DeWalt miter saw doesn't cut straight

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Forum topic by MarkLTX posted 02-22-2020 11:14 PM 614 views 0 times favorited 12 replies Add to Favorites Watch
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6 posts in 222 days

02-22-2020 11:14 PM

I recently bought a DeWalt DWS779 12” sliding miter saw. My first test cuts have been with 1/2” plywood about 7” – 8” wide, which can be cut without sliding the saw at all. Instead of cutting a straight line, the cuts are always somewhat convex. That is a straight-edge placed against the cut edge will rock slightly.

I tried replacing the factory blade with an 80 tooth Freud and the results are the same, maybe worse. Both blades are rather thin with ~0.071” plates.

Is this normal? I mean it would be fine for rough carpentry. Maybe 12” saws aren’t really suited to precision woodworking and I should get a 10” saw instead, or just return to using my table saw sled.

12 replies so far

View CausticGuy's profile


12 posts in 1204 days

#1 posted 02-22-2020 11:22 PM

Mine does the same, but i always figured it was the Dewalt blade that came with it deflecting a bit.

View JackDuren's profile


1005 posts in 1643 days

#2 posted 02-22-2020 11:38 PM

I use the Dewalt blades on my 708. It could be a blade but it won’t be every blade. A continued problem will either be the way it is used or defective saw…

View Walker's profile


272 posts in 1156 days

#3 posted 02-23-2020 02:45 AM

hmm… when you say convex do you mean it’s short on both ends of the cut or just one? If just one, is it the edge closer to the fence or away from the fence? If you’re not sliding at all, then the blade is making contact first with the middle of the work piece and then progressing towards the edges. Just trying to visualize physically how it could happen. If there was some blade wobble involved, the deflection would be more exaggerated towards the perimeter, and less closer to the arbor. When the saw is all the way down, the center of the workpiece would be closer to the arbor, with each edge sitting closer to the blade edge. So that would make sense.

Things I would try to improve results and/or further determine cause…
-make sure workpiece is clamped/held down properly.
-make a first cut wide of your mark. Then a second cut taking off just the width of the blade.
-Try making a sliding cut, even on a narrower workpiece. It might mitigate the effects of the wobble (if that indeed is the root cause).

-- ~Walker

View CaptainKlutz's profile


2716 posts in 2178 days

#4 posted 02-23-2020 02:05 PM

My comments on blades for straight cut with miter saw are in this recent thread.
New Blade Advice for 12 inch Dewalt Compound Sliding Miter

Cut accuracy on sliding miter saws is a source of much debate.
IMHO – Sliding miter saws are not as accurate as non-slider or TS (using a jig). YMMV

Best Luck.

-- If it wasn't for bad luck, I wouldn't have no luck at all, - Albert King - Born Under a Bad Sign released 1967

View Lazyman's profile


4855 posts in 2071 days

#5 posted 02-23-2020 04:56 PM

Are both sides of the cut convex? I am just thinking out loud but if so, that would indicate to me that there may be a wobble in the arbor. The only thing that makes sense to me is that arbor can only toggle in the horizontal plane which might be a problem with the housing that holds the motor and arbor bearings in place. See if you can manually toggle the blade horizontally. I think that I would take the saw back because there may be no way to fix that.

One last thought…Are you releasing the trigger while the blade is still down in the cut? Blades tend to flex and warp (due to harmonics or something like that) as they spin up and down so maybe that would explain what is going on. Retract the blade before releasing the trigger and see if that fixes are at least improves the cut. A thicker blades would probably flex less during the spin down.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View tvrgeek's profile (online now)


658 posts in 2333 days

#6 posted 02-23-2020 05:42 PM

I inquired about this problem on my Ridgid. Pretty uniformly responses are to use a thick quality blade and go slow. I took off my 80 tooth Diablo thin kerf and put on a Ridge Carbide 40 tooth. Not quite as smooth as it is not the right tooth for a miter saw, but sure enough, my compound miter was strait not curved. Saving up for an appropriate miter blade. Thick kerf, 80 tooth and low hook.

