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

Rob,
Re-reading I should have worked out that it wasn't the spindle motor but it was the only thing I could imagine moving on a halfway logically assembled machine :-(
I suppose all the vendors of ball screws should have a recommended shopping list of Ball Screw, Ball Screw Supports, Ball Nut and optionally Motor Mount, Motor Coupling and Motor, with a brief explanation of how and why, the market has clearly moved to a point where there is no research prior to shopping,

- Nick

The Chinese vendor sent me a reply to an email that I sent them telling them about the issue.
Their response was:
"Dear friend.
Please can you buy such a coupling to change it. The problem will be solved then. This coupling is cheap only about 3usd. Can you get it in USA local? If yes. We can refund you 10usd. Then you can buy it. If you can not get it local. We can ship you 1pcs. It will cost about 12days. Please check if ok for you.
Waiting for your confirmation. Thanks a lot.
Nice day."
___________

I am unsure about what coupling I can buy in a hardware store to fix the issue.
Does anyone else have any recommendations on where to get similar couplings?

Thanks...

Mark

You didnt take a look at the fixed end of that ballscrew, which is on the bottom. There are two bearings on either side of the bottom plate, secured with a 17mm locknut on the ballscrew. The top bearing near the stepper motor is the floating end.

Quote:

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Originally Posted by xoneeleven

The Chinese vendor sent me a reply to an email that I sent them telling them about the issue.
Their response was:
"Dear friend.
Please can you buy such a coupling to change it. The problem will be solved then. This coupling is cheap only about 3usd. Can you get it in USA local? If yes. We can refund you 10usd. Then you can buy it. If you can not get it local. We can ship you 1pcs. It will cost about 12days. Please check if ok for you.
Waiting for your confirmation. Thanks a lot.
Nice day."
___________

I am unsure about what coupling I can buy in a hardware store to fix the issue.
Does anyone else have any recommendations on where to get similar couplings?

Thanks...

Mark

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They are talking about this type of coupling https://buildyourcnc.com/images/rigi...01-4%20200.JPG
As we have said - this uses the stepper motor bearings to fix the position - it is not the proper solution - the Z axis leadscrew needs to be in a fixed bearing at one end and a floating bearing at the other end.

A better compromise would be to have a collar on the top end of the leadscrew, but we would need better pictures of the leadscrew mounting arrangement to be able to advise you properly.

Best regards,

Rob

I assume I have the same machine, and this is where the ballscrew is fixed.

Atleast on my machine, no motor-coupling would have anything to do with it.

Yes, we really need to see what is going on at the lower end of the Z on the OP's machine

Hello All,
Here are some photos of the bottom of the screw, the mid point, and another of the upper section per your request.

Thanks again for these tips and info.

Attachment 18237 Attachment 18238 Attachment 18239

Check the bottom bearing is not loose in the plate. It just looks like a push fit and it could have gone sloppy.

In the photo in post #4 there's a gap between the red buffer and the driven section of the coupling, either the ballscrew moved down or the driven section of the coupling moved down.
If the driven section of the coupling can move down then it's not tight enough and would also slip in rotation, and because more torque is required to lift Z than lower it the axis would slowly slip down.
If the coupling driven plate is slipping then stripping, cleaning, then reassembling with a stud & bearing fit compound may provide a temporary fix,

- Nick

I cannot see this being the problem more likely the simple options, ER11 thread damaged and not locking collet properly, material insufficiently secured if clamped at edges acrylic will lift in centre with any sort of up cut helix, wrong collett size used, whatever is causing a 3.75mm shift is gong to be measurable so measure collet protrution, relative position of ballscrew, deflection in frame or material plane with dial indicator etc and identify movement.
Before trying to remedy a problem identify it and save a lot of unecessary speculation.

Quote:

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Originally Posted by lucan07

I cannot see this being the problem more likely the simple options, ER11 thread damaged and not locking collet properly, .

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Could you speculate on how this would cause the bottom section of the Z axis motor coupling to move into contact with the screw underneath, leaving a gap in the coupling assembly which should not be there it as can be seen in the video?
:D

Quote:

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Originally Posted by magicniner

Could you speculate on how this would cause the bottom section of the Z axis motor coupling to move into contact with the screw underneath, leaving a gap in the coupling assembly which should not be there it as can be seen in the video?
:D

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The coupling can slip down onto the ballscrew and off of the motor shaft without the screw or the axis actually moving poor grip and gravity with a little help from vibration, also the video of the screw is someone elses problem not stated by this poster as far as I can tell.

If a betting man I would look at first item discounted as the problem, thin double sided pads probably applied towards edges with a little give amplified at centre as a helix screw into piece pulling it up or pulling the bit down with forces generated to clear the chips, the solutions are being given without identifying the problems which is illogical.

Post #1 (Quite obviously by the OP)

Quote:

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Originally Posted by xoneeleven

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

Mark

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Post #5 (Clearly by the OP - the clue is in that name's the same)

Quote:

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Originally Posted by xoneeleven

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

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Quote:

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Originally Posted by lucan07

also the video of the screw is someone elses problem not stated by this poster as far as I can tell..

