Table saw miter slots not the same width all the way

[Dennis Aspö]

I noticed this last night as I was making runners for a jig, both slots are affected but the right slot is much less so, that's mild enough to not be an issue I think, the left slot though makes it impossible to get a good fit because it's wider in the middle and narrower in both ends. My table top is cast iron by the way.

What's the best way to correct this kind of issue, I've heard of solutions like taking the top off and having it machined. That's a solution that's a bit too involved and expensive for me personally.

Now I suppose maybe I am just doing it wrong, the slots on my table saw are not T-shaped as most saws, but they're dovetails:


I have made the runners to index against the very top of the slots, where there is little material. This is far easier than making a runner that's a dovetail, and I can just lift the jig up from the saw, but I suppose I am indexing against a bit that is most likely to get worn down and this saw is 40 years old now. Maybe I need to try and make dovetail runners, but it's been a real annoyance before to get everything to line up properly.

Any tips or suggestions would be welcome.

[Peter Quinn]

I worked on an old PM 66 before where the top of the slots for the first 10" or so on the left side was worn way more than the rest of the slot or the left. Somebody had used a sled with shallow steel runners for years that referenced only the top 3/16" of slot, wore out the cast iron. Solution was to use runners that went almost to the bottom of the slots where the widths were still consistent. In your case I'm afraid that may mean using dovetail runners as the narrow throat at the top is even more likely to get worn.

[Tom M King]

I've seen table saws before like this. It's fairly common on older saws, and something I always check when buying an older saw, after buying one like that once. I always thought it was wear from using a miter gauge.

[glenn bradley]

I use two runners on nearly everything. I even added a second runner to my Incra i-Box jig. This helped minimize the effect of problem miter slots on an old contractor saw I had. Maybe this can help you as well. If the width does not fit after-market stuff, you will always be making you own runners anyway so you may consider pulling the top and taking it to a machine shop. They could maybe give you a solid 1/8" of depth on both slots that was true and parallel between each other at the same time ;-)

[Dennis Aspö]

I dunno if it would be worth it given how little I use the slots, if there was some reliable home operation I could prhaps have done it.

I don't even own a mitre gauge, I get so much done work with my sliding table that I have not needed the slots until now. I'm building a box joint jig now however so I'll get more use for the slots. Suppose I will have to see how it fares.

[scott spencer]

Your miter slots look like they're dovetailed. Did you shape the runners to fit the dovetail or just the top opening of the channel? You might find that just the upper portion of those channels is worn a bit, but I doubt the wider section at the bottom is.

[john bateman]

If you can make your jigs so that they use two runners...
Then make both runners narrower than their slots, but position the runners on the jig so that each one rides along the innermost edge of it's slot. Unless the slot edge is really bad, you should be able to get something that glides acceptably.

A few thousandths of an inch of wobble will feel loose, but the cut won't suffer all that much.

[Phil Thien]

Your miter slots look like they're dovetailed. Did you shape the runners to fit the dovetail or just the top opening of the channel? You might find that just the upper portion of those channels is worn a bit, but I doubt the wider section at the bottom is.

That is a really good point. The top edge of the slot would be fairly easily abraded, so the runners really should match the profile of the slot for best performance.

[Lee Schierer]

I think the slots were originally intended to keep the miter bar from tilting when you have it almost fully out the front side of the table. There would have been a washer on the leading end of the miter bar. I would get precise measurements of how much wear there is before deciding what to do.

[Phil Thien]

I think the slots were originally intended to keep the miter bar from tilting when you have it almost fully out the front side of the table. There would have been a washer on the leading end of the miter bar. I would get precise measurements of how much wear there is before deciding what to do.

I think at least some of the Euro saws had dovetail miter bars.

[John Donhowe]

Just a random thought- how about filling in with a thin layer of JB Weld where the slot is too wide? If your runner is straight and true, you might be able to coat it with wax (so it doesn't bind to the epoxy) and use it as a mold in the slot. Or, you might need a way to file or machine the slot smooth and to dimensions. Mind you, I've never tried this myself, but it might work. Good luck with whatever you do.

[mike mcilroy]

A different angle on John's fix with the JB weld. Make temporary runners extra deep and narrow enough to fit in an aluminum track then JB weld the track to the bottom of the slots using your jig runners to get the track parallel to blade.

[Keith Weber]

You said that the right slot wasn't an issue. Is there a reason you can't just make you jig so that it uses a runner in the right slot instead of the left? After all, the blade is in the middle of the slots, so it shouldn't really make a difference, right?

Another option (which you may or may not be comfortable with) would be to make a jig without slots and run it along your fence instead. A little more work, as you'd have to pay attention to keeping the jig firmly against the fence, but it would get the job done. If you were worried about pulling the jig back in this manner, you could always add an element of safety by pushing the jig all the way through the blade, and then carry it back to the front so you don't have to pull the jig back past the blade. That way, it wouldn't be any less safe than a standard ripping operation.

If you had a friend with a milling machine, that would be about as simple a job as you could get to fix that permanently. About 10-15 minutes for clamping to the table and alignment, and about 2 minutes to do the milling.

[Dennis Aspö]

Well it did feel more stable with two runners as per the plans I was building it from. I could also perhaps attach it to the sliding table, but I think my next step is trying to make precise dovetailed runners. And yeah this saw is european (as am I) so that explains the dovetails.

Removing the table would be a lot of work, the motor mount and everything is attached to its underside, I am hoping to avoid doing that kind of service for a few decades yet.

[Larry Edgerton]

If you can make your jigs so that they use two runners...
Then make both runners narrower than their slots, but position the runners on the jig so that each one rides along the innermost edge of it's slot. Unless the slot edge is really bad, you should be able to get something that glides acceptably.

A few thousandths of an inch of wobble will feel loose, but the cut won't suffer all that much.

This....... You could make the runners run on the outside as well, whichever side is the straightest. Making your runners as long as possible will make up for some of the discrepancy as well.

Larry