Second Momus Build (Pro Version)

[OCNC]

OCNC, there's a thread somewhere entitled, Make your gantry rock solid, that might be a cool retrofit for your larger format MOMUS. It uses pulleys and cables in a "moving loop" formation that resists gantry twisting.

Yeah, I've thought about that too...it's the same way a drafting board works. I wasn't sure I would be able to find a cable that wouldn't stretch too much. I'll look for the link. Right now I'm leaning towards a slaved stepper.

Chris

[bjesson]

I want to build one eventually, to use in my office, when I need to do some work where my other CNC wouldn't be convenient. With its speed I think it would make a great reprap machine too. It would mostly be for engraving and foam prototyping.

OCNC, there's a thread somewhere entitled, Make your gantry rock solid, that might be a cool retrofit for your larger format MOMUS. It uses pulleys and cables in a "moving loop" formation that resists gantry twisting.

with the added bearings to the left side of the gantry the only chance for twisting is poor alignment, bearings too loose, or the rails being out of alignment/too loose. I'm approaching this part very soon. Picking up the material for the base on friday and will take me about a week or so to build. i would lean towards slave motor. Never saw a CNC with pulleys

billj

[louieatienza]

with the added bearings to the left side of the gantry the only chance for twisting is poor alignment, bearings too loose, or the rails being out of alignment/too loose. I'm approaching this part very soon. Picking up the material for the base on friday and will take me about a week or so to build. i would lean towards slave motor. Never saw a CNC with pulleys

billj

I saw a cool build from a guy in Mexico that used cables and pulleys in lieu of timing belts!

That said, I too would probably go with a slaved stepper or using a jack shaft or belts to run both sides. My thinking is that the amount of torque applied to the right side carriage will be proportional to the gantry length, so the right side carriage should be lengthened to counteract this as well. Problem with that is your actual axis travel will shrink.

[louieatienza]

Here's a pic of a ballscrew fixed end support block. Basically there are two angular contact bearings in the block bore, face-to-face. The inner ring is very slightly wider than the outer ring. The bearing plate has a ring that prsses against the outer bearing ring, which creates the preload. This ensures that the screw does not move axially or radially, thus adding accuracy to the drive system.

For light (or possibly even medium duty) application you could construct one with two regular bearings and a very light preload (may need a spacer between the inner rings). A possibly simpler way would be to tension the screw between two fixed blocks with angular contact or thrust bearings. This would increase the stiffness of the screw and allow for higher RPM befoe whipping.

[bjesson]

Purchased the wood for the new base on friday, had all the pieces cut out by friday night, used saturday to make the ribs. Used a different technique to make the slots in the ribs this time and it worked out perfect. For each axis of ribs I clamped them all together, marked the slot locations, using a radial arm saw with a dado blade attached i cute the slots. I also marked the ends of the clamped ribs so I could lay them out the same way just in case any of the slots were off by a bit. I test fitted them together and they were perfectly squared. Total time to cut the slots was about 20 minutes working at a slow pace so I wouldn't lose and body parts. Sunday night I started building the base and should be close to done with it tonight..more pics will follow of that process.

Billj

[bjesson]

between family and work they are impacting my timeline here. Started assembly on base beginning of the week, almost done as of 1am this morn. Here's a few pics on progress for those who are interested...

billj

[louieatienza]

between family and work they are impacting my timeline here. Started assembly on base beginning of the week, almost done as of 1am this morn. Here's a few pics on progress for those who are interested...

billj

Family.. they just get in the way!

[bjesson]

Been working on this as time permits and have made a considerable leap in progress since my last post. Got the base completed as far as I need it to setup the rest. Rails have been installed, Gantry is installed and squared, most leadscrew setup is done on both axis. The extra bearings on the left side along with making it a mirror of the right side has worked out quite well. I expected the adjustments to be a little difficult but it went fast. The gnatry moves very smooth and feels a bit more solid. Hopefully I will have all the leadscrew parts complete in the next day or two and hoping more to apply power this week. I have also added the second support bearing as suggested by another member of this forum. Here's some pics of where it was at yesterday...

