vibration / resonance

[boyvox]

I am a newbie and I'm fiddling with an old CNC (thermwood 5ft wide, 5ft tall, 9ft long) that I'm trying to rebuild for woodworking. I have some Slo-Syn steppers on it and some Superior Electric Slo-Syn ss2000d6 drives. The cutting table is 2ftx4ft and the gantry is a pretty beefy unit. I am running it at 3200 step per rev and it just glides like an ice skate at 150 ipm; it's wonderful. When I slow it down, as it gets to around 20ipm, it starts to vibrate and resonate. Then when it drops down below 10ipm it's fine. I am assuming that this is just the resonant frequency of my unit and that's why it starts vibrating around those speeds. My question is what can be done about this? Will different settings, steppers, drives, etc help or modify this. How does one go about dealing with resonance and vibration on a machine? Would using servos instead of steppers cut down on this phenomenon? Is the start/stop of the stepping action a large contributor and would linear motion with a servo work better? Any information on this subject would be greatly helpful. I would like to know how to analyze and tune my machine from the perspective of low vibration and smooth running. I have a pretty good grasp of how to control speeds and resolutions with various hardware and sofware configurations but a very small understanding of what the pros and cons are regarding smoothness of motion.

[jeffs555]

Don't know what rpm on the stepper that ipm equates to, but it sounds like it could be mid-band resonance. Mid-band resonance usually occurs somewhere between 300 and 900 rpm. Some drives have circuitry to try to dampen this resonance, not sure about your drives. There is a little discussion in this thread about it. http://www.cnczone.com/forums/showthread.php?t=13678

[keyne]

Servos don't have a resonance and run generally smoother, quieter and more efficiently than steppers. way to go if you want "ice skate" like performance.
An old thermwood sounds like a *really* nice starting point for DIY-CNC... !!! sounds like it is a moving table design.

[ger21]

INcreasing acceleration can minimize resonance, if your motors are up to it.

[boyvox]

It's a very musical machine right now. Lots of beatiful notes coming from it!

So here's some info on my machine. It's and old Thermwood Cartesian 5 that has a moving gantry. It's pretty worn out but I acquired it for free. It has thompson blocks on 1" dia. rails. The ball screws are rolled 1/2" with 1/2" pitch(inches per turn right?). The ballnuts are crappy warner electric. Z axis had a big dovetail block that I'm not going to use. I will make a new Z axiz more suited to my needs. 2ftx4ft cutting area.

I tried switching my drives to some gecko g201's that I have and first off the motors (Superior Electric Slo-Syn 200 steps per rev, 3.3v 5A, 570oz) are much quieter. When I power up with the Superior Electric drives there is a kind of hiss noise and when they run there is a squeeky rubber quality to the sound. With the Gecko's the motors are dead silent. I liked that. I know the Gecko's have some resonance dampening features built in and at 2000 steps per rev both the Superior Electric and the Gecko's drove the same. But I much prefer the Gecko's as they were much quieter.

I adjusted the steps per rev on the Superior Electric from as low as 1000 steps per rev, to 50,000 steps per rev. I adjusted the acceleration, velocity, etc. No matter what I did with the either of the drives right around 18 inches per minute the machine just starts to vibrate. The whole body resonates. This happens a lot on the long axis and to a lesser extent on the short axis. (Z axis is not on right now). I think that since it doesn't matter what steps per rev I am using it just must be that speed that hits the resonant frequency of the machine. The whole bottom of the machine (from the red line down) is a hollow base with metal walls about 1/8" thick. It BOOMS when you tap it with your foot like a big dumpster. It seems like it wouldn't take much to get that chamber vibrating. I am pretty happy with how the machine runs. The ballscrew on the short axis has terrible axial play and some backlash so it needs to be replaced (if I can find a .5 x .5pitch screw and nut that I can afford!). Despite that it runs pretty smoothly. I am just concerned that when I go up through that 17-19ipm range I get this WOW vibration that feeds back into the table and to the spindle to some extent. I am only doing woodwork so I think I can live with it. I would still like to try to achieve as much of a vibration free system as I can someday, so I am trying to figure out how I will get rid of this. If it is indeed from the base just resonating perhaps I can make a new open base for it. I also would like to try servos and see how that changes things. I also realize that maybe I need to get better ballscrews and make sure all the linear components are as smooth running as possible first. The long screw is about 7 ft long and that would be pricey. The gantry is so heavy that its impossible for me to get in there and really take it apart and check out the ballscrew. I'll have to get something rigged up to take it apart. If anyone has advice on how they would tackle this particular machine, let me know. Any and all advice regarding machine vibration and how to eliminate it would be great. Thanks for the help so far. -Ben

[keyne]

thanks for the descriptions and picture of your machine. lucky you! nice machine for the price! it looks big for the cutting area.
what kind of rapid speeds do you get?
Is the long axis only driven on one side of the gantry?

that spindle motor is enormous...
would it be possible to brace the base, and/or fill it with sand?

[jcc3inc]

Sir,

Do you possibly have excess backlash in your leadscrews? With 16x microstepping, I would expect NO resonance in operation. Can you run the motor thru the speed range with the motor disconnected? Particularly at the resonance frequency?

Regards,
Jack C.

[boyvox]

Keyne,

Yes the machine is very large for the cutting area. Kind of rediculous! It has a
coolant trough around the cutting area. The whole top is a cast aluminum piece. The long axis is only driven on one side of the gantry. The spindle motor is a 1.5hp GE 3 phase induction motor. I'm not using it. Behind that you can see the z-axis stepper, which was huge. I've taken all of that off and right now there is no z-axis at all. I am going to build a new Z axis with THK linear rails and blocks and a wood routing spindle. I have thought of bracing or filling the base, and I may try that. It seems like it would be hard to fill well enough to deaden it. This thing weighs so much that I hate to make it heavier! I could try stacking 25lb sandbags in it untill its filled up. There is a door on the side and I can actually crawl inside the machine! So if I put bags in it I could take them out later if I want to move it or if it doesn't do anything.

Jack,

Yes I think it is very likely that I have backlash in my leadscrews. I think that is one of the most likely problems and the first thing I should take care of. I know for sure that the short axis has backlash and quite a bit of it. I need to set up and test the long axis. I really need to get a new ball screw, but I just don't have the money at this time. The motors seem to run fine by themselves so it's probably some aspect of the machine. Probably a combination of the ballscrews being worn out and the giant trash can body. Perhaps if I fix the ball screws the hollow bottom won't be an issue at all.

Thanks for the suggestions. I don't have a lot of time to work on it, or money so it will be a while until I can try some of these things.

Thanks,
Ben

[bryanrabb]

You can always send out your ballscrew and have it rebuilt