View Full Version : Thread cutting questions
11-18-2005, 07:02 PM
Now that the old lathe is running (see post 17: http://www.metalmeet.com/forum/showthread.php?t=3404), I have managed to cut some clean threads with some homemade cutting tools. First project was a spindle adapter for a 4 jaw chuck, 2" X 8 TPI. I tested my outside thread cutting tool cutting 1.25" X 7 TPI on a length of 1.5" hot rolled bar. Worked great. The reason for me to cut larger (than the average tap & die set) threads is that I am not likely to buy dies at those $ize$ for occasional needs. It is however a slow process.
Question for any of our machinist members:
When cutting standard 60 degree inside or outside threads, how do you determine the clearances? For example: 1.25" diameter shaft, 7 TPI, my thread gage lists a double depth of .186", advance tool cut in small increments to a depth of .093". In reality there needs to be clearance for mating threads to work smooth together without being too loose. How do I figure how much off the nominal shaft OD and how much deeper to cut the thread? On the spindle adaptor I was able to unscrew the face plate that the work was mounted to and test the threads right on the spindle. It took about three more fine cuts to get it perfect. What happens if there is nothing to test with?
Here is a photo of the spindle adapter, my homemade boring/inside thread tool, the outside thread tool, and some test threads.
I found out that learning to correctly grind the HSS cutting tools is essential for good results and I'm getting the idea one step at a time.
11-18-2005, 07:13 PM
Really nice looking tooling. Nothing like making it yourself, save a few $$, and having the satisfaction you did it correctly.
when cutting an external thread you can use the three wire method
with an outside micrometerto measure the thread then use that as
a gauge when cutting the internal thread
that way the two will fit together
any old machine shop text book will cover thread cutting
and the math involved
11-18-2005, 08:08 PM
Hi Ernie, Machinery's Handbook has two hundred pages of threading information. If you only had one reference book, it would be my choice. If you're cutting many threads, find some used insert tooling that is sized right for your lathe. A lot of stuff that you thread for tooling is best made from prehard materials and threading with inserts will save a lot time.
11-20-2005, 04:36 PM
Thanks guys for your responses.
I would like to get a copy of the " Machinist Handbook" to get to the finer details and go beyond the scope of the two books I used to get me this far: The Amateur's Lathe, by L.H.Sparey, and Textbook of Turning, by Hurcus. These books have some good lathe information for a beginner such as myself, but don't go far enough.
The next question: Which Machinist or Machinery Handbooks should I look for? I see older versions on Ebay for reasonable prices. I'm not looking for a collector's item nor do I need the latest state of the art. I would be more comfortable bidding on a book if I knew what issues or years would do the job. If any members here have a older copy that would work for me and want to part with it let me know.
11-20-2005, 06:45 PM
Thanks guys for your responses.
Which Machinist or Machinery Handbooks should I look for? I see older versions on Ebay for reasonable prices. I'm not looking for a collector's item nor do I need the latest state of the art. I would be more comfortable bidding on a book if I knew what issues or years would do the job. If any members here have a older copy that would work for me and want to part with it let me know.
The later ones have deleted some of the older data related to pre hi speed tool steel tools and have more related to carbide, inserts and cnc work. I have a 23rd edition I bought new and it serves me well. I also picked up an early edition that lists the original owners service assignments to various european aerodromes in the 19 teens! Much of the info is the same.
I'd pick up the most reasonable I could find. There is also a companion book available that tells you how to use it that is quite useful.
A formula that I use to get threads extremely close is to subtract the pitch of the thread from the diameter of the screw being chased.
Example: The pitch of a 20 TPI is .050 (divide 1 by 20). Subtract .050 from lets say a 1/4" (.250) .250-.050=.200. OK. so .050 needs to be taken off the diameter. Thats .025 per side.
This formula works for any size bolt and thread.
Now here is the kicker. Threads are chased using the compound set at 30 degrees and the compound is used to move the tool into the work so you need the compound to move in .025.
In order to move the tool in .025 on a 30 degree angle you need to multiply .025 by 1.1547. .025 X 1.1547= .029 .029 is the depth the tool needs to cut into the material per side.
And to think that I hated math in school. Just didn't understand or have a need for it until I started machining. I still get splinters in my finger from stratching my head.
If this isn't clear as mud I'll try again.
Yes, get a machinist handbook. It will do a much better job of explaining than I. Any year from the 50s to the 70s will do if you want to learn to grind your own tools etc. The newest version has a lot of CNC stuff in it.
Best of luck,
11-21-2005, 03:57 AM
Most times the angle of the tool is set to 29 1/2 degrees and the threads are not that critical even in this application as the small collar at the end is what aligns the chuck to the spindle, this is the area where the fit is very very critical if your going to mount a three jaw of collet holder or any other "indexed tooling" to a threaded spindle.
The four jaw isn't that critical as all the jaws adjust independent, but having said that you don't want to take a chance of it moving on the alignment area as your turning as this will cause some really hard to trace out of spec parts at times.
When turning threads into steel use the correct type of oir lube or it will have the tendency to rip the threads, this is where most people cheap out and just use whatever they have. If your threads come out ragged looking it is tearing the thread tops.
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