[JFettig]

Hey guys, I have a homier mini lathe and thinking about CNCing it.

After using the Bridgeport EasyPath CNC lathes at work and turning thousands of parts I have gotten an urge to convert mine, I woudl like it to be fully cnc and manual.

The main questions I have are what size steppers?

I had an idea for turning threads too, disingaging the spindle and driving the leadscrew and having the gear train to drive the spindle. Although it seems that would take tons of power.

Would a 300oz/in motor be enough for that? I would be hooking this up at the end of that screw and always have the halfnut engaged.

And for the X axis, I think i woudl only need about a 100oz/in motor right?

I was also thinking I might want to use a servo for the Z axis because if I miss steps, that would mean possibly throwing a rapid right into the chuck, that wouldnt be pretty for this I woudl definately use limit switches, kinda like how I should have on my mill


Let me know what you think.

Jon

[kong]

I converted my 8x14 lathe to servo powered cnc, I using 360 oz-in motor on the z-axis, on a 3-1 reduction ratio. It has far too much power for that little lathe, but it was I had lying around at the time. The 3-1 reduction still gives me rapids of around 85 IPM, which again is far too fast. So yes, 300 oz/in will be fine, perhaps go for a 2-1 reduction depending on what speed you need.

[JFettig]

alright, cool. I appreciate it. thats pretty much what I was thinking.

What do you think about threading like I have mentioned? will it have enough power to turn the spindle and move the axis?

I want to set mine up so that you can use it completely manually and completely cnc, thats what my goal is. when I get my mill set up again, Ill probably start working on it.

What did you use for your X axis?

-edit- The cnc lathes at work have like 3-400IPM rapids or thats what it reads before it says rapid on the screen.


Jon

[kong]

Lol, 400IPM rapids on a mini lathe, I wonder how long the bed would last!
Seriously though, I am using Mach2 to control the lathe, with the aid of a tachometer fed into the software, cnc threading is possible. Mach2 will adapt the feed speed to maintain the proper pitch on the thread. Of course, I have not got that far yet since I have had no need to turn threads, but you may be able to use the info so the servo does not have to drive the spindle too.
As for my x-axis, I am using an unknown servo motor I picked up on ebay for about £5. It is a slow speed type - about 500 rpm, but bags of torque. I run it 1:1 ratio, and have never had it stall, even though I have some accidents with the work being pulled from the chuck. I guess 100 oz/in would be fine, our little lathes don't cut that fast, so you should be able to maintain the torque. Obviously, if you can get a more powerful one, it will be a bit of insurance against lost steps if you are going the stepper route.

[JFettig]

thanx a ton man, Ill take a look at servos but if nothing comes up for a decent price, I will probably be using steppers. I know of a stepper motor that is 5v 1a but doesnt say anything about torque nor can I find it when I search the model number, if you know anything about how much it would have, let me know, otherwize Ill be looking around.

Jon

[balsaman]

Those are fairly common numbers for 50-60 oz-in steppers.

I would say there is no way you can use the gears to run the spindle via the lead screw. You need to do it the way Kong mentioned by putting a simple tach on the spindle.

There used to be a fellow selling 114 oz-in steppers on ebay...can't seem to find him now. Someone else will pipe in with his ebay name...

Eric

[balsaman]

Kong, I would be very interested in your 8x14 conversion. Is it documented anywhere on the zone? Pictures etc. I think my lathe is very similar, but I don't have the quick change gears. Either way I would love to see how yours was converted.

Eric

[kong]

I think the guy is called deepgroove1 - I bought some steppers from him over a year ago now, so don't know if hes still around. Balsaman, ok I will get some piccies up, but it is a bit of a bodge job to be honest. I started buying the pulleys, then the company stopped selling the timing belts, so ended up buying bigger belts and using some crude bearings-on-bolts to take up the slack. I will get some photo's in the next couple of hours.

[kong]

Here we go, some photos of my conversion. I hope you dont mind the thrad crash Jon, but hopefully they will help you out too.