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Really?
I'll Speculate That You're Speculating Again! ;-)

OK so I mixed up the poster of one part of thread it does not change the point that whatever is moving 3.75mm should be easily identifiable, if the ball screw is dropping the retaining nut on base would be 3.75mm below expected position and obvious. this is obviously a poor design the retaining of ballscrew appears to be on wrong end but unless ballscrew dropping 3.75mm an easily visible amount another fault is likely to be the problem.

And the clearly visibly out of place coupling lower half in his video would suggest?

OK I'll give you this one, either the ball screw has moved down or the driven side of the coupling has moved down -
Discuss
:D

Having seen similar problems with thin acrylic on several occasions that were put down to software issues which is why as a non machinist I got involved, I identified the cause in every case as the helix screwing action pulling material up or mill down I have noticed same thing happen with acryilic since I started to use a CNC flexibilty and its tendancy to grab the mill or get a little soft and sticky whichever cause it seems most likely to me. It just strikes me that this is another situation where the obvious has been discounted at the outset by poster a way to eliminate would be try a straight router bit no helix and see if problem still occurs.
"Material Table = very thin stickypad mounted to the aluminum CNC table (this is not likely causing the issue)"

Quote:

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Originally Posted by xoneeleven

Bit = fine needle bit (not sure of the lingo to describe)
Intended engraving depth = 1.25mm

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Obviously the tool pulling down in the chuck or the material pulling up then?

If it has a helix its a good bet, not exactly clear exactly what bit is being used from that description but if a cheap PCB micro mill with a tight helix turning fast enough to soften and grab then yes my bet.

Another possibility on the 6040z (especially with the standard electronics) is missed steps that occur on upward Z travel. Not sure if that can be ruled out in this case as I haven't watched the video, but could apply in other cases.

Today, we are going to pull off the coupling, and wrap a sheet of aluminum onto the spindle, and re-attach the coupling.
This may add enough friction for the coupling to stop slipping.


Right now, we believe it is a coupling issue where it is either damaged or badly designed. We are located in North Idaho and nowhere near a place to take the bad coupling into a store to replace it. So we have to rig it until we can find an online store to order a replacement coupling.

I will post an update with news on this failure or success. :)

It may be better to run a drill though the coupling and tap it put an m2 or m3 screw in for each side to lock on as an addition to clamp would be better than a shim but as that coupling is two pieces held together by a piece of rubber which is a just a friction fit I still think it may not be your problem and nothing to stop rubber coming away if ends stay in place. Check mill not slipping in collet and put a dial guage on top of acrylic to make sure not lifting seen acrylic do this many times, if using a bit with a helix try a straight router bit, also if it misses steps on upstroke it will get deeper. Anything movinng 3.75mm is going to be obvious so material or bit moving or missing steps sound much more likely to me.

Quote:

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Originally Posted by Fred

Another possibility on the 6040z (especially with the standard electronics) is missed steps that occur on upward Z travel. Not sure if that can be ruled out in this case as I haven't watched the video, but could apply in other cases.

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Good point I seriously doubt this slipping coupling is the problem its two pieces held together by the friction of the rubber alone so if ends stay firm nothing to stop rubber moving.

In fact that's the only thing that makes sense if everything else is secure the rubbing coupling is causing enough friction for the stepper to miss steps so that the Z axis is losing its integrity.

What you talkin' 'bout Willis?

Some thing is pulling that coupling apart. If it were just loose on the stepper shaft there is no force to pull it downwards. I would recommend running the program but just cutting air and looking at the Z axis to see what exactly is moving.

If that red bit in the middle is the least bit squidgy you and drive it in either direction and it will push the two metal sections apart ;-)

We checked the nut at the bottom, and it was fine.

So, we took both the couplings off and examined them further.

The first thing we noticed was that the couplings appeared to be mounted slightly too low. So, when we put the motor housing back on, it may have been pushing the couplings down on the cylinder.

Here is what we did to patch/remedy the issue:

1: Used a dremel tool with a cutoff wheel to lightly score the cylinder (to create some friction for the couplings to adhere to)
2: Moved both coupling halves up slightly on the cylinder.
3: Reassembled and ran a job.

So far, the results are positive. There is no slippage of the couplings.

NOTE: We did originally try to wrap a tiny sheet of aluminum around the cylinder to use as friction, but the tolerances of the couplings to the cylinder was too tight. It would not fit. So, we came up with an alternative idea in scoring the cylinder to create friction.

Anyway... Thanks for the tips. The tip on the aluminum sheet is what mainly led us to success. (that idea eventually led us to consider creating friction using a different method)
Let's hope this stop-gap method works over time.

Originally you said the cutter bit was being dragged down into the job I don't think fixing the coupling will have anything to do with the first fault as the two halves of the coupling are not fixed together. So either the bit was loose and came out of the collet or the fixed bearing is faulty. Only time will tell.

If the bottom coupling was slipping on the shaft most slip would have been when working against the greater force required to lift the Z axis rather than when lowering, resulting in Z gradually slipping down,

- Nick