Low spots on bed

This is an issue I’ve had with my machine since day one. I haven’t tried to tackle it yet because I’ve been lazy and worked around it. Now I have the time and would like to put it out to the community here and see if I’m alone. If not, then maybe someone has a fix.
First thought, maybe I’m expecting too much? Is this just something I should expect from a machine made out of extruded aluminum?
Second thought; The low spots are located at the general area of x9 y73 and then along the x axis at y94-96 ish.
When I run a flattening toolpath for a spoilboard I have to run it at at least .045 to get a bite in these areas. I then flip the board over and do the same on the other side.
Machine setup is shelac primer coated mdf vacuum bed with a hurricane black box. and mdf spoilboards. pro 4896

What I have checked is the tracks are level and not warped.


Yep, there are many reasons you slap a piece of MDF on the machine and cut it flat.

It’s not just that the 8020 is “close enough” but that dern MDF is also imperfect.

I wrote a program that takes the probe and finds the highest point on the sheet. Then I use that as my starting point and work down from there. It can take several passes to get it leveled.

This is the only time I ever probe the top of stock. It is the one exception to the rule.

I have been writing a program to probe the highest and the lowest and then generate the g-code to do the complete surfacing operation for me. It will be part of the support subscription on my website once I get it debugged. Kind of a single button spoilboard maker. The bolt holes were easy, the surface op is a bit harder.

It’s not your machine it’s the MDF it look nice an flat at the store but it still has hills and valley!! :wink:

Did you mean you are flipping your entire spoil board over and resurfacing? That part didnt make sense to me.

The part you missed is “vacuum table”. The skin of MDF blocks the vacuum but the core lets it through, so you have to remove both sides of the top sheet.

Thanks fellas. I should probably add that this is not my first spoil board. This is #4 I think. They all seem to be low in the same spot. Seems like too much of a coincidence.
I’m really interested in the surface probe program. I saw how it’s done on youtube but the video leaves out what probe to buy.

Then i would check the crossmembers with the sides they may not be even at the tops.:roll_eyes:

Maybe a beam could twist under fastener forces and create a low spot.

It might not be worth chasing down if you can just shim it.

I had a low spot in my spoilboard and it turned out there was a small dip in one of my linear rails. Fixed it and everything gets flat now. Might be worth a look if you haven’t yet.

Just wondering what those surface probes do? Do they fix the dip or confirm it!!

they map out your bed and mark the height variations based on points recorded from a mapping tool in mach. There is a youtube video about it but it doesn’t give any information on what brand probe to by or how to connect it to your machine.

My 48x48 bed on the Avid leg set has an issue of it not being perfectly flat. So far I just put up with it, attached an mdf bed, surfaced it and attached a spoilboard, also surfaced.
Curious if anyone has run a surfacing cut on the aluminum bed to fix the issue once and for all. Stupid idea? Pros and Cons?

There are a few variables here:

Your cross members might not installed perfectly flat, also just about any sheet good you use for a surface is also not flat.

Assuming your spindle is trammed correctly if you plane down your surface with the machine itself, the surface is going to be flat to the bit, which is really what you want.

Technically there could be a low or a high spot if you put a straight edge across it, but if you run a bit over the surface that you planed with the machine, that bit should follow over top of that surface? Make sense?

Now all of that being said, it’s in your best interest to get everything mechanically as flat as you can, but that last tiny bit of bowing either from the metal frame or from your sheet goods you can deal with by surfacing the spoilboard.