Trouble with M2 thread

I'm drilling a 1.7 mm hole in aluminium and threading it M2 x 0.4. When I screw in the M2 threaded rod which fits an M2 nut perfectly the rod will not hold. The rod measures 1.8 mm in dia. Am I trying to screw a fine thread rod into a course thread hole Also the tap appears to be too big to screw into the M2 nut.

Edited By Eric Cox on 20/09/2014 09:03:59

Hi Eric,
The chart I have I have suggests a 1.6mm tapping size drill. If you are using a 1.7mm drill (And assuming it does not drill oversize) then as the threaded rod measures 1.8mm then there is only 0.05mm (About 2 thou) engagement. I suggest also checking the pitch of the threaded rod by measuring how far the nut moves when it is rotated 10 revolutions. Just use a digital caliper to measure from the end of the rod to the nut.

Les.

Eric: I drilled and tapped a few M2 threads last week for a client project. I used a 1.7mm drill, which should be about 70% engagement and had no problem with the threads holding. A value of 1.8mm sounds a bit small for an external M2 thread; I just measured one of my M2 screws and it is a tad over 1.9mm. The other possibility is that your drill is drilling over-size. What make of drill are you using?

Regards,

Andrew

Never had any problem but I always use a 1/16” drill (I have a lot of them) and they always seem to drill oversize so seem to give the correct engagement

Eric - to state the obvious, if the tap is too big to fit the M2 nut that your trying, then either the nut isn't M2 or the tap isn't! - You've got a mismatch between the tap and bolt/threaded rod. The problem now is to identify what it is that you've actually got and then sort out the way forward.

1.8mm is a standard metric thread, so you could have a M2 tap and a 1.8mm rod and nut - which sounds distinctly possible. Very easy to confuse by eye. It's also just possible that you might have 9 or 10 BA threaded rod and nut - 10 I think would be more likely that 9, buit the rod should have an od of 1.70mm in that case.

It's also possible that the tap is M2.2 - also a standard thread - but I think you'd have found that tapping the hole required more effort than expected as the tapping drill size for M2.2 is 1.9mm rather than 1.7.

Checking the pitch of the rod will help but you need to be aware that M1.8 has a pitch of 0.35, M2 can come with pitches of 0.25, 0.40 and 0.45. While M2.2 has a pitch of 0.45. Unfortunately 9BA has a pitch of 0.39mm so very close to the nominal M2 pitch, and 10BA has a pitch of 0.35 - the same as M1.8, so the comments from Les above about checking over 10 revolutions (at least) is very pertinent.

Good luck

Keith

I've just finished an engine that had about 40 M2 holes tapped in aluminium, all done with 1.6mm drill and the screws hold firm.

Is it a bit of off the shelf M2 studding you are using as some can be quite poor quality and undersize. If its a rod you have threaded yourself try to expand the die a bit more to make the male thread OD larger.

Eric,

It's just possible that some ally may have galled up onto your tap and as you came out it tore the thread, making it oversize,It's not easy to spot at these small sizes and it is important to keep some cutting fluid around specially with aluminium

Bob D

A good quality -SHARP- HSS tap makes a world of difference in most metals but especially aluminum.

JD

Thanks for all the comments. bare with me and I'll be back in the shed on Monday and let you know how I got on.

By the way, the threaded rod was off e-bay and the nuts and taps were from well known ME suppliers.

Edited By Eric Cox on 21/09/2014 08:29:08

That deserves an award for "Outstanding Patience"!

Steve

Problem solved, drilled 1.6 not 1.7 as stated in handbook.

.

I see this chart gives different tapping drill sizes for different materials

Link to chart :- **LINK**

Nick

Raised metric one of the first things I learned at school in the sixties was that the drill size for metric taps is 0.8 times the nominal tap diameter, It has and still works fine for me. Niko.