1220 × 2440 build

[HullMark]

Here's design that will allow you to be more flexible with the frame. Although the base here is in one piece it could easily be made in 2 or 3 pieces and bolted together.
The important part is the full-length profile section running full length down each side. This will allow you some flexibility in adjustment so can get both rails on same plane without messing around epoxy resin etc. Must stress thou the accuracy will only really be good enough for cutting woods and softer materials. It will also take lot of setting up and tweaking until spot on.
In my experience profile is relatively flat and straight enough for wood router and does make fitting rails, ball screws etc much easier. The downside is cost so mixing steel and profile gives good balance of strength, costs and accuracy with ease of building which doesn't require too much machineing or timely and costly techniques for leveling etc.

I guys hope all is well. So much to do with so little time! When I've been looking at doing something like this, I've been concerned about how to attach the profile to the steel so it's strong and minimises lateral forces. Are you really sure just bolting it on the bottom via those brackets is strong enough?

I don't know how clear it is in the pictures I've sent, but I've used 90mm box section with 160 x 40 profile sat on the top. This gives 35mm overhang either side and would allow me to bolt the profile from the top down on the inside and outside edges of the outer steel lengths. Also does the gantry look strong enough now? Cheers.

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