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devonwoody

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Yesterday I decided to put a replacement wooden (iroko) handle to this kitchen knife.



I changed the blade down to a 6mm and removed all the guides off the bs except the back bearings.

Switched on and the loudest machine shop bang I have heard, checked my pants were OK , thought the blade had broken, switched off, opened the top door, blade in one piece but behind the wheel, opened the lower door but blade was intact.

It appeared that the blade had spun off the rearside of the top wheel and was intact. Replaced blade back on wheels, same again and another bang. Several more times, it would not stay on drive, previously this blade has performed ok, but to get the blade to stay on course it needed the centre of the top wheel axle connection to be changed to a different setting. Then got machine to cut without those bearings as per picture above.

Put the 18mm blade back on and bearings back, cut a test piece and it was way out. So bad that the fence angle has gone through around 15% and the opposite fence angle.

(Cutting straight again but took some setting up)
 

kirkpoore1

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I've got to wonder whether something on the tracking adjustment or tension mechanism has broken. Otherwise the sudden change in behavior isn't easy to explain. Try putting the 6 mm blade back on, and tension it properly. Then, with the wheel covers off, hand turn the upper wheel and watch what the blade does.

Kirk
 

devonwoody

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Thanks Kirk, I will have a word with the suppliers tech dept. there is a lot of movement with the top wheel, It does seem to have a larger movement. The top wheel takes a long time to come to a halt I found at the time as well, I had to slow it down by using a batten to brake the motion.
 

Eric The Viking

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I think small blades get less 'auto-centering' from the crown of the wheels than bigger ones, because they can twist more (for a given blade length/wheel diameter). It's odd - if blade twisting was causing it, intuitively it ought to come off the bottom wheel, because it'll be mis-fed onto that wheel.

I don't understand this:
... to get the blade to stay on course it needed the centre of the top wheel axle connection to be changed to a different setting.
That sounds like the tracking adjustment - is that what you mean, in which case it's normal to have to do that (I'd have thought).

There is another possibility: When I bought my SIP from OPJ, he gave me a large quantity of blades with it, some of which were Ian's (Tuff Saws). I've never had a problem with any of those, but by accident I picked out another blade recently, about 12mm (1/2"), roughly the size I needed and very sharp.

I found out immediately why it was very sharp: it had a twist in it. I've never seen anything quite like it before (about 5-10 degrees of 'wobble' - like wobble washers on a table saw!), and it definitely wasn't the weld (about 1/4 round from that), and didn't seem to be related to it. Could your blade be similarly twisted?

You mentioned it took a lot to get the 18mm blade to run true -- what did you have to do?
 

devonwoody

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Thanks Eric. that small 6 mm blade had run correctly on two previous occasions. This time it ran to the backside of the top wheel and any tension would not keep it on, and on the lower wheel it ran to the front edge it just about stayed on.

All side bade bearings top wheel and bottom wheel off.

To get it to run that tension winding wheel/nut that pushes the top wheel forward and backwards on the axle needed unwinding.

When putting back the wider blade and all bearings, I left that axle tension alone and set blade tension the normal way that stretches the blade. However the blade angle changed. The cutting/fence line went to the left at back of table , whereas it was slightly to the right previously.

When no blade is fitted, the top wheel has a lot of movemnet forward and backwards which I have not noticed up to now, perhaps someone can confirm this is normal.
 

Eric The Viking

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Yes, it's normal. The top wheel, when not carrying a blade, should be loose enough to tilt a bit top-bottom, as the shaft is held on a hinge (the pivot is above it). The blade tension pulls it down at the top outer edge and the tracking adjuster behind the shaft (the one you mention, I think) controls how much it tilts. when it's set up properly, it's almost parallel and coplanar with the bottom wheel, how parallel/coplanar depends on how well the blade tracks 'naturally'.

The tracking adjustor thread behind the top wheel shaft (usually a knob on the casing, opposite side to the blade teeth) should tilt the wheel, it doesn't move it straight in/out. That's the adjustment you need to get any blade to run true. If you didn't re-adjust it when putting the bigger blade back, I'm not surprised that didn't run true!

Strong recommendation: get Steve's DVD on bandsaw setup. It makes sense of all of this, far better than anyone can using text, and there *is* a lot of duff gen out there on the subject!

The bandsaw geometry is really too complicated to diagnose from a distance. You need to start from a known good setup (wheels co-planar, etc.) and go from there. I suspect something has got knocked or bent out of alignment. It's possibly a bearing failure, but that's unlikely unless you've had it really highly tensioned for a long time, and anyway, you'd likely hear the racket from the bearing.

HTH,

E.

PS: did I mention Steve's DVD? :)
 

Steve Maskery

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All that sounds perfectly normal to me. Two different blades, even of the same spec, will not necessarily track in exactly the same position, that why the machine has adjustment. You can't just take one off and put another in its place and expect it to run true, I'm afraid.
Bandsaw setup is not actually difficult, but you do have to set things right and it helps to do them in the right order, too.
S
 

devonwoody

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Thanks Steve for your assurance, it was the terrific bang (I suppose inside a tin box) and it running ok on previous occasions.

Do you take your side blade bearings off when using a narrow blade and doing shaped cutting?
 

Steve Maskery

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No I most certainly don't. If the blade is too narrow for the standard guides I fit guides that can touch the blade without damaging it, or themselves getting damaged.
S
 

devonwoody

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Steve, on reflection I would put guides on when using that 6mm blade normally but seeing I was only cutting a length of 6" and around 18mm thick, what the heck. If I was cutting some depth I would most probably make the effort and spend more time setting up the guides, but 45 minutes setting up and taking off for that job seemed OTP.

(My old Record bandsaw the guides were about half inch off the blade (alright no where near the blade :mrgreen: ) and it used to cut upto 6" depth no problem).
 

devonwoody

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Steve Maskery":1s35t1a5 said:
If it takes you 45 mins to set up your bandsaw you are not doing it right! :)

It takes me ten minutes to get off the floor each time I go down there. :)
 

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