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I'm trying to cut grooves, 5/8" deep and 1/2" wide (well, actually 15/32 because it's to fit plywood) in finished 2x4 stock (so, some kind of pine). I have a new set of stackable dado blades from Freud (so, sharp). I use a tall featherboard against the fence. Everything seems to be set up straight, and the bottom face of the groove has the correct width, but the opening is 0.5 to 1 mm wider, so the plywood fits loosely (picture slightly exaggerates the difference).

What am I doing wrong?

Diagram

My setup:

Saw setup

Blade

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    I think movement of the stock is the explanation here, although I can't see how that could happen uniformly. The drawing has me confused as to orientation here, aren't you cutting a groove rather than a dado? Anyway, either way if you didn't catch this on a test piece but instead this is on a project part it should be no biggie to rectify — you can shim them out with a plane shaving, strips or card stock or whatever. Or, just fix the ply in place with pins/brads or glue.
    – Graphus
    Nov 28, 2021 at 13:34
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    Is your stock dead flat on the face, or perhaps slightly cupped? Do you feel like a taller additional fence would help keep your stock more perfectly aligned? Nov 28, 2021 at 15:27
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    Stock faces seem flat, to the extent I can measure. The fence is almost as tall as the boards I'm using so I don't think additional height will change anything.
    – mustaccio
    Nov 28, 2021 at 16:25
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    Case hardening of wood? Is that possible? So far I cut four test pieces, they all show the same defect.
    – mustaccio
    Nov 29, 2021 at 13:13
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    The outer dimensions of the stock don't seem to change, so probably that wasn't it. @AloysiusDefenestrate.
    – mustaccio
    Nov 30, 2021 at 4:17

4 Answers 4

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I believe that your fence is not perfectly aligned with the face of the dado blade. The out of alignment will cause the groove at the table surface to be wider. The deeper part of the grove will be narrower.

  1. As others have commented, first ensure that the dado blade is assembled properly and square to the arbor.
  2. Check the fence alignment at both the beginning and end of the cut against the tip of blade. Distance A and B should be exactly the same. Any deviation between A and B will produce the slanted wall of the groove you are describing.
  3. Make 2 or 3 multiple cuts versus one deep cut. Pass the wood in the same direction.

To check the alignment of the blade to the fence, raise the dado blade to the highest level. With the blade at its highest, check that Distance A and B are the same. With the blade at its highest, this will exaggerate any difference between Distance A & B. Once A and B are equal, lower the blade to the proper depth.

enter image description here

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    The blade is NOT mounted non-square, the blade is mounted square to the saw arbor and does not wobble. The fence face is not truly parallel to the dado blade, therefore there is a slight difference in Distance A and B. In OPs setup, the difference would be about .5 mm, giving a total 1mm wider width at the surface of the groove. Nov 30, 2021 at 20:21
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    Looks like you were right, the fence wasn't exactly parallel to the blade. I was able to bring it to about 0.1 mm precision (I guess the best I can do with my basic Ridgid saw), and, together with reassembling the dado stack and adding a second featherboard, I now can cut truer grooves -- they are still about 0.3 mm wider at the top than at the bottom, but that's good enough and I suspect unlikely to improve on my hardware anyway.
    – mustaccio
    Dec 1, 2021 at 0:05
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    OP - .3 mm is better than a 1/64" deviation and is in the realm of fine carpentry. Dec 1, 2021 at 17:35
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    +1 for answer, but 0.3mm is not acceptable error for some joints. Does not matter for this joint because op says bottom of groove is correct, but think of mortis and tenon: Cutting tenon 0.3 too thin = loose fit and weak glue joint. 0.3 too thick = tenon does not fit.
    – Volfram K
    Dec 2, 2021 at 6:48
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    @Graphus put a regular blade in your table saw. Clamp a straight edge at a 45° angle to the blade. Run a piece of 1x4 along the straight edge over the barely raised blade. Raise the blade a fraction. Lather, rinse, repeat. You'll get a cove molding, wider at the edges than in the center. With the factory fence not square to the blade, you're doing the same thing, just at a smaller offset.
    – FreeMan
    Dec 3, 2021 at 14:05
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My guess is that when you stacked the blades and chippers, you didn't quite get them perfectly vertical, then when you tightened the nut down, it clamped them tight, but that one or the other of the outer blades isn't perfectly perpendicular to the table. I'd say it's especially likely to have happened if you've got any shims in the stack to get to your exact desired width. Since the shims are so thin, they can fall into the threads of the arbor and sit crooked.

Raise the dado all the way up and use a square to check. You may have to rotate the blade up to 180° to find where it's out. My personal procedure is to unplug the saw before doing so, even though it's Really Tough™ to accidently hit the power switch. I'm kinda fond of my fingers.

