Threading with a tap

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Sandman north
Posts: 41
Joined: Thu Jan 29, 2015 8:02 pm
Location: Parry Sound Ont

Threading with a tap

Post by Sandman north »

Not the first time I have run into this. I was doing this on the lathe.

I was making a thread in some aluminum 1/2 X 13 TPI according to the chart I should drill the hole to 27/64 .

All good so far, then I get the tap in about half way and OMG is it tight. get it threaded, and try a bolt and it's really tight too.

I have run into this problem with other tap sizes too.



As always look forward to the great input

Sandman
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GlennW
Posts: 7284
Joined: Sun Apr 15, 2007 9:23 am
Location: Florida

Re: Threading with a tap

Post by GlennW »

It sounds like you are using fairly dull hardware store grade carbon steel taps.

I believe that a new HSS tap and some tapping fluid would make a huge difference in your results.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
Sandman north
Posts: 41
Joined: Thu Jan 29, 2015 8:02 pm
Location: Parry Sound Ont

Re: Threading with a tap

Post by Sandman north »

Yes it is a hardware store tap, I did use cutting fluid.

Thanks
Magicniner
Posts: 532
Joined: Thu May 30, 2013 4:40 am

Re: Threading with a tap

Post by Magicniner »

29/64 for 50% thread in iron and steel.
Any tapping chart worth referring to will tell you what % of thread the drill size will yield, going for 100% in steel isn't going to do your tooling any favours ;-)
earlgo
Posts: 1794
Joined: Sat Jan 29, 2011 11:38 am
Location: NE Ohio

Re: Threading with a tap

Post by earlgo »

27/64 is what the chart in the Machinery's Handbook says for a 75% thread. What grade of aluminum were you using and what was the wall thickness around the tapped hole? Aluminum is not known for spring-back but some of the tougher grades present all kinds of issues. WD-40 is a great lube for machining aluminum.
When I was writing programs and specifying tooling for a Cincinnati Milicron T-10 Horizontal mill, one of the things learned was that using a thread cutting tap in aluminum castings almost always broke the tap after a few holes. We switched to a thread forming tap and cruised on to success. Of course the thread forming taps require a bigger tap drill and quite a bit more torque to form the threads.

Switching to a new cutting tap that might be sharper may be your answer as was mentioned.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
John Hasler
Posts: 1852
Joined: Tue Dec 06, 2016 4:05 pm
Location: Elmwood, Wisconsin

Re: Threading with a tap

Post by John Hasler »

Are you rotating back to break the chip after every turn? Is this a blind hole?
johnfreese
Posts: 219
Joined: Thu Jan 21, 2016 2:10 am

Re: Threading with a tap

Post by johnfreese »

I favor spiral point taps for through holes and spiral flute taps for blind holes. Both have cutting rake angles that are much more favorable than conventional hand taps. Neither requires backing up to break chips. Actually backing up is undesirable with these taps.
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