What is causing this?

[cxixer]

I'm having issues with my cnc. Everything was working great, but it started doing this on some letters I've been cutting. It seems intermittent as it will also do it on some ovals I cut as well. I've ruled out the Gcode as I can use the same Gcode to cut perfect ovals, reset with a new peice of wood, and it will screw up. It seems to step over, but only sometimes.

As you can see in the pictures, the "r" turned out fine, and the "s" got messed up.

I know I have some backlash issues, but they're not bad enough to cause this. There's over 1/4" difference on the "s" from the top to the bottom of the piece of wood.

I'm real tempted to scrap the rack and pinion and redo all axes with acme.

Any help is appreciated.

[harryn]

I am still pretty new at this, so this might be dead wrong.

First - why are you cutting in three passes - why not one full depth of cut ?

A 1/4 inch off is probably not from being a rack and pinion - something has come loose, and it has to do with the left - right axis.

[Mike Stevenson]

Where the ball screw attaches to the motor On my K2CNC Router there is a junction box that houses the two pullies and a belt. Those pullies attatch to the ball screw and a the servo motor with a small set screw. Make sure the screws are tight against the shafts. If they are not the motor "slips." Also try to turn your ballscrew with the servos on. You should not be able to turn in by hand at all.

[ger21]

Looks like you're loosing steps on one axis. Could be mechanical, though. Loose pinion maybe? A sharper bit wouldn't hurt, either. A bit that cuts that rough puts a lot more load on the machine than a sharp new one.

[cxixer]

OK, I don't cut a full depth pass because: A. I've broken a lot of bits by hand like that, and B. I'm using a PC 690 router, and would be cutting at .5ipm so it wouldn't die.

The pinions are direct drive, and yes, they are held on with a set screw. This did come loose in the past and it was a disaster. I've checked and rechecked these screws and they do not seem to be loose. I'll check again. (they're steppers, btw) I have turned the motors on and tried to move the axes and they're stuck.

New bit... that's for people who make money with these things! I am, in fact, going up to buy a few more bits on Monday. I have quite the collection of them so far, I found I use them up much more quickly with the CNC than by hand. Anyone know where I can get them sharpened?


On the axis in question, I have a (not pretty) pre-load on the pinion pulling it toward the rack. I'll check that to see if it needs adjustment too. I appreciate the quick replies.

[ger21]

Originally Posted by cxixer I am, in fact, going up to buy a few more bits on Monday. I have quite the collection of them so far, I found I use them up much more quickly with the CNC than by hand. Anyone know where I can get them sharpened?

Cut faster and they'll last longer. At work the other day, we were hand routing sink cutouts in 3/4" plywood countertops. Full depth with a PC 690, at an estimated ±100ipm. No problem with the right bits

[cxixer]

Originally Posted by ger21 Cut faster and they'll last longer. At work the other day, we were hand routing sink cutouts in 3/4" plywood countertops. Full depth with a PC 690, at an estimated ±100ipm. No problem with the right bits

The best bits for MDF I can find locally are PC spiral upcuts, and I've been through a lot of bits.

What bits are you using? And where can I get them online? How long do they last?

[ger21]

What size bits are you using, what rpm and what feedrate?

[Steve-Tee]

Howdy Cxixer,

As you are using steppers on your machine, perhaps you are seeing the effects of resonance in them.
This has some very undesirable effects, including unexpected stalling.

One way to look for this is to jog the affected axis back and forth, all the way between limits, at varying speeds.
Under certain conditions, for a given speed, the motor may begin to vibrate. The effect builds up over a short period of time, and the motor suddenly stops.

This caused havock with my machine, until I discovered the curative powers of 'resonance dampers' .

Here's hoping this helps.


Best wishes,
Steve.

[cxixer]

Gerry,

I'm using 1/4" bits, and whatever RPM the new PC690 routers spin at, they're no longer variable rpm. I always cut them at max rpm on my other hand router anyway because they slow down too much if I don't. And on the CNC, cutting at around 30ipm.


Steve - I've never heard of this resonance issue. I'll give that a go tomorrow. I usually jog a while anyway when setting up new pieces, and havn't noticed anything. What are these dampers you speak of? And did you notice similar results in your cutting when this happened?

I did tweak the machine today. I went out and checked all my set screws, everything was good and tight. I loosened my x axis (single motor) pre-load adjustable springs, in case it was putting too much load on the motor, and then I added some springs to each motor on the y axis for a pre-load against the rack. Before I just had screws tightened down, and I was always afraid the rack was slightly bowed in some areas because of the welds which would cause backlash. This should help the situation, although I may need to go get tighter springs.

I'll see if anything had an effect tomorrow as I have more letters to cut. Hope they turn out nicer than my other ones! I keep getting tempted to just do vinyl and quit cutting letters because I get nervous selling things I'm unsure if I will be able to cut (I have a very quick turn around time on most items), but I have too much money wrapped up in it and really enjoy my CNC!

[ger21]

I think a 690 is ~23000 rpm. 30ipm is way too slow to get good tool life at 23000 rpm. 100-150ipm would give you better tool life. MDF is extremely abrasive and dulls bits quickly at any speed. In a production environment, if i was cutting MDF non stop for a full day, I'd probably use a new bit every day.

If 30ipm is your max, you could try to find a single flute spiral, which should last a little longer, but probably won't be cheap. I think the most cost effective bit for you would be to use carbide tipped straight bits like these. http://eagleamerica.com/straight-bit...ng/p/102-0102/

One thing that may also help is to clean them after each use with a bit cleaner. Keeping them clean will reduce heat, which is a major cause of dulling.

[cxixer]

I could cut faster, but the router slows down and starts tearing up the wood if I do. What bit would allow me to cut that fast?