So I purchased a 1" surfacing bit from Amazon. I have some things that will be very picky for level, so I’d like to be able to know that I can go only a small way into the material but have it as consistent as possible.
Most of these things are going to be small, and not require a lot of depth. Things like milling circuit boards, or some engraving. Honestly, they’d probably be fine on my spoilboard as-is because of small size.
I’m considering a few options:
Bolt down a piece of stock to the spoilboard in a specific spot and surface that. Make it, say 300mm square and outline it in sharpie with a key shape so I know what orientation it is level, then just bolt it down when I need that “absolute level” section. I might give up 20mm or so of Z using that, but for circuit board etching, that’s completely fine.
Surface a small section of the spoilboard. Have it near centre. It will be a tad lower than the surrounding area in that spot, or maybe some edges of it, but with creating placement, I’m sure that it can be kept reasonable.
Damn the torpedoes… Surface the whole region that I can address with the router. This of course leaves the edges that I can’t address, so either make those lower with my plunge router or leave them be. The vast majority of what I do is entirely inside the addressable area, or within the 1/2" that the 1" piece will allow me. There’s less than 1mm of variation (that I’ve measured) across the area. Nothing that I’ve done has had a problem if I cut .3mm into the spoilboard.
Any of these options are really me being a little OCD about things being level. I shimmed the table so that the X and Y rails are level, and the tool is plumb. It bugs me a bit that the spoilboard has some variation in it at all, but I can accept that the bed isn’t perfectly flat. So long as I can stull cut accurately, it’ll be fine, but I do want to be able to use the router for higher accuracy work as well.
#1 and #2 seem like easy to achieve options, depending on how much of your usable surface you regularly use. While my LowRider can handle 4x8 feet, 95% of the time I’m using a 2x2 area or thereabouts.
I’ve been too lazy to surface my spoilboard, but that’s what I would do.
I had the “pit” problem with my Burly. My solution when I went to the Primo was to mount the legs on a base board and then mount my spoil board on the base board. The spoil board was sized and positioned to my cutting area, so I can surface the entire spoil board. This does make my legs 18mm taller, but so far no issues from this added length.
If you are 1) reasonably sure that the things you need precision for are small, and 2) if your spoil board is reasonably level without surfacing, then I would go as small as possible. Large items that might go beyond the cutting area likely can span a small “pit.”
There is another choice. Small items are likely to be short. You could put another, small spoil board on top of your current spoil board and then surface only the small spoil board. You could then remove/re-install this spoil board as needed. This would have the added benefit of reducing the leverage on the bit when cutting.
My legs are mounted on 3/4" strips so that I could get them perfectly square, a removable spoilboard mounts between the stringers. It’s a little larger than the addressable area of the CNC though, so I can’t surface the whole thing.
When I tried doing circuit boards, I went with option 1. I didn’t bother keeping a particular board in a particular spot… I just re-surfaced a new sacrificial board each time I tried to cut a new circuit board.
Then after doing that a few times, I just surfaced the entire spoilboard. This was necessary as I was about to try a project that was v-carving most of the cut size. V-carving is pretty particular about Z depth into the board being flat or else the points don’t come out right.
First #3 and then #1. I like having the freedom to not think too carefully about minimizing the cuts into the wasteboard. Even though the spoilboard is cheap to replace, I find myself reluctant to slice it up with sloppy overcuts. With a small second wasteboard, I am comfortable going to town on it without second thoughts.
Both my lowrider and my mpcnc have spoil boards that are the size of my cutting areas, so I surface the whole top. Well I surface the whole top on my mpcnc. Found my lowrider top had a twist in it when I surfaced it, so I still need to resurface it after I got the twist out.
I needed to V carve for signs and have to have a flat bed. I alternated spoil board with t-tracks. I drove the LR2 around and had ~ 2mm of height difference which would be a problem for V carving, but not much for cutting plywood. In the end, I surfaced 3 ft x 4 ft with a 1 inch bit. My feeling is to get it right before committing to wood.