Last weekend I built a cross cut sled, following the techniques used in The Wood Whisperer, Epsiode 146. If you scroll forward to 8 minutes, you will see the portion of the video where Marc attaches the fence and then aligns it using the 5 cut method.
My fence was made from a 4" tall piece of 3/4" melamine covered composite m aterial that my neighbor gave me. He uses scraps he has for sacrificial fen ces since the material is perfectly flat and smooth.
I went through the alignment process and then test cut a few pieces of 1 x 6 poplar. The fence was initially aligned using the left side of the kerf a nd all boards were fed from that side i.e cutting off small strips and smal l ends from the right side of the boards. I was quite satisfied with the sq uareness of the cuts.
Both ends of the fence were screwed to the sled as part of the alignment pr ocess, and then when I was satisfied, I added one more screw just to the ri ght of the kerf, planning to add more screws once I was sure that everythin g was aligned to my compete satisfaction. I'm glad I waited.
Last night, I decided to make a few zero clearance insert blanks from some true 1/4" hardwood plywood left over from some old drawers. Since the panel was ~14" wide, it spanned the kerf in the fence more than anything I had u sed for my test cuts. That's when I noticed that there was a gap between th e fence and the far right side of the panel. As first I thought there was a n issue with the panel, but I decided to check the fence with a straightedg e. There was indeed a slight bow in the fence right at the kerf.
I bent down and looked under the sled and saw the single screw I had placed in the center area of the fence. As soon as I removed that screw, the bow disappeared and the fence is now once again flat across the face.
I went back and watched the video again and I didn't see any mention of Mar c checking the face of fence once it is secured to the sled. My next step w ill be to carefully pre-drill for more screws, starting from right, working towards the kerf and checking the flatness after each screw is secured.
Just something to keep in mind...