Reasonable expectation of flatness

[aarvidsson]

I'm just about to tram the spoilboard of my newly rebuilt Shapeoko, and it occurred to me: how flat can I reasonably expect this board to stay? It's an 16mm MDF that i'll remove 0,3mm from - how flat will it stay with changes in temperature, humidity and such?

For you guys that want a very high consistency in the Z axis (0.1mm and such) - how do you do it? Metal bed? Tram whatever piece of sacrificial material before running every job?

[CastIrony]

Yeah, I've noticed some significant swelling from the piece of thick MDF I use as a spoilboard/mounting board. In the short term I'm probably going tram it, but eventually I would like to replace it with a material that won't warp or swell with changes in humidity, like HDPE.

[WillAdams]

If you spray it w/ spar urethane or some other sealant before (top, bottom, sides and into all the holes) and after tramming (touch up where you cut), it should stay reasonably flat.

[DanMc]

I use mdf. If I'm working on something critical I'll tram a second sacrificial board specific to that job, but then I usually mill a jig specific to each project or series anyway with holddowns where I need them. Even with something that won't swell is the table the machine is resting on stable to <0.1mm? There's too many variables and it takes no time to skim the surface before a project.

[aarvidsson]

Good points from all of you, thank you very much!

[WillAdams]

For an example of such jigs, see the calipers which I made: http://www.shapeoko.com/projects/project.php?id=154 --- they also made use of sacrificial aluminum and nylon screws.