# 7 x 12 Mini Lathe CNC Conversion



## TomS (Feb 25, 2019)

I'm not doing anything that hasn't been done before but at the request of brave_ulysses I'm starting a thread on my conversion of a 7 x 12 mini lathe.  The basis of the conversion is a run-of-the-mill Asian mini lathe that I picked from a lady that was disposing of her recently deceased father's possessions.  The lathe was new but absent the accessories except a three jaw chuck.  The stepper motors (270 oz in), drivers (DQ542MA) and power supply (350W/36vdc) are from Wantai Motors.  I've had good reliability with the Wantai equipment I purchased for my vertical mill conversion five years ago so decided to go the same route on the lathe.  The breakout board is a leftover from my mill conversion as is the UC100.  Ball screws and ball nuts were purchased from Linear Motion Bearings.  Here are a few pics of my progress so far.

As you can see the X and Z motors are mounted and are coupled to 1605 ball screws with double ball nuts.  Another ball screw bellows is on order and should be here in a couple of weeks.  Please disregard the wiring mess.  I just started the wiring to get everything working.  Once I'm satisfied with the results I'll tidy it up.   




A closeup of the X axis mounting brackets.  The only original remaining moving component is the saddle.  The picture doesn't show the new saddle gibs I made nor the machining I had to do on the saddle to get them to fit correctly.  Evidently the factory deemed it not important to machine the gib mounting surfaces and used a hand grinder instead.



Here's a shot of the Z axis mounting brackets.  This picture doesn't show it but I setup the lathe bed on my milling machine and machined the end of the bed so the bracket had a flat and perpendicular surface to mount to.  Setting up the lathe bed in the mill was a real PITA.



Here's a back side pic of the X axis screw and nut.  A cover will be made to keep chips off the ball screw.  The cross slide riser is made of 7075 aluminum.  I'm mentioning this because it's the first time I've machined this stuff.  It's a lot tougher than 6061.



Another view of the Z axis mount.  If you look closely you can see a groove cut in the face of the stepper motor mount.  This was a "doh" moment when I assembled the motor and realized I couldn't get to the coupling clamp screw and set screws.  



Still a work in progress but a pic of the control cabinet.  Most of the test wiring is complete.  Still need to wire in limit switches and the e-stop.  Powered up the system to see if the motors would turn but got driver red alarm lights.  I need to go back and double check all my control wiring.


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## brave_ulysses (Feb 26, 2019)

very clean install! thanks for the pix

any plans for a spindle encoder for threading?


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## TomS (Feb 26, 2019)

brave_ulysses said:


> very clean install! thanks for the pix
> 
> any plans for a spindle encoder for threading?



Yes on the spindle encoder once I get it up and running.  I'm also looking at using gang tooling for multiple tool setups.  Turning CNC is all new to me so I'm taking it a step at a time until I get familiar with it.  Then it's on to the next step.


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## brave_ulysses (Feb 26, 2019)

will be following!

i like the top of your cross slide. mine is bending up slightly when moving in negative x and losing about .2mm. i need to plan a rebuild with sturdier components

how uneven were your ways? the prism side on mine isn't bad; the back side needs some attention


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## TomS (Feb 27, 2019)

brave_ulysses said:


> will be following!
> 
> i like the top of your cross slide. mine is bending up slightly when moving in negative x and losing about .2mm. i need to plan a rebuild with sturdier components
> 
> how uneven were your ways? the prism side on mine isn't bad; the back side needs some attention



If you are referring to the ways being flat/straight not so good.  I found this out when I set the lathe up in the mill to machine the end of the bed for the Z axis mount.  Most of the variation, about .007 - .008) is due to the headstock and tailstock mounting feet not being flat and in the same plane, IMHO.  Shimming the feet got me pretty close.  

What I did find is the saddle gib surface (the bottom side of the ways) was reasonably consistent along the length of the bed in relation to the top of the ways/prism.  I shimmed the saddle gibs to where I could get a .002 shim between the gib and gib surface on one end of the bed.  Checking with feeler gages at six locations along the bed length the clearance didn't vary more than .001.  I was pleasantly surprised that it was that close.   

As I was building my motor mounts and ball nut mounts rigidity was a major concern.  I had a lot of 1/2" aluminum flat bar so I went with that for the motor mounts.  Probably way over-kill but it's what I had.  For the X axis my first thought was to bolt the ball nut mount to the cross slide and add a simple spacer for the tool post.  When I went to the metal store they had a chunk of 7075 laying in their remnant rack and it got me thinking about a one piece solution.  And the price was right.  It evolved from there into what's on the lathe now.  Plenty rigid for sure.  We'll see how it works out under load.


