How to most easily square a miter saw?

January 18, 2013 11:35 AM

I've got this Dewalt miter saw, and it seems to be slightly out of square -- neither the 0º miter or bevel is exactly square, at least with the door casing I've been cutting. Does anyone have any tips (or links to good resources) on how to square up this sort of saw? For what it's worth, I think I've figured out how to adjust the indicators & scale on the saw, so I'm not looking for advice on how to adjust this particular saw. Instead, I need help figuring out how to determine which direction -- and to what degree -- I need to move the scale to square it up. Advice on how to cut and match a board to test the square is just the sort of help I need.
posted by jacob to home & garden (7 answers total)

I don't have an answer for you directly, but if AskMefi comes up dry, I'd post this same question to lumberjocks.
posted by jquinby at 11:38 AM on January 18

Advice on how to cut and match a board to test the square is just the sort of help I need.

Take two straight boards of a width that the saw can cut, i.e. pieces of 1x6 dimensional lumber. Use really straight boards, not some warped junk. Set the saw for a 90-degree cut. Mark one side of each board. Chop the ends off of both boards, with the marked sides facing up. Use a piece of sandpaper to knock off any fuzzy edges left by the saw on the bottom of the cut. Stack the two boards, one on the other, marked sides together. Stand them both up on a very flat and clean surface, like a saw table or granite countertop, with the cut ends down. Loosen your grip and allow them to settle onto the flat surface. If the saw is cutting square, the long edges of the two boards will be nicely aligned and parallel. If the saw is out of square, they will lean in opposite directions and the long edges will not be parallel, in which case you can see which way they're leaning and deduce which way to adjust the saw.
posted by jon1270 at 11:45 AM on January 18 [2 favorites]

The instruction manual has fairly detailed directions on page 5. You'll need a good square. If you have a square that's been dropped or otherwise roughly treated, you'll want to check it first:
1. Find a board or other markable surface that has one good straight edge.
2. Lay down the square and mark a line.
3. Flip the square over. If the edge doesn't line up exactly with the previous mark, your square's not square. Get a new one.

I would start by testing your square, then following DeWalt's adjustment process, then testing on a board like jon1270 suggests. Be sure to blow out any sawdust that might be hiding in the corner of the fence; that'll mess up your cuts for sure.

You'll want to avoid using the sliding function on that saw if you can; more moving parts equals less accuracy. Only use the slide when the board's too wide to cut otherwise.

It's also possible that your saw is fine, but your doorframe isn't square. In that case you'll just need to fiddle around a bit to find the cut that fits. Your local library will have a book on trim carpentry that's packed with handy tricks. The ones from Taunton Press are generally quite good.
posted by echo target at 11:56 AM on January 18 [1 favorite]

It's more direct to just check the blade position. You'll need something like a machinist's square or precision square. There'll be a bunch of them at your local Home Depot. WITH THE POWER CORD DISCONNECTED, lift the guard and lower the saw to check how the blade lines up relative to the back fence. For mitre cuts, I use "A" in this photo. It's good because it's small enough that I can press it tight to the saw blade without being thrown off square by the teeth. It may help to elevate the square a bit using a flat scrap of wood or you might need to lay a board across the back fence to have something solid to register against. Check the vertical as well. I'm kind of OCD and never trust the gauges when I'm making something like a picture frame.

Also, use a straight edge to check that your fence is straight/aligned to itself on both sides.
posted by bonobothegreat at 11:57 AM on January 18

Yeah, when squaring up my chop saw and table saw WITH THE POWER OFF AND UNPLUGGED/DISCONNECTED, I just use a small square between the blade and the deck or fence, and make the necessary adjustments until 0° really is 0°. Tighten up, double-check, and you're good to go. I use a small combination square for this, and will not only check that 0° is correct, but will use the 45 on the square, and set the table or fence to 45°, and re-check it there, just to be sure.
posted by xedrik at 12:53 PM on January 18

....also, if you decide you really like nice, tight joints, you may want to eventually build yourself a few shooting boards to use with your hand plane. They can be made to fine tune sawn ends, mitre joints, box mitre joints and square long edges for gluing boards together to make panels.
posted by bonobothegreat at 2:25 PM on January 18

If the saw is fine and the door is not plumb, what I've seen carpenters do is build the frame as a unit outside of the doorway and make sure it is square. Then install it as a unit, shimming to maintain square. Then your trim will line up nice and square as well.

I like jon1270's answer the best, because it involves the rule of not measuring anything unless you have to. Using boards two feet long, you can get far more accuracy than eyeballing a square will ever get you.

(if the boards show as being off, then you split that angle in two and that's how much you adjust the blade by.)
posted by gjc at 4:00 PM on January 18