SCMS - extreme burning and lots of breakout - please help

[AES]

Dear all,
As a relative newbie I’d appreciate help with the burning and breakout problems shown in the pic below. Thanks in advance.

For reasons I won’t bore you all with I needed to angle-cut some ply spacers at an angle of 3 degrees across the material thickness – as accurately as possible! These spacers are 80 mm square (3.14”) by 21 mm thick (0.83”) and will not fit my new bandsaw so it seemed to be a job for the Sliding Compound Mitre Saw - I certainly didn’t feel able to plane or sand them by hand with sufficient accuracy.

As per the advice from various kind people here when I first bought my new SCMS, I’ve invested in a couple of decent quality blades for it. The saw itself is a “badge-engineered Chiwanese” cheapo from Aldi Switzerland. But the blade that’s giving me the burning & breakout problems shown below is a Summit from UK (as recommended by the kind folks on this Forum), Part No: 214. It’s called Premium Quality, Extra Fine for Aluminium, Wood, & Plastics, is 210mm dia (8.26”), 64 TPI, TCT, with probably a zero or slight negative rake tooth form. It’s marked for max 7,500 rpm and the stated max no load speed of the saw is 4,800 rpm with motor power of 1,800 Watts.

The workpiece shown (on the Left) is good quality birch ply (as above 80mm x 80mm x 21mm thick). Calcs showed that starting at as close as possible to the max (21mm) thickness position at the front of the cut, I needed approx 15mm thickness by the time the blade had reached the rear of the 80mm width of the spacer. As these cuts would be across the “vertical thickness” of the material I needed to rig up a sacrificial fence/support device which gripped the workpiece. After some “faddling” I came up with something that gripped the spacer from underneath, plus only for a very short fore and aft depth to the rear of the spacer – i.e. the blade would NOT be “trapped” or otherwise impeded or pressured until the very last part of the cut. Due to the height of the spacer versus the depth of cut of the saw I realised I wouldn’t be able to cut the full 80mm depth of the spacers but could get a cut of just over 45mm deep, so I planned to finish off by using the tenon saw (by hand!) to follow the kerf made by the machine saw.

The result is the extensive burning plus breakout and fuzz you see in the picture. Please note that the burning starts immediately the cut starts – i.e. WELL BEFORE the blade has even reached my holding jig at the back of the spacer. Also please note the traces of blue paint at the top of the cut (also well before the holding jig is reached). This is the blue indent markings on the blade, now virtually unreadable.

Also. On the right of the picture is one of several pieces of scrap softwood I used to check the initial 3 degree angle set up. Spot on, so I seem to be able to do something right ;-) ! It’s probably not clear in the photo but even this scrap piece shows the beginning of some burn marks and I think the breakout on the lower surface is very clear in the pic. The dimensions of this scrap piece are 40mm wide by 15mm deep and the test was of course cut in the “usual manner”, NOT across the thickness of the material but across the width.

Definitely NOT an expert, so I did carefully follow the advice of various folks here re operating the saw – i.e. draw the saw forward first, then descend to the required cut depth and push the blade into the workpiece in a rearwards direction. Once I saw (and smelt) all the smoke I also deliberately made these cuts in three passes of about 15mm each. Also I did NOT push the blade into work too quickly and at no time did the motor seem to be labouring.

So what have I done wrong, and how do I fix it please?

All hints & tips will be greatly appreciated.

Cheers
AES

 

[Chrispy]

You have got the blade in the right way round haven't you?

 

[CHJ]

First thoughts, likewise is the blade fitted the correct way round ?

 

[Bradshaw Joinery]

MY crosscut would burn a little on that along grain cut, and it sounds like you have a very fine blade in which wont help. For along grain thr the more agressive blade that probably came with the saw.

Plus one for blade direction, does look to be the case.

 

[devonwoody]

You are cutting into a depth of 80mm ply, is that what you are meaning.

So you would be cutting into a glue line almost continuously?

If so you are not cutting timber, you are sawing glue perhaps?

 

[AES]

Gents,

Thanks for all the comments.

I'll check the blade direction first, then try again if it does needs reversing, & if no go, then try the original (much coarser) blade and advise Monday (today's visitors will NOT appreciate the row the brushed motor kicks up).

Re cutting glue rather than wood - yes, that's true, it's (almost) an 80 mm cut. As you can see, this ply is multi-lamination with the wood veneers all being rather thin. And sorry if I didn't explain it clearly enough, the laminations are all standing vertical with the blade cutting into them vertically. Only the 3 degrees setting takes the blade "across" the laminations at all. But (as I think they say in the trade) "that's the job"!