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## TomS (Feb 27, 2019)

I've run into a problem that seems easy to fix but the obvious changes haven't cured the problem.  The problem is the X and Z stepper motors won't turn.  The drivers power up with a green light (measured 36.2vdc at the input terminals) and the motors lock up, as expected.  When I press the keyboard arrow keys there is no motor movement.  I've triple checked my wiring and tried alternate Mach3 configuration settings and still no movement.  I'm using a UC100 for motion control and a licensed copy of Mach3 version 062.  The UC100 green and blue LED's are on and steady.

On the Port Setup screen I've enabled Port #1 with a port address of 0x378.  Kernel Speed is set at 25,000Hz as is the UC100.  On the Motor Outputs screen I've enabled X axis and Y axis (you will understand why I selected the Y axis when you see how I wired the BoB) and input Step Pin #2 and Dir Pin #3 for the X axis and Step Pin #4 and Dir Pin#5 for the Y axis.  I tried checking/unchecking Dir Low Active and Step Low Active.  Step Port and Dir Port is set at 1.

On the Motor Tuning screen I set X and Z velocity to 100 and acceleration to 50.  I tried Step Pulse and Dir Pulse at 3 and 10.  Made no difference.

Here's a pic of my BoB.  I connected my X and Z axis wiring from their respective drivers to the terminals with pin numbers 2/3 and 4/5.  I measure 3.5vdc across the ground and pulse+ terminals which I think is correct.  When I press the keyboard arrow key the voltage drops to 3.25vdc.  IIRC it should drop to 0vdc.  Is this the root of the problem?  Got a few other ideas to try but any input on what I might be doing wrong is appreciated. 



Edit - There is a Red LED next to the terminal block for Pin 1 and Pin 14 and just below the letter "E".  My first thought was this is a power on indicator but now I'm not so sure.  Could this be a fault indicator?


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## brave_ulysses (Feb 27, 2019)

hi tom,
what do you have in the way of diagnostic eqpt?

the step/dir outputs (any many more low-ish speed signals) can be monitored with a cheap usb interface and sigrok/pulseview sw
www_youtube_com watch?v=z8Tdz7eQ

captured this while testing grbl on a nucleo411re


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## TomS (Feb 27, 2019)

brave_ulysses said:


> hi tom,
> what do you have in the way of diagnostic eqpt?
> 
> the step/dir outputs (any many more low-ish speed signals) can be monitored with a cheap usb interface and sigrok/pulseview sw
> ...



I have a multimeter and that's about it.   I'm thinking it's the breakout board but not sure.  If I knew what that red LED is indicating that would be helpful.


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## brave_ulysses (Feb 27, 2019)

do you have another way to toggle the stepper driver step/dir inputs? i'm linux-only, so i won't be any help with mach

can you verify that the parallel port output voltages change with keyboard input?


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## TomS (Feb 28, 2019)

brave_ulysses said:


> do you have another way to toggle the stepper driver step/dir inputs? i'm linux-only, so i won't be any help with mach
> 
> can you verify that the parallel port output voltages change with keyboard input?



I can check that but I think that's the issue.  The output voltage is not changing when pressing the keyboard direction buttons.  I'm a mechanical person so when I encounter electronic problems I struggle.  

I don't think Mach3 is the problem.  The more I think about it's wiring or BoB related.


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## TomS (Mar 1, 2019)

TomS said:


> I can check that but I think that's the issue.  The output voltage is not changing when pressing the keyboard direction buttons.  I'm a mechanical person so when I encounter electronic problems I struggle.
> 
> I don't think Mach3 is the problem.  The more I think about it's wiring or BoB related.



Problem solved!  It was a wiring issue.  I removed the Enable - wire from the BoB and the motors moved as expected.  What's interesting is I wired the motors, drivers and power supply the same as my mill when I was using this same BoB and UC100.  Must be a difference in the driver internals between my mill and lathe.


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## brave_ulysses (Mar 1, 2019)

good news! glad you got it working

on the drivers i have if enable is low or disconnected the motors are powered, enable high - the motors are unpowered and can be turned manually

what is next?


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## TomS (Mar 2, 2019)

brave_ulysses said:


> good news! glad you got it working
> 
> on the drivers i have if enable is low or disconnected the motors are powered, enable high - the motors are unpowered and can be turned manually
> 
> what is next?



That's how mine work also.  Took me a week of tinkering to find that out.

Next up is limit switch installation, guards to keep the chips out of the lathe internals, motor tuning, and cleaning up the wiring.  I ran some gcode this afternoon just to watch it run.  Operationally it seems smooth but the proof will be cutting metal.  Hopefully in a few days.


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