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    Default Yet another mini lathe CNC conversion

    This mini lathe is branded by Proma (a Czech toolmaker), but it is a pretty standard SIEG lathe.

    Yet another mini lathe CNC conversion-proma-jpg

    I purchased it couple of years ago and since then I was implementing my CNC conversion on the weekends (with long pauses for running other projects between the steps). In this blog I’m going to describe conversion solutions I used step by step.

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    Attached Thumbnails Attached Thumbnails Yet another mini lathe CNC conversion-proma-jpg  


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    Default Re: Yet another mini lathe CNC conversion

    The first stage was a “proof of concept” with Z-axis only. I had already some experience with CNC routers, and implemented 4-axis router with Mach3 control previously, so I did not have initial problems with electronics – my router setup had two 3-axis controller/driver (based on Toshiba TB6560 chips), driven by a 2 LPT-ports computer, of which only 4 channels were used for the router, so I had 2 axis available – just enough for the lathe. However it was a challenge to implement a stepper-driven axis with minimal metalwork – so I started with removing change gears and putting a simple plate for stepper installation. Z-stepper was driving a standard leadscrew, and moving the existing carriage apron.

    Yet another mini lathe CNC conversion-1-proof-jpg



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    Default Re: Yet another mini lathe CNC conversion

    Well, it worked. The next step was to find a simple way to automate X-axis. I used a trivial scheme replacing an X-feed handle with a stepper – so I had a fully functional CNC lathe since then…

    Yet another mini lathe CNC conversion-2-xaxis-jpg



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    Default Re: Yet another mini lathe CNC conversion

    You may see limit switches used for homing both Z- and X-axis. Some wiring was done on the Z-axis stepper plate for limit switches and steppers connections.

    However, backlashes were terrible, especially on Z-axis. It was possible to adjust X-axis for approximately 0.1 mm backlash, but on the Z-axis it was 0.5+ mm. The standard half nut on the leadscrew of mini lathe is absolutely not meant for CNC driving.

    Yet another mini lathe CNC conversion-3-zaxis-jpg



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    Default Re: Yet another mini lathe CNC conversion

    So the next step became replacing the Z-axis leadscrew by a ballscrew. I ordered a 16-mm ballscrew SFU1605 of overall length 650mm and BK12/BF12 bearings on eBay. After removing the standard leadscrew, pinion bar and carriage apron I had to drill and thread 4 M6 holes in the machine bed for installation of the ballscrew. The construction of Z-stepper connection was changed as well – I connected it to the bearing holder on the headstock side of the ballscrew.

    Yet another mini lathe CNC conversion-4-overall-jpg Yet another mini lathe CNC conversion-5-index-jpg Yet another mini lathe CNC conversion-6-tailstock-jpg



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    Default Re: Yet another mini lathe CNC conversion

    One can mention couple of other improvements on these photos. First of all, the compound rest is replaced by an aluminum cylinder (50 mm diameter, 40 mm height), as it is not needed for CNC lathe. But another (and much more significant) change here is the aluminum box and display on the headstock. This is the spindle automation PID controller, and it was probably the most complex part of the project.

    I designed a USB-driven controller based on Arduino Nano, and programmed a control plugin for Mach3 to report status, coordinates and spindle speed to this controller. Arduino drives a digital potentiometer (MCP41010) that replaces a standard potentiometer of the mini lathe and a block of relays, which replaces a standard direction toggle switch. It adjusts the digital potentiometer to achieve speed setting and displays status and current coordinates on the small LCD screen.

    Yet another mini lathe CNC conversion-7-pcb-jpg

    Well, there are actually two Arduinos here – a Nano, which interfaces with Mach3 through USB, and Mini, which drives a digital potentiometer. They are optically isolated, as the SIEG control board is not isolated from the mains. The relay on the board isolates Mach3 eStop signal from the control board power – when emergency stop button on the lathe is pressed, Mach3 goes to Reset state.

