Cuts are not straight, has a wiggle?

[Brett1970]

This just started happening recently. I was cutting black melamine with a double flute 1/4 compression bit, file I've used for the last 5 years, new bit, etc cut it with success many many times. My lines are not cutting straight.. line wavers left/right about 1/32 all the way down the cut. Seems more prevalent on Y axis cuts, but is visible in either.

Today I was cutting Birch ply, using a new double flute straight bit, have used this combo before with no issue. I can still see the wiggle but not as much as the melamine. I had an inside corner that looks awful. I double checked the file, it's a nice arc that flows well, and the machine looks like it treated it as a ton of small straight lines. There are no points in the arc on the file either.

So far, I've reset my X and Y motors, no play found before or after. Checked all my V groove bearings, all tight. I am using a yaskawa spindle, iv'e wiggled everything I could think of with the machine on, but can't find any play anywhere. Totally stumped!

http://i68.photobucket.com/albums/i24/dptwiz/shopbot001_zps2c634b1e.jpg (http://s68.photobucket.com/user/dptwiz/media/shopbot001_zps2c634b1e.jpg.html)

[jerry_stanek]

When I got cuts that look like that it is because one of the steppers on my x is unplugged of tripped.

[srwtlc]

David,

You didn't say what machine you have, but if it's a PRS, check to see if the lower v-bearings on the Y car are adjusted properly. Grab onto the bottom of the spindle and give it a firm pull/push perpendicular to the gantry to feel for any play. Due to the weight of the spindle and Z axis, it does take a fair amount of force to wiggle it. Any play there would be more prevalent in the Y direction.

[Brady Watson]

If you have a PRT, check for a loose pinion. This happens a lot.

If it is a PRS - check lower YZ bearings & adjust as needed. Not too tight...

-B

[Brett1970]

thanks for the suggestions, now that the holidays are wrapping up, i'll be back in the shop again! I just had a customer find a problem on a project he picked up. I made an arcade control panel, had lots of button holes in it, buttons are 1.125 so I make the hole 1.3 and pocket it out. He called and said the buttons didn't fit, I had another control panel I had cut at the same time still here, I checked it, and sure enough, the hole size is 1.06 !! I double checked in partworks, it's a 1.3 circle on there. I am baffled on this one.

my machine is a 2008 or 2009 prs standard, upgraded control card, yaskawa spindle, and vacuum hold down system.

[Brady Watson]

First, make sure you are using the same diameter bit that you programmed for in PW. I know that seems obvious, but mistakes can happen.

Second, pull up the VU command and make sure that you unit values are correct. They shouldn't really change - but it won't hurt to check.

The 'fish tail' that you show in your cuts is indicative of a motor or motors stalling or a pinion being loose. How fast were you cutting both the circles and the part that shows the fish tail?

-B

[Brett1970]

I am using a double flute 1/4 compression, .405 per pass, 3.5 inches per second. Been running that for 2 years or so now.

[Brett1970]

I did just check my partworks file again, I should say the hole size is 1.13 not 1.3 as I mentioned. Still losing about 3/32 of the actual size. Bit was set correctly to 1/4 bit.

[Brady Watson]

Next chance you get - do some cuts in some scrap and see if it is still happening. Check sizes. Then SLOW those speeds down and check dimensions again. See if you are any closer to sneaking up on spec.

.4" deep @ 3.5 IPS is pushing it a bit on a Standard tool. Your inside cuts should be a separate toolpath at a slower MS than large sweeping panel cuts. There is no way the tool is going to cut a 1.13" hole @ 3.5 IPS accurately. SB3's ramping will kick in to a certain extent, but it will still be too fast for that little hole.

Do some testing...

-B

[Brett1970]

I was thinking the hole size was part of the issue with it not cutting straight. The holes do look perfect, not out of round or anything. I will make a series of holes from .25 to 1.0 and try it at 3.5 and i'll do a set at 1.5 and see if it makes a difference. thanks!

[Brett1970]

Believe it or not, today was the first chance I had to get back to the cnc. I noticed something today.. the holes that I believed to be undersized, are actually slightly oval shaped. The way I had it oriented, the oval is longest on the 4 ft (Y) axis.. when measured with a tape measure, it looks to be right on the 1.125 on the 8 foot (X) dimension of the oval is 1.04 or very close to it.

