Widening table saw miter slots

[azk404]

Hello

Ive noticed on my cast iron table saw the front and back ends of the mitre slots are maybe 0.3mm narrower then the middle which makes sleds stick when being pushed to the end, and its very frustrating.

Has anyone widened these slots a bit before any advice on how to do that?

Cheers
A

 

[ChaiLatte]

0.3mm narrower covers a lot of possibilities.

Relate the slot position and width to the blade and fence position before you touch them.

Is the 0.3mm reduction all in the right side of the slot? All in the left side of the slot? 0.15mm taper (parallel taper) equally in each side?

Draw a map at an exaggerated scale of the slots, the blade and the fence. That map will rule in and rule out various remedial measures open to you.

 

[azk404]

Ok sounds like a weekend activity so will take a look and see what im getting

 

[Robbo3]

0.3mm narrower covers a lot of possibilities.

Relate the slot position and width to the blade and fence position before you touch them.

Is the 0.3mm reduction all in the right side of the slot? All in the left side of the slot? 0.15mm taper (parallel taper) equally in each side?

Draw a map at an exaggerated scale of the slots, the blade and the fence. That map will rule in and rule out various remedial measures open to you.

Nice summary.

 

[Sideways]

Ultimately I see two ways forward.
The table comes off, goes on a large milling machine and gets machined straight, parallel, even.
That is probably expensive and hard to organise.

DIY method will need an accurate straight edge clamped to the table, parallel to the blade, as your reference. Then you will need to correct the slot by hand wherever it needs it, using a fine file with a safe edge, a diamond sharpening stone or other abrasives.
You will need to map the error as @ChaiLatte describes above to know where to remove metal.
It will be a bit slow. Take a bit off, measure, take some more.... until you have the slot as straight and parallel as you want it.
Concentrate on keeping the sides of the slot vertical, not sloping or undercut.
Minimise the amount you remove and then adjust the bar of your mitre gauge to fit smoothly but snugly in the slot. Some bars are adjustable width, albeit to a limited degree.

Your saw may have some way to adjust the mitre slot parallel to the blade. They usually do, even if just some slop in the bolt holes that connect the top and the spindle assembly. If so, you only need to get the mitre slot straight/parallel. You can realign it to the blade afterwards.

Oh, and sharpie marker on the sides of the slot, then file etc. Repeatedly so that you can see that you are removing metal where you intend to.

 

[--Tom--]

If you were feeling brave I’ve heard of people machining cast iron with a router. The bit won’t last long, you can’t take much off per pass, you need to keep the speed down, you need rigid jigging and surface finish would make a proper machinist cry, but could be made to work.
Example of someone doing it here:
https://www.woodworkforums.com/f11/machining-cast-iron-normal-wood-router-yes-239003

 

[MorrisWoodman12]

Errr! This may be an incredibly stupid suggestion but..........
If the difference in the width of the mitre slot is only 0.3mm is it necessary to correct the slot at all? I would have thought that was an acceptable maximum clearance to allow the sledge to run and if the sledge runners are 300mm long then the slop is much less than 0.1° (assuming my math is correct). The result of these thoughts is that you could reduce the width of the runners by 0.3mm which may be an easier task. The slop may be a little greater as I haven't taken into account any existing clearance but it's still going to be minimal.
As I said it's a random thought and possibly quite stupid.

 

[Amplidyne]

Errr! This may be an incredibly stupid suggestion but..........
If the difference in the width of the mitre slot is only 0.3mm is it necessary to correct the slot at all? I would have thought that was an acceptable maximum clearance to allow the sledge to run and if the sledge runners are 300mm long then the slop is much less than 0.1° (assuming my math is correct). The result of these thoughts is that you could reduce the width of the runners by 0.3mm which may be an easier task. The slop may be a little greater as I haven't taken into account any existing clearance but it's still going to be minimal.
As I said it's a random thought and possibly quite stupid.

It'd be nice to have it right, but I don't think it'd be on my urgent priority list.
For 0.3mm I'd use a file personally. As already said, a "safe edge" one. Shouldn't take that much doing really.

 

[ChaiLatte]

The slop may be a little greater...

I have never seen it, but would there be some way of making part of the runner spring loaded or slightly elastic to accommodate the varying width without slop?

I am thinking in principle of how a finger board works.

 

[deema]

IMO, unless you’re an apprenticed tool room guy, your only option is to have it machined. Getting anything like 0.3mm accuracy on a vertical sided pocket is just not going to happen any other way. I would go so far to say, that anything you do will make the errors larger.

 

[XTiffy]

In my engineering apprenticeship one of the test pieces was a mild steel hexagon fitting into a cast Iron plate, the hexagon had to fit with fine tolerance when rotated for each face. Filing the cast iron was much easier so much so that it was too easy to go too far.
Absolutely need to find where the error lies but still a reasonable hand tool job. One tool not mentioned is an engineer's bearing scraper.