View Full Version : acme tap drill size

12-02-2005, 09:52 PM
Posted: Fri Dec 02, 2005 7:44 pm Post subject: Acme tap drill size


I am a hobbiest but lack only one course to complete a 2 year program at the local tech college. I know this is a good foundation, but the more chips I make the more I realize how little I know and the greater my respect for the gray haired machinist.

Now my question: Today I was called upon to make 2 acme threaded nuts: 3/4-6. With tap in one hand and the machinery handbook in the other I set about to calculate the correct tap drill. Hmm, several varieties of acme thread - who knew? I turned out 2 parts that worked (without breaking the tap), and I am sure of my arithmetic but not of the calculations I used and so wonder what my %thread was. Any advice/pointers would be greatly appreciated

Much work remains to be done before we can announce out total failure to make any progress.

12-03-2005, 01:12 AM
I may not be able to answer all your questions but I will try and answer a little. The majority of ACME treads are "General Purpose" not "Stub" or any other type. At least you were lucky enough to have a tap but not lucky enough to have a "Roughing" and a "Finishing" tap as they make it alot easier. Some people will rough ou an ACME thread first with a 60 degree threading tool to cut away most of the "meat" to make it easier as the thread form takes quite alot of pressure to cut as you probably found out. ACME threads are designed for the most part to be loose, depending on the class and application, of course. I have the Machinery HandBook 23rd Edition and on page 1561, Table 2Bit gives the Major, Minor and Pitch Diameter starting at 1/4 inch dia and going up to 5 inch diameter. The Chart before 2A describes how to figure it out mathamatically.

Internal threads,
Minor Dia, Minimum K:
Minor Dia, Maximum K plus .05p (min 0.005);

K= Dia of thread, .750 in your case - p (p being the pitch of 6 TPI or 0.16667.

0.588 Tap Drill size or 37/64 (0.578) ( the drill will drill oversize slightly)or 15 mm (0.590)

Hopefully the nuts you made were special in size or shape, as Enco sells them for $1.79 each.


[This message has been edited by coles-webb (edited 12-03-2005).]

12-04-2005, 10:16 PM
<font face="Verdana, Arial" size="2">I may not be able to answer all your questions but I will try and answer a little.</font>

Thanks Mike. I guess I muddled through it correctly. I suppose it is better to be lucky than good.

Oh, yea, these were not standard nuts. The customer provided some 1.25 hex bar for the base material. Electric camper leveling jacks.

[This message has been edited by romoshka (edited 12-04-2005).]