Here are files for cutting 1/4" MDF to make the guide rails as shown on my phlatprinter video tour http://flashsphlatprinter.blogspot.com/2011/01/flashs-phlatprinter-video-tour.html

The MDF was cut using a 1/16" bit using the multipass option.

Even if your machine tracks perfectly straight, aligning a 48" piece of 4" Balsa is no picnic. This mod takes the guess work out of getting the material lined up.

Simply use a builders triangle to align one guide rail, place your balsa up against the fence and slide the second guide rail up to it and tighten the thumb screws.

I used 8/32" tee nuts and thumb screws for hardware and glued the pieces together with Tightbond glue.

Be sure you check the size of your drill in a piece of scrap to make sure the tee nuts fit snug into the top plate. Attached files GuideRail.skp (219.6 KB)Â

 

While these will obviously keep the material from skewing, I think the more important thing is how it helps you get the material square on the machine.
While my machine was running, that was the part that annoyed me the most, I'd get the foam in it and then as it was feeding I'd notice that it wasn't straight.

 

Exactly! Squaring up a 4" wide piece of 48" long balsa piece in which you want to cut within an eighth of an inch from the edge was always tricky.

A 4 foot wide FFF is not so critical nor is the need for guide rails to align it since you can align the edge with the front of the machine.

If you are getting skew on a large sheet of FFF, there is something else wrong with the alignment of the machine.

I once tried a fence on a large sheet of 3/4" Blue Foam when my machine was not tracking straight and all it did was put so much force on the fence that the stepper motor stalled.

The use of guide rails assume you already have a well tracking set of drive rollers.

 

Actually I was talking about after I got my skew to go away. And I did run some MDF through it also(With masking tape on the edges). Another thing was trying to hold the piece as square as I could while dropping the pressure rollers, it always seemed to want to shift right before I got the rollers down. This would help alot with that. I like the idea of the fences for the harder materials, but since I have some leftover bearings laying around from some other projects, I think I'll try to incorporate them to lower any friction I can.

 

Interesting! I did not realize that. I rarely ever cut FFF that close to the edges so I would probably never have noticed.

Depron on the other hand should be cut to closer tolerances I would think.

As for testing skew, I would not think it would matter much since you are just moving material (could be of any shape actually) along the axis and it should still return back to the starting gate if everything is trued.