Blade Clearances

[Chris Parks]

I have a new type 18" jet bandsaw with roller bearing guides. I cannot see the reason why there should be a clearance between the blade and the bearing. The bearing turns so why shouldn't it run against the blade? The idea of a clearance was a good one when the guides were a static block which rubbed against the blades and is not relevant to bearing guides. Comments and thoughts would be welcome.

[eddie the eagle]

Still need a small gap Chris,

About the thickness of a plastic $5 note - I wrap one around the back of the blade and then set the guide position to be a neat snug fit with the note in place (both side and thrust bearings,) then remove the note and the thing's set perfectly.

Avoids bearings wearing out too quickly.

CHeers,

eddie

[Chris Parks]

Eddie, these bearings have a multi thousand RPM rating and things like alternators and water pumps under far greater load don't wear them out in years of service. I just think the clearance thing is a hangover from static guides. When the deflection happens as cutting starts the bearing starts turning whether it has a clearance or not and not having the clearance prevents this deflection in the first place thus helping eliminate blade drift.

[eddie the eagle]

No dramas Mini,

You did ask for an opinion.

Cheers,

eddie
(who's had to replace worn out thrust bearings and guide bearings and can't see the point in spinning them unnecessarily, especially if they catch the blade when it deflects 0.3mm or so. However, there are far more important things to die in the ditches about.)

[Skew ChiDAMN!!]

I use a cigarette paper, but it's the same principle as the $5- note.

I've had the occasional blade with a dodgy weld (good enough to cut clean & straight without letting go under tension, but off just enough to get that annoying "tick tick" every time it passes the bearings...) and the small clearance allows it to pass through the bearings without putting undue load on them. Unless you want to waste time looking for the weld and rotating the blade so that section's between the bearings every time you set them?

I've no idea whether this is "the" reason it's done... but it's why I set them that way. :)

[RETIRED]

Another reason that you need clearance (do the same as both Eddie and Skew) is that sap build up on the blade can reduce the gap considerably.

This didn't worry the block type of adjuster as it sort of self cleaned because it was static, not so with a bearing. The load just gets greater until they disintregrate.

There is nothing to be gained by zero clearance as the blade will still wander a bit anyway.

[Chris Parks]

Eddie, thanks for your reply, I must have sounded like I didn't appreciate it, for that I apologise. and Skew, they are good reasons, but those reasons go out the door when actually cutting due to the clearances disappearing anyway so why have a clearance in the first place?

[silentC]

I think the point is that there is no need for them to be touching the blade when it's not running, so why do it? Given that a bearing has a limited life, and that is a factor of how many revolutions it does, why have it rotating when it doesn't need to?

[Chris Parks]

My thinking went something like this...Blade deflection leads to drift, if the blade deflection is reduced so should the drift? The bearings are relatively large for the job and their life if turning continuously (SP?) should be in the thousands of hours so why not reduce the clearances. If they were not meant to be run continuously then why change to roller bearings when the old rubbing blocks have been doing a great job for a long time. I wonder if our thinking is not catching up to the newer machines?

[silentC]

My copy of The Bandsaw book by Lonnie Bird just says to leave clearance, he doesn't go into the details of why. It's one of those things I suppose that we do without really thinking about why. You could be right that it is a hang over from guide blocks but and Skew have given a couple of reasons for why they do it.

You certainly don't want the guides, especially the side guides, putting any pressure on the blade because that will contribute to heat build up and wear, so at the very least setting them with a gap ensures that contact is only made when it is needed.

At the very least, it does no harm to have the gap. However having them too tight against the blade will do more harm than the small amount of deflection you get from having the gap.

You can look at it as erring on the side of caution.

[Chris Parks]

Silent, I did not expect any definitive answers and as you said the texts we read don't give any. I am going to try it and see what happens. Seeing as I am going to be busy getting a new business off the ground the bandsaw will be taking a break for a while I reckon.

[silentC]

That's OK, it's a slow day in the office and these little mysteries of life are more interesting than the other things on my computer screen at present.

Let us know how you get on, although I suspect it will be difficult to determine any conclusive results. For what it's worth, I set mine by eye, so the setting is different every time, yet I've never noticed any significant difference in operation. Tracking and the state of the blade seem to be much more important factors in controlling drift.