Rant. I know we've all been there but damn.

[rlukens]

Just snapped a 6-32 tap off in the last of 16 blind holes that I was tapping into an aluminum part. Dug it out, made a mess, but I think I can plunge an endmill into the part and plug it. Guess I'm going to bite the bullet and buy some decent spiral flute taps. Whoa, 20 bucks apiece!
Rant over.
Russ

 

[pickleford75]

Just snapped a 6-32 tap off in the last of 16 blind holes that I was tapping into an aluminum part. Dug it out, made a mess, but I think I can plunge an endmill into the part and plug it. Guess I'm going to bite the bullet and buy some decent spiral flute taps. Whoa, 20 bucks apiece!
Rant over.
Russ

Unfortunately I know EXACTLY how you feel had the same problem on my Lee Hodgkins radial build.... got to the last hole for the cylinders after replacing the tap for the second time, and SNAP broke one off... I guess after 140ish holes it was inevitable. I was using the spiral taps they still break but are better

 

[kuhncw]

Breaking a tap is always a worry as you get to that last hole.

Thread forming taps are, in my opinion anyway, a good option for threads in softer metals such as aluminum. These taps have one very shallow flute, so they are quite resistant to breaking. Also, you don't have to clean chips out of the hole. Forming taps do require a slightly larger hole than thread cutting taps. The manufacturers publish drill size charts.

Balax is one brand, there are others.

http://www.balax.com/catalog/thredfloer-taps

Chuck

 

[rlukens]

Thanks Chuck.
I've used thread formers back in the day... nothing smaller than 1/4 20. Hadn't considered them for this small stuff. Most of my parts are aluminum or brass. I can imagine they work fine in aluminum. How about brass?
Russ

 

[kuhncw]

Brass is no problem.

Chuck

 

[rcaffin]

Thread forming works fine on softer aluminiums - 6000-series are usually OK. 5000-series no worries. I find drilling to (metric) OD - (pitch/2) is normally OK. If this puts you between two drill sizes, go up in size.

But you need to the VERY careful with the harder cast aluminium alloys - tooling plate, Fortal, 7000-series. These alloys can snap even good thread-formers. You need to go a bit more over-size on the tapping holes.

In either case, LUBRICATE!

Cheers
Roger

 

[tornitore45]

When I was smart and knew everything I use to break lots of taps because I calculated exactly the hole diameter based on the thread geometry. Then I learned that unless I was into thin aluminum a 70% tread was plenty strong and if I had 2 diameters depth then even less was fine.
Also learned to use a mechanical alignment, lathe, mill or drill press but never start free hand if mounting on a machine was possible.
A bit of lube and have not broken a tap in many years all the way down to #1 in steel.