Dave Trendymiddlename Starr

Table saw cut narrow in the middle?

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I was making some miter slot runners out of a plastic cutting board today, and I was getting some weirdness in my cuts. Using my calipers to measure the strips of plastic I was cutting, the front of the strip and the back were only off by about 0.05mm in thickness, which I can live with. But for some reason, all the strips get narrower in the middle, by as much as half a millimeter. Which, on miter slot runners, means there's some wiggle to them until both the front and back edges are in the slot, which I'd really like to eliminate. But I'm not sure what would cause a ripped piece to be essentially equal in width at the start and finish, but be narrower in the middle.

Any thoughts? Thanks!

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Welcome to the forum.  I'll start the ball rolling with some questions.  Sorry if these seem obvious to you.

1.  is your fence straight?

2.  Is you blade parallel to the fence?

3.  Is the edge of your work piece that is against the fence straight to begin with?

I am sure that others will chime in soon.

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My bet is #3 on Ronn's list. If the factory edge of the cutting board had a curve, it would telegraph through the cut. Attach the plastic to a piece of plywood or other material with a confirmed straight edge, and make the cut with that known straight reference against the fence.

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For a miter slot, .5mm might as well be a centimeter :( 

I know if you don't watch for it, it's easy to let the board wander away from the fence a tiny bit in the middle of a cut. I've had this happen before myself. It just requires constant attention and pressure to keep the piece tight to the fence.

Of course, as already pointed out, you need to start with a perfectly straight edge

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Is the plastic sticking to the blade? Is the edge you are cutting getting any melted plastic on the edge and, if so would a faster or slower feed rate get cleaner cuts. I have experienced melted material on the edge of some acrylic glass (Plexiglas ?) anyway I found a faster feed rate eliminated the melting on the edge 

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