Major Diameter Before Threading

martik777

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How much LESS than the major diameter, of the required thread, should work be turned to before threading:

With a die?

Single pointing?

For example a 3/8 NC bolt is always approx .367 not .375 and when one tries to cut the threads with a die, .375 is too large for the die. No charts I have seen specify this, so I usually just turn the shaft a few thou under.
 
I bought a 1/4-50 die some time ago, as delivered it cut .013 undersize. Is this what you're working with? I had to open up my home made die stock allow the die to enter it. It now cuts about.003 under, and the screws made screw easily into a hole tapped 1/4-50.

As to 3/8 bolt being undersize, those threads were very probably rolled, and who knows how accurately they were made. Measure the OD of you 3/8 tap. It's not undersize.
 
There are specs on threads that are undersized on purpose. Infact I forget the exact terms or lettering they use. Ck your tap chart it tells you percentage of thread with different drill sizes.
 
You should pick up a copy of Machinery's Handbook. It does not need to be a new one, and used ones are not expensive. It is about 2500 pages, and it has charts that tell you minimum and maximum outside diameters for screws and bolts, among many, many other topics. Charts galore. I just used my handbook a half hour ago to see what size I should turn drill rod to for a 10-24 machine screw thread (class 2A threads, 0.1818" min., .1890" max.). Very useful book, and every machinist should have at least one. The newer versions cover metric and CNC stuff better and the old ones cover stuff pertaining to older machines and methods better. Either is fine, get one! I am using the 22nd edition, published 1985.
 
For example a 3/8x16 tpi thread has a pitch of 1 ÷ 16 = 0.0625". Take the pitch 0.0625 × the magic number 0.859 = 0.0537 rounded up. Now take 0.0537 ÷ 8 = 0.0067. Take this number and subtract it from the bolt diameter. 0.375 - 0.007 = 0.368" diameter. The 0.007 is the peak (H8) in Bob's drawing but I find this works for any tpi i have tried. The full method is in the Machinist Handbook a I too recommend getting a copy for much more than this. Enco seems to have the best price on a new one. The book is a must have for cutting a thread using the 3 wire method and tons of other stuff. Hope this helps.
Dave
 
Thanks, I have that ebook, just a little overwhelming to look at sometines.
 
Thanks, I have that ebook, just a little overwhelming to look at sometimes.

martick777,
Definitely overwhelming. Yet, very doable.
I had to take one concept at a time, look up the terms and attempt to understand both the theory and application.
I'm still working on it! But, I am functional.
You can do this... I was fortunate to have both HM and some local mentors who patiently answered my questions.
What specific questions do you have? Let's work through this!
Daryl
MN
 
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I am with Daryl. First, there is plenty of knowledgeable help here if and when you need it. Second, there is the satisfaction of knowing you can do it yourself. We want to help you get from the first to the second. There is an old proverb: Give a man a fish and you feed him for a day, teach a man to fish and he will be out with his buddies drinking beer all day! 8^)
 
A really nice companion to the Machinery's Handbook is the Machinist Calc Pro.
It has all the common thread sizes programmed in, you can input any custom size thread, and adjust thread percent. It will return all the info you need for drill sizing including close size drills if you are missing the correct drill, wire gauge numbers, etc. It is really handy if you are making an odd ball thread.
It also does instant metric to imperial conversion and a boatload of other things like hole patterns, trig, etc.
I all but stopped using my Machinery's Handbook for reference once I got it.


 
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