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  1. #1
    Join Date
    May 2016
    Posts
    2

    CNC/manual benchtop lathe

    Hello all,


    I am planning to build a reasonably compact table top lathe.
    This is designed for a Ø100mm chuck (that I already have).
    Axial (Z) travel about 250mm.
    Radial (X) travel at least 50mm (to match the chuck).


    I designed this around 1204 ballscrews and HGW15 linear rails. I tried to keep the saddle height as low as possible (68mm, which feels still quite tall compared with traditional prismatic/dovetail lathes).
    The spindle is high enough so the saddle can slide a bit under the chuck. This is done to have the cutting force mainly as compression in the middle saddle and avoid cantilevered loads that would create rotational forces.


    I am planning this machine to be working first in manual operation and add CNC capability for threads, chamfers and anything that require precision or more than one axis .
    Handwheels are accessible on the front. Steppers, limit switches, wires and associated clutter fits in the back.


    I geared the ballscrew to make the X axis behave like a left-hand leadscrew. There is room for belt driven stepper on the back of the saddle, but this is not an immediate priority.
    The Z axis is also geared with bevel gears to make the handwheel face the user. Still a bit of work to be done there as the handwheel direction is inverted (maybe add a new gear stage to reverse direction?).
    A side effect of using gears is that it will reduce the impact of stepper cogging in manual operation. With direct drive ballscrews, I have 1.8° cogging, resulting in 0.02mm "indexing" of the sliding movement. Using a 1:2 reduction brings that down to 0.01mm (although that will add some backlash).


    For the spindle, I already have a 25mm silver steel rod with a faceplate welded on one end, mounted on pillow bearings and driven by a treadmill motor. On the long term, this would be upgraded to a minilathe CJ18 spindle with proper bearings .


    What do you guys think of this?

  2. #2
    Join Date
    Aug 2009
    Posts
    230

    Re: CNC/manual benchtop lathe

    A few thoughts...

    If you can afford the $$$, servos don't have step detents.

    Don't put gears between the CNC motors and the ballscrews; or you will be fighting backlash & inaccuracy of the gearing.

    You might consider tilting the bed up at an angle to help with chip management.

    You are going to want to make some covers for the mechanical bits to keep chips & coolant out of them (yes, the ballscrews and linear ways have wipers on them; but covering them is better).

    What are you going to make the base plate out of?

  3. #3
    Join Date
    May 2016
    Posts
    2

    Re: CNC/manual benchtop lathe

    Hi Britt,

    Thanks for your feedback, you confirm my concern over gears between motor and ballscrew. I have seen quite a few builds using belts, would that be better?
    I have a mini mill converted to CNC (steppers in direct drive) and the cogging is quite noticeable when turning the wheels manually. I thought this would be more of an issue in a lathe, especially in the radial direction.
    Than I guess, the precision work can always be done by CNC with microstepping.

    I'm not ready to commit to servos at this stage, but it might be an upgrade in the future. Given the length of the travel, it seems a bit overkill.

    I did consider slanted bed, I really like the look of it, but couldn't figure out a way to have easy manual operation in that configuration.

    I have a thick board of melamine plywood that I am planning to start with. It's easy to work with and experiment with.
    Again, the long term plan would be to upgrade to welded steel and/or epoxy granite.

  4. #4
    Join Date
    Aug 2004
    Posts
    780

    Re: CNC/manual benchtop lathe

    For a metalworking lathe---
    I think the spindle shaft is far too slim.
    Use 40 MM tgp or drill rod.

    Steppers will give so-so accuracy direct driven.
    They are springs, with 1 step boundaries.
    Mostly, this will not matter with small-rise leadscrews.

    Use a vastly more rigid base plate.
    A local stonecutter making granite countertops will give or sell 10$ offcuts, 20 or 30 mm thick.
    Epoxy 2-3 together.

    Strength is not the issue.
    Rigidity is.
    Lathes and mills use 2% of strength, at most.

  5. #5
    Join Date
    Jul 2018
    Posts
    6254

    Re: CNC/manual benchtop lathe

    Hi Larry - Good project. If its going to be a CNC machine forget the handles and design a good CNC machine. Then you can take advantage of various things that limit you when handles are not needed. Cheers Peter

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