if we were building an aircraft engine full size And if we were going to fly it would you be in it with nuts and bolts at 60% engagement ?...on the other hand look at the lack of broken taps and lower stress levels...

John.

Martin: From memory the thickness of the test piece was around 12mm. However, as I understand it when a bolt is tightened it stretches slightly, so that only two or three threads carry the load, irrespective of the overall length of thread. The key calculation is the shear strength of the internal thread versus tensile strength of the external thread core. Generally shear strength is greater than tensile strength, so the bolt breaks before the thread strips. That's why you can use an ordinary nut with a high tensile bolt. It also means that small diameter, coarse threads are especially weak; the classic is the US 6-32UNC thread. Remember also that taps tend to extrude slightly as well as cut, so the engagement is often a bit larger than might be expected.

As always there are caveats; one also has to look at the crushing load per unit area on the thread surface, particularly important for fine pitch threads, where the bearing area may be small.

For fine pitch threads I may tend towards 65-70% engagement, particularly for the ME 40tpi series. On the other hand, for stainless steel I'll often go to 50% as it can be difficult to drive the tap otherwise.

Regards,

Andrew

It's reckoned that a thread will distort under load, only about 3 threads carry any load. I was taught this years ago, may not be entirely correct. Think of the thickness of a nut, and how much clearance there is on a standard nut and bolt

As for only a few threads carrying the load, that is very dependent upon how the joint is designed.

In a nut/bolt combination the bolt is in axial tension, with the nut is in axial compression so the threads tend to mismatch a little, first thread takes most load, there is little contribution to strength after the first few threads.

Bolt in a tapped hole is very different - axial stresses in both parts are in tension, so thread pitch gets stretched in both members. There will be less induced pitch mismatch, so stresses are more evenly distributed.

Andrew

Thanks for the detailed reply it certainly clarifies things a lot for me. I have to admit that I have tended to use the 'specified' drill sizes for tapping even though the subject of clearance has been raised in other threads on this forum! As a result I have tapped threads using, if this is the correct terminology, a 'starter' then 'intermediate' and 'finishing' tap for each hole. Thankfully as most of the tapping I do is fairly small I could use the mill or lathe to power drive the tap.

Gordon & David, thanks for your replies too.

Cheers

Martin

An ancient but still learning and enjoying it.

Edited By Martin W on 16/10/2013 11:50:56

Thank you all for such detailed answers to my original question, sort of makes me think about the bloke who won his clubs competotion by entering a real cracker of an engine build...he much later told one of his friends (the whole thing was held with engineering glue...no threads at all)

John,

Read the book David Littlewood recommended. It's inexpensive. Read it from cover-to-cover. It will explain all .... and a whole lot besides