Making accurate cuts on a SCMS

I use a makita 12" sliding compound miter saw for 99% of the crosscuts I have to make. I've set it up to accurately make 90 degree cuts (theoretically, at least), and I keep it at that setting nearly all the time. I've noticed, particularly on wider boards, that there's just a little bit of variance to how accurately it cuts a 90 degree line (maybe half a degree). I've read that some people feel this is caused by which way the user puts pressure on the handle as he makes the cut, anyone have any test results from fooling around with this?

My goal is to get to where I can consistently use it for accurate 90 degree cuts and not have to resort to a table saw sled and frequent blade changes. Any advice on improving technique or possible tweaks to the saw would be greatly appreciated.

Are sliders inherently less accurate than regular miter saws for this?

I probably have the same saw and use it the same way. The saw took more time to set up when new than I thought it would - especially the bevel angle stops. But, mine has held settings good for a about a year now. When I set up for 90 I cut the board using the left fence then flipped the board over so the same side was against the right fence and made about a 1/2" cutoff and measured the thickness at the front and rear - this should magnify the error. I know you probably know this but I have seen multiple posts where some people didn't - you should be pushing the blade toward the fence. Also, are you using a thin kerf blade - some have said they get blade flex on the 12" saws. I don't know if this helps for square cuts but I read somewhere that to reduce top side tear out to make a slight skin cut before cutting through the board.

Mike

I just tried the test that Mike mentions--cut a 5" wide board, flipped it to the right fence, re-cut and measured. Mine is a 10" Makita, not the 12". Cut varied by .007" over the length, so actual variance is half that or about 1/300" in 5". Stock blade. You could be experiencing blade deflection in the 12" blade--main reason I went with the 10" over the 12".

also using the stock hold down, will help. But beware of tightening it too much as it will push the fence out of square.

Check both sides of the fence for square to the blade. Also take a straight edge and make sure the fence is perfectly flat all the way across the face. A common problem is that in a rush, we start to plunge toward the workpiece before giving the blade a chance to spin to full speed before entering the workpiece. Net result, the blade catches the workpiece, and tries to fling it through the fence. The fence is aluminum, and the center has to duck to get out of the way of the blade. Net result there isn't too much meat in the center of the fence. It is so slight that even with regular use, when cutting let's say the last inch off of a 2x10 you are causing pressure on one side of the fence, without causing the same pressure on the other side of the fence. Normal cuts it is not a problem, but if the blade catches, it can bends real easily. If it does have a valley in the center, or is off in any way, you have to straighten it. Removing it by taking out the 2 bolts, straightening and putting back on is the answer. Adjusting for a perfect 90 does require some tweaking, but the problem is that there really isn't any adjustment in the saw, other than a tiny, and I mean tiny amount of play in the bolt slots in the fence. I talked this over with the rep from Makita, and mentioned that my LS1013 came through with 1/10th of one degree off on a 90 that I could not get out with adjustment, measured with a Bosch digital protractor, so I reamed the bolt hole with a drill slightly, and was then able to get it perfect. He thought casting the fence with a minor amount of play in the bolt hole would be a good idea, because there wasn't any under table adjustment.
I have the LS1212 also and use that one a lot. I found the same problem, but tweaking the straightness of the fence, does it for me. (I do adjust after that for the perfect 90 just using the tiny play in the bolt hole.

I don't seem to have a problem getting reasonably accurate 90 miter cuts but I have a problem when the saw is cutting 45 degree bevels. I have a Makita 1013 and I notice that when the saw is set to cut a bevel the cut is not straight or flat. I don't know if this is due to technique or a problem with the saw. I also noticed that when the saw was tilted to the left it seemed to burn the wood even if the wood was clamped.

I was doing some test cuts the other day on a 3" wide piece of fir. I ended up giving up and making the cuts on the tablesaw as I couldn't get satisfactory results.

I would be interested in any ideas anyone has.

Greg