Miter Cut Dilemmas

[Chip Clark]

I have this ongoing problem with achieving perfect 45 degree angle cuts on my sliding compound miter saw. The bizarre thing is when I check the blade for square and with a 45 degree speed square,
the blade is set perfectly. But when joining the wood (such as for a picture frame) I have gaps at the outside corner - as much as 1/16". I've checked with more than one speed square, so I know it's
not the tool. This is driving me nuts! Any suggestions??

[Dan Friedrichs]

1) Is the wood moving (even slightly) as the blade pushes it? Put a sacrificial scrap behind it to make sure it's held tightly and not "falling" into the opening in the fence. Also, clamp the work tightly to the fence - you might be letting it move a bit if you're using your hand to hold it

2) Is the angle still correct as you move the saw through the slide? Are you sure the torque from cutting isn't causing it to move a bit?

3) Is the fence straight and perpendicular to the table?


SCMS's are notorious for being "sloppy" - there are a lot of moving parts. Even a cheap non-sliding miter saw will likely do a better job (once set up correctly).

[Jerry Thompson]

I can get 45's with my SCMS. The pieces have to be exactly the same length. That is the two rails the same length and the style the same length. I use a stop block. They need to be perfectly flat. The piece(s) need to be held so as not to move.
To get them right on the money with little kerf showing I use a home made 45d shooting board with a low angle jack plane.
Make up several test pieces and have at it. Remember that using the wood you want to make the frame out of first will be your test piece.

[Marty Schlosser]

There are several possible causes of this dilemma - even if your saw's mitre setting is exactly on the money. The following is based on the assumption you're using stock of the exact same width. While you're at it, why not ensure the pieces are all the same thickness. I'm also assuming they're from the same species of wood. Here goes...

First of all, the two sets of opposite pieces must be exactly the same length as their corresponding opposite piece. If one of these two pairs isn't, then it's impossible for the four corners to come together, even if all 8 cuts are exactly at 45 degrees. Even if you start out with all 4 pieces correctly cut to length, when it comes time to cut the opposite ends, if you shove some alreay-mitred board ends harder against the stop block (or whatever means you're using to ensure correct lengths), it may slightly deform and therefore compress the already-cut mitre and therefore result in some of the four boards being of different lengths. Many craftspersons will go to the trouble of making a mitred stop, which helps alleviate this problem. You also need to chamfer the underside of the mitred stop block, as that'll provide an area for sawdust to collect out of the way and not interfere with the proper functioning of the stop block.

Another area of error may be caused by blade flutter. Many use thin kerf blades, with the assumption that it'll be easier on the saw motor. Although that may be a concern for some, in most cases if you take your time and allow the blade (it is sharp, isn't it?) to do its job instead of going too quickly, this shouldn't be a valid concern. I stopped using thin kerf blades years ago after eperiencing this dilemma - especially when ripping coarse grained woods such as oak. This same problem with blade flutter can rear its ugly head when cutting at angles slightly cross grain and slightly with the grain (which is what a 45 degree mitre is, right?) and especially moreso when cutting coarse grained wood species.

You can also run into trouble simply by unknowingly placing pressure on one or the other side of the handle as you're bringing the saw down (and sometimes forward, depending on the width of the boards being mitred). This of course can result in an angled cut that's incorrect either all the way along but usually just in one section of the cut. That can be enough to give you poor results.

Last, there may be a question of rigidity with the saw's slide and/or pivoting mechanism. You'd be surprised to know how much slop there can be, even in an otherwise "tight" saw. You can verify how accurately your saw is cutting by very carefully measuring the resultant cuts, using a proven to be accurate angle measuring device. I hold my cut pieces up against my precise mitre gauge, and sight while holding it up to the closest ceiling light. That way I can see if there are any gaps and re-cut accordingly.