This tells me if the distance is short on the X, I would assume loose pinion, that's where I'm losing my accuracy. Not the case.. Pulled both X motors, and Y motor, all pinions were tight, I checked the two set screws on all of them. Also checked the wiring, nothing amiss there. I can feel a little play on one of my X motors, the one closest to my PC, or the side of my machine with all the wiring on the side.

You can't see the movement, and the movement is not in and out on the shaft, but as if you were turning it. I can feel the play, and it's minor. I can't tell if the play is in the pinion to the shaft, or if the play is inside the stepper motor. I had also cleaned all the pinions, looked over all the teeth, looked at the sides of the machine, everything is fine there. Here's the odd thing.. when I unplug both X motors, and I push the car back to the mechanical stops, verify both sides are touching the stop, I hook both X motors back up, and power on the control box to square the car, and I tried to wiggle the car on the X axis.. the far side of the machine has no play whatsoever. The side closest to me, I would estimate it has about a 1/16 of play.

I do not have anywhere near that kind of play on the pinion or shaft, so I don't know why I would have that kind of play with the motor installed. Motor is all the way up as far as it will go, and tightened. I also had powered up the machine with the X steppers off and verified the motors are locked, if I could still turn one by hand, that would tell me wiring problem, but not the case.

Sorry this is so long, just want to make it clear exactly what it's doing. A while back I had noticed when I was cutting straight lines in melamine, the line would waver back and forth + or - 1/32 either way. it looks like hell, but wasn't sure what was causing it. I'm pretty sure it's related to the hole issue.

My plan is to pull my Z motor off the machine, and replace it with the X motor, and put the z motor in the x position. This should in theory solve the problem with the holes being oval shaped, and will tell me I need a new pinion or a new pinion and stepper. If it does solve it, my problem will now transfer to my Z axis.

Anyways, please share and ideas/suggestions, i'll be giving this a go tomorrow morning. thanks!

[Brett1970]

well, that was it. Swapped the X and Z motors, now my circles are round again. I tried it at 3.5 and 2 ips and there was no difference. I started hunting for a motor, and the model number of Vexta A7328-9412KTG doesn't come up in any searches except to the shopbot forums. I was on oriental motors site and couldn't find it. No offense to shopbot, but I know its about 1/2 price to find it elsewhere.

[steve_g]

David…
You said you checked the set screws… I don’t know about your motor, but often it’s the practice to use two stacked set screws, one to lock the other. You might completely remove a set setscrew to see if there’s a loose one underneath.

SG

[Brett1970]

All the pinions were tight, they wouldn't move on the shafts. All the set screws were tight, and the ones I did break loose to swap the pinions for the X and Z were on there good. The X motor had a lot of play you could feel.. now with it in the z position, I'm not seeing any issues, and everything seems good. All the set screws were just single set screw per hole, 2 holes per pinion.

[bleeth]

FYI that motor is custom made for Shopbot and you can't buy it direct.

[steve_g]

What am I missing here… is there a gearbox built into this motor? Or how else is there play?

SG

[Brett1970]

Well hmm.. that's not good. I can't tell for sure, but I'd assume the stepper is attached to a gear reduction box?? It has to have bearings in it somewhere.. shaft wear, bearing wear, not sure? All I know is out of 4 steppers all had a little play and I mean very very minor. One had more play than the rest, noticeably.. maybe 1/32 or slightly less, but with the multiple back and forth on a 1.13 circle, and you lose that on each side as it changes direction, in theory you'd be losing about 1/16 on the overall size. On a 1.125 button shaft, it wont fit. Plus with the hole being oval shaped, that's somewhat telltale right there.

[bleeth]

Yes, there is an internal gearbox. The gears are tapered hob and in that particular stepper the factory standard gears are straight. Straight gears will typically show more backlash effect (which is very possible what you are seeing) over time, but both do wear and it does get worse. I went through the same thing and eventually changed out the motors. In my case I started with the idea of using straight no gear motors and an external reduction gearbox, but eventually just bit the bullet and upgraded to Alpha. That did cost a bunch, but for me it was worth it.
If you want to test the theory, set up a test circle or two and have a bungee cord pulling on the gantry while cutting. (This suggestion came to me originally from Ted Hall). One end to your frame and the other to the end plate, one on each side of the mill. Similar on your Y.