Timing belt and critique drive concept please

[slow-poke]

I'm now thinking about how I'm going to add CNC capability to the knee joint of my A1S mill.

I want to use a similar 2.4Nm continuous torque 7.2Nm peak ac servo, except this one has an integrated brake. I have a beefy double nut ballscrew waiting in the corner.

From my measurements and reading about knee conversions it seems I will need a 3:1 torque multiplication to be on the safe side. Aim is to have < 0.001" error, so driving through the bevel gears is not an option. There appears to be plenty of room to mount this servo just inside the front edge of the knee and then drive the new ballscrew via a suitable timing belt as shown in the concept drawing below. The servo moves with the knee and the upper end nub of the ballscrew simply pushes up against the existing bearing (more likely on a spacer that fits in the existing bearing).

Can anyone spot any obvious problems with this concept?

Suggestions for suitable timing belt that will accommodate about 20+ Nm of torque?

 

[Maker Mike]

Not sure if it is something you would consider, but this is an option. Go to about 5 minutes into the video to see what he did with the Z axis.

 

[slow-poke]

Thanks for the link, I actually watched that one a while back, a couple of interesting points:

1) the gas struts, I'm hoping to avoid that by using a servo with an integrated brake, basically the servo is locked up mechanically whenever it's not being commanded to move, that includes with no power.

2) fortunately it appears I have room to fit the servo towards the front of the knee so I don't need to chop a hole in the knee. The belt and pulley setup will be near identical except to the front instead of the side.

I will be sizing up the servo mounting bracket today. I found a suitable (probably overkill) AT10 belt and for initial test fit and perhaps a couple of up and down test runs I will print the pulleys (found step files at McMaster-Carr).

 

[DavidR8]

@jcdammeyer has powered knee on his mill I believe.

 

[jcdammeyer]

@jcdammeyer has powered knee on his mill I believe.

Oh boy oh boy one of my favorite subjects!
Where to start...
Here's my timeline.
I pulled the shaft that turned the pinion gear which turned the acme screw that lifted the table assembly. I made a new mount for the shaft at the handle end to have bearings instead of bushings and at the pinion end made an adapter that holds a ball bearing and was able to move the pinion closer for less backlash. Attached is a PDF of the drawing.

Then mounted a toothed pulley and size 34 500 oz-in stepper motor and ran it from my Electronic Lead Screw for positioning and motion. Worked fine but very very very slow before it would lock up. Some torque testing and upgraded to about 1200 oz-in and 65VDC power supply. Now I got 25 ipm motion.

Better. Here's a short video of Y and Z motion. Y is a DC Servo, UHU Servo drive. Z is 1200 Oz-In Stepper with Gecko Stepper Driver.

It's been run with a BeagleBone Black, MachineKit (LinuxCNC).

Still not fast enough though. When I started having DC servo problems on the X axis where it would lose position I changed the XY to AC servos from Bergerda. Very happy. A while later upgraded to an AC Servo (750W) on the knee. Still with the same 3:1 reduction. Now 150 ipm.

Meanwhile during that whole time I had the idea to get rid of the right angle drive and change to turning a ball screw directly. This photo shows the ACME style where the screw is turned and the bronze nut is held in the post.



But what if I made a new post out of some pipe and a base designed now to hold a ball nut assembly.




Now mount a motor onto that pedestal and turn the pulley which turns the ball nut. That moves the ball screw up and down and the top is anchored to the table.



The only other thing it would need is some sort of brake that holds it in place when power is removed or the weight of the table will spin the nut and motor assembly.

So that's where I am at the moment.