acme screw to ball screw question

[Billw]

Hello All
I built my machine with acme screws at first , worked good .
I then added ball screws and nuts from Mcmaster .Seems to be a bit sloppy now .Iam cutting flanges out of .5 acrylic .
My main question is , If I push on the x or y axis I can backdrive the screws pretty easy , Is this normal ? Where can I find anti-backlash ball nuts ,will this solve the problem?, Cant seem to get them from mcmaster .My table is 48x 36y is a 1/2 inch.5 lead srew good for this .
Thanks for the help .

[ger21]

All ballscrews can be backdriven. This is not the same as backlash. To eliminate backlash, you can use 2 nuts fastened together, or replace the balls in the nut with larger ones.

[wizard]

Being able to back drive ball screws is normal with power off to the serovs. If you can do this with the power up and the servos or steppers enabled then you might have a problem. On a CNC system, with Ball screws, the motor holding torque is what keeps everything in postion. That is why vertical axises often have brakes on them. This has nothing to do with backlash.

One thing you can do to measure backlash on a machine with well fitted gibbs is to drive an axis forward at least an inch or so with the CNC controls in manual. Install a dial indicator and step the axis back in the other direction a few thousands at a time. Observe the dial indicator when doing this and note the distance jogged when the dial indicator starts to indicate slide movement. I would doubt that your ball screws are worst than the Acme, but it is not impossible.

As to sloppy what do you mean. Are the parts coming out worst than before? If they are you may have issues with the controls.


Thanks
dave

Originally Posted by Billw Hello All I built my machine with acme screws at first , worked good . I then added ball screws and nuts from Mcmaster .Seems to be a bit sloppy now .Iam cutting flanges out of .5 acrylic . My main question is , If I push on the x or y axis I can backdrive the screws pretty easy , Is this normal ? Where can I find anti-backlash ball nuts ,will this solve the problem?, Cant seem to get them from mcmaster .My table is 48x 36y is a 1/2 inch.5 lead srew good for this . Thanks for the help .

[carlnpa]

To get decent accuracy with ballscrew you need to be into at least precision rolled (+-0.001/ft) with zero backlash ballnut. The standard ballscrew may be (+-0.004/ft) and the ballnut (+-0.007). Ger21 has written the solutions available to create a zero backlash ballnut.

[Billw]

Iam using the Fet-3 board and stepper motors from stepperworld .
When I turn on the power supply for the motors there is not noticable resistance to rotating the shaft but the motors sem to work well .
Shoul they be hard to turn ?

Thanks Again

[LeeWay]

I used the same ball screws as you from Mcmaster Carr. That is if you bought the 5/8" type on the low cost scale. I can push my gantry in either direction with just a little force. I can't do that with the motors powered up.
When I turn on the Xylotex drive, I hear slight thunk sound. This is the motors locking into position as they power up. At this point I cannot move the gantry under my power. It then takes input from Mach 3 for me to move them.
I find that the tolerances I get from the ball screws are just fine for my needs. I can shave off 2 mils of powder coating without actually milling much steel underneath. Thats precise enough for me.

[Billw]

Iam starting to thing there is something wrong with my steppers , I have the wired up as the instructions say but they still move very easy .
Iam overdriving them with 12v with a 1.5 ohm power resistor on the black and white wires .
could my steppers be shot ?

[ViperTX]

Billw....I would suspect that they are low on torque....what's the torque curve look like for your steppers. Also, explain what you mean by "sloppy".

[bunalmis]

Backlash simulation

[wizard]

I'm not familiar with your hardware so I can't comment directly on it.

That however doesn't mean that I can't provide some input. The first thing I would do is to determine what features your drives have. Things such as enable and and current fold back. If you have the machine moving underpower and you can overcome the axis easily then there may be problems with current settings also. Unless you have extremely small motors, what you are describing should not be happening.

So I would double check your current settings and make sure the drives are actually delivering said current. Hopefully they have a test point for that. If you still have issues, see if there is a current fold back or standby option that can be disabled. If so disable it and see what happens.

One thing I forgot to mention is that you should make sure your power supply is up to snuff. Place a DVM across the supply and measure voltage under load. If the supply can current limit that may be an issue. Ideally your supply should be simple and not have these issues.

As to your question about turning the motor YES when pwoer is supplied to the stepper there is a holding torque that should be noticable. Over powering this torque could cause stepper damage and in a servo is very likely to cause issues. This is why I believe you have a problem possibly with the current being feed to the stepper motor.

You can't rule out software issue either. Your motion control software could cut current back on some dirves right after a move. That of course demands a drive that can actually drop into standby mode.

Thanks
Dave

Originally Posted by Billw Iam using the Fet-3 board and stepper motors from stepperworld . When I turn on the power supply for the motors there is not noticable resistance to rotating the shaft but the motors sem to work well . Shoul they be hard to turn ? Thanks Again