Vic,

For that size I think the slightly larger drill would be the better choice with less likelihood of tap breakage. The difference isn't much admittedly, but these are the sort of sizes where 'every little helps'

Regards

Brian

Measure the hole and if it is 2.0829mm send it back

try uk drills i use them all the time very helpful if you ring still have to order through ebay i wanted i think 5.7 mm not listed on ebay one phone call not a problem order 5.6 and will send out 5.7 and thats what i got

Geoff

Vic,

That is known as supplying within the tolerance band.

No process on earth, real or imagined, will produce anything dead on theoretical size 100% of the time, it couldn't afford to and hold to a price people will pay.

Bazyle just now touched on the absurdity of expecting such perfection. Jason's experience confirms that perfectly good results can be had without being unrealistic or dogmatic about the preparation. You may be paying a premium price for your new drills, treat them with care. They will be just as prone to breakage as more bog standard versions I'm sorry to say. The final hole size will in any case depend on how they have been sharpened. I have experienced things as useless as sharpened nails!

Regards

Brian

Vic,

Despite what is printed in Zeus or similar tables here is not a "correct" single tapping size for any thread. There is a range of tapping sizes for any particular thread and the one to be chosen depends on a number of considerations such as the material being tapped, the application of the fastening, dimensions of the male thread etc. Tapping sizes in such tables are usually based on providing 70% thread engagement suitable for "general engineering".

I recommend anyone who wants to know how to select which tapping size to use should buy a copy of Drills, Taps and Dies by Tubal Cain (Workshop Practice series No 12). This has extensive tables showing the range of suitable tapping drills together with the % engagement for each. There is whole chapter (8 page 62) on selecting tapping drills.

Eric

I always, for metric taps. take the pitch from the diameter and round to the nearest 0.1mm. Then drill and tap the hole in the same material but not the finished part to test. When I used to design tooling if the bought tools, and we could be talking of 100's, didn't cut to size they were sent back to wherever they came from to be re-ground.

If one cutting edge is slightly longer than the other, the drill will cut oversize.

The drills used by C A V to drill holes in Injector sacs were sized by drilling a hole and measuring the hole with wires, rather than measuring the drill.

A slight difference in % thread engagement, because a drill produces a hole marginally oversize, is unlikely to caused much of a problem.

A nominal 2.00mm drill may well provide the 2.05 mm diameter hole for which you yearn. Maybe even the 2.1mm for slightly reduced engagement and reduced risk of tap breakage.

Howard

.

Get the Workshop Practice Series "Drills, Taps and Dies" by Tubal Cain and read it from cover-to-cover like a novel.

In particular, look at the tap-drill tables at the end of the book. It will tell you why your #45 will make essentially zero difference (and why) …. other than making the tapping slightly easier if it's noticeable …. plus the book will give you scads more useful information.

Edited By Bandersnatch on 13/12/2018 22:07:58

Tubal Cain's chart in ME Handbook recommends 2.2mm drill for 2.5mm thread.

No point in measuring shank size. The shank, and first parts of the flutes, are made to be smaller diameter than the tip, for clearance. Maybe more meaningful to measure the tip diameter. But the size of hole depends also on how the tip is sharpened. If it's a bit off-centre, the resulting hole will be slightly larger than the tip diameter, as is usually the case.