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Martin
03-03-2003, 08:48 PM
I recently bought some tooling. Included with the deal was a box of various taps.
These taps have no flutes, but if you look straight on at the end you will see that they have 5 or 6 sides. I asked the seller about them and he said that his shop used them all the time. They don't actually cut threads, they form the threads.
My question is where would I find info on what size of drill do I use for these taps ?
And now that I typed this, it just came to me to maybe check out the manufacturer of the taps.......duh....
Martin

DR
03-03-2003, 09:47 PM
The required drill size for fluteless taps is near or slightly larger than the pitch diameter.

Tap drill sizes are much more sensitive with forming taps than with cutting taps. So we generally start on the big side and work down until we get the specified percent of thread. 65% percent thread is the usual rule of thumb for forming taps since the threads tend to be forged therefore stronger than cut threads.

The relationship between tap drill size and percent of thread can be so tricky that sometimes there won't be a drill in the correct size available, then you may have to ream to size. For small diameters this can be a problem.

Run the form taps at a much higher rpm than that recommended for cutting taps. Use an extreme pressure cutting fluid.

Forrest Addy
03-03-2003, 09:58 PM
The formula to determine tap drill size for thread cutting taps is nominal diameter minus the thread pitch. Example 1/2 - 13. Pitch equals 1/13 or 0.077. 0.500 - 0.077 = .423" or 27/64" drill.

Thread forming taps require a "tap" drill somewhat larger because the metal has to have a place to flow to. I use nominal diameter minus HALF the pitch and try it on a couple of holes in scrap material similar to the workpiece. There's a table somewhere but I use thread forming taps so seldom I lose track of it.

The trick is to not push the envelope when using forming taps. Gaining a few extra percent of effective thread at the hazard of breaking a tap doesn't make sense. Removing a broken thread forming tap without damaging the thread its stuck in is very difficult.

Here's a link:

http://www.osgnet.com/select/taps/japanese/pdf/tech-nuroll.pdf

rustyherman
03-03-2003, 10:45 PM
Martin,your taps are called EXPRESS taps and I would consult your Machinest Handbook for tap drill sizes.

Ragarsed Raglan
03-04-2003, 08:40 AM
Are these cold forming taps not for sheet metal work? We used some similar taps for direct threading into Al sheeting, I believe they were invented during WWII for speeding up aircraft production. From what I can remember we drilled quite an undersized 'tapping' hole to allow the thread to mushroom out on the reverse side, thus giving a longer thread engagement.

RR

John Stevenson
03-04-2003, 09:43 AM
RR,
Fluteless taps can be used on sheet metalwork but they really come into their own in materials like alloy castings where the metal can be friable and lead to chipping in the thread. Fluteless taps 'roll' the thread into the metal much like cold formed bolts and so the thread is far stronger that a cut thread.
Another advantage is that because they have no flutes the tap itself is far stronger than a conventional tap. This pays big dividends in the smaller sizes as regards breakages.

John S.

Martin
03-05-2003, 10:44 PM
Thanks for the info.
My hand book is from the 80's and it may not have info on them, I have yet to look.
I figured that breaking one of them in a hole was not really worth the risk, because trying to remove it would be almost impossible without an EDM.
Thanks again.
Martin

Thrud
03-06-2003, 02:00 AM
Martin:
Forming taps require carefull attention to hole size as has been mentioned. check with the manufacturer as to theri exact spec on thir taps. If you do not use the proper sized hole the tap will seize and break - the slight grooves you see is to relieve hydralic pressure from extreme presure lubes used with these taps. They are usually used is difficult to tap materials like Lead, Aluminum, Magnesium, etc. They form beautiful threads in Aluminum and the taps are less likely to break than a fluted tap in the smaller sizes.

[This message has been edited by Thrud (edited 03-06-2003).]