[CNC] Tell Me About Ball Screws

C

cjones6108

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Ok, so after reading for several hours on the Hobby-Machinist forums, I have taken the plunge and ordered a PM-932M mill, the very most basic model - no power downfeed, no z-axis motor, no x-axis table feed. The idea is to begin immediately planning to convert it to CNC.

I have noticed other members doing this conversion, using 1600 oz.in steppers on the X and Y axes and 4200 oz.in for the Z-axis. These look like pretty hefty motors, and I'm in the process of sourcing the motors and controllers. I'm thinking Ethernet Smoothstepper or similar, particularly because computers haven't shipped with parallel ports or DB-anything ports for the last 10 years or more.

I realize ball screws are going to reduce friction therefore required motor torque, but it seems like with such large/hefty motors it wouldn't matter that much. So what is the reasoning behind replacing brand-new acme screws with ball screws? And do you replace the X, Y and Z screws - All of them?

caj
 
The biggest reason for ball screws is reduction in backlash.

Imagine cutting a circle. As the cutter follows the circular path, depending on the quadrant of the circle one screw will be turning clockwise while the other maybe turning counter clockwise. When moving into the next quadrant of the circle those screws may now have to reverse and turn in the opposite direction. Any backlash will affect the roundness of the circle being cut.

Ball screws have much lower backlash, and preloaded double nut ones have even less.

Hoped this helped.
 
+1 what @DAT510 said. I highly recommend ball screws. Software backlash compensation is a fallacy. It works ok for hole patterns and some other linear moves, but for accurate profiling and circular interpolation you need as close to zero backlash as you can get. Also near zero backlash is required for climb cutting, and most CNC work is climb cut. Better finish, easier on the tool, and normally more accurate. Also a ball screw has about 1/15 the rotational resistance of an acme screw under the same load conditions.

You might look at the ClearPath motors https://www.teknic.com/products/clearpath-brushless-dc-servo-motors/

I did a router retrofit for a customer using these motors and was very happy with the result. They are a drop in replacement for stepper motors, and have a nearly flat torque curve. Stepper motor torque drops off very rapidly with increasing RPM, especially the 1600 and 4200 oz/in motors. The other advantage is that the drive is built into the motor, just connect a power supply and the step & direction signals and you're off and running.
 
OK, that makes sense. Thanks. I'll check out the Clear Path motors.

Now, about "preloading" -- Another term I'm not familiar with. Can somebody explain that?

caj
 
+1 to everything above.

The Clearpath servos are very nice and having the drivers right on the servos is pretty awesome for reducing your electronics enclosure clutter. However, I'd suggest you read the "So, What's The Catch" page on Teknic's web site that describes some of the compromises they made with the Clearpath servos. You'll see that there's no encoder output. Not a show-stopper, but I eventually want to option to close the feedback loop to the controller and the Clearpath drivers are not capable of that.

The one killer for me is that the cable connections at the servo are not waterproof - not mentioned on that particular page. I run fairly high pressure flood coolant and didn't want to have to cobble up some sort of coolant-proof shield for them.

Otherwise, they're awesome. I saw them demonstrated (and met some very nice Teknics guys) at the NYCCNC open house this year. I simply can't say how professional and helpful the Teknics reps were. And you can't beat the 3-month money-back return policy.

Another option to consider for axis motors are DMM Technology servos. Looks like they're roughly competitive with Clearpath on price for a given motor power/size/speed, but the DMM drives have significantly more features than does the Clearpath. The DMM are more in line with Teknic's Hudson servos and Eclipse drives. I've heard/read nothing but good stuff about them.

My situation is not your situation, of course. You may be doing this on the cheap, so you'll be sticking with steppers - they'll work fine at low speeds. If you consider your PM-932 as basically a Tormach 1100 then you can figure out what size stepper motors the Tormach uses. Your spindle head weighs quite a bit more because of the gearing, of course, but you'll be ditching the gears sooner or later and moving to a 1 or 2 speed belt drive so the weight will be back to (about) the same.

Pay attention to what Jim wrote above - ball screws have much less rotational friction than acme screws, and the weight of the head can back-drive the axis screw. Figure on adding some gas springs or a counter weight to the head because the stepper (or servo) may not be able to hold the head up when they're not powered once you have a ball screw on the Z.

What you don't have is Turcite on the dovetail ways like the Tormach. The friction on your ways will be higher than the Tormach, so maybe add 10-15% of stepper (or servo) motor 'umph' to overcome this deficit. Otherwise I think the Tormach motors should give you a good starting point for choosing a motor.

I'd just caution you that trying to save a few bucks here or there has it's place, but eliminating backlash is not the place to compromise. Ballscrews and pre-loaded ball nuts are your friend.

While you're at it, you might as well fantasize about substituting one of those DMM Tech 1.8kW servos for the spindle motor and gears you currently have. A 2.5:1 belt drive and you're good from 0-7500RPM. Plus rigid tapping, 'cause it's a servo and you don't have to add an encoder to the spindle motor. Hoo-ya.

Keep us in the loop how it's progressing, and when you get to certain decisions or options on the electronics. There's more out there than just Mach3 and an ESS...

-Spumco
 
Thanks for those insights. I'll check out these other things you mentioned. Gas springs on the z-axis is an interesting idea, and fairly inexpensive I would think.

You've pretty much sold me on the ball screws, and in the brief check I did on those, they don't seem terribly expensive either. I'm not trying to do this on a shoe-string budget, but I would like to get a quality job that's not too wildly out of proportion to the machine I decided on. My original purchase of a Bolton lathe/mill has been an education, primarily in what NOT to do.
 
Next question: Can someone tell me the exact dimensions for the ball screws and nuts I need for the 932M? My machine won't be delivered until later in the week.

I did find the motors (steppers) on EBay and got them on order, so I'm committing to this project for better or worse.
 
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