chatter in relief?

[woodshopcnc]

I recently got a used shopbot prs alfa.

When milling a relief there is chatter or mill marks in the relief that are not shown in software (Artcam pro). I think it is from vibration "somewhere". It's worse in the X axis. When the machine runs on the X left to right, there are vertical lines (top to bottom across the relief). Almost looks like when you run a piece of molding in a molder, where you have slight mill or chatter marks.

Is there a quick fix as far as a loosen / re-tighten procedure to lessen the vibration? Or is it more the table not still enough?

If anyone responds, I am an extreme newbie on this machine so don't assume I know the real technical things yet.

Thank you, Stephen

[woodshopcnc]

Here's a pic of what I am talking about. Tool is running left to right and chatter marks are vertical so something is loose.

[woodshopcnc]

Gene,

The final pass is a raster rather than an offset at about 9% stepover on an 1/8" ball mill. I never offset the final pass with hardwoods due to the crossgrain issue. I don't think the stepover is the issue, even at a 50% step there should not be the vertical lines, a greater step would see more horizontal ones. I've ran this same part 100's of times on a thermwood, which is really stiff so it's got to be a worn part / play somewhere.

[Brady Watson]

Grab machine a shake each axis feeling for slop. Off the cuff, it appears that the 2 lower V roller bearings that attach the YZ car to the X car (large beam) are loose. These are on an eccentric hub that can be turned to tighten. Do not overtighten - just enough to snug it up.

You could also have some incorrectly tightened motors, where the motor pinion is not fully engaging the rack. Feel for it, drop the afflicted motor and push it back up while tightening the 3/16" hex head cap screws.

-B

[woodshopcnc]

Brady,

Ordered replacement pinions and installed them (25 tooth), cleaned the rails, greased, etc.

If the motors are dropped the car glides down the rails nicely. When I re-set both x motors and move using the keyboard there is noticeable vibration only in the x axis. Especially if I drop the speed to .5 you can really hear it in the X. Maybe it is just low speed resonance.

If i grab the car and shake it back and forth there is a slight tick, tick sound. Is this normal for a Vexta ASM98AA-T7.2 motors. Noticed the tick, tick as well when I put the new pinions on the motor (when they were not engaged).

I'll try and mill something tomorrow to see if the chatter is better.

Is there anything else I can do to quiet the vibration? I think the vibration equals the chatter in the relief problem.

Stephen

[Brady Watson]

Stephen,
It's OK to hear a little bit of 'ticking', but you should not FEEL any slop. That light ticking is in the gearbox and is normal.

You might want to tweak your VR settings if you have not done so already, and make sure that your Z axis is nice & tight. Be sure to lube those racks. Dry is bad...steel on steel.

In ArtCAM, set your machining tolerance to .0025", unless you are doing very, very small reliefs or details with a tiny (.0625 or less) bit. It will speed up the tool (.0025 makes a file 5x smaller than one @ .0001" tolerance) and it will 'cushion' small 'noisy' parts of the relief, making the tool less prone to slowing down to resolve them.

-B