    Yet another mini lathe CNC conversion-8-arduino-jpg

    PCB is designed in DipTrace and drilled and engraved on the CNC router. The overall setup looks like following (well, it’s not exactly the same version, but the idea is the same).

    Yet another mini lathe CNC conversion-9-pid-jpg

    So actually there are three programs running when controlling spindle speed: Mach3 plugin on the computer, interface and PID on Arduino Nano and digital potentiometer driver on Arduino Mini. Mach3 is talking to Nano through serial over USB, and Nano is talking to Mini through optoisolated proprietary channel. It could be implemented much simpler if I had fast enough optoisolators to drive digital potentiometer SPI interface, but I had not, so Mini serves as a simple buffer to convert slow proprietary serial interface to relatively fast SPI for setting digital potentiometer value. Two 10K digital potentiometers are combined in parallel to achieve 5K total value (the standard SIEG potentiometer is 4.7K).



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    Default Re: Yet another mini lathe CNC conversion

    Now I had a pretty stable speed control, index signal on Mach3 and good enough Z-axis (with backlash less than 0.01 mm). The next step was to convert X-axes and to increase positive cross distance (everybody who have experience using SIEG mini lathe knows that the standard toolpost position is too close to the spindle center – the overall thread of cross feedscrew is just about 40 mm, which is much less than maximum diameter of stock over the cross-slide). So I planned to move toolpost about 30 mm (it became possible after removing the compound rest), and replace the feedscrew with a ballscrew. It is definitely a challenge, as the cross-slide of mini lathe is pretty compact and a standard ballscrew does not fit it. I managed to order a small SFU0802 ballscrew on eBay, and used it in a bit unusual way – secured the ballnut on the fixed part of the cross-slide (I had to drill the existing screw hole to 16 mm and drill and thread 4 M3 holes for fixing ballnut), and the bearing on the moving side. In this case the stepper is moving together with the moving part of the cross-slide, and to keep balance and scale you need to place it under the slide.

    Here it is shown how the construction looks like from the headstock side:

    Yet another mini lathe CNC conversion-10-x-head-jpg

    And this photo shows the same from the tailstock side:

    Yet another mini lathe CNC conversion-11-x-tail-jpg

    The 10 mm stepper plate is milled on the CNC router and has pockets for the Nema23 stepper and ballscrew bearing of different depths to get the same level of the shafts on the other side. The toolpost is sitting on the horizontal 12 mm plate and is shifted 30 mm from the original position, with toolholder rest reduced to 28 mm height to keep the standard height of 40 mm as it was in the original design. The horizontal plate is fixed through the standard holes plus 2 additional M6 holes threaded in the moving part of the cross-slide.

    Yet another mini lathe CNC conversion-12-xplate-jpg



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    Default Re: Yet another mini lathe CNC conversion

    Total cross distance with such construction becomes more than 70 mm with 52 mm in positive direction, which is exactly enough for the maximum diameter of stock over the cross-slide.

    Here you can see how the overall construction looks like:

    Yet another mini lathe CNC conversion-13-overall-jpg



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    Default Re: Yet another mini lathe CNC conversion

    Reducing the stepper speed in 3:1 ratio was a mistake – there is no real need in increasing moment, however, it significantly reduces maximum X-axis speed, especially taking in account that the small ballscrew has a 2 mm twist – I plan now to change timing belt pulleys to 2:3 ratio, it will make a 3 mm advance per stepper rotation (comparing to 5 mm per rotation on Z-axis).

    Another improvement plan is to increase resolution of the digital potentiometer to 12 bits comparing to 8 bits in the current setup – 256 grades is not enough to stabilize PID on the exact speed defined by G-Code “S” command. It turned to be easy enough – just to add one more digipot chip and a couple of balancing resistors, using the following scheme:

    Yet another mini lathe CNC conversion-14-digpot-jpg

    http://www.analog.com/media/en/techn...tes/AN-582.pdf



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    Default Re: Yet another mini lathe CNC conversion

    I would like to extend my gratitude and appreciation to the members of CNC Zone Mini Lathe forum, whose posts gave me a lot of ideas and strongly influenced engineering decisions of this conversion. Though I did not directly replicate any of published conversions, I used your experience shared in this forum. Especially I would like to thank Simon C for his thread “7x14 cnc conversion”.



