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Drill size for form taps

PhillipM

Cast Iron
Joined
Aug 19, 2008
Location
Rotherham, UK
Never used a form tap before, but I've got a job that would benefit from them so have ordered a dozen OSG taps, as my existing suppliers don't stock forming taps, 1/2-20 UNF, what size drill should I be using with these, as the initial hole needs to be larger IIRC, and I don't have an OSG catalogue.

Cheers.
 
What machinists bible? ;):D

12.1mm looks spot on for the thread engagment I'm after, and easy to get hold of over here, thanks for the help.
 
#1 is for a 1/4-20

1/2-20= 75% thread .475 65% = .477 55% = .480.




Yes it is! :eek:

That's what I thought I read. :nutter:

It's still Monday Morn here.... :o

For 1/2-20 their chart reads 31/64ths.

But just making the hole to the pitch D or slightly larger is really the name of the game enyway... And a decimal reamer may be a good idea to make good and repeatable threads...

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Sorry for that. :bawling:
Ox
 
Machinery's Handbook

My book says 0.4531 (29/64") or (11.501mm) for a 75% of full thread. Of course my handbook is a 1943 12th edition -- maybe things are different now.:cheers:

Assuming 75% is what you want then I would follow the book.
 
Last edited:
I'm not sure

Hmm, sounds far too loose for a rolled thread? :confused:

I have never used a (roll) form tap and have been on Google now for a while and really don't find anything but advertising for tap dancing and also some pictures of roll form taps.

There is an obvious difference so I will continue to find more info -- the process, speeds and feeds, and size requirements may be of significant importance. I just not sure.:confused:
 
Old handbook again

MHB says the diameter of the blank for 1/4" thru 1/2" should be 0.002" to 0.003" less than the pitch diameter.

Major dia for 1/2"-20= 0.5000
Pitch dia. = 0.4675"
Minor diameter = 0.4350"

So 0.4675-0.003 = 0.4645 Slightly larger than the 29/64" I said earlier. So the options based on this are: 29/64" (0.4531), or 15/32" (0.4688), Maybe reaming the 29/64" to size would work.

I'm gonna have to do this at least once just to say I tried it. Not much call for it in my world tho.
 
MHB says the diameter of the blank for 1/4" thru 1/2" should be 0.002" to 0.003" less than the pitch diameter.

Major dia for 1/2"-20= 0.5000
Pitch dia. = 0.4675"
Minor diameter = 0.4350"

So 0.4675-0.003 = 0.4645 Slightly larger than the 29/64" I said earlier. So the options based on this are: 29/64" (0.4531), or 15/32" (0.4688), Maybe reaming the 29/64" to size would work.

I'm gonna have to do this at least once just to say I tried it. Not much call for it in my world tho.



I think your gunna have a very bad first time experioence running those numbers.

I like to generally run my holes a bit tighter than recommended, But I don't think I ever end up with a start hole SMALLER than PD. I recommend that you re-think that.


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I am Ox and I approve this post!
 
I will be trying

I think your gunna have a very bad first time experioence running those numbers.

I like to generally run my holes a bit tighter than recommended, But I don't think I ever end up with a start hole SMALLER than PD. I recommend that you re-think that.
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I am Ox and I approve this post!

I'm gonna try it as soon as I can and be assured I will follow your advise -- I respect the fact that you have sucessfully done this before so I will start with your numbers and work back to my calculations just for fun -- Thanks this is an interesting thread.:cheers:
 
What material are you tapping; it makes a difference as there is some 'springback' when using form taps. Aluminium is quite soft so it forms quite well. Stainless OTOH will spring back a little so a larger drill is needed for tougher materials. Remember that you can only form tap ductile materials so cast iron is a no-no.

Form taps are actually made to a 6HX tolerance which is larger than a 6H; this is to factor in the springback.

You had best do some testing and use the largest drill that will still give you a good thread; these taps use a lot more power than a cutting tap and you should run them faster as well.
I usually check with a go/no go gauge to allow me to use the largest drill possible while staying within tolerance. You will break less taps and have longer tool life the bigger you drill.

I am also an advocate of ER tapping collets; they have a square in the bore to hold the tap securely They are expensive but since starting to use them we have never broken a tap. Just a few degrees of slippage in a collet will put enough strain on the tap to break it. WNT have them.

I know you were just asking for drill sizes but I hope the above info. is of some use.

I assume that you are rigid tapping in a VMC...

Regards,

Mike.
 








 
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