Can you help diagnose this poor cut?

Hey guys,
I'm doing test cuts with a radius cutting jig that I made, and I often get a cut that looks like this - little diagonal ridges where the blade has wobbled side to side. I can see it happening as I feed, and even slowing the feed way down - say 1mm per second - the wobble keeps going. At that point it looks like the blade is snaking side to side.

Here's a picture - the cut started from the right of the piece, so you'll note the ridges started smaller and closer together and get wider.
http://i131.photobucket.com/albums/p...pspac1cqlk.jpg

To answer a few of the obvious questions:
Brand new blade, 6TPI 10mm
Radius of the cut is 5.5" (ie not tight)
16" bandsaw with 2HP cutting 45mm thick pine (ie enough power not to break a sweat)
Upper and lower guides set very close using feeler guages - 0.1mm as per a Popular Woodworking article
Thrust bearing set about 0.4mm back

I suspect blade tension may have something to do with it - I don't have a tension gauge nor does the saw have a tension indicator. Tensioning instructions always use wildly vague instructions like "push on the blade a bit and it shouldn't flex more than 1/4 inch" - in this case the blade barely flexes more than 1/8" with the guide post all the way up and a pretty strong push.

Any thoughts?

Will

Measure the run out for the old band and then this one. - i.e. how much it snakes side to side - do that by holding a ruler behind the band and turning the wheel slowly by hand.
Mark the greatest points of deviation side to side on the band - one of those gold or silver texts works really well for this
Then take the band off the saw and lay it on a flat surface and see if you can find a bend or uneven section on the band near the marked areas.
If you can, put some gloves on tweak the band by hand to see if you can get it even and smooth al the way

While you are turning the wheel manually look for a rogue tooth or two sticking out of line.
If you find one put it back into line with a saw set - small multiple increments at a time rathe than one big go.

Check the join is smooth - if not sand it smooth with a fine flap sanding wheel

Hey BobL, thanks for the quick reply
No wobble on the blade at all when turning by hand - not even on a dial indicator, much less a ruler (that even surprised me, and I'm used to expecting the worst from what is a cheaply constructed saw).
I'm not inclined to think it's a blade problem as I've noticed this phenomenon on many different blades over the years, and it seems to happen when the blade is under some degree of sideways load - ie. cutting a radius like right now, or ripping bigger boards against a fence that's maybe not quite adjusted for drift. Surely a brand new 6tpi 10mm blade and 2HP is enough to cut a 5.5" radius though?

Gidday Wsal

You might find these useful:


Diagnose Problems "How-To's" for all Band Saw Blades - Allbandsawblades.com

band saw blade trouble chart and terminology

How To Diagnose Band Saw Blade Problems – WooDesigner

Regards Lou :D

Quote:

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Originally Posted by wsal

Hey BobL, thanks for the quick reply
No wobble on the blade at all when turning by hand - not even on a dial indicator, much less a ruler (that even surprised me, and I'm used to expecting the worst from what is a cheaply constructed saw).
I'm not inclined to think it's a blade problem as I've noticed this phenomenon on many different blades over the years, and it seems to happen when the blade is under some degree of sideways load - ie. cutting a radius like right now, or ripping bigger boards against a fence that's maybe not quite adjusted for drift. Surely a brand new 6tpi 10mm blade and 2HP is enough to cut a 5.5" radius though?

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How does it cut in a straight line?

While the radius of curvature V width of blade charts says a 10 mm wide band should be able to cut a 2 " radius circle it doesn't say anything about the finish you might or might not get.
I use a 6mm wide band to cut lots of 6" radii of curvature and I notice it does not give me quite as good a finish as a straight cut.

You could try removing a small amount of the set. This can be done by using a fine stone and holding it against the band and turning the wheels.

I was also going to ask what a straight cut looks like.

The aspect of your set up I do not like is the 6 tpi. I would much prefer 3 tpi. The higher tpi creates smaller gullets, and these are more susceptible to filling with sawdust. When they are loaded, the teeth struggle to cut, and that is when you get the blade wandering.

Can you try with a different blade?

Regards from Perth

Derek

I think half your problem is that the blade is too wide for the radii you're cutting, or that the teeth have an incorrect set.
The other half of the problem could be where you have the centre pivot point in relation to the blade. The centre should be at a point where the blade is cutting true, not trying to go wider or narrower, and would be approx about a mm behind the teeth, I found it varies from blade to blade, so there is no definite rule on this. I set up the jig as to where I think it should be, then fine tune on some scrap, so that it runs true.
Attachment 1 is the Radii X Blade size, Attachment 2, shows what happens with incorrect size blade.
Hope this helps,
Kryn
Attachment 405411
Attachment 405410

Thanks guys, a little update - I think we're all getting towards the same answer here:
I think it's to do with the blade clogging - I'm noticing left over sticky sawdust that's a bit compressed and stuck to the parts after cutting. I had just put this down to it seeming like a particularly gummy, resinous bit of pine, but maybe it's a case of the gullets not clearing out quickly enough. I also backed off on the thrust bearings as I guessed maybe the blade was bearing too heavily on them and was being forced to zig zag - this has given me a little improvement.
I have a 1/2" 4TPI and a 10mm 4TPI (both of which are older, a little rusty and maybe a little less sharp, hence having the brand new blade on hand). I'll give them a go

Ok, straight cutting on another scrap of pine (not the exact same piece, but same thickness) and the same blade - performs beautifully. Crosscut by hand/eye (no gauge) and rip with a single-point ripping fence (ie. the corner of a block of wood clamped to the table!) and it's surely the best cut I've ever had with this saw - just about glue-ready. Probably due to the hours I've been spending on the guides/tension/tracking!

Check the position of the pivot point of your workpiece to ensure that the back of the blade is not rubbing heavily on the wood as in the second attachment that Kryn has shown. If it is rubbing heavily, that could explain why you start off okay and the ribs get bigger as you progress. The blade may ride against a small rib and create a larger one where the teeth cut, then the back of the blade rubs on this bigger one to create an even bigger ridge and so on getting worse and worse.

I've seen this. Brand new blades, set to factory specs with feeler gauges. Wrong.
It turns out that the wheels run on roller bearings.
Some how, some time, one of the damn rollers has decided to take a flat.
You will never feel it by hand, you will never see it by eye but the dang thing
comes over with a jerk on a fairly regular basis.
Get the wheel bearings changed.

I'm only guessing at this because I've had 2 different saws go out, same fix.