Table Saw Miter Cut "U" Shape

[Mike Geig]

Howdy, so I am racing to get some boxes made for Christmas gifts. I am trying to cut the 45 degree miters on my table saw using a cross-jig. I clamp the work piece in place and use stop blocks, but I get an odd U shape to the cut. The board is milled flat and square prior to making the cut. Any idea what is happening here? My blade is calibrated to the miter slot and my rip and parallel cuts are fine. It is just these miters that are killing me. Ideally I would use a shooting board, but I don't have one and I don't have time to make one before Christmas. Any help would be amazing.
https://i.imgur.com/XEehco0.jpg

[Andrew Hughes]

Too much movement in your crosscut jig would be my guess.

[glenn bradley]

It is possible that the jig could move away and then back to the desired feed path. I feel that it is unlikely that it would be consistent. Have you checked the blade alignment at 45*? Folks often go to great lengths to get the blade parallel to the miter slot at 90* but, don't finish up by adjusting when at a bevel. Everything is great until you start making bevel cuts.

The material is clamped to the fence and the sled rides in the miter slot so they are fixed in position. If the alignment is off the (crooked) blade plane could be flexing as it tries to conform to the unforgiving and misaligned feed path of the material. Easy to check. Just set your blade to 45* and check it like you do at 90*. A thou or so won't hurt you too much but, 4 or 5 thou starts to get problematic.

421595

[Andrew Seemann]

Looks like your board is going though at a slight arc, rotating presumably with the axis on the miter bar. I'd check that your sled is not loose as mentioned above and that it is rigid enough to not flex. Beveled cuts are have more blade engaged compared to a 90 cut, and what works at 90 may strain at 45.

Also, any chance your blade is getting a little on the dull side? That could cause you to muscle through the cut and could give you the a little pivot against the miter slot. Again it would show up at 45 more than 90.

[Lee Schierer]

What blade are you using to make the cut? Is the blade dull? When is the last time you checked the blade alignment to the miter slot? Slop between the miter gauge and the miter slot can also cause this problem.

[Tom Bender]

Don't race, nobody wants a box with your bloodstains on it. Better to make excuses and deliver later.

[Todd Mason-Darnell]

What blade are you using to make the cut? Is the blade dull? When is the last time you checked the blade alignment to the miter slot? Slop between the miter gauge and the miter slot can also cause this problem.

Lots of good advice here. I was also wondering about the blade and if you are getting deflection because it is dull or a thin kerf.

[Steve Eure]

I'm with Todd on this. I've had some problems in the past while using a thin kerf blade and having the blade raised up for a higher cut, i.e., while on a sled for example. While using the same blade, I had to install a deflection plate to avoid this. The simplest thing would be to check blade deflection first.

[Mike Geig]

Thanks for the info everyone. Follow up:

-The blade is sharp, though it is a 10" 60th finishing blade (full kerf) so there is a little pushing involved
-It's a good point about checking for square while at 45 degrees. I've only tested for square at 90 degrees. I will check on that
-The whole thing is on a sled, but I will check for slop / flex with the rails and fence
-Something I forgot to mention, when using this sled, there are burn marks on the back-half (closest to fence) of the cut. I agreed that this would indicate an alignment issue, but seeing how test cuts came out fine I wasn't sure

Thanks again for all the help

[Mike Geig]

It is possible that the jig could move away and then back to the desired feed path. I feel that it is unlikely that it would be consistent. Have you checked the blade alignment at 45*? Folks often go to great lengths to get the blade parallel to the miter slot at 90* but, don't finish up by adjusting when at a bevel. Everything is great until you start making bevel cuts.

The material is clamped to the fence and the sled rides in the miter slot so they are fixed in position. If the alignment is off the (crooked) blade plane could be flexing as it tries to conform to the unforgiving and misaligned feed path of the material. Easy to check. Just set your blade to 45* and check it like you do at 90*. A thou or so won't hurt you too much but, 4 or 5 thou starts to get problematic.

421595

Unfortunately, it seems I'm not cool enough to view images on this site.

