Accurate Miters

Tips on achieving good miter joints when working to fine tolerances. October 25, 2006

I recently had a lot of trouble building a simple square picture frame. The problem was with the miters. Does anyone have any tricks on how to cut perfect miters, to form a perfect square, with no gaps? I was using Oak trim and a Dewalt tilt and slide miter saw.

Forum Responses
(Cabinet and Millwork Installation Forum)
From contributor A:
Make your first cut on two pieces then check with a square to make sure your saw is cutting properly. If it is, then make the second cut on each piece of identical length stock from a stop.

From contributor B:
I use a Hitachi 8-1/2" slider. It is old, going on 16 yrs now. It is the best saw I've ever owned. But it still won't cut a perfect miter in material wider that 3" without a small bow in the cut. I have tried to correct it to no avail. So I spent 2 hours making a perfect 45º jig for my tablesaw. It seems like an easy jig to make but 45º is hard to get. When you are making the jig, start with a metal runner for the slot piece. Then use a piece of 1/2" Baltic/Russian birch for the bed. Take another piece about 2" wide, make sure it is straight! And using a 45º guide or compass, screw it to the bed. On the end nearest the saw blade try to get the screw in the center of the board and close to the end as this is going to be a pivot point for fine tuning your jig. Now make a 4" strip of material, and again, make sure it is straight! Cut it into 4 pieces using your out of alignment jig. Start with the pieces at about 6-7" from point to point. Use a stop to make them all the same length, flipping them to get opposite miters. Now assemble your test frame. Is it perfect? Probably not, that's OK. Depending on if the inside is open or the outside, adjust the fence by removing the screw from the end farthest from the saw blade and pivot the fence ever so slightly in the direction that will close the gap. Now re-install the screw in a close but different section of the fence, and try again. Keep at it using the same test pieces, just trimming them slightly and using a stop to keep them all the same length (important). Sooner or later you will stumble upon 45º. At this point load the fence with screws so you never lose the alignment you worked so hard to get. Now you have a perfect 45º jig. Don't lose it or you will have to do this all over again. By the way, you'll probably need to do it again anyway for the opposite 45º so you can cut profiled material that you cannot flip. I love mine, it is so nice to make 8 cuts and assemble.

From contributor B:
I read this same question asked of Jerry Metz in Wood & Wood Products magazine. The questioner was in furniture production and had problems with miters in trim applied to cases. Jerry suggested getting the miter cut set to a perfect 90, and then tweaking it just a breath so that the points of the miter touched just slightly before the inside corner. I have used this advice successfully for years. Also, as any fan of Jerry will know too well, be sure the moisture content of the material is correct. Shrinking width plays hell with a miter - hardwoods, 6-7%, softwoods 10-11%.

From contributor C:
Your blade must be sharp and the correct blade, and your stock must not move. A dull blade causes the stock to move ever so minutely. Do it 4 times and you have a poor fitting joint. If you are trying to get the perfect mitre - whats perfect? This is wood, not metal. Professional picture framers have dedicated miter saws that are fine tuned to the hilt. And let’s not forget - it takes practice just like anything else. If you do it enough, soon it will be second nature.

From contributor D:
Professional picture framers actually use the Morrso machine for mitering. One other point about miter saws is I think the blades should be thicker and stiffer to avoid deflection.

From contributor E:
I have had the same problem for years and finally gave in and bought a used Omga chopsaw, 252lbs of cast iron, no deflection just dead on miters every time. I think for the smaller budget though the tablesaw sled described above is a good way to go, or even one of those Lion miter trimmers. You make the cut a little oversize with your chopsaw and then shave a whisker of with the trimmer to get it just right.

From the original questioner:
Thank you very much everyone for the tips. I have been in the trade for close to ten yrs. and I have finally started working in a shop where the standard is very high, (the owner made instruments for yrs) and I am not used to this and still adjusting. I have never measured anything in my life to the 64th, but I am having a lot of fun learning.

