CNC lathe

I have a Craftsman Atlas 6″ lathe that has been a workhorse for years. Recently I have a couple of lathe projects that are difficult or impossible with a manually operated lathe. So I have been modifying the lathe to add CNC capability. Parts to convert the lathe include stepper  motors, stepper driver, power supply, computer and modifications to the lathe to attach the motors.

I bought the pro driver board kit and a pair of steppers from Hobby CNC. It is fun to build up boards like this and it saves a bit from buying a complete board. It also needs a case for the board. I decided to keep the power supply and driver board assemblies separate so I can swap things around if needed so the power supply is in its own case.

It turned out that the transformer I bought for the power supply put out a little too much voltage. The maximum input voltage to the stepper driver is 42 VDC and the the transformer was putting out just about that when connected to a rectifier and filter capacitors. I think they rated the transformer output with 110 volts input rather than the 120 volts which it the line voltage around here. Actually it runs a bit higher than that most of the time. I really did not want to get another transformer so I decided to take a few turns off the secondary.

transformer2

Transformer Modification

 

This is the transformer opened up with a few turns removed.

transformer1

Transformer Test

 

Here the modified transformer is hooked up to check the output voltage.

On a lathe there are two axis that need to be driven, the main lead screw and the cross slide screw. To attach the pulleys  for the stepper motors I added and extension shaft on the ends of the lead screws. To do this I drilled a hole into then end of the screw and turned down the extension shaft for a close fit into the hole. This holds the extension straight while I silver brazed the extension to the screw.

lathescrew1

Slide Screw Extension

 

Here is the cross slide screw and the extension shaft before brazing. Of course the machining work was done on the lathe.

lathescrew3

Cross Slide Screw

 

Here is the cross slide screw just after the extension was brazed on. After I brazed on the extension I discovered it was slightly bigger than 1/4″ in diameter and the bearing and belt pulley would not fit. So I mounted it in the lathe and turned it down with a file and sandpaper.

lathelead

Lead Screw Stepper Motor

An extension was added to the lead screw in the same was as the cross slide screw. The mount for the lead screw stepper motor is simple, just an angle bracket to bolt it to the table. The pulleys provide a 2 to 1 reduction from the stepper motor.

lathecross

Lathe Cross Slide Stepper Motor

To mount the cross slide stepper motor two holes were drilled in the carriage. The back mounting plate has a ball bearing to support the lead screw extension.

So far I am pleased with the operation of the lathe. One of the difficult without CNC parts I have made is a brass rod with a long taper. That only took a few minutes to machine. Currently I am working on adding a spindle position encoder to the lathe. When that is complete I will be able to cut threads under CNC. Threading is difficult manually and the lathe does not have any gears to do metric threads. CNC will solve these problems.

 

Here is a YouTube video of the lathe in action. https://www.youtube.com/watch?v=vzmgTqwl5tA It only took a few lines of G code to do this operation.

References:

Hobby CNC http://www.hobbycnc.com/

Linux CNC http://www.linuxcnc.org/

 

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2 Responses to CNC lathe

  1. Ray Taylor says:

    Hello Jim, You’re lathe conversion seems simple and nicely done. I have a similar lathe that I’ve been considering converting to CNC.
    I was going to start by replacing the drive screws to minimize endplay. It looks like you modified the existing screws. This is a great cost saver. Are you having any endplay problems? Seems like this could impact threading operations.
    Thanks for the good information and video.
    Best regards,
    Ray

    • jimhannon says:

      Ray,
      The lathe has the same backlash that it has a manual lathe. I deal with the backlash in the G code the same way I would when using the lathe manually. For example establish the home position by moving in the same direction you are going to be cutting. Threading is not a problem as far as backlash. But I still need to add a spindle encoder to do threading as the CNC controller needs to coordinate the tool movement with the spindle rotation. Backlash would be a problem when doing cutting that involves curves that go in an out. Supposedly LinuxCNC has some build in backlash compensation but I have not tried it yet.

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