[Bradley Gray]

Unfortunately, it seems I'm not cool enough to view images on this site.

It's not coolness, it's the $6 a year you don't pay.

[Mike Geig]

It's not coolness, it's the $6 a year you don't pay.

Ah, I didn't know that was a thing.

[David Utterback]

Others with comments are right. I also wonder if the piece is possibly lifting slightly from the surface of the sled. I had an opposite problem with small pieces I was cutting for boxes but it turned out the thin would was cupped after resawing and planing.

[johnny means]

Your banana cuts and burn marks would indicate that you blade is skewed to the right when tilted. Thus your board is being forced upwards, ramping up onto the blade. As you exit the cut the board is able to drop back down, hence your arc. If i was pinched for time, I would cut my miters a 16th long, then trim to final length on the other side of the blade. If I wasn't pinched for time, I would calibrate my saw.

[Mike Geig]

Your banana cuts and burn marks would indicate that you blade is skewed to the right when tilted. Thus your board is being forced upwards, ramping up onto the blade. As you exit the cut the board is able to drop back down, hence your arc. If i was pinched for time, I would cut my miters a 16th long, then trim to final length on the other side of the blade. If I wasn't pinched for time, I would calibrate my saw.

Good idea. I will give that a try in the short term

Turns out my blade is .0004 off at 90 degrees and .0012 off at 45. After Christmas I will get to fixing this but need a faster solution for now and this might be it.

[John K Jordan]

Ah, I didn't know that was a thing.

The forum changed earlier this year to require a small donation - with it you can view photos, send/receive private messages and more. The motivation behind the change was not to rake in the cash but to keep the forum operating. I signed up for a monthly recurring contribution so I don't ever have to think about it.

https://sawmillcreek.org/payments.php

JKJ

[Brian Tymchak]

Good idea. I will give that a try in the short term

Turns out my blade is .0004 off at 90 degrees and .0012 off at 45. After Christmas I will get to fixing this but need a faster solution for now and this might be it.

If you got the number of 0s right in your measurements, then I'd say that is pretty good accuracy and don't touch a thing. The problem is most likely in your sled. Do you have any play of the miter bar in the miter slot? Or another thing that comes to mind is that the sled might be sticking on the saw surface when you push it through the cut. Clean and wax both the sled bottom and saw surface, and wax the miter slot and bar.

[Andrew Seemann]

Good idea. I will give that a try in the short term

Turns out my blade is .0004 off at 90 degrees and .0012 off at 45. After Christmas I will get to fixing this but need a faster solution for now and this might be it.

if your blade is really within 4 ten thousandths of an inch and 12 ten thousandths, you won't get that better. That is well below the machining tolerance of the machine as a whole. Even 4 thousandths and 12 thousandths probably would not cause what you are describing. And even if they are off by that much, it won't cause the work to rotate, the angle would just be off, and not enough to notice.

The issue is with how the work is going past the blade. Either it is rotating slightly , or the blade isn't aligned with the miter slots, or you have too much slop in the miter slots relative to the jig. My old saw had that problem, the miter slots were so worn that they would be tight at the ends but sloppy in the middle.

[Mike Geig]

I've rebuild the sled with no slop at all and aligned with the blade. The blade is a little off from the miter slots (as indicated) but I agree that it shouldn't be causing as much of this motion. If nothing else, this has been a good learning exercise. Unfortunately, my wood has started to cup a little. I started this project with 8/4 hard maple. I brought it into my shop and let it set. Then I rough dimensioned it and again let it set (about 2 weeks). Then I dialed in the jointing and planing. I was hoping that it milling it is stages I would catch any warping. My shop isn't heated though and I live in Ohio, so I think the cold has caused more warping now which, even though I tuned up the sled, it introducing more slop into the cuts. It never ends :)

[Lee Schierer]

Any time your wood is not actively being worked it should be stacked so air gets equally to all surfaces. Never leave your wood on a surface where the air can't get to the faces. 2 weeks is not a long time for 8/4 wood to a acclimate.

[Thomas L Carpenter]

I've had something similar happen when I rush the cut. Slow and easy seemed to solve my problem.