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Thread: Mini-Lathe Improvements, A WIP Thread

  1. #1
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    Default Mini-Lathe Improvements, A WIP Thread

    Been saying id do it for a while, and here i am! Im the proud(ish) owner of a Grizzly g8688 lathe, and have always been of the opinion that while it works, it could work a lot better. I know the general opinion is that this class of machine is more a toy than tool and isnt appropriate for much of anything, let alone spending the time necessary to improve, but my opinion is the bones are there, and getting excellent performance (inside what can be expected for this class of machine) is just a matter of putting in some elbow grease.

    First, lets meet the victim:


    Pretty bog standard mini-lathe, almost no modifications. Now, it does work okay at the moment but ive had some issues with it. The first and most obvious is the plastic headstock gears, I broke the low-speed side a while back so that needs to be replaced. Still works in high gear though, barely. Sticking with the headstock, id also like to change out the stock radial bearings with a set of better quality tapered roller bearings. Ive had some serious issues with workpiece deflection, .005" of taper or more on ~1" parts, and im fairly certain half the problem there is just bearing runout/slop.

    The next problem im looking to correct is the movement of the cross and compound slides. Both work, but not particularly well. Im forced to either keep the gibs so tight movement is next to impossible, or loosen the gibs for easy movement but plenty of slop. Compounding the issue is the fact that even when the gibs are locked, theres still an unpleasant lack of rigidity in the setup. Im pretty certain that this particular problem can be attributed to poor contact at the ways, and to correct that ill be trying my hand at scraping in the ways. Backlash on the 2 slides is also a problem. A minimal one to be sure, but one ill be attempting to correct by the addition of Delrin feed-screw nuts.

    The rest of what ill be doing to this lathe is a touch more standard. A carriage lock, carriage stop, and cam-lock tailstock are also on the list, as is a fine powerfeed mechanism separate from the lathe drive train

  2. #2
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    So, to recap what ill be doing to this lathe:

    -Scraping in the ways of the compound- and cross-slide
    -Delrin nuts for the cross- and compound-slide
    -Replace the plastic headstock gear with a metal one
    -Replace the stock bearings with a set of tapered roller bearings

    That completes what ill be doing to try to address the mechanical accuracy. The separate list to improve usability is:

    -Carriage lock and depth stop
    -Tailstock camlock + improve tailstock alignment
    -Fit fine powerfeed mechanism, separate from the main drivetrain

    I may or may not add to either of these lists. The first thing ill be starting on is scraping in the ways on the slides, so i can get out the most difficult thing first.

  3. #3
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    It sounds like a good endeavor. I use my 7x12 a lot, and found that adjusting ALL the gibs was important to allow easy, consistent and accurate use. By putting dial indicators on the bed, apron, cross-slide and compound I was able to find out exactly where the movement was.

    May I suggest that before scraping the ways that you straighten and lap the gibs? Sometimes they pick up a banana shape that causes erratic contact.

    I don't worry much about the backlash. The proper technique of infeed is needed whether you have .001 or .010 backlash.



    I look forward to your thread.
    Measure twice. Cut once. Weld. Repeat.
    ( Welding solves many problems.)

  4. #4
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    So, part 1, scraping in the slides! Ill be starting with the compound slide, just cause its the smallest and easiest to replace if i screw up. First things first, a disassembly and cleaning to see what im working with:


    So im going to call the bit on the left the saddle and the bit on the right the base. So, to start with im going to scrape in the bottom of the saddle. Initial bluing:


    Yeah, not too great on the whole 'flatness' thing. In case you cant really see it, theres only blue on the very corners, and not much at that. Few rounds of scraping:


    Getting a little closer. Few more iterations:


    Id say thats a pretty good bearing surface. Better than the start at any rate.

  5. #5
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    Now, onto the base, but there we hit a problem:


    It shows just a bit in the photo, but the right side is nearly 5 thou lower than the left. Honestly, not sure at this point how im going to correct that, it seems like a little much to attempt to correct via scraping while keeping everything in plane. I might throw it on the mill with a dovetail cutter and just skim it. Of course, the other problem is my present scraper is too thick to scrape into the corners of the dovetails. Both problems for another day!

