Fixing bandsaw blade drift?

[Steve Maskery]

Ooh I do like a happy customer!
S

 

[RogerS]

Ian

I also have the 5-4. Are you sure the noise isn't coming from the bearings...in particular the rear thrust bearing ?

Roger

 

[ondablade]

I'm not sure Roger. Presume you mean the roller behind the blade. Once the blade is tracked I've been setting both of these to lightly touch the back of it.

Is there something i'm likely missing? It'd be nice to find that was the case. Is there a setting that works for you?

More on blades. I tried the 5/8 x 3tpi Tuffcut with the raker teeth that Steve mentions tonight on resawing the same piece of pine again. Another very nicely made blade. Again no kick or wobble. A bit more aggressive/faster cutting again than the 1/2 x 3tpi M42.

It actually cut smoother most of the time, but with the odd patch of vibration showing in the cut surface in places out and a similar but from time to time louder squeal from the saw.

Going off to have a play with the side and back bearing clearances now in the hope of finding something...

 

[RogerS]

This is what I'm referring to. I can't help feeling that the wiping action of the Basato arrangement is wrong and does cause screeching on mine.

 

[ondablade]

I guess it depends on how firmly the blade is pushed against the thrust wheel when it's cutting Roger. For sure there's a sideways wiping action there that surely has the potential to trigger all sorts of vibrations in both the blade and the saw.

I spent some time playing with thrust wheel clearances tonight to see if it made any difference - from definitely touching on all three sides, through variations of clearances up to about 0.5mm. (by eye mind you). No obvious effects that i could tell.

The thing is perfectly quiet even at the tighter settings when not cutting - it runs straight and true and tracks 100%.

It cuts thinner material beautifully, these Tuff blades really do cut nicely compared to anything else i've used. It even rips/resaws quietly up to about 80mm depth of cut in pine and the ply i had to hand.

By time it's up to 100mm the screech is starting but intermittent, leaving patterns on the cut, and from much above that it's more or less continuous when cutting.

Feed pressure doesn't seem to be a big factor, but with over 30 coarse hooked teeth cutting at a time it probably is almost self feeding anyway.

Placing hands on different parts of the machine the heavy vibration seems to be coming from within the cut. It's only a guess, but my sense is that the actual chassis is not vibrating all that much much while this is going on.

Which could suggest that the drive and top wheels are maybe getting going with some sort of torsional vibration relative to each other which might surface as a sideways flutter as the blade is alternately compressed and tensioned by this. Your sideways wiping action of the thrust wheel could help trigger that.

It's perhaps not by accident that industrial saws usually have really heavy cast iron wheels with lots of inertia if this is the case - the effect could be to stop any possibility of resonance getting going at any frequency the blade would naturally have.

It's all speculation of course, the fact is that there could easily be some set up step that sidesteps these issues...

 

[paulm]

Could it be the blade itself resonating perhaps, although I take the point it isn't noisy whilst not cutting at depth ?

Does it worsen or improve by changing the blade tension ?

Cheers, Paul

 

[Jeff Gorman]

ondablade wrote:

:: ...........my Scheppach Basato 5-4 it's drifting badly - by about 25mm in say :: 400mm relative to the rip fence.

For some jobs it might be best to accept this as a fact of bandsaw life and use a point fence.

This one http://tinyurl.com/386bqt9 was made for a small bandsaw, but should be adaptable for larger machine.

Jeff
www.amgron.clara.net

 

[Sgian Dubh]

For some jobs it might be best to accept this as a fact of bandsaw life and use a point fence. Jeff
www.amgron.clara.net

I agree. The fact that the drift can vary from blade to blade, and from one piece of wood to the next frequently makes setting up a fence to account for it pointless. Here's another image of a single point fence that might help illuminate the principle. Slainte.

 

[Lons]

I'm with DickM on this one Ian. The old rule of 3 teeth minimum in contact with the work will be impossible to meet with any saw blade on thin ducting.

An angle grinder with one of those new (ish) thin kerf discs like this

http://tinyurl.com/24h4ppv

0.8mm thick will make an easy job.

To mark the pipe, take a piece of newspaper, fold in half such that the crease is longer than the pipe circumference. Wrap round the pipe once and pull tight. The crease will automatically describe a straight line round the pipe. Mark with a felt pen.

Bob

Very good advice

I use these and they do an excellent job, safely and create very little swarf. I mostly cut stainless kitchen extractor chimney ducts but have used a number of times on s/s chimney liners which are similar to your ducting.

Bob

 

[ondablade]

It seems Paul like the blade is vibrating/resonating (chattering) within the cut as you suggest. What i was getting at was that the very heavy cast wheels on a bigger bandsaw probably play a role in stopping or reducing this this. Point being that a blade inclined to vibrate lengthwise will probably (unless the frame is deflecting like mad) have to move the wheels to get going.

