3d Mold divot issue - SOLVED - Loose screw holding pinion gear
 

Looks like it is time to chase some ghosts.....

My build partners and I have racked, raked, or is it whacked our brains in order to get a plan to go chase this ghosts.

The divot is 0.030 +- 0.006"

We are about 90% complete but this is a killer issue for us.

Does it appear in the same place everytime you run the file - possible code problem, if not check for a loose pinion or rack on your Z axis (if you have belt drive check all the pulleys on each shaft), possible loose spindle mount or even Z motor mount.

Are you using a vacuum holddown?
Foam has a tendancy to "climb" when shaping like this.
It has a stability issue and when you release the tension on the fibers on the top surface, parts will concave or convex on the release.

"short of like if you weld, the material shrinks and changes the shape of the base metal"

Have you tried cutting the mold out of something more solid?

After a divot, did the machine recover to the correct position by itself, or did you have to do a manual reset? For the time being, I assume that it recovered by itself, and that points to good electrics, good programming, bad mechanicals.


My guess is that the red line is the deepest part of the mold, the point where the z-axis has to reverse direction. That would be another indicator of a mechanical problem related to the z-axis.

First suspect is the grub/set screw in the pinion. Then the rack mounting to the slide, the router mounting to the slide. Maybe even the hold-down roller at the back of the gantry is not doing a good job (stuck against y-car)

Let's see if I can get all the answers...

The problem doesn't appear in the same place/same time. This points away from gcode. Haven't checked the rack/pinion on the z, but this is a good/easy check.

The stock is RenBoard 440 which is more like a big slab of bondo (polyester body putty). While it is "technically" a foam (syntactic to be precise), it is very stable dimensionally. But the answer is no there is no vacuum in use on this part. BTW, when cutting this part out of pink eps (extruded polystyrene) the problem seems worse. So this line of reasoning is still plausible.

The "deepest" part is just to the right of gerald's line.

and now for the "piece de resitance" (don't you just love a southern boy who can't even speak proper english tries french :D )

No manual reset. The machine recovered itself, this is very evident in the "scratch" in the last picture above.

Can we rule out: z-axis motor/driver, wiring, interference as I don't want to look where the problem is not ;)

stan

what about checking the rails for build up or mis-alignment? A mis-alignment of the rails would only cause the gantry to rise not gouge or or drop as shown in your photo. It appears to be mechanical though...

What about putting an indicater in the spindle and run it along a pc of flat stock on the top of the surface. place the stock in the same area as you had your mold sitting and see if you notice anything dips or drops from the machine.

Usually when I see marks like this, it is due to not leaving enough stock from the roughing pass for the finisher or the rougher path has some issues with programming. The rest of the mold looks beautiful though.

I will put my money on the workpiece not secured 100% to the table. I had this before.

Did you LEAN on the table or pushed on the piece during the cut ?

To my eye the Y-axis is wonky.

Look at the vertical line, its as though the piece moved to the right causing the cut to be deeper.

Well spotted John, I think you're onto something there.

Update.. We have removed and sent the spider to a local machine shop to be surfaced (on the 6 posts that support the eccentric bushings). We were planning to do this when we move/paint the machine. This will eliminate issues there. So until next week, more thinking and thinking...

Thinking of ways to test various scenarios.... I'm going to create a simple gcode to move the bit back and forth with small steps (simulating the same steps as in the mold), first using 45 degrees but without the complex z, then 0 degrees (mainly x) and then 90 degrees (mainly y) to try to get a particular axis nailed down.

Yes, the y did look wonky in that spot. I'll look at the other spots, but it could the result of something like a sticking gas shock that really pushes in the x direction but appears y because of the 45 degree finish cut (again back to some slop in the z mechanicals).

More thinking and tinking..... We are 90+% home on this so I am trying to check this in perspective. The last 10% is more difficult and may be more rewarding in the long run.

Stan

Still thinking as the machine shop didn't call me today (will be next week now).

John's pointing out of the "wonky y" has been causing me to do some thinking... So I will ask a question...

Could a build-up of crap on the z roller (running 4 rollers at the top) cause enough of a shift of the bit to cause this issue? If so can I tighten the rollers (will be going back to 6) or rig up a wiper/scraper to keep it cleaner?

Cut a fair amount of styrofoam with pink bits everywhere especially the z rollers. Ted and I do not believe this occurred when the machine was "new."

Stan,
When I was cutting HUGE blocks of foam on #5, I did notice this problem.
Though the skew of the tool bit was not an issue with the parts I was creating - so I didn't pay attention to it.

