Drill size for form taps
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.
From the OSG poster on the subject
I am Ox and I approve this here post!
Originally Posted by Ox
#1 is for a 1/4-20
1/2-20= 75% thread .475 65% = .477 55% = .480.
go to WWW.balax.com and download there thread former pdf file. the sizes are also in your machinist bible.
What machinists bible?
12.1mm looks spot on for the thread engagment I'm after, and easy to get hold of over here, thanks for the help.
Machinist bible = Machinist hand book
Yep, like I said, don't have one!
Normally I just calculate what size I need but I wasn't sure how to do it for a form tap.
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.
Assuming 75% is what you want then I would follow the book.
Last edited by ietech; 10-06-2008 at 01:31 PM.
Reason: Corrected transposed numbers.
Hmm, sounds far too loose for a rolled thread?
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.
I'll let you know after I've broken half the taps
Originally Posted by ietech
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.
I am Ox and I approve this post!
Machinery's Handbook recommends working to 50, 55, and 60% full thread for form taps. Just like this chart:
Originally Posted by apestate
Thanks for the link to the chart I will be saving it in my favorites.
Originally Posted by allez
This info i posted is from the Balax book.
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...