Tapping Drill Sizes

Found a reference to this handy chart on another forum (Practical Machinist) that I thought may be of interest.

http://littlemachineshop.com/Reference/TapDrillSizes.pdf

The interesting thing for me is that they specify "oversize" tapping holes of 50% for steel / stainless etc. This reminded me of the instructions of one of the past greats (can't remember which one - Tubal Cain perhaps), who suggested that Model Engineers should be using slightly larger diameter holes to aid tapping.

The discussion string also stated that larger diameters should also be used for deep holes. It also said that this principle is used for the data in Machinery's Handbook. It may have cropped up before on this forum, but I cannot remember reading it.

I have occasionally drilled oversize, but do not normally have any big issues tapping steel, although I have always winged it with Stainless and guessed a slightly bigger diameter to save damaging the tap. But after looking at this link, I will try as they suggest and see how it works for me.

Cheers - Will

Yes, it was Tubal Cain in the Workshop Practice Series No 12 "Drills Taps and Dies". He gives tables of tapping sizes for each thread size for a range of thread engagements.

Actually, if you have - or get - this book, it's worth reading from cover to cover. Lots of interesting stuff in it.

Thanks for the LMS Tap drill chart Will, comes in handy for those who don't have access to Tubal Cains book. Harold Hall has some similar Tap drill pages on his website that I find useful.

Thor

I know this is an old thread but it saves starting a new one and the links above are still useful.

I always use the recommended sizes that most charts list so for metric coarse threads these are, from M2.5 to M8:

2.05

2.5

3.3

4.2

5.0

6.8

Having now got the Tubal Cain book mentioned above I’m now going to try going up .1 of a mm for each size.

But what do other folks use?

If you can’t buy the book and have a ‘puter to browse on here, they are all listed on numerous websites. I remember the common sizes I use and look up the rest on the internet when I need them. This thread is long past its use by date!

Apart from M2.5 which I use a 2.1mm drill for I drill as you have listed 95% of the time, may go a bit larger if it is just a quick thread to hold something that does not rally take any load.

I use the sizes given in the Zeus book,unless I find it a bit tight,then I go a little bit bigger,since taps are not a throw-away item,not with me,anyway.

Hi Vic,

The tapping drill size I use depends on the material, for tough materials like stainless steel I use larger dill sizes than the ones you listed. For free cutting materials I use the drill sizes you listed. Using a drill size 0.1mm larger will reduce the chance of the tap breaking and the reduction in strength is very small.

Thor

Dimension standards available for all classes of thread Int. & Ext.
Look them up!

If you are stuck for metric tapping drill sizes, just subtract the pitch from the diameter.

Regards

Having worked for Leica for 25 years all I ever calculated was the, 'K' factor for tapping drill sizes. Multiply the thread diameter by 0.8 and it will give a nominal drill size for that thread. We were expected to know this without recourse to wall charts etc.

Clive, how would that work with various pitches of thread, can't see you being able to get a M10 x 1 tap into an 8mm hole and would not even work for standard metric coarse at M10 x 1.5.

I have all the tapping drill sizes on my cell phone. There are several apps available for machinists. Unfortunately none of the apps I found cover BA threads.

Hi Jason, those threads were calculated individually. Bear in mind the threads used were for Theodolites and Levels and mainly only up to 8 mm and in the normal series. Where we had larger threads like jacking screws on plotting tables it was up to the engineer to work out the tapping drills. Also everything was pre-made so it was modular change and we only machined transit bearings that were worn. Being R & D I had to make sure all threads and pins etc were DIN Norm acceptable, There were finer Metric threads used, plus some odd ones like two start 0.7 pitch used on eyepiece threads. I always assumed this was to stop the opposition from copying our kit.Very hard to match up.

Clive, Merry Xmas by the way.

That still seems tight even on the smaller sizes in standard metric coarse.

M5 would be 4.0mm hole (5x0.8), but minor diameter of the female thread is 4.14

M3 would be 2.4mm hole (3x0.8), but minor diameter of female thread is 2.46

Both would give over 100% engagement?

.

This is a handy little tabulation: **LINK**

http://www.engineeringsupplies.co.uk/threads-tapping-drill-sizes-i-22.html

... just save the image onto your 'phone.

MichaelG.

.

P.S. ... In general though ... if struggling to select a suitable drill size, look at the fit of the taper tap in a drill gauge.

Edited By Michael Gilligan on 25/12/2018 08:54:32

Jason, I think that you misunderstood the "Rule of Thumb" for Metric threads. M10 x1 would be drilled 9mm and so on.

For the Coarse series of threads, I use 5mm drill for M6 (1mm pitch), 6.8mm drill for M8 (1.25mm pitch) and 8.5mm drill for M10 (1.5mm [itch) and so on.

It has worked for me down to M3, (Says he, tempting fate!)

Clive was probably working in Aluminium, and the threads were lubricated, with a special grease, and needed to have minimum, or no slack.

Howard

Howard I know and use the rule of thumb for dia-pitch.

My query was that Clives method would give results that make the drill a lot smaller than the rule of thumb as the examples I posted show. Could you get a M8 tap into a 6.4mm hole regardless of material? (8 x 0.8 = 6.4)

Was absolutely amazed to think that you did not use the rule of thumb, Jason!

Yes a 8 mm tap into a 6.4mm hole would be a tight fit! Especially since Sine 60 degrees is 0.8660. This would seem to give a minimum thread diameter of 6.928mm (8 x 0.8660). The only way that I could imagine this working would be a first cut Tap with a plain minimum "diameter", to act as a pilot drill, and open up the hole to allow the tap to enter.

Being E Leitz, they may well have used specialised Taps.

Or have I misunderstood things again?

Any further enlightenment Clive?

Howard

Edited By Howard Lewis on 25/12/2018 13:08:38