From contributor A:
If you are measuring to the 64th you may be interested in a system that evolved through working with a guy for a bunch of years, first on high end interior trim, and later on high end cabinets as well. We wound up eliminating all eighths, quarters and halves and would holler a measurement out like 98 and 7, meaning 98 and 7/16ths inches; 98 and 12, or 98 and ¾; 98 and twelve and a half, or 98 and 1/32 more than ¾; 98 and 12 strong, or 98 and 3/4 plus 1/64th; 99 shy or 99 minus 1/64th.

It's a lot easier and quicker to find 98 and 14 strong than to find 98 and 57/64ths. I can't even see the 64ths lines, but I do pretty well cutting half a sixteenth in half if the tape doesn't have all those damn lines on it. When writing down these measurements we would write the whole number of inches then put the 14 strong (14+) or 14 shy (14-) up where the inch mark (") would go surrounded by a circle. We used no inch marks at all since everything was in inches anyway - plus we would never confuse inch marks with 11/16ths.

Addition was also a snap in the fairly rare cases it was needed since the strongs and shys cancel each other out and two of either becomes a 1/2 or a -1/2. Basically we dispensed with the C hair, and made far fewer mistakes.

Another clarification: addition - two of the same sign (+ or -) becomes either 1/2 or -1/2.

From contributor F:
Has anyone ever used a disk sander to fine tune a mitre? It works well on smaller dimensioned material.

From contributor G:
Cupped material won't give perfect joints. It must be truly flat with parallel edges.

From contributor H:
If you are using a miter saw you can swing the blade back and forth from 45 to 45 on opposite sides of the blade. Make all your cuts for one side of the joint from the left angled setting and all cuts for the other side of the joints from the right angled setting.
Doing this will cancel out any misalignment in the miter saw. If, for example, the saw is off by one degree, all of one side cuts will be at 44 degrees and all from the other will be at 46 degrees. Net result is tight joints. One caution: this will only work if you have a good straight fence.

From contributor I:
To all this I will add - cut slowly. If you saw nice and slow most deflection will be eliminated.

From contributor J:
To answer contributor F's question, yes, I use a 20 inch disk sander quite frequently with excellent results. Mostly for very small work, very large profiles and miters that are segments of curved mouldings. It is very nice to be able to sneak up and get a perfect fit. But it is an extra step, thus not a production solution.

From contributor K:
Also, I like to use a small strip of 150 grit sandpaper applied with spray adhesive to the fences of my mitre saws. This stops the stock from any creeping during the cuts.

From contributor L:
I hate mitres. I agree with most comments made on the subject. The problem is often the tendency of the saw blade to follow the grain in the timber. There is a solution and it involves giving up ideas of tweaking saws and blade types and what have you. In my youth I first came across a Danish mitre guillotine that was operated by a wheel, similar to the wheel on an old time sailing ship. Since then I have used variations of this guillotine trimmer and the best was foot operated - no noise and no electricity - but by far the most reliable with perfect mitres every time. The last thing is something of a personal rant and that is the references to taking sizes such as 78/ 7 or what have you. What a system! Give up all your prejudices and get into the metric system. It works and I started with silly old feet and inches. Metrication is the only way to go. You will be embarrassed when you find out how simple and easy it is.

From contributor E:
Unfortunately it is really not easy to give up a system when every product you deal with comes in the inch system. As a cabinetmaker I cannot call up and order material in mm. Nor can I get a set of architect’s drawings in metric measurements. It's not that I don't think it's an easier system - it's just that our country is not geared to it. Besides, we are pretty clever over here and don't have too much trouble with feet and inches.

From contributor M:
I wish someone would make a tape measure that was in tenths of inches. It is quite easy when looking at tenths of an inch to guess at what the measurement would be to various amounts of precision. For instance if it was a hair over 98.5 one could easily tell the difference - for halfway to 98.6 you could say 98.55; a quarter past would be 98525 or the like.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor N:
I am a trim carpenter in Wisconsin. My method for a closed miter is using my Dewalt dual bevel miter box and when on the 45 degree setting, I tilt the saw 1-2 degrees to back-cut the joint and then put a thin skin of Titebond trim glue on the joint about 12" down the jamb side of the trim to hold it solid. The bevel is so slight that you really can't see it, and it makes for a nice tight fit. It just takes a little bit to get to know your saw and how to finesse the joints.