threads

[teddy52food]

The depth of thread was .096 How would you know when using it at an angle?

 

[teddy52food]

Sorry! I was thinking of reverse on the main drive .Feed is reverseable. I guess that would work but then you have to stop the motor each time or the gears will clash.

 

[4wdtom]

Was .096 the double depth? That would be the depth for a fairly course thread. If it was 8 threads per inch, divide 8 into .75 = .094, infeed with the compound set at 30 deg. Using the compound set at an angle method you use the compound to advance the tool and the cross feed to remove the tool at the end of the cut and then only to advance it to where it was, the zero setting. Always at zero. To make it easy I retract the cross feed one turn and advance it one turn and I set it up so zero has the handle on the cross feed wheel at the bottom. You can do this by adjusting tool in the tool holder or by adjusting the compound before setting the compound dial for zero when the tool just touches the work. Then at the start of the thread both dials are at zero. The cross feed goes in and out at the end and before the start of the next pass, and the compound starts at zero and you advance the tool with it to the specified depth, being careful to sneak up on it by trying the thread or measuring with a thread mike or the 3 wire method. Mostly for general use trying the thread on a nut or what it will be required to fit is fine.

 

[teddy52food]

That was on the thread gage. 18 tpi & .096?

 

[4wdtom]

.096 is the V form double depth of a 18 TPI thread. Standard threads are not V shaped. They have a flat on the point that is 1/8 of the pitch. My example above of a 8 pitch thread would be .125 x .125 + .0156 or .015 or .016. Therefore the depth of cut is less as the point of the tool is not there to extend it. The DD for an 18 pitch USS (American Standard or United States Standard) with the flat is .072. and the amount of compound infeed is .042, 75 divided by 18 + .041, as explained above. Generally for us who do non critical/to spec. work just stoning the sharp point off the tool is good enough although you could compare the flat to a feeler gauge of the right thickness or the set gap in a mike or caliper. There gauges for this buy I haven't had any luck finding one with years of keeping my eyes open for one at swap meets, flea markets, etc. Also when you cut a thread it will likely have points that also should have a flat of 1/8 pitch on them too. Usually a light pass with a file on the points while the lathe is running will do this. The points file off easy. And while the lathe is running wire brush the thread to remove burrs. And make sure you lube the threads before trying fit as dry threads can become stuck and that's not fun. The DD measurement is what you would subtract form the nominal diameter to bore for a nut or internal thread you are making. After the first thread and sneaking up on the fit I am after I find that I can generally depend on the setting of the compound to produce subsequence threads to the same size. Because of all the variables I find the sneaking up on the fit to be necessary. But the pitch into 75 gets you very close. Don't be afraid of threads. after a few it gets easy. I do it all the time for parts I make mostly for tractor/equipment I am working on. Threads cut on a lathe come out true to the part without the risk of getting a die started crooked. And if you don't have a complete set of dies you aren't stuck, and setting up the lathe is quicker than going out to buy a die or waiting for it to be delivered.

 

[MarkB_MI]

> Can't do it sitting in a wheel chair. Can't stand up for long either.Make a few cuts & sit down again. Takes twice as long to get only half as much done.

I thought you mostly use a wheelchair, Teddy. You very rarely mention it, so I don't think most folks here are aware of it.

 

[teddy52food]

Thanks for all the info. I am 80 & this may be the last threads I may have to cut? It was a challange & I got it done! I will try your way if there is a next time.

 

[teddy52food]

Had a bone spur in my back that damaged my spinal cord. Surgury in 1993, 30 years ago. Muscles of my legs are there as always but the nerves are not there to control them. Worse than a drunkard. No help available. Been to Mayo twice. 1973 & 1994.

 

[4wdtom]

No Problem, hope I helped you (and maybe others) to get threads. The South Bend Book, How To Run A Lathe, is a great help in all this. I understand it can be found on line to view but I haven' looked.

 

[John Garner]

For those who are interested in digging deeper into the geometry of the most common North American and European screwthread forms, I'm attaching a link to an essay that I wrote decades ago.
Screwthread Geometry Essay Posting 6 of link