Tuesday , February 20 2018
Home > Shop Tips > How to Set Your Lathe Compound to Remove “Tenths” (0.0001″)

How to Set Your Lathe Compound to Remove “Tenths” (0.0001″)

“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!

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About Tyler

Tyler is a hobby machinist and 3D printing aficionado. He teaches computer programming and web development at Highline College near Seattle. Tyler founded Projects In Metal in 2008 because he was frustrated by the lack of free plans available for hobby machinists.

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  1. What a great tip Tyler!  I've never heard of this but it makes perfect sense.  I guess the only thing I would say is the carriage, cross slide & compound need to be tight enough and you'd need to have a sharp enough tool to take a tenth cut and leave a good finish. Hitting that last .0005″ is hard enough using the cross slide alone on an old machine.

  2. You're right Fabrickator, I mention in the video that this method shouldn't be used to take a tenth at a time. Even with a very sharp tool and a perfectly rigid setup the tool is likely to rub instead of cut of you're taking suck a light pass. Instead, I use this setup to get to a very accurate final dimension by taking cuts of at least a half thou or greater.

    For example, lets say you want a part that's .050″ exactly and you measure and find that it's still a bit oversized at .0513 inches. That .0013 can be nearly impossible to remove accurately using a traditional setup. But if you had your compound set at the proper angle you could take that last 1.3 thousandths by moving the compound 13 thousandths. 

    So anytime you have a part with a critical dimension (like the OD of a piston or the ID of a cylander bore) start out with your compound slewed to the proper angle. Use your cross slide dial to get you to within one or two thousandths of the final dimension, and then finish your final pass or two using the compound (but make sure your taking at least .0005 at a time when using this method). 

    And just a reminder, HSS is your best option for this type of critical cut because when properly sharpened HSS can actually shave a half thou at a time. Most inserts cannot. 

  3. My appologies for not watching the video prior to commenting.  I don't have the ability to watch videos on my work computer, which is where I do most of my forum surfing.  You videos are very good and your explanation was thorough and accurate, easy to understand.  I have a whole series of videos on You Tube for my Honda stuff and I know how hard it is to make a 3-5 mintue one in one take, non rehearsed.

    Great Job Tyler.

  4. No problem Fabrikator, no need to apologize. You made a very good point, one that I should have addressed in the post as well as in the video. Furthermore, I'm glad you have an idea of how much work it can be to put even the shortest of instructional videos together. But they're worth it if they help people. What's your Youtube link? There may be some Honda fans on here that might be interested.

  5. I'm not sure that anyone is into restoring and modding old Honda Noped's/ 50-70cc 2-strokes, variated drive system (circa 1983) but you can find a bunch of my instructional/informational videos at my “alter-ego” (read – woodwoorking) You Tube “Routabit's Channel”.  I get a kick out of taking the little engines to the max with custom intake, pipes and huge carburators. I'm known as “Extreme Rick” on the forum I frequent because I call my hot rod bike the Extreme Honda Express.  I clocked it at 58mph @ over 10,000 rpm (23mph normally in stock form) with my 220 lb fat a** on it.  I had a couple of Brits pay me $300 for a photo shoot of this bike for an advertising agency to make commericals.  Another Brit professor documented it for the modifications I made to increasing HP in a 2-stroke.


    Here's a pic of it loaded and ready to launch.


    Image Enlarger

    Yet, I digress and highjack your post…

  6. “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”

    A common term most usually 'formally' used in reference to the movement of cranes, it means to rotate around an axis, and usually a vertical one. Technically though it can be applied to anything rotating from one position to another, rather than just going around and around. So one can slew a swivel chair for instance.

  7. @Fabrikator, thanks for sharing your links! You hijacked with permission, so no harm Laugh

    @ Jerry, thanks for filling me in. I always wondered if the term was slang or legit. Now I don't have to worry about using it in front of an “ol' timer” for fear he'll correct me.

  8. My Colchester lathe moves the cross feed twice the amount shown on the dial, so if I move in toward the center 0.001 inches, I take off 0.002 inches on the diameter.  Instead of setting the compound around 5.75 degrees like for a machine that takes off the same from diameter as the dial amount (5.73917 exactly),  I calculated that I should set my compound 2.865 degrees rotated of compount pointed toward and parallel to chuck centerline, rotating to point in 2.865 degrees, or about 2.9 degrees. I will try this with sharp HSS to take off the last few thousandths instead of trying to get tenths with abrasive cloth etc. Thanks for the suggestion.