View SMP's profile


1892 posts in 589 days

#7 posted 02-23-2020 09:13 PM

These saws are built for construction, where judging by tract homes, +/- 1/4” is ok, and as long as corners look somewhere around 90 to the average person from 10 feet away, thats “good enough”. They are built for profit overseas, so you kind of get what you get. This guy does a series on how to flatten and square everything, worth watching the series and see how much work you want to put into this:

View MarkLTX's profile


6 posts in 222 days

#8 posted 02-24-2020 12:33 AM

Thanks for all the thoughtful responses. I tried a Freud Ultimate Cut-Off blade, which is noticeably thicker and heavier than the stock blade (and very expensive). The results were somewhat better but still not acceptable to me. As SMP said these saws must be primarily intended for construction. I should have known better.

I returned the saw to Lowes. The guy at Lowes got excited and said he was going to call his dad to see if he wanted it. I wonder what kind of discount he’ll get.

BTW, Lowes will price-match Amazon which in the case of this saw was a $200 difference.

Now I have two blades to return to Amazon. I feel like a bit of a schmuck returning all this slightly used stuff.

View tvrgeek's profile (online now)


658 posts in 2333 days

#9 posted 02-24-2020 02:35 AM

Framing only? Don’t buy that as they are intended for trim work too. Crown mouldings etc.

The slower I cut, the straighter the cut. Clamping helped. Watched the video and will check that on my Ridgid. I don;t buy all of what he says as if the wings are off, then any long board will be off.

View Foghorn's profile


262 posts in 70 days

#10 posted 02-24-2020 02:50 AM

I have a Bosch 12” slider, the old style, and couldn’t be happier with the accuracy. Sorry to hear about your problems.

-- Darrel

View tvrgeek's profile (online now)


658 posts in 2333 days

#11 posted 02-24-2020 03:28 AM

OK, checked ny saw. Center table was about half a thou high on one edge. A bit if work with a diamond block ( we woodworkers of course have them) and I am flat to less than half a thou. Barely passes light. Side tables were uniformly low but flat. Low by dead on two layers of foil tape on one side, one on the other. Now, less than a thou end to end.

Lower fence was dead 90 degrees, but I had to put a thumb on it to get both ends in line to no light gap. Alignment of the blade to the center slot was dead on, so aligning the fence to 90 to the blade was easy, confirmed by flipping a cut.

But, upper fences were setback by 20 thou at least and tilted forward by at least half a degree! That is not going to cause curved cuts, but sure could throw the angle off. Fortunately, correcting is removing material from the upright, so I will do that tomorrow.

FWIW, to support long boards, my saw sits fixed to cabinets and I have blocks ( not exactly a 4×4 ) to be very much level with the saw. More level than a bit of crown that’s for sure.

This video was talking thousands of an inch when we are talking baseboard and crown. If the cut is strait, we can deal with ten times that. Maybe not making doll house furniture but I would not be using a 10 inch power saw to do that anyway.

Last bit, my old first generation Delta non-compound was flat in the center and lower on both sides by about the same as the Ridgid. I propose that is intentional. It is still amazing how a spinning blade can make a curved cut.

I am betting this will not have any effect on the curve cuts I was getting, but always nice to know one’s tool is as good as you can get it.

View tvrgeek's profile (online now)


658 posts in 2333 days

#12 posted 02-24-2020 05:38 PM

OK worked a bit with a 19th century milling machine ( file) on my fences. Not perfect, but far better. Actually, before is one step better than it really was. It also skewed away from the fence closer to the blade. Again, better, not perfect. All out of old crown so nothing to test exactly, but I’ll do some compounds to see if the are any straighter and closer to 90 degrees. Lunch first. Still tempted to just go to a jig and sell the Ridged. My old Delta seems more rigid than a Ridgid.

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