Y-axis "ringing": stiffen up the table?

[lgm]

I sent some of these questions directly to SB, but thought it would be helpful to get some opinions here as well.

We have had our PRT Alpha with extended Z-axis for quite a while, and last week we finally got around to upgrading our Z-axis and the motors. Because we have such a tall z-axis (16" or so) we always had some chatter while cutting thin, but denser material (plywood). The upgraded Z-axis seems to have made that much better, but we still have one minor remaining issue: that is some "ringing" in the y-axis.

Things appear much stiffer now, but the y-axis still moves a bit when the cutter changes directions. For example, if we are cutting a rectangle (say in plywood), it moves fine and does not chatter when moving in the Y, but when it rounds a corner and changes to the X direction, the gantry has some momentum that makes the X rails actually shake a bit. This results in a wavy cut.

My thought is that there are two ways to fix it: One would be to
adjust the ramp values so the machine slows down more significantly
before it changes direction. The second would be to somehow stiffen
the long X-axis rails so they don't move.

So for ramp values: We are just using the defaults. What kind of
numbers should we be considering to help with this issue? Typically
we are either cutting plywood or urethane foam. For a 2 fluted bit,
we keep the spindle RPM's around 10K, using moves speeds of 1.5-2
(x,y) for wood, and 4-6 for the foam (x,y).

For stiffening up the X rails: We need to keep the ends open for
loading/unloading material and access to the cutting area. So our
thought was that either adding another leg set in the center might
help, or filling in the void between the X-rails (the entire length of
them) would stiffen them up over all. I have attached a picture to
help illustrate these ideas.


http://i99.photobucket.com/albums/l318/jdbwrx/Ringingquestions.jpg


Thoughts?

[Brady Watson]

Jason,
Your 18" Z PRT was primarily designed for cutting foam. You'll get some deflection with the Z down that low. If you don't need the 18" Z, you can raise the table back up to the top table side and get better results using only 6" of the Z.

Yes - adding a center leg would stiffen up the table. You can either add a full leg, a partial that just bridges the 2 table sides together or add a 10' long C channel table side in the middle and bolt the center one to both the top & bottom one. My preference would be to add 2 legs, gussets and a 3X5 to the center.

-B

[lgm]

Jason,
Your 18" Z PRT was primarily designed for cutting foam. You'll get some deflection with the Z down that low. If you don't need the 18" Z, you can raise the table back up to the top table side and get better results using only 6" of the Z.

Yes - adding a center leg would stiffen up the table. You can either add a full leg, a partial that just bridges the 2 table sides together or add a 10' long C channel table side in the middle and bolt the center one to both the top & bottom one. My preference would be to add 2 legs, gussets and a 3X5 to the center.

-B

Thanks Brady. Appreciate the feedback.

I fully understand and expect we will get some deflection due to the depth of our Z-axis. We still primarily use it for milling terrain forms from foam, so usually we need all the travel. We have raised the table surface before (i.e. block of 6" foam) for cutting ply, but depending on what we are cutting (sizes and parts) that is usually more trouble than it is worth. It also didn't make the cut significantly better.

I will look more into the extra legs.

I would also like to hear feedback from anyone who has adjusted or has suggestions about adjusting the ramp values to help with this though too. Even when we cut foam, the ringing can sometimes cause some surface issues.

[Brady Watson]

Sorry....forgot to post a link to tuning your ramps using the VR Command (http://www.shopbotblog.com/index.php/2008/03/a-ramping-the-vr-command-and-how-to-tune-your-tool-for-maximum-performance/)

I would change the following values & leave all the rest @ default:

3D Ramp Threshold: 150
Minimum Distance to Check: .08
Slow Corner Speed: 35
If the setting is present in your VR, Change Keypad Ramp Rate to: .2

Slow Corner Speed will influence ramping more than any other setting. A valid/useful range is between 25 and 65.

-B