![]() He also points out higher engagements are needed when tapping sheet metal - at least 85%. This ranges from 50% for Stainless Steel (the nasty tough type, not the free-cutting variety) up to 70% in Brass and 75% in Cast Aluminium. There's also a table of recommended engagements by material. Thus he recommends 7.1mm for M8 rather than the usual 6.8mm. Therefore his tables of tap drill sizes in Model Engineer's Handbook have been designed to give at least 65% engagement in small sizes (below 3/8", and up to 75% for larger sizes. He says: ' The tables are designed to accept the higher thread engagements found in production workshops are not suitable for hand tapping.' In Section 4, he explains that most tap drill tables are designed for tapping machines fitted with slipping clutches and automatic reversers. Oodles of good advice in Tubal Cain's Model Engineer's Handbook. To save me reaching for the book every time - I've stuck a reminder from Tubals book on the box. These sizes work very well and makes tapping a far more relaxed process - and I've never been concerned about the strength of anything I've tapped using this thread engagement. Instead I use the 65% flank engagement recommended by Tubal Cain in his ME Handbook (my copy is the 1993, 8th version - page 63) which for an 8mm (1.25mm pitch) thread is given as 7.1mm. ![]() However, I found these tapping sizes to be hard work in mild steel - so I don't use them. They also came with a matching set of "tapping" drills & the one provided for the 8mm tap is 6.8mm. Imperial particularly BSP and American pipe threads with their taper options are a different kettle of threads entirely.įor my bread & butter metric tapping in mild steel, I use an inexpensive set of taps (purchased from Lidls) and they seem to work very well. The question was asked regarding M8 coarse, ie standard thread, and my reply was regarding metric threads. It'd make a bloody poor BSP or NPT thread seal. That will give a 50% thread depth (actually 47% for a 60° thread), but one might not want that amount of engagement. I have a few heli- coil sets for just that reason. Smaller sizes often give trouble when being clever and drilling bigger to avoid tap breakage. If drill cuts big you have room to manoeuvre, if it’s over tight you can open up. On the first test the bolt broke before the thread stripped and that hole was drilled for 50% thread engagement.Ħ.8 mm. Many years ago I did some experiments tapping 6082 and using high tensile (12.9) bolts. While not universal the bolt usually fails in tension long before the internal thread fails in shear. That's largely determined by the tolerances of the tap and mating screw. The drill size has little or no effect on the fit of the thread. The foregoing applies to coarse threads, for fine pitch i'll go down a tad in size. In percentage terms I aim for 60% to 70% in most materials, may be 75% in softer materials and 50% in tough materials. In brass or aluminium 6.9 to 7mm and in stainless steel 7.3mm. 8mm - 1.25mm = 6.75 or round up to the nearest preferred size 6.8. I normally use the diameter minus the pitch for tapping drill sizes in metric threads i.e. my method of getting the appropriate drill size, I generally aim for 60% thread form. ![]() But a loose fit also means low strength: don't deliberately weaken load bearing fastenings. Taps in bigger holes last longer and are much less likely to break, especially in small sizes. When strength doesn't particularly matter, and often it doesn't, I generally use a larger than recommended tapping drill because bigger holes reduce the work the tap does. Given the application is T-nuts which can be expected to be stressed, best to go for a small drill leaving the maximum amount of metal for a strong tight fitting thread - 6.8mm. The difference in tapping drill sizes you have found may be due to different pitches, an M8x1.25 requires a tapping drill of 6.75mm where an M8x0.75 needs 7.25mm. The tapping drill size for metric fixings can be calculated simply by deducting the pitch from the diameter. Don’t think I’d want to go above 7.0 unless I was experiencing problems with thread quality. I’ve always used 6.8 and have had no problems. I personally use a letter "J" drill for general use M8 threads which is 7.05mm. I appreciate the small drill will produce a tight thread and the large one a loose fit, so is approximately the mid point between the sizes the optimum, given that the application is some T nuts. From published information it seems the tapping drill can be anything from 6.8mm to 7.5mm.
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