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Threading and Change Gears on a Mini-Lathe

One of the better kept secrets of cutting threads on a mini-lathe is the angle of advance.

When I first started cutting threads on my lathe, I had the lathe setup so that I could advance the threading tool into the job using the cross slide. When cutting threads I often found that advancing the tool into the work would cause the tool to jam in the job. I also found the tool would easily chatter and make a very rough thread.

Then I discovered the secret of cutting good threads on a mini-lathe! Change the angle of advance!

Let us consider a thread. A standard metric thread has a angle of 60 degrees. Your threading tool will have an angle of 60 degrees at its tip. During each cutting pass you advance the tool into the work. If you make this advance using the cross slide, you are moving the tool into the work at right angles to the tool tip. The tool tip should, of course, be at right angles to the job.

This means BOTH SIDES of the threading tool will cut into the job. This is a very unstable way to cut on a lathe. If you just try and advance a threading tool straight into the job it will soon jam. This method of threading puts great strain on the threading tool. It will chatter, jam, and, if you are unlucky, break the threading tool

We do not like having luck enter into things. The trick is between each cutting pass, advance the threading tool at an angle. How do we do this? Simple - we ignore the cross slide, and use the adjustable topslide on the mini-lathe.

With the angle of advance set to between 25 and 29 degrees, the tool will only cut on one (the leading) side. In fact, it now becomes viable to grind a very slight rake into the threading tool with this fact in mind.

The picture below shows my internal threading tool setup on the lathe. During threadcutting I DO NOT touch the cross slide. All the in-out action takes place in the direction of the arrows using the top slide control.

For an external thread, place the top slide pointing out to the right. This assumes you are cutting your thread right-to-left.

correct angle of advance for threading tool

Some fiddling is required to setup the lathe in this configuration. I use a simple protractor to set the rough angle. It does not have to be accurate. So long as it is just a little bit less than 30 degrees.

This configuration is well worth the hassle to setup - since using this method I actually look forward to threading jobs!


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