So, in no particular order, the z-axis servo is mounted to a ali plate, which is bolted ontp the back of the lathe. I have used nuts as standoffs to align the pulleys. The large ali block in the foreground houses 2 deep-groove bearings - thrust would have been better, or even angular contacts. this conversion was done on the cheap!




The z-axis nut is delrin, 3" long, which should stop it from "squishing" undeer pressure.

Next is the extension I made to hold the x-axis bearings. the top was milled with my hacksaw and files to allow the x-axis to slide over the top. The leadscrew was replaced with a longer one, the nut was replaced with a bronze nut, and again, 2 deep-groove bearings on each end.




And next, the x-axis motor is mounted to an ali plate, which is offset to allow the motor to travel below the bed. The ali plate is mounted to a round ali block, which holds the leadscrew bearings. This, in turn, is bolted to the cross slide.




Lastly, a very simple control box holding two gecko 320's. Nothing fancy, simple engage/disengage switch, and power on/off.


Hope this helps. but like I said, could have been better if I had a few quid to spend. Perhaps when I get the wheels a rollin'

[balsaman]

Nice! So I take it you can no longer run it manually.

Eric

[kong]

Nope, I use fly-by-wire! Hehe, jog it in Mach2 at a low feedrate.
You will know more than i do, but I think there are dangers of driving the servo motors with handwheels, don't they become generators or something? Are steppers different?

[JFettig]

I know steppers are like that, I dont think servos are, they are mostly just dc motors arent they?


Jon

[balsaman]

I want my pie and to eat it too, so I am planning ways to have both...

DC motors do beme generators when turned.

Eric

[kong]

So, can you simply unplug the motor from the drive, or should there be some sort of resistor in there to soak up the generated voltage? It would be nice to have a manual cnc lathe, but I would want detatchable handwheels too.
Actually, there may a way to do it, but it would involve using rotary encoders as digital handwheels, so even then, you don't have the "feel" of the lathe.

[balsaman]

As far as I know it does not hurt the 320 drives when the motor becomes a generator, so isolating the drives from the power supply should do it. Check with Marriss first.

I was thinking of ways to "disengage" the x axis from the drive mechanically, possibly using a new leadscrew to drive it and keeping the original intact allong with it's half nut and rack and pinion....not sure yet. Z-axis the options are more limited...

Eric

[Hobbiest]

I was told that a DC motor would only become a generator if magnets were placed on the armature. I can't remember the last time that I took apart a DC motor, but I seem to remember that the armature is already magnetic, and that the windings are around it. Could anybody explain how the motor is made up so that I don't have to take any of my toys apart?

[Chagrin]

A "permanent magnet" motor does have magnets (obvious) and will act like a generator. Servo motors are of this type. The Ametek tape drive motors that many use in their CNC projects are also collected by renewable energy (windmills, etc.) folk.

You might be thinking of universal motors which are frequently retrofitted with magnets to become generators.

[JFettig]

I made a small amount of progress on my lathe I drilled a hole in the end of the leadscrew, 1/4" and I made a 3/8 shaft with a .251" end that I press fit into the end, I made it long enough so I can still cut it down, turn it down or whatever I need to do. I also plan on putting a handwheel on the end of it along with a pully. I will be doing a 2:1 reduction for more torque with a japan servo motor 300oz/in stepper, same ones I have on my mill.

I need to find a 1-200oz/in stepper for the X axis still, I will do a similar thing to the x axis as I did with the z but I will be doing a 1:1 ratio and milling under the table thing for room for a small pully and offset the stepper downward or to the right.

Here are 2 pics of my progress so far:




I will be making either a brass bushing for the end plate so it has something to support the end with the pully and such, be using a ball bearing.

I plan on having the motor offset to the other side of the lathe and behind it.


-edit- what drives do you think I should get? I was thinking something along the lines of xyoltex but I need one that can do 4 amps for my 300oz motor. I only need 2 axises too.

Jon

[balsaman]

Will you use the stock half nut, or a new nut to drive the x axis?

[JFettig]

Im not really sure, maybe at first but I really should have a new one, I have the nuts from my mini mill still and I think they are the same, maybe Ill preload them just slightly or something.
-edit- dont forget to read my edit on the post before this one

Jon