Engraving small font - the bit digs deeper and deeper as the job goes on. 6040z

Hello All,
We have begun testing engraving some plexiglass with small text. (1/4inch plexiglass, with 10point font)

Here is the issue:
The job starts out ok.
However, as the engraving job progresses, the bit cuts just a little deeper each time it engraves a line.
So, half way through the job, the bit has punched a hole through the plexiglass and is ripping apart the stickypad underneath.

Here are the specs and variables:
Machine = Chinese 6040z CNC
Toolpath design Software = VCarve 8
Machine software = Mach3
Material Table = very thin stickypad mounted to the aluminum CNC table (this is not likely causing the issue)
Material being milled = 1/4 inch plexiglass
Bit = fine needle bit (not sure of the lingo to describe)
Intended engraving depth = 1.25mm

I guess my question is, has anyone else ever ran into a similar issue, and what are the most common causes?

Thanks...

Mark

We figured out the source of this issue. It turned out to be either a design flaw or manufacturing quality control issue related to the clamps that hold the Z axis motor shaft in place.
I will explain more on this and post photos/video of what I am talking about.

In a nutshell, the clamp that is supposed to hold the z motor shaft in place, tends to slip even when tightened very well.
We added washers underneath the motor so it would not slip down too much. When we added the washers, and re-tightened it, it still slipped, but the washers are keeping it from slipping far enough to rub on the screw heads under it. the photos and video will explain it all later.

If it's a clamp which holds the motor cylinder cut a strip of beer can to line the clamp and "shim" it tighter,

- Nick

Here is a YouTube video of the issue, and I have attached a photo.
Attachment 18222

http://www.youtube.com/watch?v=vKDzOAWZZvk&feature=youtu.be

We will certainly try that route.

Thanks

It looks to me that the leadscrew for the Z axis is not clamped at one end (it should be floating in one bearing and fixed in the other)
Here's a a couple of photos to show what I mean.

Attachment 18223Attachment 18224

It looks as though your machine is using the stepper motor to 'fix' the Z screw. However the type of coupling you are using negates this option.
It is bad design to use the thrust bearings in a stepper and you should post some pictures of the leadscrew bearings at each end for us to check this out.
The 'Oldham' coupler you have is not designed to resist being pulled apart, which is what is happening and the whole leadscrew is slipping downwards.

Quote:

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Originally Posted by magicniner

If it's a clamp which holds the motor cylinder cut a strip of beer can to line the clamp and "shim" it tighter,

- Nick

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Nick, I think it is the leadscrew bearings that are design deficient.

Rob

Quote:

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Here is a YouTube video of the issue, and I have attached a photo.

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As Rob suggests I think you have a problem with the bearing holding the screw in position the motor should certainly not be doing that job. It will further damage your machine without sorting this problem out.
Take the motor off and see if you can check for end float in the screw
Attachment 18226

Quote:

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Originally Posted by cropwell

Nick, I think it is the leadscrew bearings that are design deficient.

Rob

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Rob,
probably so, my response was to the fault as described, before photos were added.
Having watched the video there seems to be a glaring absence of end-float bearings on the ballscrew, unless you count the screw the coupling rubs against ;-)

- Nick

Quote:

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Originally Posted by Clive S

Take the motor off and see if you can check for end float in the screw

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It isn't end float. More like end sink. That has got more slop in it than an old Beetle crankshaft. I would bet that there is no fixed bearing at all.

Nick - I agree with you the OP was confusing us talking about the Z Motor.

Happy Days !

Rob