Finally (I guess I wasn't really done...), the glueing-up can sometimes be problematic. What I have is a band clamp designed with four square corner blocks, which greatly facilitates the process of getting everything held precisely. I always do a trial dry fit to ferret out any problems before reaching for the glue bottle.

Now, even if all of that's been done and to the greatest of your attention, there can sometimes be one corner that's giving you problems. This is whre a well-sharpened block plane comes to the rescue. Simply determine whether you need to take a smidgen off the toe or the heel of a joint then make one - repeat, only one - pass with your lightly set block plane, then recheck. You will be astonished how effective this planing can be to deal with one wayward mitre.

Be patient and attentive and before you know it, you'll be making perfect mitres in no time.

All the best.

[Prashun Patel]

Dumb question: Are you talking about bevels @ 45, or a miter @ 45?

The piece needs to be clamped or stopped to prevent it from moving during the cut.

Do you have the same problem even on narrow pieces?

[Michael Zerance]

When I have to cut precise miters on the miter saw, I usually will cut the piece about a 1/16" to an 1/8" long, then take a few slivers to sneak up on the line. I think it helps by reducing the amount of work the blade is doing which eliminates movement in the work piece. Or, as mentioned, you could clamp it.

For long cuts that require the use of the slide, I doubt that you will be able to get perfect miters. Close, but not perfect.

[George Bokros]

Unless you need to have the saw slide because of the width of the material being cut lock the slide mechanism and use ad a non sliding saw.

[Jim Dwight]

First, let me saw that you are not the only one who struggles with miters. I think they are harder to make than most joints. I would much rather cut machine cut dovetails, for instance, in drawers than cut miters.

But with good technique, I get success. On my current project, I cut the pieces for both sides and both ends to length at the same time. That helps with the error of having pieces that are different length. It also helped me cut rabbet joints in the sides. But I still could have messed it up by cutting the miters wrong. So I snuck up on them. I was using a 12 inch non-slider. It is pretty hard to deflect but I also have used it a lot and move it straight down. On wide pieces, I use my RAS (but almost always for 90 degree cuts). It has to be biased to the left to cut correctly. Both my cross cutting saws have negative hook blades to minimize their tendency to climb cut. So when my technique is good and my tools are set up right, I can get good results. But I can also mess it up and have to cut another piece too (or make the project a little smaller by recutting).

Shops that do picture frames have little manual cutters to trim to final length and angle. That would be nice to have but a little shoot board with a block plane is a lot more reasonably priced - but also somewhat harder to use.

On my most recent project, I decided pins would not look too bad so I pinned the joints after applying glue, and then I clamped. If the pins will be an issue, it would be a good idea to put a dowel or spline in the joint to help with assembly. That is another opportunity for things to go wrong.

Overall I think the problem with miters is there are just so many ways to get them wrong. Cutting pieces to different length, wrong angle (or beveling them slightly), and putting them together improperly. The latter is easy since the pieces want to slide when you try and clamp.

[Alan Lightstone]

I cut slightly long, and perfect with an accurate shooting board and a Lie-Nielsen 51 shooting board plane.

Not a cheap approach, but very accurate.

[pat warner]

I can saw x hand in a miterbox to a close 45° .
But like you, I'm chasing the wind for a perfect fit.
So I clamp the common pair(s) and sand them (in pairs) to net.
My edge/radius sander can be set precisely in x, y, & z.

[glenn bradley]

I've gone through a lot of jigs and sleds and so forth but, finally ended up at the tablesaw with a V-27 miter gauge.

346090 . 346089 . 346091

Get your tablesaw dialed in and square up your miter gauge with the 5-cut method and you will get some direct results. I did trick out the gauge a bit but, just some sandpaper to avoid shifting and a mitered stop to hold pointy ends better.

346092

[Frank Martin]

Looks like I may be in the minority but I can cut perfect miters on a sliding table saw and did so on a Unisaw before that the first time without further tweaking. The key for me is to make sure opposing pieces are exactly the same length by using a stop and the cross cut fence (or miter gauge) is set exactly at 45 degrees to the blade.

