Mitre Saw, True 90 Degree Cut Problem

I have a Bosch GCM 8 SJL Mitre Saw and, I guess this topic has been done to death in various places for Mitre saws in general but, I really couldn't find any successful threads so here is hoping.

I have attached some pictures where I have calibrated the saw, squaring the blade to the left and right fence and then vertically to the table and right away I am getting far far better cuts than I was before when trying to do true 90 degree cuts.

But, as you can see from the images, it is still a tad out and I can't figure out what else there would be to adjust. Any other ideas what could be giving this slight out of 90 degree cut?

I have tried letting the blade spin up to full speed before pulling down, I have tried pulling down and through the wood really really slow through medium pace through fast and still get very similar results so I am not sure it's "user input".

One thing I do notice is, if I slide a bit of wood across the left fence, keep it pushed to the fence and keep sliding right so it slides over to the right fence, ie, so the wood will span both the left and right fence it catches ever so slightly the right fence before sliding over, it catches literally by a hair (or maybe two hairs) but could that "hair" be enough to send it out like shown in the picture? If so, I will struggle because, no matter how I adjust the left fence, soon as I tighten it pulls into that position.

Have you tried clamping the left fence down before tightening? Or clamping a piece of wood across both fences before tightening?

The other option is , if the right fence gives you the cut you want then clamp the wood to that before making your cut although that's awkward unless you are left handed!

Another thing.............is your square defiantly square?

put a strait edge on the base touching the back fence full machine width and look for gaps or bumps

Had a cheap chop saw and repeated 'kicks' from cutting large timber sections caused the fence to crack and bend very slightly in the centre.

loopy66 said:

I have a Bosch GCM 8 SJL Mitre Saw and, I guess this topic has been done to death in various places for Mitre saws in general but, I really couldn't find any successful threads so here is hoping.

I have attached some pictures where I have calibrated the saw, squaring the blade to the left and right fence and then vertically to the table and right away I am getting far far better cuts than I was before when trying to do true 90 degree cuts.

But, as you can see from the images, it is still a tad out and I can't figure out what else there would be to adjust. Any other ideas what could be giving this slight out of 90 degree cut?

I have tried letting the blade spin up to full speed before pulling down, I have tried pulling down and through the wood really really slow through medium pace through fast and still get very similar results so I am not sure it's "user input".

One thing I do notice is, if I slide a bit of wood across the left fence, keep it pushed to the fence and keep sliding right so it slides over to the right fence, ie, so the wood will span both the left and right fence it catches ever so slightly the right fence before sliding over, it catches literally by a hair (or maybe two hairs) but could that "hair" be enough to send it out like shown in the picture? If so, I will struggle because, no matter how I adjust the left fence, soon as I tighten it pulls into that position.

Click to expand...

You've shown the square on the vertical and horizontal but have not said which one is producing the inaccuracy.
Its easy enough to get a slight non square cut with curved timber not registering with the fence as it should.

Ummm... a bit worried about my square being true and square. It's square against my speed square but I picked up a bigger framing square and it's not against that. This could just mean my framing square is crap rather than the square I am using for to align the mitre saw but, still got me needing to properly triple check this. I'll try this tomorrow, correct me if wrong but the best way to check it is to find some wood that is true 90 degrees and use my square to draw a line from each edge and if they don't line up, my square is shot?

Regards putting a straight edge along both fences, they line up apart from about a hair off on the left fence.

I am also not totally convinced about this wood either so, i'll dig out some proper straight stuff and do some cuts on that too.

Many thanks for the suggestions so far.

get your square and draw a line at right angles on a bit off board flip though 180 degrees and draw a line on top off the first one the square is out by half the difference

loopy66 said:

It's square against my speed square but I picked up a bigger framing square and it's not against that. This could just mean my framing square is crap rather than the square I am using

Click to expand...

TBH neither speed squares nor framing squares are that accurate (after all both are designed for first fix work where tolerances are fairly generous). Have you checked the square against itself and a straight edge?