Cheers
AES

 

[devonwoody]

My next comment would be that I can see no way that a piece of ply like that could be held and be sufficiently stable whilst blade is cutting. ???????????????

Perhaps glued to a longer length of any timber of same dimensions and then clamped in the normal manner of a long batten being cross cut, and a final parting cut either manually or perhaps the SMCS saw

The forcces exerted are tremendous by the blade to the timber and the timber takes some firm holding and will need clamps in my opinion.

 

[AES]

@Devonwoody,

Thanks for that point. As said in my original post I did spend a LOT of time "faddling" up a jig to hold the workpiece and it did seem to work pretty well. Later on today I'll try to re-make that jig and photo it, but I should say that I was quite satisfied that the workpiece was very firmly held, with no perceptable movement at all.

Thanks for all the help gents.

AES

 

[9fingers]

It only takes a very slight misalignment between the plane of the blade and the axis of the slide to cause a burn.
Trying to cut a quickly as possible should help a bit by minimising time in contact with the blade.

Bob

 

[andersonec]

I think that is just too small a piece of wood to be putting through a cross cut, it is obviously starting to twist as soon as the blade touches , not much but enough, if it were secure you wouldn't get that much burn even if the blade was dull, also if were to come loose the result would be an uncontrollable piece of wood flying round the saw.
It will be a problem to hold such a small piece, especially thin side on, firm enough to make a clean cut, it only needs to twist a fraction to produce the results you are getting and because of the amount you have got I would stop right there and come up with a different way, hand saw and plane perhaps.
Please don't take this the wrong way, but I think machines are relied on too much for some of the things we need to produce, think of the amount of time you have spent making jigs plus the wood wasted, now you have to almost start again., my advice is to use a tennon saw and a hand plane, you can use the layers of ply as a guide. A sharp blade in your plane and thin cuts making circular movements with the plane, you may surprise yourself.

Andy

 

[AES]

@ Chris; CHJ; Olly; Devonwoody:

You can all go to the top of the class, while yours truly (AES) stands in the corner with the “Dunce’s” cap on his bonce. The blade IS on backwards. What a pillock I am (a trade term I believe)!!!!!

But thanks to all of you at least if I ever see this problem again I’ll know what to check first – it never even occurred to me to check the blade/direction of the teeth.

I’ll refit the blade right was round tomorrow and advise the results.

@ Devonwoody:

Here are a couple of pix of my “wonder jig”. Neat it definitely isn’t but it does definitely hold the workpiece VERY firmly. In “Jig 1” the X on the LH side shows where the SCMS clamp bears down vertically on the jig. Part 1 is simply double-screwed to the vertical “fence” part of the jig. The workpiece is then clamped as hard as possible against Part 2, and the resulting assembly is then cramped to the existing fence of the SCMS, front to rear, at point X . But the most important point is the strength of the “wooden spring” (Part 3). The horizontal surface of the jig is only 60mm wide as measured from the forward face of the back of the jig – i.e. 20mm LESS than 80mm “length” of the workpiece. By using a pair of VERY large screws, the LH of which screws Part 3 fully home against the horizontal edge of the jig, the RH screw can then be tightened up as much as possible, as shown with the workpiece in position in “Jig 1”. As said, pretty it ain’t but it does hold the job VERY firmly indeed. As said previously, please note that Parts 1 & 2 are only 21mm thick - i.e. they don't start to compress onto the blade until it's reached about 60mm of the 80mm length of the cut.

@ 9fingers:

Your point about cutting quicker is interesting – though very much a beginner here, I do know a bit about cutting materials generally, both by hand and machine. I’ve always heard that you don’t force a blade (of whatever type) into the work but just let it cut “on its own”. That was precisely what I was trying to do this time, but (especially having unwittingly fitted the blade backwards) I guess that however gently I fed into the workpiece I was going to have problems. But thanks for the comment. When the blade is re-fitted tomorrow I’ll try varying feed rates (I clearly need to make some more spacers, hopefully they'll unburnt this time)!

@ andersonic:

Yup, from the start I was concerned about the relatively small size of the workpiece, especially in relation to its thickness. But please see the comments to Devonwoody re the jig, and the pix below – I reckon that part of the problem is licked.

Thanks to all for the helpful comments and suggestions. Sorry to be such a dummy as to fit the blade backwards :-(

AES

 

[9fingers]

I'm sure the blade 'rotational adjustment' will improve things no end.