I didn't notice it on the z axis, but it was evident on the x axis and caused the gantry to rise up on one side only that was full-of-crud. Thus, showing about a 1/64" change in depth of cut.

SO, it is entirely possible you need to add cow catchers to your machine.

I added them to scape the rails. a little 2" x 4" piece of delrin profiled to match the rail and "scrub" it. Fixed the problem instantly.

The static from the machine really grabs the foam particles,...I never noticed this issue with wood.

Sean

Sean, got pictures of the cow-catchers especially mounting?

http://www.mechmate.com/forums/showt...96&postcount=4

http://www.mechmate.com/Forum/messages/2/185.html


I don't have the actual pictures, but this should get the point across.

I just used a piece of 3/16 plastic (UHMW or Delrin - can't remember) and bolted with 2 - 1/4-20 UNC socket head cap screws to the Gantry end mounts. Total of 4 units installed on the machine in each direction.

The one that mounted (under) the cable chain entry point required an extra tab of metal added, but was easy. Depending on your cable chain mount, you might not have a problem.

My machine #5, was pre-mamba parts and the end caps had a slightly different profile that did not accommodate the cable chain.

ok fellows, I think I found the problem. The z pinion gear on the transmission has just the right about of play, combine that with a sticky z strut..... and as if by magic, z-boo-boo's!!!!

Ted, aka Crashingsucks, indeed found the problem...

We're VERY HAPPY now...

Moral of story.... check the grub/set screws...

Thanks to everyone who helped.. It was a team effort.

Well I feel a little stupid, if I would have read stan and my own thread I would have seen that Gerald D pointed the grub screw out on post #4....


One should learn from the master!

Thanks all, Ted :)

Alan was the first to suggest that you had a loose pinion ;)

Funny thing is that the grubscrew issue occurs close to the point of the motor changing direction, but never at that exact point.

...and now I don't feel so bad that I TIG welded the end of the pinion to the motor shaft on my z-axis because it kept moving :)

Sean, you are a brave man! :cool:

what about taper pins / spring pins? They would be a pain to get in BUT a bit more stable than the screws and less drastic that TIG.

Snag is that you may want to swop motors and pinions around the various axes, where the pinion position on the shaft varies a bit. Or you simply want to replace a pinion 5 years down the line. Then a tapered pin through a hole in the shaft is a problem to line up.

The set-screws do work if you follow the basic "rules":
- bigger diameter screw
- finer pitch of thread on screw
- minimum of 2 screws, 90 degrees apart
- good steel quality of screw (wrench hole must never want to strip out)
- use good wrench and tighten until the wrench deforms
- Loctite screws and tip of shaft where it mates in the pinion (A bit of heat loosens the Loctite)

I have double grubs screws (one on top / backing the other) plus locktite on all my pinions. Have never had an issue with any loose pinions.

greg, by double do you mean two screws 90 degrees apart or two in each hole (really 4 screws)?

Two in the same threaded hole. 2 screws per pinion. Thats it.

If you have large pinions (on geared/belted motors) the screw holes are deep enough to get a pair of screws into the same hole, on top of each other. But there is no harm in a second hole, 90 deg off, with another pair of screws in there as well.

The small pinion (direct-drive) guys won't get 2 screws per hole, therefore they are obliged to go to two holes.

could it be that our flat spots on the shaft are too deep?

how long is a piece of string?

If it shifts after using high strength loctite, either you don't have enough area on the threads (too small a diameter or too thin a wall) or you haven't cleaned the threads on the pinion and screws of grease and contaminants. Bit of acetone works wonders.


Loctite 270 is strong enough. You could use 290 but it penetrates, be careful it doesn't get onto the pinion or shaft, it will wick its way between the shaft and pinion and you'll have a job removing it without overheating the motor shaft.


There is an oil tolerant threadlocker from loctite but afaik its more that once it sets it is tolerant to oil, not that you can use it on an oily surface.

John

We do wet the shaft & pinion with Loctite, in addition to the threads, and a puller has always shifted the pinion later, without applying heat. But I can't remember the specific number of the Loctite in our arsenal - I think it is "medium" strength stuff.

However, I have a personal belief that Loctite does not help that much for something that wants to start coming loose. Sure, it stops a loose screw from falling out, but it can't help for the contact metal between screw and shaft relaxing and developing a bit of slack. First prize is for a mechanical joint to work without adding Loctite.