Wobble at the start of diagonal cuts

[lcolburn]

Hi all,

I'm a new ShopBotter, just got our PRSAlpha 48x48 up and running a month ago, and have run a few jobs for myself and clients. I'm running into a problem with some wobble at the start of diagonal cuts. (see pix below). When the bot starts making a cut diagonally, wobble shows up for the first inch or so of the cut, but it then evens out to a neat, straight cut along the diagonal. No problems on cuts parallel to the X and Y axes, just when cutting diagonally.

I made a quick test file in PartWorks, with lots of diagonals and corners. When running it as an air cut, all the bearings in the Z and the X-Car were moving normally and engaged with the tracks, and I didn't hear any binding sounds in the gantry. When I cut the test file, though, the wobble showed up.

This only seems to happen with diagonals located near the lower end of the x-axis and the higher end of the y-axis- I think that's corner 1 in ShopBot talk.

Any ideas what might be causing this? My instincts say something mechanical, but I can't seem to isolate it. FYI, the file was drawn entirely in PartWorks, but I encountered the problem with files drawn in Illustrator and then toolpathed in Partworks as well. Any tips or advice would be great. Thanks!


7138

7139

[bleeth]

Lewis:
Are you ramping in? Also are you using a good quality bit? Although MDF is pretty soft you could be getting a bit of deflection at entry. Maybe a slower plunge could help as well.

[richards]

You're seeing the classic 'chatter' problem that seems to affect all of us.

I've been fighting that problem since 2004, when my PRT-Alpha first arrived. Since then, I've added a spindle - no reduction in chatter. I've added belt-drives - significant reduction in chatter. I've added the Alpha 7.2 geared motors - about equal to my 3:1 belt drives. In short, I've spent over $5,000 in upgrades and enhancements to to my machine and I still have some 'chatter'.

What I have learned is that is that a few strokes with some sandpaper takes care of the problem. The Shopbot takes me most of the way and sandpaper finishes the job.

[Gary Campbell]

Lewis...
As Mike says, many of us experience what you are. However, if you have one of the Gen2 (newer) PRSalphas, there should be no problem reducing it to unnoticeable.

Check that your X, Y and Z pinions are very snug. When properly adjusted you will hear a very slight noise when you pressure back and forth against the motor. There will always be a slight amount of backlash in the gear reduction.

Next snug up the YZ car rollers. Then do the same for the Z rollers. This should cure virtually all of your cut problems.
Gary

[lcolburn]

Thanks for all the suggestions and advice- I'll start by trying to ramp into the cuts. This cut was made with the Onsrud 1/4" downcut that comes in the starter set (just what I had set up after running another job).

I'll take another look at the pinions as well- that could be my problem. More later.