"Z compensation" on 4x8 foot CNC router
Posted: Fri Mar 08, 2024 7:14 pm
I would like to do some miter folding on my CNC router, but this requires leaving .005" of material at the bottom of the V-groove. While my machine is very accurate in the X and Y axis, I have never really been able to dial in my Z axis and get the table perfectly flat...
I am running Centroid Acorn on an Avid Pro 4896. This is a 4x8-foot gantry-style CNC router made of mostly 80/20 aluminum extrusion. I knew the aluminum would be a weak point, so I welded my own base using 1.5" diameter Schedule 90 pipe:
On top of this, I have an MDF vacuum table made from two 4x8 sheets of MDF. The bottom platen was coated in wood glue after this image to reduce leakage through the bottom:
I'm running an Airtech L160B rotary vane vacuum pump which is a bit undersized for this size of router, only pulling 112 cfm. While hold-down is good, the MDF top is actually too porous and kerf leakage also contributes to parts lifting up. The somewhat counter-intuitive solution I found was to add another sheet of 3/4" thick, 4x8 "normal density" MDF, not skinned, on top. For a total of three sheets of 3/4" MDF making up the entire vacuum table. This results in good part hold-down and minimal leakage.
I mention the vacuum table construction because I think it might be one contributing factor to the challenge I've had getting accurate Z-height across the entire table. Of course, I am flattening the table using a 2.5" SlabSlayer flattening bit routinely, and while the vacuum is turned on during this operation, there aren't any parts on top of the table to apply downward clamping force. I suspect that the spoilboard is being flattened correctly, but once a real sheet of plywood is put on the table to be cut, the vacuum table pulls the table harder toward the middle of the table, likely creating an valley in the middle of the table.
It could also be that the MDF is compressing slightly, and maybe inconsistently, forming Z height irregularities.
I was wondering if anyone has had this challenge before, and if there's been any creative solutions to solving for it? Is there a software solution that could incorporate Z-height measurements at different locations of the table, say every square foot, and use that to compensate for Z height irregularities across the table surface?
I am running Centroid Acorn on an Avid Pro 4896. This is a 4x8-foot gantry-style CNC router made of mostly 80/20 aluminum extrusion. I knew the aluminum would be a weak point, so I welded my own base using 1.5" diameter Schedule 90 pipe:
On top of this, I have an MDF vacuum table made from two 4x8 sheets of MDF. The bottom platen was coated in wood glue after this image to reduce leakage through the bottom:
I'm running an Airtech L160B rotary vane vacuum pump which is a bit undersized for this size of router, only pulling 112 cfm. While hold-down is good, the MDF top is actually too porous and kerf leakage also contributes to parts lifting up. The somewhat counter-intuitive solution I found was to add another sheet of 3/4" thick, 4x8 "normal density" MDF, not skinned, on top. For a total of three sheets of 3/4" MDF making up the entire vacuum table. This results in good part hold-down and minimal leakage.
I mention the vacuum table construction because I think it might be one contributing factor to the challenge I've had getting accurate Z-height across the entire table. Of course, I am flattening the table using a 2.5" SlabSlayer flattening bit routinely, and while the vacuum is turned on during this operation, there aren't any parts on top of the table to apply downward clamping force. I suspect that the spoilboard is being flattened correctly, but once a real sheet of plywood is put on the table to be cut, the vacuum table pulls the table harder toward the middle of the table, likely creating an valley in the middle of the table.
It could also be that the MDF is compressing slightly, and maybe inconsistently, forming Z height irregularities.
I was wondering if anyone has had this challenge before, and if there's been any creative solutions to solving for it? Is there a software solution that could incorporate Z-height measurements at different locations of the table, say every square foot, and use that to compensate for Z height irregularities across the table surface?