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    Default Re: Yet another mini lathe CNC conversion

    Quote Originally Posted by IgorA View Post
    I would like to extend my gratitude and appreciation to the members of CNC Zone Mini Lathe forum, whose posts gave me a lot of ideas and strongly influenced engineering decisions of this conversion. Though I did not directly replicate any of published conversions, I used your experience shared in this forum. Especially I would like to thank Simon C for his thread “7x14 cnc conversion”.
    Now in a couple of years the project is finished - I milled another lathe plate and changed X-axis speed ratio to 1:1 - see the photo. Now I have stable CNC turning and threading with different rates. It was a lot of fun to do this project

    Yet another mini lathe CNC conversion-img_0134-jpg



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    Default Re: Yet another mini lathe CNC conversion

    Quote Originally Posted by IgorA View Post
    Now in a couple of years the project is finished - I milled another lathe plate and changed X-axis speed ratio to 1:1 - see the photo. Now I have stable CNC turning and threading with different rates. It was a lot of fun to do this project

    Yet another mini lathe CNC conversion-img_0134-jpg
    Im just now coming across this thread . It is 7 years old but I sure could use some of your ideas would you be interested in sharing /selling your plans for the conversion like drawings , blueprints , wiring diagrams ? Just thought I would ask because this is one of the best 7x conversions WELL DONE sir !!!!!!!!!!!



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    Default Re: Yet another mini lathe CNC conversion

    Thank you, I would definitely be glad to share some of the drawings and program code I used in the project, but unfortunately I do not have a full set - many simple parts were created "as is", without any documentation. I'll look in my archives and publish what I have in the next few days.

    Frankly speaking, if I would start this project today, I would definitely go other ways in some decisions - the main mistake I've done was trying to use drive and electronics of the original design. It would be much simpler and functional to drop the original electronics and spindle drive and replace it with a powerful stepper drive - there are Nema 34 motors with virtually same size and power as the original DC drive. The control scheme would me trivial in this case, as well as the significant increase in functionality could be achieved.



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    Quote Originally Posted by IgorA View Post
    Thank you, I would definitely be glad to share some of the drawings and program code I used in the project, but unfortunately I do not have a full set - many simple parts were created "as is", without any documentation. I'll look in my archives and publish what I have in the next few days.

    Frankly speaking, if I would start this project today, I would definitely go other ways in some decisions - the main mistake I've done was trying to use drive and electronics of the original design. It would be much simpler and functional to drop the original electronics and spindle drive and replace it with a powerful stepper drive - there are Nema 34 motors with virtually same size and power as the original DC drive. The control scheme would me trivial in this case, as well as the significant increase in functionality could be achieved.
    Yes thank you any notes or suggestions would be appreciated and I agree 100 percent with the spindle the electronics are a bit out of date on these type/style "mini lathes ". I am looking forward to seeing the material you are able to share I like the style of your machine and again great job



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    Default Re: Yet another mini lathe CNC conversion

    Quote Originally Posted by Michael82 View Post
    Yes thank you any notes or suggestions would be appreciated and I agree 100 percent with the spindle the electronics are a bit out of date on these type/style "mini lathes ". I am looking forward to seeing the material you are able to share I like the style of your machine and again great job
    Well, to start with here are DXF files for the X-axis holder. 12 mm for the top horizontal plate and 10 mm for the main vertical plate. Depths.txt shows depths of deepenings in the vertical plate (in mm's).

    Attached Files Attached Files


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    Quote Originally Posted by IgorA View Post
    Well, to start with here are DXF files for the X-axis holder. 12 mm for the top horizontal plate and 10 mm for the main vertical plate. Depths.txt shows depths of deepenings in the vertical plate (in mm's).
    Wonderful yes this is perfect to start with thank you



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Yet another mini lathe CNC conversion

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