If that's the problem, loosen the locking nut, wiggle the blades a bit to get them to sit square and snug, make sure any shims are sitting squarely on top of the arbor threads (and haven't fallen into a thread) then hold the outer blade against the stack to keep them all tight while you get the nut finger tight. Then tighten the nut as appropriate with your wrenches. Double check with your square to ensure this setup is now properly squared.

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    Thanks, good point. I do have shims in there. I'll give your advice a try tomorrow.
    – mustaccio
    Nov 27, 2021 at 22:43
  • This is I think the obvious first thought..... but wait a sec, what about wobble washers or wobble blades?
    – Graphus
    Nov 28, 2021 at 13:27
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    Not a wobble setup here... (and though I don't have direct experience with wobble blades, I suspect the walls would still be square -- the top would have a curve to it). Nov 28, 2021 at 15:29
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    @FreeMan a drop of oil on the shim will create a suction against the blade/chipper and prevent it from sliding into the threads.
    – gnicko
    Nov 29, 2021 at 13:30
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    @AloysiusDefenestrate, sorry in case it wasn't clear, what I was saying is if you do have a 'wobble' blade (i.e. it is mounted at an angle to the arbor) you still get parallel sided-grooves, but that is a different kind of non-square. Anyway, from Comments above the issue does appear to be with the dado stack; given it's Freud I would have presumed it's an installation snafu rather than a fault but it seems the fault recurred when reinstalled so we'll see.
    – Graphus
    Nov 29, 2021 at 16:20
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Not to take anything away from the accepted answer (or the others) I think it's important to mention how to go about fixing the problem although the original question was concerning what's causing the problem.

Looking at the photos in the original post (especially the second one) seems to show that the saw blade is not square to the rest of the table saw. The distance between the teeth and the sides of the throat plate cut-out are plainly not the same at both ends of the blade.

By setting the blade and the fence to the common reference point of the miter slot, you will have your saw dialed-in to a specific "landmark" and close to the precision limits of the machine. That miter slot is never going to move so it makes a good "zero" to calibrate everything else from. This should eliminate this problem and other similar ones.

You can do this (both moves, actually) by eye, but you'll get better results using a gauge like this, for example. Using a gauge like this allows you to measure from the center of the miter slot rather than the walls of the slot. I doubt that both walls of most miter slots are exactly parallel along their length.

What you need to do to correct this:

1. Set the arbor/blade square to the miter slot on the saw table.

Specifically the miter slot on the right side of the blade. Adjusting this usually entails loosening some bolts on the bottom surface of the saw table and "tapping" the trunnion mounts one way or another to move the heel of the saw blade to the right or left and bring it in line with the miter slot.

There are aftermarket attachments that you can get to dial this trunnion adjustment in, but they can be kind of tricky on some saws or not available for others. Here's a video of a guy showing how to make this adjustment on a saw similar to yours.

A couple of points: Use a "regular" blade (not the dado stack) and measure from the flat part of a single tooth (not the carbide tip) to cancel out any deformity in the blade itself. Get this as accurate as you can but a couple of thousandths of an inch are probably as good as it gets.

2. Set the fence square to the same miter slot.

Measure the distance from the front of the fence to the miter slot and the rear of the fence to the miter slot. Depending on your particular fence, shim/adjust to make these measurements match or nearly match.

Many people will intentionally set the trailing edge of their fence a couple thousandths away from the blade to prevent kick-back. I don't know if that's entirely valid, but it makes sense. If you're going to have it slightly out, it would be better to have it open up after the blade instead of narrowing against the blade. (I'm sure there's lively discussion on this topic elsewhere here....)

Next steps could very well be to look into a replacement/upgrade to the fence system. I haven't used the saw you have, so I don't know how good the fence is. The problem with lower-end, factory fences is that they are not terribly "repeatable". In a nut-shell, there's too much difference between one cut and the next that are supposed to be the same.

There are many after-market, and plans for "DIY" fences, that you can replace the stock fence with and see vastly improved precision and ease of use. Problem is, building a DIY fence is at the mercy of the existing fence... so you may end up having to make multiple homemade fences as the accuracy improves.

The saw I use most is a 30+ year old Craftsman contractor saw which someone gave me to get rid of. After tuning and aligning it using these steps, I can make repeatable cuts within about .005" accuracy... which seems to be good enough for what I'm doing.

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One possibility is that the dado blade is not square, and in that case I would expect to see circular saw marks in opposite direction on the interior faces of the slot you are cutting.

It may also be the case that removing the material is causing the wood to warp. If that is what is happening, then would probably also see variation in the slot along its length. Assuming this is happening in a rip-wise cut, you could see whether the same thing happens in a cross-cut. If it happens with cuts in either direction, it's probably an alignment problem, otherwise, it's wood warping, and you could get around that by making smaller cuts, i.e. not trying to cut the full width of the slot in one pass.

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