“How do I set my lathe so that I can take very fine cuts?”
This question comes up occasionally on the forum and I’ve seen it addressed in books, but I’m not sure if I’ve ever seen a video of the process, so I decided to make one.
Setting up your lathe to take very fine cuts is a simple process. The quick version is this: By setting your compound at an angle of between 5° and 6° you’ll be able to use the dial on the compound as a very fine feed, advancing the tool in tenths for every thousandth you turn on the dial.
This is sometimes referred to as “Slewing the compound”. Although I’m not sure how technically accurate the term “to slew” is when talking about lathes. This could be slang for all I know and to make matters worse I’m not sure if it’s American slang or Brittish slang (as I have read dozens of books from both sides of the water), so be careful if you decide to break out the term in a shop full of machinists. You might get some funny looks.
I’ve seen this process described in multiple books, including the Machinist’s Bedside Reader by Guy Lautard, and in Lathework a Complete Course by Harold Hall. Both of which are excellent books. The first book by Lautard shows you the math behind the process I’ve outlined here and uses imperial (inch) measurements. The second book by Hall describes a slightly different method and is written for those who work in metric.
Anyway, if your compound is set at 5.75° and you advance the dial on your compound .001″, the tool bit advances toward the part .0001″ thus taking a very fine cut.
That’s as easy (or as complicated) as it gets. For most of you the image above will be enough of an explanation, but for those who require a little more reinforcement of the concept, here’s a “short” video of the process. I took 8 minutes to explain what should have taken 60 seconds. It seems I need to work on being succinct and not sounding deadpan. But hey, we all have our things to work on, right? Bueler … ?
Setting your compound to exactly 5.75° isn’t critical, somewhere between 5° and 6° will get you very close. You can also use this process for metric cuts.
Do you have a different method that you like to use? Please leave a comment on the forum. We’d like to hear it!