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Thread: Drill size for form taps
10-06-2008, 10:14 AM #1
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.
10-06-2008, 10:31 AM #2
From the OSG poster on the subject
I am Ox and I approve this here post!
10-06-2008, 10:54 AM #3
10-06-2008, 10:58 AM #4
10-06-2008, 11:04 AM #5
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.
10-06-2008, 11:29 AM #6
Yes it is!
That's what I thought I read.
It's still Monday Morn here....
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...
Sorry for that.
10-06-2008, 11:57 AM #7
Machinist bible = Machinist hand book
10-06-2008, 12:05 PM #8
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.
10-06-2008, 12:27 PM #9
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 12:31 PM. Reason: Corrected transposed numbers.
10-06-2008, 12:36 PM #10
Hmm, sounds far too loose for a rolled thread?
10-06-2008, 12:48 PM #11
I'm not sure
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.
10-06-2008, 01:06 PM #12
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.
10-06-2008, 01:10 PM #13
I'll let you know after I've broken half the taps
10-06-2008, 01:25 PM #14
10-06-2008, 01:28 PM #15
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!
10-06-2008, 01:36 PM #16
10-06-2008, 01:45 PM #17
I will be trying
10-06-2008, 01:49 PM #18
10-06-2008, 02:31 PM #19
10-06-2008, 03:03 PM #20
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...