To do that you take a machined reference edge (e.g factory edge of a sheet of MDF), set the stock of the square against the factory edge and draw a line using a sharp pencil against the outside of the blade of the square. Flip the square over so the stock is still against the factory edge, but points the opposite way (e.g.left instead of right). Align the blade to the pencil line and draw a second line. If this diverges from the original pencil line, then your try square isn't square (which is sort of what B-A said.....)

Note that with lower cost and non engineering quality try squares the inside edge of try square blades are often not exactly parallel with the outside edges

Before attempting to square your blade you need to ensure that the fences on the saw are coplanar. This is done using a reference straight edge (a steel.strsight edge or even a good spirit level) and feeler gauges. Most mitre saw horseshoe fences are a bit out, so it may be necessary to make up plywood sub fences screwwd to and packed off the horseshoe fence with paper or card shims to correct discrepancies. The fences also need to be vertical to the bed of the saw (checked with a steel.engineers square, ideally, and the sub fences shimmed to adjust)

Any timber you use to test the cuts must be accurately prepared, strsight and square edged

I was supposed to come back and update this but, just forgot. I am since this original thread happy with the cuts. I was doing a few things wrong to be fair, the following is what I do now when doing cuts I need perfect (or almost perfect)...

*** Test my squares are still squared against a known good bit of machined edge
*** Test my speed square (if needed 45 degree) against a known good 45 degree
*** Set the mitre saw against the square, never in a rush. (also adjust the mitre detents to the mitre scale if it's moved enough to annoy me.

*** Ensure the wood is good (the wood when doing this original post was actually dodgy and kinda rocked against the fences when I held them, I didn't notice this at the time).
*** Put the wood against the fence, ensure it is lined up flush along the whole length of the wood to the fence(s). It is surprisingly easy to have it a hair out at one side, especially if sawdust is on the table.
*** Ensure to hold the wood firm.
*** Let the blade spin up and bring the blade down without adding any other inputs. I practiced on some scrap wood since the original thread and found I was unknowingly adding some input which moved the blade. Important to get a feel for the saw before expecting top end results.

Much appreciate the inputs into this thread in put me on to the right tracks.

NOTE: This is necessarily a detailed technical post. Skip this post if you are not interested in the minutiae of accurate saw set up

There are a couple of "self tests" you can do.

If you want to check for plumb cut, assuming that you have already straightened or checked the back fence to the blade, make a square cut (0° mitre, 0°bevel) in accurately machined timber, then rotate the right hand piece ONLY through 180° around its' long axis (so basically the front becomes the back) and push the two cut ends together. Viewed from the front a "v"-shaped gap indicates that the blade is tilted slightly to the right, an inverted "v" shaped gap indicates that the blade is tilted slightly to the right.

Similarly, when viewed from above a "v" shaped gap indicates that the 0° mitre setting is out with the blade pointing a bit to the right whilst an inverted "v" shape indicates the opposite

The larger the section of timber you use, the clearer the indication you get. Note that this technique actually doubles the discrepancy making it more obvious

So whilst squares have their uses in initial setting up (in fact I'd say they were essential), they aren't always the best arbiter of absolute accuracy accuracy.

Also, rather.than depend on a Speed Square, combi square or even a mitre square check for accurate 45° (mitre) cuts, simply cut a piece of something like 3 x 2 planed, square edged stock at 45° and place the timbers on a flat surface. Rotate ONE of the pieces through 180° (around its' centre axis) and push the two mitre cut ends together so that they form a right angle then check with your verified try square that it really is square. If the corner formed this way has a gap at the outer end of the blade of the square when the stock is hard against the flat, your indicated 45° is actually less than 45°. Conversely if the square has a gap on the blade where it meets the stick your 45° is greater then 45°. Check both mitre cuts (so left and right) as one side may be more accurate than the other

Bevel cuts can be checked in exactly the same way, but tilting the head as opposed the using the mitre adjustment

I'll bet any money your saw is out a bit. These tests just help you find out where your saw is out. In reality for jobs like skirting being 0.5 or 1° out can be easily corrected by the judicious use of a block plane. Jobs like picture frame make up, however, may require you to get the saw absolutely bang on, so it can be worth knowing how to check your saw's accuracy. Just don't beat yourself up if your saw is slightly out - they all are!