[bjesson]

That looks like 1/2-10 acme? If so, you've got a combination of poor choices.
1) 1/2-10 acme is slow. Most now use 2 start or 5 start to get 4-5 times more speed with no other changes.

2) A 48" 1/2-10 screw will whip at high speeds. The 2 start and 5 start can minimize this by slowing the rpm of the screw, thus reducing whipping.

3) The 495oz motors lost torque faster than the smaller ones, so it's possible that the 270oz motor will spin much faster. Combine this with 1/2-10 acme, which requires higher rpm for higher speeds, and you may find that the 495oz motor will perform worse than the 270oz motor.

On my 30x45 machine, I use 1/2-8 2 start acme with a Xylotex and 250oz motors. I can get 190ipm reliable on my 40" Y axis screw, but my 60" X axis screws whip badly at high speeds, which limits me to 150ipm on the X. That's 600rpm, which would be 60ipm with 1/2-10 acme.

ger21,

since you have been down the road I am on already maybe you can help me out. My machine is finished. Powered it up tonight. I went with your suggestion on using the 270 oz in motors as master/slave on the Y axis driving the 1/2-10 single start lead screws. X axis is using a single 495 oz in stepper driving a 1/2-10 5 start lead screw. Z is using a 495 oz in stepper driving a 3/8-10 lead screw. X and Z run great, but Y is having issues. Everything is set to 1/16th microstep. For X and Z i have mach setup to 6400 pulses per inch and Y is set to 32000 pulses per inch. When I move on the Y axis the steppers seem very weak. I can grab the lead screw and the motor will stall. If i try this on the X or Z axis it doesnt even hickup just keeps on spinning. The odd thing is if I lower the pulse rate on the Y steppers to 6400 then the torque seems to be fine but then Y will not travel the correct distance obviously. I'm thinking maybe the Velocity/Acceleration settings may need to be better tuned...any thoughts? anyone?

Thanks,
Billj

[louieatienza]

ger21,

since you have been down the road I am on already maybe you can help me out. My machine is finished. Powered it up tonight. I went with your suggestion on using the 270 oz in motors as master/slave on the Y axis driving the 1/2-10 single start lead screws. X axis is using a single 495 oz in stepper driving a 1/2-10 5 start lead screw. Z is using a 495 oz in stepper driving a 3/8-10 lead screw. X and Z run great, but Y is having issues. Everything is set to 1/16th microstep. For X and Z i have mach setup to 6400 pulses per inch and Y is set to 32000 pulses per inch. When I move on the Y axis the steppers seem very weak. I can grab the lead screw and the motor will stall. If i try this on the X or Z axis it doesnt even hickup just keeps on spinning. The odd thing is if I lower the pulse rate on the Y steppers to 6400 then the torque seems to be fine but then Y will not travel the correct distance obviously. I'm thinking maybe the Velocity/Acceleration settings may need to be better tuned...any thoughts? anyone?

Thanks,
Billj

Have you tried setting a smaller microstep vavlue for the Y axis drive? Like 1/2 or 1/4? Since you havve a finer lead on that axis you may not need a higher microstep? Also, microstepping comes at a cost of torque decrease, the higher the microsteps the more torque lost. The GeckoDrives get around this somewhat by morphing to full steps at a higher RPM.

[marsh]

Hey there I'm pretty new to the forum. Been watching the momus builds as I like the design. I want to build a small machine to mill fiberglass pannels for surfboard fins.

I really like your adaptation bjesson with the ball screws, seems like it would have higher precision?

Just wondering why Vee Bearings weren't used in the Momus design? You wouldn't have to machine so many intricate parts? less wheels to adjust and better for dust issues?

Also for the racking issue there is a thread on cnczone about a draw string method to fix this, maybe it could suit momus?

http://www.cnczone.com/forums/diy-cn...ock_solid.html

Just a few thoughts, I really don't know anything about cnc but I want to build a small machine like this.

[ger21]

What drives are you using, and what voltage are you running at?

I'd try as Louie suggests, but I'd try 1/8 stepping first. COuld be that your PC is having trouble with the high pulse rate. You might also try increasing the pulse width.