servo positioning problem

[Dirkos]

Hello there,

Help!! I hope this is the good place on cnczone to ask this question.
I have a Deckel GNC1 with 3 CNCteknix drives and Mach 3.
There are encoders (512pulses/round) on each axis.
When I'm milling something, for example 10mm aluminium, after each z-axis movement, there is a 0.1mm edge on the right and bottom side of the workpiece. The mill moves downwards and to the right after each turn, after cutting for example a rectangle with round corners. This is very hard to explain, you have to see photo's. I hope someone can help me with this. In the attachment I made a drawing of the problem and I will post a photo. Greetings, Dirkos

[Zig]

Hi..

I use CNCTeknix drives with BLDC motors and EMC2.

No problms of the kind You are experiencing.

Do you have any free movement ( backlash) in Your drive mechanism?

Does MACH3 have facilities to account for backlash in software?
If so are parameters set correctly? In other words is it possible backlash is overcompensated for in software?

[Dirkos]

Hi Zig,

Is it possible that there is backlash only vissible on on side?
There are step overs of 0.05mm, on the right (positive x-axis side) and down side (negative y-axis side). These step overs occure only after a z-axis downwards-movement. So you get layers of the workpiece (each layer is exactly the same) but the layer has moved 0.05mm to the right and downwards. So this 0.05mm is cutt of of the top and left side. (I hope I explain this properly) This gives a tapered wall on the right and front side.
I really hope someone can help me with this, it is a really big problem.

When the power is on the servo's, I can move the axis a little (+/- 3 degrees turn). The servo is moving to then. I tried to compensate this with the proportional gain, but with no succes. The machine (a Deckel GNC-1) has a very fine axis-pitch. (I guess 1mm) Maybe the encoders (original) dont see thr movement? (encoders have 512 steps/turn)

I think it is trange that on this forum only you replied on this question.
Is this a unique problem?

Thanks - Dirkos

[Zig]

Hello Dirkos,

I am using Tek 6 BLDC servo drivers with Renishaw linear scale and a resolution of 0.005mm.

The motor is directly coupled to the recirculating ball screw drive with a slight preload so there is no discernible backlash.

Is Yur motor directly coupled to the screw or are you using a gearbox ( say belt drive) between motor and leadscrew?

If You are using belt drive between the motor and leadscrew are all pulley locking screws tightened down so there is no free movement?

If none of the obvious mechanical problems exist You can always talk to Pete Dunster at CNC teknix or post Your enquiry at CNCteknix section of this forum.

Is the rotary shat encoder mounted on the motor shat or the lead screw?
If not mounted on the motor shaft try mounting it on the motor and then in the scaling factor of MACH3 account for effect of a gear box if You are using it. If You are not using a gearbox make sure the flexible coupling between the motor and leadscrew is rigid enough. There were instances of people reporting the sort of problem You are seeing iwhen the flexible coupling was too soft ( plastic ) and it did not return the screw back fully.


When setting up the loop are you using the an oscilloscope to monitor the error signal?

Sorry i can not be any more specific.

Regards

Zig