Thread: Sliding compound mitre saw cuts bowed

The only thing i would try is to just skim the board 2-3mm deep with the blade and check the cut for straightness. Are you cutting with 1 drop or doing a few passes? you will need to play around a bit with different cutting methods but if it was me i would be buying a quality blade.

Most likely the boards are cupped

Try a rough cut just outside your line then take the last 1mm off to the line.

My initial reaction is the same as Riverbuilder’s. I would get some nice flat test stuff: a piece of the pre-oiled merbeau decking at Bunnings is cheap, about the same height and width, and you can check across the piece with a square for any warping. Make some test cuts with that: if they’re still bowed, then it is some weird machine issue...

So, just an update.
I bought a new Diablo blade and I still get the same issue.
I've done a few tests along the lines the replies suggested and it seems when the cut is light or the timber is soft the cuts are fine and when I did a light 3mm deep cut across a flat board the cut was straight and cutting plywood, thin mdf or pine gives a straight cut too.
When I do a single sliding cut through one of the h/w skirting boards (19×85mm Tassie oak) clamped in place and then drop the blade back down it takes a definite extra cut out of the centre of the board.
Also if I do a slow drop cut, the edge of the board shows 2 or 3 blade marks at different depths but doing a really fast drop cuts the best and straightest but over stresses the saw.
I made a zero clearance insert out of 3 mm mdf and just cutting through that alone left a dead straight line through the whole sliding range and after several other slide and drop test cuts through the insert alone the slot was still perfect but after slide cutting a single h/w board the slot in the insert below the board was more than 4mm wide and really rough.
There's a pic in my next post.
So there would seem to be a definite issue with cutting harder timber.

What size saw? 10 or 12"?
how many toothed blade?

12" 80 tooth.

This is after one sliding cut through the h/w.
Saw Insert.jpg

Check the saw collars, (the big washers that go on each side of the blade) they should be flat and smooth. Even the tiniest burr or damage or even dust on them will throw the blade out of true. Rub them on a flat diamond plate or some emery paper on a flat surface to clean them, check the shoulder and also, have you fitted the correct bush inro the blade to fit the shoulder of the collar? I know this sounds obvious, but is the blade in the correct way around? Seen this before.

Originally Posted by sneem26

12" 80 tooth.

This is after one sliding cut through the h/w.
Saw Insert.jpg

I can't see how the timber can be held firmly in place on this sort of cut.
In fact it is very risky to use a mitre saw to make these sorts of cut.
One of the chaps at the mens shed had a fair size chunk taken out his thumb attempting this sort of cut.

Let's see a cut with the wood being firmly held down.

Hi S26. A couple of thoughts. You can cut without using the slide function. 85mm and less should be fine to cut just with the dropsaw, maybe pulling back a smidge on the 45° to finish. This will get the job done but not fix the problem.

Do you have 2 rails for the saw to slide on? This is normally pretty stable but the further the saw is pulled out the more flex can be introduced. If you are leaning to one side to see cut you may also be pulling the saw in that direction too. RB has highligted the other potential of blade wobble.

Do you have insert rings for the blade to reduce the diameter of the pre cut hole? Be careful of the difference between 25mm and 1". In a previous life we had a few dropsaws on site and 2 Makitas (one with 25mm and one with 1") and people assumed they were the same or at least near enough is good enough. Not so.

One other thing to check. We also had a Hitachi saw that had gaps under the collars that the insert rings could fall into. But you knew when that happened, the saw literally jumped around

Another possibility... probably unlikely, as you say you changed the blade, but...

Are the saw blades thin kerf?

I've found that the larger the blade diameter, the more likely a thin kerf blade is to buckle/deflect under heavy cutting loads or hard going.

I know that we should let the blade cut at it's own pace, but when in a rush one often can't help but try to 'assist' it through the cut, leading to less than ideal results.

Originally Posted by Skew ChiDAMN!!

Another possibility... probably unlikely, as you say you changed the blade, but...

Are the saw blades thin kerf?

this is the next question I was going to ask too, quite possibly the blade is spinning on the collars and has taken up a burr on the collars and thrown the blade out of true.
but I still reckon it’s actually the timber. Need to put a straight edge across the board, it will tell you straight away.

I check new saw blades (both sides) before installation for flatness in several places using a precision straight edge ruler or square.
Accumulative error happened with my radial arm saw. A two inch wide cut was off by 1/32" making the error greater as the lumber width increased. The linear bearings were worn out and no parts were available. Nothing could be done to repair the machine.
Try making several test cuts using absolutely flat lumber. (maybe particle board). Using different lumber widths and lumber thickness make a few straight cuts and several angles, compound cuts checking each cut for accuracy. Also secure the miter saw to a solid work bench to minimize vibrations transferred to the wood being cut.

We still don't know if your board is flat. This is the most common reason for your problem.