  6. #6
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    Every time I see or hear of these small lathes I think of this video.

    Clank clank, screech, thunk thunk thunk, "looks pretty good".


    https://www.youtube.com/watch?v=IwJiyFiqxTE
    Andy

  7. #7
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    I watch that video and see that none of his gibs are properly adjusted. You can see the toolpost tilt to the left as the cross slide lifts on the right side. Later in the video it looks like the compound gibs are loose too, as it looks like the tool post is tipping forward under load too.

    Of course, as soon as the tool breaks off a bit of aluminum it's no longer under load it springs back to normal and starts all over again. It's a perfect training film of what not to do.


    Dan
    Measure twice. Cut once. Weld. Repeat.
    ( Welding solves many problems.)

  8. #8
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    Quote Originally Posted by epicfail48 View Post
    rly 5 thou lower than the left. Honestly, not sure at this point how im going to correct that, it seems like a little much to attempt to correct via scraping while keeping everything in plane. I might throw it on the mill with a dovetail cutter and just skim it. Of course, the other problem is my present scraper is too thick to scrape into the corners of the dovetails. Both problems for another day!
    The horizontal surfaces being at different heights doesn't much matter....the do have to be parallel though. You have to make small scrapers to fit in small dovetails and you will need to relieve those dovetails....can't scrape them properly otherwise. Relieve with a heavy old power hacksaw blade or in the mill. Something with a radius is nice, no stress riser (this cutter was originally used for fluting taps and Bertram (P&W)) but not critical, lots are done with straight cutters

    And to put in oil passages and cups while its apart, every machine should have them and its surprising how many don't





    Last edited by Mcgyver; 12-21-2017 at 02:06 PM.
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  9. #9
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    Quote Originally Posted by Mcgyver View Post
    The horizontal surfaces being at different heights doesn't much matter....the do have to be parallel though. You have to make small scrapers to fit in small dovetails and you will need to relieve those dovetails....can't scrape them properly otherwise. Relieve with a heavy old power hacksaw blade or in the mill. Something with a radius is nice, no stress riser (this cutter was originally used for fluting taps and Bertram (P&W)) but not critical, lots are done with straight cutters

    And to put in oil passages and cups while its apart, every machine should have them and its surprising how many don't





    Oilcups, thank you! I knew I was forgetting something on that list. May not end up with them on the compound, but I'd definitely like to put them on the cross slide and carriage ways.

    I'd like to have the surfaces at the same height to prevent an extreme amount of tilt from interfering with tool clearances. As it is now, if I were to flatten the 2 ways on the base, I'd imagine I'd end up with 3-4 degrees of tilt left-to-right. May not actually affect anything in real life, but still something I'd like to address.

    As for the relief groove, I knew I needed to put one in but I'm still trying to make up a way how. A mill with a slitting saw would be my first choice, but thatd require a non-insane setup and a slitting saw, neither of which I have. Given the low criticality, I was honestly thinking of going in there with a cutting disk on a pencil grinder

    For the scraper I was considering just making a carbon steel one. I know the edge wouldn't last very long, but the dovetails are small, I've got a lot of carbon steel hanging around and I'm just not set up for grinding carbide

  10. #10
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    Quote Originally Posted by epicfail48 View Post
    I'd like to have the surfaces at the same height to prevent an extreme amount of tilt from interfering with tool clearances. As it is now, if I were to flatten the 2 ways on the base, I'd imagine I'd end up with 3-4 degrees of tilt left-to-right. May not actually affect anything in real life, but still something I'd like to address.
    I'm not sure I follow. The top of the stack where the tool post mates has to be parallel to the lathe bed horizontal way, but that doesn't mean the cross slide horizontals have the coplaner. Parallel yes, but not coplaner. You'd like that them to be coplaner and its convenient to have them so, but if you're concerned there is so much to scrape, you can (with zero detriment to objectives) scrape them parallel but not coplaner and get back to flat to the bed on the next surface
    Last edited by Mcgyver; 12-21-2017 at 05:42 PM.
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