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I recently got a cabinet saw that has a cast iron bed, as most such saws do.

I want to make a sled for it which requires fitting rails to the guide slots in the bed.

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One issue I see is that the guide slots are not particularly deep (they are a shallow T-slot style shown above) and they have a somewhat rough surface. If I run my fingers along the side of the slot, there is some roughness.

So, my question is whether I should be filing and scraping the slots to try to make them as smooth and straight as possible before fitting the rails, or do most people accept the slots to be a little rough and its not a problem?

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  • Sorry I am not familiar with the term, is a cabinet saw a large table saw?
    – Volfram K
    Commented Sep 10, 2021 at 7:22
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    @VolframK yes. Reference this review (first one I found, no affiliation or assurance of quality) - The 1st saw (DWE7485) is a "portable" saw, the 2nd (PCS21230) is a "cabinet" saw, and the last one (CNS175) is a "stationary". Cabinet saws tend to be bigger with heavier, higher HP motors designed for high duty cycle in a pro shop, stationary are smaller, possibly moveable (mine has wheels) with smaller motors more for DIY/home craftsman, while the portable is lightweight aimed at the construction site.
    – FreeMan
    Commented Sep 10, 2021 at 12:01
  • Treow- while a flat bar for the runner is handy in that you can drop the accessory in/pull it out from any position, you might consider looking for T shaped material (or making your own out of wood or UHMW plastic) to help hold better, particularly if you're making a crosscut sled and it's going to be deep.
    – FreeMan
    Commented Sep 10, 2021 at 12:06
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    @FreeMan I've never seen an entire T-shaped miter bar; using one would be a huge pain because you'd have to feed the entire thing into the slot from the end. Normally there's just a washer or similar flat piece screwed into a notch at the far end of the bar, and that's all that's needed to keep the bar in the slot. And it means that you can easily drop the bar into the slot with just the end protruding beyond the back edge of the table.
    – Caleb
    Commented Sep 10, 2021 at 13:38
  • @Caleb IIRC, mine has a washer at each end - I'll have to double check that, though. Never thought about trying to set it on the saw and pull it backwards. Fair point, though.
    – FreeMan
    Commented Sep 10, 2021 at 13:39

2 Answers 2

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my question is whether I should be filing and scraping the slots to try to make them as smooth and straight as possible before fitting the rails, or do most people accept the slots to be a little rough and its not a problem?

It really depends on how rough it is. It's pretty normal for miter slots to have a somewhat rough surface, and it's usually not a problem at all. For example, I can feel machine marks on the inside of the slots on my saw, whereas the top has been ground to silky smoothness. Remember that the miter bar that rides in the slot will contact many of those high points all at the same time, and the fact that the surface isn't perfectly smooth can actually reduce the friction because there's less contact between the bar and slot. A rough surface is only a problem if it's so much that it causes the miter bars to wear down, or to not slide smoothly.

Put the miter gauge in place and run it back and forth a few times. Does it slide smoothly? If yes, don't worry about it and move on.

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If your rails will be made from wood and if the roughness is enough to hurt your fingers then I would smooth it out a little. It not eventually the slot will wear down the rail and cause a loose fit.

I don't think you can scrape such a surface so you could do this running a file along each side. If your file does not have a "safe edge" attach tape to one edge, or maybe you can use folded paper as a sled? An option is a DIY sanding stick with sandpaper attached to sides only, leaving edges bare wood. P80 grit one side, P150 grit other side.

This will widen the slot a very small amount but I don't think this is important even for stock accessories. Metal rails are easy to modify to fix loose fit with hammer and punch.

Make wooden rail after such work, using new measurement.

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