Squaring a miter sled

[Matthew Joe]

I am building a new miter sled and dialing in the fence for accuracy using the five cut method. On the left side of the blade, the cuts are deadly accurate---0.0005 over 56". On the right side, however, the cuts are unacceptably inaccurate---0.035 over 56". I thought that the discrepancy was caused by fence flatness problems, so I jointed the fence three times to remove any warp or bend. Didn't help the problem. No matter what I do, I cannot seem to square both sides of the blade.For what it's worth, the sled is 3/4" mdf and the fence is a lamination of quartersawn white oak.Any ideas?

[shane lyall]

Just a shot in the dark but are you sure your miter slots are both parallel? I'm no expert but I can't think of a reason it would be off on one side only? Hope someone can help.

[John TenEyck]

I'm assuming your miter sled has a runner in both miter slots. If so, and your fence is indeed flat from one end to the other, then I'd say your problem is the blade is not parallel to the miter slots; probably has the trailing edge to the right.

[Matthew Joe]

Yes, there are runners in both slots. I like your thinking on blade position, but I just confirmed that the blade is parallel to both miter slots within 0.002 front to back over the width of a fully raised blade. Maybe my jointer is tapering the fence...('course, I had the same problem before I jointed the fence, too).

[glenn bradley]

How much fence length do you have on either side of the blade. Maybe the support isn't quite adequate to control a little deviation(?). Just a guess.

[Matthew Joe]

16 inches on the right, 18 inches on the left. The test piece is fully supported on either side of the blade.

[John TenEyck]

Lay a really straight edge against the inside of the fence, with a flashlight below it. That will tell you if it is indeed straight. Also, do your runners fit the miter slots snuggly? If there's any wobble that might be what's doing it. One last idea. Are you sure your workpiece isn't sliding during one or more of the cuts? Gluing some sandpaper to the fence would solve that issue, if it is one.

[Kevin Presutti]

Is one test piece used for both test cuts with the same edge against the fence? If you were to alternate sides against the fence or use a different board entirely as the test piece, could it be the sides were not parallel? .0005 on the left and .035 on the right with the mitre slots .0002 difference seems like it could be the board. Just a thought.............

[johnny means]

What happens if you cut a board in half? Seems if one side was square the other side would also have to be square. I would guess that the variable is somehow related to the user. Maybe the sled racks or cocks slightly when pushed from a certain position. If the fence is straight both sides should be off by equal amount in opposite directions. If one side cuts at 89 degrees the other should be 91.

[Michael Peet]

What kind of blade? Is it sharp?

[Andrew Hughes]

Hi matthew,Thats the kinda stuff that happens to me when i build sleds.Dont stress too much the right side will probably cut square in the summer and the left will be out.Sorry i am not more helpful. Andrew

[Alan Schwabacher]

If the rails were a little loose in the miter slots, it might be possible to get the result you mention if you push the sled more consistently to one side (so that edge of the miter slot guides) when the workpiece is on one side than the other. If this is it, purposely pushing the sled to one side as it cuts would help.

Could it be that the workpiece slips a little bit as you cut, and one side of the sled has more friction than the other? Sandpaper or hold down clamps might help.

Since you mention rejointing the fence face several times, I assume you have checked the squareness of cut several times -- have you gotten the same reading? Have you rechecked the good side each time as well? That's useful to distinguish variability in the measurement itself from variation in angle.

By the way, I'm assuming you're talking about what I'd call a crosscut sled, designed to cut at 90 degrees. What I generally refer to as a miter sled allows me to cut 45 degree cuts for mitering picture frames.

[andre graves]

Any update to this problem? I'm having the same issue.

[Carl Beckett]

I would suspect the piece is moving slightly (for whatever reason), when making this cut. Even if the slots arent parallel to each other, if they are both guiding then should be the same from the right to the left. Ditto on fence squareness.

I like the idea of cutting a piece in half, and comparing to each other (if you still have one side square and the other side not - that would be a real puzzler)

If you are cutting on the left and not square - can you shim the piece where it contacts the fence and get it square each time? (this would suggest the sled is running consistently at least, and would suggest fence problems)

Even if the blade isnt perfectly straight, it would affect cut the entire length of cut.

[Matthew Joe]

I stopped sweating the problem and try to cut from the left side of the blade most of the time. However, the 'error' on the right side is small enough that I can live with it in most cases. I will try to remember to remeasure the accuracy in the next week or so...by now the fence or runners could have shifted for all I know.