2.05mm or #45?

I tried to buy a pack of 2.05mm drill bits on eBay as I snapped my second to last one the other day. I only got a couple of hits and they were silly money like £8 each plus postage from Germany. I double checked the required tapping drill for an M2.5 thread and I actually got two answers online - 2.05mm and #45 which appears to be 2.0828mm. I searched eBay for #45 drill bits and got lots more hits so I bought a pack of 10 #45 for about a fiver inc postage. Which is right though and will it make much difference?

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

2.1mm does for me when tapping M2.5

At a fiver for 10 bits you will be lucky if they cut spot on size anyway.

Edited By JasonB on 13/12/2018 12:28:01

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

That’s what I was hoping Brian, thanks.

The bits are ground Cobalt HSS from RDG so I’m hoping they’re reasonably sized. To be honest though I’ve measured top brand drill bits with a micrometer before and they often differ a little from the size marked for some reason.

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

What’s the postage for sending a hole anywhere?

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

Drills are ground fractionally undersize because they tend to drill oversize. The plain shanks are definitely slightly undersize.

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 was aware of the percentage engagement idea but generally stick with the normal recommendation. Thanks for that though Eric, I think that’s a good idea. I’ll take a look.

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.

+ For UK Drills,Bought many drills from them.

Good prices,good service,and no silly delivery costs !

What more could you want ?

Same here but they don’t list a 2.05 drill that I could find on their website, and certainly not on eBay where they do free postage.

actual minor for M2.5 is 2.013mm to 2.138mm

Ø2.1 would be my choice

Jim

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

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No doubt it will differ from drill to drill but I have the impression that with these smaller sizes, if you first use a drill 0.1 mm undersize and than finish with the size you actually want, it gives you a hole diameter closer to the nominal. Niko.

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

That is entirely logical if the cutting edges of the first drill are of slightly different length - the finishing drill point doesn’t need to follow the centre of the hole and be thwarted by the point being non-concentric to the drill (like the original drill might be). So OK as long as the drill flutes are the nominal size.

Yes that makes sense. I’ve not had any problems though with the 2.05mm drill I’d been using until I snapped it!