Stupid question on turning

[pbunnstf]

I have had a small lathe for a year or so and have learned by trial and error.

This is likely a dumb question on measuring - suppose that you are turning down an ACME screw from 1/2" to 3/8" to fit a coupler.

How do you get a real good accurate measurement of the final 3/8".

I have a set of digital calipers that I have used but how is it done with high precision?

[Shotout]

Originally Posted by pbunnstf I have had a small lathe for a year or so and have learned by trial and error. This is likely a dumb question on measuring - suppose that you are turning down an ACME screw from 1/2" to 3/8" to fit a coupler. How do you get a real good accurate measurement of the final 3/8". I have a set of digital calipers that I have used but how is it done with high precision?

I assume you mean turning down the the end of the screw to a shaft, not turning it down and re-threading it. If so to measure the OD use a quality micrometer 0-1in range and for length use a dial indicator with enough travel to accomidate the length, touch off and lightly face the end of the part, set the indicator to 0 and turn the length back using the indicator to measure the length. I like to leave a small finish allowance at a shoulder to face after the last longitudal cut to give a nice clean shoulder.

If you are re-threading it use the above to turn the OD determined by the class fit, calculate you minor dia and cut the thread, before finishing use a thread micrometer to assure that your crossfeed indicated depth is true and your thread is to tolerance. Ex your calculations say infeed by .008 to finish the thread, your mic reading may say infeed by .011, use the mic reading for an accurate precision thread.

A lot of this information is avilable in detail in the Machinery's Handbook, not just classes but actual procedures on how to cut a thread, measure a thread etc. If you are going to be doing precision work it might be worth the investment.

Scott

[pbunnstf]

Thanks Scott. Set up and measurement seems to be the hardest thing to get info on.

[Shotout]

Originally Posted by pbunnstf Thanks Scott. Set up and measurement seems to be the hardest thing to get info on.

Glad it helped. Lots of people on here have helped me so...

You might want to check out some machine tool tech text books. I still refering back to my school text, even a year after graduation, on occasion. Sometimes it is quicker than the handbook, sometimes it is just more plain in language. Since you said you were learning along these types of books are an excellent source of info, not overly technical but they explain the hows and whys of machine work. Amazon.com has a more recent edition of my school text Machine Tool Technology for 98.00 new down to 21.95 used. If you think it would be helpful it is a thought.
Scott

[pbunnstf]

I just ordered four books from Amazon. Machine Tool Technology may be another one. I'll check it out

Thanks again.

[mhtom]

Calibers are great for rough work or ballpark measuring. Nothing beats a micrometer. Same cost but more accurate.

[yoopertool]

try using an optical comparitor or a toolmakers scope. Google either of those and you will see what I mean. With those you can measure any dimension on the thread. Good luck!

[JROM]

Were I come from calipers are called very nears! Get yourself a good set of mikes. You can do .0001 with a good mike. (and a little practice)