Odd spoil board flattening pattern.

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Odd spoil board flattening pattern.

Postby DavidT » Tue May 28, 2019 10:04 pm

I ran a pocket pattern to flatten my spoil board and it created and odd, non-flat pattern (see attached picture). First time machine has run and the software and machine seemed to work just fine.
New 1" diameter spoil board bit with 40% overlap on each pass.
0.0625 cut depth
Offset, conventional pattern

The concern is how un-flat or non-flat the surface is in certain areas. I drew a pencil line where the surface transitioned from really flat to not flat at all. The area on the inside of the hourglass was not flat in the y direction with ridges that were created during the left to right x-pass. During those passes, presumably the bit is not at 90 degrees but it varied quite symmetrically as the machine operated. The area outside the hourglass on either side was flat in both the x and y direction and felt smooth.

This is an odd pattern which makes me believe the gantry is not staying true when it moves from side to side.

Thoughts, suggestions, help?

David Tuinstra
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Re: Odd spoil board flattening pattern.

Postby Scheffy » Wed May 29, 2019 11:14 am

I run into the same issue when doing cuts along the X axis. My thought is that it's due to the gantry just being way more flexible with fore/aft movement. With the machine off just grab the bit and pull it toward and away from you vs. side to side and you'll see the difference. Side to side mainly just wants to push the gantry along the rail but the gantry stays mostly level. Fore/aft makes the gantry and bit actually rock toward and away from you a good amount. Extend that angle out over the surface of a wide surfacing bit and you can understand the little ramps you see. With those wide bits you can also imagine the extra force on the spindle because of the longer lever arm whenever the bit hits new material, just making the flex worse.

It's not much of a fix, but to get smoother spoilboards I just do a raster cut along the Y axis and that at least makes the surface smooth. When cutting an actual project I'm almost never using such a big bit, and while the flex isn't eliminated it's at least much less of an issue.
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Re: Odd spoil board flattening pattern.

Postby DavidT » Wed May 29, 2019 3:01 pm


I get what you're saying but this was not a fast cut. Took 16 minutes to run the entire 24"x24" spoil board. The machine never bogged down. A 1/16" cut in MDF wasn't much of a challenge. That said, why an hour glass pattern. I would have expected a more square or rectangular pattern for the area that wasn't flat.

I'll try the raster cut. Likely will explore tramming the gantry too, just to see if it makes a difference.

Appreciate the feed back.

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Re: Odd spoil board flattening pattern.

Postby Bob » Wed May 29, 2019 7:09 pm

Hello David,
It appears that you are using an offset clearance cut.
Try the same pocket using a raster cut at 90 degrees...on scrap first.
Or try it at 0 degrees...on scrap first. You should eliminate any X axis flexing (if any) by cutting along the X Axis.
Either one will make your hourglass disappear.
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