As a general point, increasing speed of cut might not seem right but applied within reason does help stop burning when sawing and when using a router.
Proceeding too gingerly allows frictional heating to build up with resultant burning. When routing the general rule is fine cut, high speed.

hth

Bob

 

[devonwoody]

I reckon you have got that timber secured enough.

Should be OK now, unless glue melts on blade as it revolves. :twisted:

 

[AES]

Thanks Devonwoody, I think so too. But let's not even THINK about melting the glue - please!

Cheers
AES

 

[andersonec]

I still think the jig is not holding the wood securely enough, fine now before it is cut but what will be holding it after the blade has passed completely through? the two pieces will then be loose and the one direction they can move is in towards the blade, the little spring clamp at bottom is only touching one corner and if that little piece of wood moves under the teeth as the blade is exiting #-o


Andy

 

[AES]

@ andersonec:

Thanks for the comment. You may be correct but we'll see when I try it again tomorrow (VERY carefully!) with the blade the right way round. Also bear in mind that as said in my original post (lengthy, sorry), the height of the workpiece (80mm) is such that the slide bars and casting of the saw prevent the blade from cutting deeper than about 45mm - i.e. still a third of the uncut material will remain when I've finished with the SCMS. That's why I plan to finish the cuts off by hand with the tenon saw - just cut on downwards following the cut made by the SCMS blade.

Re your earlier point about relying on machines too much, you may well be right. But I suspect you have a helluva lot more woodworking experience than I do - mine is limited to woodwork at school (which I thoroughly enjoyed but it was about a million years ago now) plus a bit of general DIY around the house. On the other hand, to plane or saw accurately by hand requires a certain skill of hand which takes time to develop. You may have had such training but apart from the above, I haven't, so tend to rely on machines which, if used carefully, do have the advantage of a certain amount of "automatic accuracy". But again on the other hand, during my apprenticeship as an aircraft engineer I was taught how to file a piece of metal flat and square accurately to within plus/minus one thou (of an inch) and although that training was also more than 50 years ago I can (to my great surprise and delight) still easily achieve that level of accuracy when working metal. I just need the time to develop that skill of hand with saws, planes, sanders, etc, when working in wood.

Sorry, a lengthy answer again, but I hope you understand where I'm coming from.

Once again thanks to you (and everyone else) for all the very helpful inputs.

Krgds
AES

 

[AES]

Dear all,

Thanks once again for all your help over the last couple of days.

As promised, here's what happened this Morning (Monday):

No one will be surprised to learn that when the blade is installed with its teeth facing the right way round it cuts MUCH better, and on this job, with NO burning! I still cannot believe I was such a dummy as to not even check before my original post.

When removed from the saw the blade had a huge build up of very fine and very black "powder" (I guess ply and glue). It was certainly all well "glued onto" the blade anyway, and as it's a TCT blade the slight difference between the width of the tips and the thickness of the blade parent material made a perfect place for all the crud to become very firmly "welded" in place. I ended up with WD40 and a steel wire brush as a first pass, followed up with the hottest water my hands could stand and used a brass (suede shoes) brush and lots of Swarfega hand cleaner to get all that crud out of the nooks and crannies. It took quite some time and effort! Apart from the fact it was Monday morning it rather reminded me of Detention at school - take a 1000 lines boy, "I shall stop pulling Jennifer's pigtails" or something. It certainly felt like a punishment which I suppose in view of my silly mistake in the first place was quite fitting really.

But the net result was excellent and I now have 4 off ply spacer blanks all nicely set up with a 3 degrees "slope", all easily completed within about half an hour (once the blade was refitted). No problems with melting glue, no smells or smoke. In the final part of the cut I did find that a rather quicker feed made a cut all the way to the back, adjacent to those two "support blocks" on the jig also made it very easy going.

So thanks once more to all of you who generously offered their time and knowledge to help me out.

I guess this thread can be closed off now????????

Cheers
AES

 

[pip1954]

hi the big problem is a little knowledge is very dangerous thing when you get more experience you may look back and think how lucky you have been, some thing are just no -no's
all the best
pip

 

[Racers]

Hi, AES

Next time try Mr Muscle window and glass cleaner, its very good at removing the resin and crap from saw blades etc.

Pete

 

[AES]

@ Pete Maddex,

We have that stuff here too (along with "Mr. Proper" would you believe). I'll bear that tip in mind, thanks, but hopefully there won't be a next time ;-)

Krgds
AES