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Discussion Starter · #1 ·
Hi,

I'm building my first crosscut sled using 3/4" mdf and oak for the runners. I've got the back fence on and the runners attached. I'm finding that something is rubbing as I move the sled back and forth thru the slots.

I saw in a youtube video someone recommended putting chalk on the top of the miter slots to transfer to the runners to show where it is rubbing and I did that and did a bit of sanding of the runners but am still getting some rubbing altho there is no longer any chalk transfer. I'm afraid to sand anymore in random spots for fear I'll sand the runners too loose.

I can see there is a little bit of friction on the bottom of the mdf in some spots but I've looked over the saw table carefully and dont see anywhere that it is rubbing. So I'm not sure if there is a tight spot on the rails somewhere or something else going on. I even removed one of the side tables on the saw to see if that would make it go away.

Anyone have any ideas for how to diagnose where the rubbing is happening?

Thanks.
 

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When I built mine, I used the method from the video on the Wood Whisperer web site. He marked the side of the runner with a sharpie, then moved it back and forth. Where the sharpie rubbed off or got fainter was the rub spot and you sand there. This worked well for me. My sled is also mdf base and oak runners.
 

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Fine Woodworking magazine has the perfect sled - build 2 and then attach them. Build a sled for the right and left miter slots and build them so they extend beyond the blade. Cut off the excess for both sides - add the fence and voila...
 

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check it then fix it

How do you check it? Use a marker, black shoe polish, chalk, spray paint anything that will rub away when under friction. You can check the straightness of the rails using a known straight edge like an aluminum level. You can measure the distance between the runners using the most accurate means at your disposal, whether it's a tape measure or steel scale or just a stick with precise marks made with a knife blade. Compare your dimension with the distance between the miter slots at the front and back of the saw table.

How do you fix it? Removing material is the only way, since something is too tight/wide, curved slightly, OR not parallel. For out of parallel or a curved runner, use a shoulder plane or a long sanding block with 320 or so wet dry paper on the runner side only. Sand a little, try it out, sand a little more until it's a smooth constant feed.
 

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Discussion Starter · #5 ·
How do you check it? Use a marker, black shoe polish, chalk, spray paint anything that will rub away when under friction. You can check the straightness of the rails using a known straight edge like an aluminum level. You can measure the distance between the runners using the most accurate means at your disposal, whether it's a tape measure or steel scale or just a stick with precise marks made with a knife blade. Compare your dimension with the distance between the miter slots at the front and back of the saw table.

How do you fix it? Removing material is the only way, since something is too tight/wide, curved slightly, OR not parallel. For out of parallel or a curved runner, use a shoulder plane or a long sanding block with 320 or so wet dry paper on the runner side only. Sand a little, try it out, sand a little more until it's a smooth constant feed.
I checked it by putting chalk on the miter slots themselves and then looking for transfer to the rails. It sounds like you and the previous responder are saying to put the marks on the rails instead and then check for where it is rubbing off? I will do that tonight.
 

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Remove one runner at a time to see which is the problem. Or if you glued them on, wax the runners but don't buff then see if you can see the rub.
 

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When I make a sled I place the runners in the miter slots, make sure they are straight so they slide and countersunk holes are drilled. Pull the runners back past front of saw table, lay base in position and fasten front with screws from bottom, slide entire assembly forward to hang over back of saw table, drive in screws to hold back end of runners. Remove and add any extra screws.
 
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