I never tried but always heard that sliding miter saws may not be very precise and accurate for this type of cut.

[Mike Henderson]

I cut miters on my CMS (compound miter saw) and get good miters. I've done this on several different miter saws (Bosch 12", DeWalt 12" and Kapex 10") Make some test cuts first and see if you can figure out what you might be doing wrong if you're sure the saw is set up accurately.

Mike

[Doug Hepler]

Chip,

1. If you have an appropriate plane and know how to make it very sharp, I agree with above recommendations to finish with a shooting board. Although it may take some time, if you can get a true 45 deg secondary fence your problem should be permanently solved.

2. Another very useful approach is to make a secondary table for your CMS. Just a nice flat piece of Baltic birch or MDF. Then attach a triangular secondary fence to that table. The triangle should have an angle of exactly 90 degrees. (This will be easier to get right than 45 deg angles), The 90 deg apex of this secondary fence should be centered on the path of the saw blade. Now, the magic is that you cut one piece against the right side and the complementary piece against the left side. The two angles will add up to 90 deg even if they are not exactly 45 deg each. Any small errors will cancel each other. This approach is actually easier if you have a crosscut sled on your table saw, as shown in the photo. 346106 (Assuming you have a table saw.)

I would guess that you are cutting every 45 deg cut from the same side of the saw. Possibly whatever small errors you have are adding instead of cancelling.

3. Finally, for accurate cuts on a CMS you should limit the slide and secure the workpiece (as suggested above) and - sometimes very important -- let the blade stop rotating before you let the blade come back up out of the cut.

I feel sure that suggestions 1 and/or 2 will solve your problem.

Doug

[peter Joseph]

I gave up trying to dial in my SCMS. 10$ worth of cabinet grade plywood for a shooting board and a bailey #6 handplane will give you results you'll never achieve with your saw; gods honest truth. You'll be able to sneak up on the perfect joint literally .002" at a time.

[John Gornall]

Chip,

You keep checking your saw with more than one square but your joints are open at the outside corners.

Lose the square and adjust the saw fence until the corners close.

To find out how much to nudge the fence try playing card shims. Put a pencil mark on the fence and lay in one card, test cut, add cards until your joints close. Then draw a line on the saw table and nudge the fence to the line or just tape the cards in place and keep cutting.

I use the card shim trick in the clamps as well to pull things in if needed.

I've cut about 8 million miters over the past 35 years.

[mreza Salav]

This is what I use for perfect miter cuts; have used it lots and lots including mitered cabinet doors 41"x18" and no issue at all.

346113

[Dave Cav]

I'll add another vote to a SMCS not being ideal for cutting precise miters; they're a construction tool. I have always had the best luck with a table saw and a well adjusted miter gauge, and I have had a couple of Incra miter gauges that work very well. Accurate stops for length are just as important.

[Ole Anderson]

I don't do many miters, mostly picture frames, but when I do I use a shop built sled with an aluminum Woodpecker 90 degree brace.

[Von Bickley]

I get my best miters with a "Incra Miter Gauge 1000HD".

[Chip Clark]

Thank you all for the great responses and the time you took to write them out. There are a lot of great suggestions here and the one that seems to have worked out the best is doing miters (not with bevels) on my table saw.
I've always gotten good miters on the table saw, but figured it should be easier on the combo slider. Unfortunately while doing a recent cut I was concentrating so hard on the accuracy that my foot slipped on some sawdust -
which caused my hands to shift the wood - and that 1x4 maple became a projectile - along with the metal blade throat and carbide teeth shards - right into my gut! now I know what it's like to survive a gunshot!!

And I know you're all wondering.........yes, the miter was not accurate!

[Mark W Pugh]

I get my best miters with a "Incra Miter Gauge 1000HD".

With an extended fence attached to it. But, I still have to make a few cuts to confirm the angle.