Problem- Z Axis slipping

[neomagik]

Hi all,

I have a recurring problem with my Z axis that I am beginning to think may be as a result of insufficient holding torque from the motor.

The problem is that during the cut the Z axis drops and continues to drop throughout the cut to the extent of approx 0.75mm after 15 minutes and as a result it cuts into the table top.

The Z axis is the only axis (on a 3 axis machine) that suffers the effects of gravity and to be honest the whole Z axis set-up including the router itself is quite heavy. I've checked the shaft/motor coupling for any slipping and it's solid.

I can only put it down to the weight transferring too much torque to the motor via the ballscrew.

I was wondering if anyone has experienced a similar problem or could help me calculate how much torque my ball screw is applying to the motor.

The holding torque for the motor is 1Nm

[mcentric]

I've had similar results from losing steps due to too fast acceleration settings on my Z axis. Every time the z retracted I would lose a few 0.01's of a mm. The motor tuning in mach3 solved it.

Mikael

[Jack000]

if this only happens during a cut I'd suspect the router bit was slipping. I've had it happen to me a few times, especially with smaller upcut bits.

[neomagik]

Thanks Mikael I'll check the acceleration settings in Mach 3 but mine is a lot more severe than yours so I would be surprised if it was that.

I don't think the bit is slipping but will check that too.

[ger21]

Reduce the accel by 1/2 and see if the problem goes away.

[neomagik]

OK I halved the acceleration from 50 to 25 and it made no difference. I tried 15 and still no difference so I dropped it as low as it'd go which was 4. Although this did seem to work it was painfully slow.

Any other suggestions?

[M250cnc]

The norm is to have the most powerful stepper motor on the Z axis.

Has the amount you are out, changed as you have slowed the Axis down ?

Is the Z axis being pulled down under cutting load ?

Phil

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[neomagik]

All my motors are the same.

The amount it is out has probably changed as I reduced the acceleration but I have no accurate way of determining this.

I've tried it with no load and get the same result.

Do I need a stronger motor?

[M250cnc]

Originally Posted by neomagik I've tried it with no load and get the same result. Do I need a stronger motor?

1Nm seems a bit on the weak side, and you have a load on the motor which is the weight of the motor and axis, but only you know what the weight is.

Take the motor off and wind the axis up so you can get a feel for the force required to move it.

Then with the motor still off but powered up turn the axis coupling to see how much effort you need to overcome the stepper motor.

You can then compare.

Phil

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[neomagik]

I will try your suggestion and maybe rig up a torque wrench to get a more accurate result.

Am I right in saying that the holding torque, in this case 1Nm, is the highest torque produced by the motor and that as the rpm increases the torque decreases?

If this is the case then would decreasing the total motor speed and increasing the acceleration to a more acceptable rate acheive anything?

[M250cnc]

Yes the motor figure quoted is holding torque.

Torque comes from amps, but the speed comes from volts.

Measuring torque is a good idea but the correct gizmo is a spring balance (what a fisherman would use to weigh their catch)

You need to fix an arm to the shaft you can read all about it here http://www.google.com/m?q=Newton+met...ni&channel=new

Phil

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[hemsworthlad]

Hi Neo,

Has this problem always been there from the day machine was built or just started happening.?
If it was ok before was it just doing simple 2d profiling etc then started losing steps or position when you've tried 3D work or Z axis intensive code like lithopains or engraving.?

Can you give some info on your setup IE: motors specs and if wired parallel or serial, drive's used and settings set, PSU size, BOB make and model, pitch of z screw, Velocity & ACC set in mach also Step pulse & Dir pulse settings in motor tuning, what are the active high/low settings in ports&pins/motor outputs.

Also are you using sheilded cable and what are you using for a spindle.?
Can you approx guess the weight of the moving portion of the Z axis inc spindle.

With this info it will be easier to help and trace if it's just under powered motor or whether you have another issue.

Cheers
Dean.