Reading the Duginske Bandsaw Book in bed again last night he seems to be saying that problems are often caused by multiple inter related issues - so troubleshooting requires informed trial and error.

He reckons that chattering (that produces a vibration pattern in the cut) though is usually the result of feeding faster than the saw can handle.

He reckons that barreling (the sideways humps i described seeing) is primarily the result of insufficient blade tension - when the force to pull the blade down through the cut exceeds the tension then the section between the cut and the top wheel is left slack and free to wander. Trouble is that in the 11in re-saw cut where this happened i was at the max tension the saw can deliver.

SD and Jeff. The major drift problem disappeared immediately with yesterday's switch to the Tuff blades. It's starting to look very much like whatever the big issue was it was in the US blade i started testing the saw on deep cuts with. (the weld checked with a straightedge along the back edge was misaligned by about 0.5mm over 100mm, so maybe that was it)

That said it's clear too as you say that even a good blade well set up does drift with changes in cutting conditions, and that the point fence makes sense. I can get the blade the cut perfectly to a line on e.g. a 20mm thick piece guided off the rip fence - the saw has always been good in this regard. I've found too though that this is not 100% consistent, but so far my cuts have typically been short enough for it not to matter much.

It's seems very possible though that minor drift could be triggering some of the issues i've seen in deep cuts against the rip fence - as the blade tries to move off line all sorts of forces would arise.

Next step i think is to try some full depth but very lightly fed (thought i was feeding lightly already mind you) freehand cutting to a line and see what happens. That way drift is not an issue, and overfeeding will also be out.

I have a Scheppach blade i'm going to try too as maybe it's in some way matched to the machine.

Thanks again for the inputs.

 

[RogerS]

It has been said that it's difficult to pile on enough tension on a wide blade on the 5-4. Maybe a 1/2" Tuff blade would work better?

 

[ondablade]

Ok, trials completed.

No significant progress, but a bit more clarity on what's going on.

In the deep 11 in rip/resaw to a line with no fence or guide test in red pine using the above 5/8 x 3tpi Tuffcut (my nice stash of pine is rapidly disappearing into sawdust) the shrieking/blade chatter starts immediately enough feed pressure is applied to start the blade cutting.

Excessive feed pressure or tracking issues are clearly not the issue in this case.

Touching (carefully) a strip of wood against the side of the chattering blade just below the upper guides as it was cutting (don't try this unless you are sure you know what you are doing) suggested that the noise/chatter is definitely coming from the blade - there's a very hard high frequency vibration in it that comes and goes with the noise.

Just for the hell of it i played around with blade speed. The saw has so far been set at the stock 420 m/min recommended for hardwood, i upped it to the 800m/min softwood setting. No real change - the chatter is still there (albeit a little softer) and it cuts faster, but it's not a fix. Those cutting speeds seem quite low by the way compared to industrial bandsaws.

The final trial was with a Scheppach blade, a new 1in x 4tpi as i didn't have the 1/2 x 3tpi i thought i had stashed away somewhere. This is a much more traditional blade than the Tuff - a much thicker and heavier traditional style band. No real improvement on noise unfortunately, very similar to the Tuff at the higher speed. Much slower and (hard to describe) but less 'free' cutting - leaving a finish no better than the much coarser Tuff either.

As ever it'd be great if somebody would appear with the instant set up answer - but I'm reaching the end of the road so far as considering the saw a real option for deep ripping/resawing is concerned - barring the emergence of a silver bullet.

 

[paulm]

I've lost track Ian but have you tried with the back bearings, top and bottom, backed well off of the back of the blade ? Apologies if that's been covered previously!

Cheers, Paul

 

[ondablade]

I tried the 1/2 x 3tpi Tuff Paul, it was pretty much much the same as the 5/8 x 3tpi. They are both excellent blades.

I had a cautious try with the guides backed off Paul, but will try it again. It would clarify whether or not they are causing anything.

 

[9fingers]

Your tests with different thickness blades and at different speeds should help identify resonances.
If the frequency dropped between thick and thin blades and was unchanged with speed then the resonance is probably in the blade above or below the cut or both. If it was a function of speed but not of blade type then I'd suspect bearings as the source. The rear guide uses the face of the bearing and effectively each point on the rim of the bearing travels back and forth across the back edge of the blade rapidly. This could also be the mechanism that excites the blade resonance.

Something also to check is the smoothness of the back guide bearing when not in contact with the blade. It should spin silently with no sense of feeling the balls rotating or grittiness.
They have a hard life and don't last long but are cheap to replace if bought from a bearing supplier (not Sheppach!).

Bob

 

[ondablade]

No luck Paul, it's exactly the same with the back and side guides backed right off so that the blade is positioned solely by the camber on the wheels.

The frequency of the vibration/squeal probably reduced a little Bob with the thicker and wider Scheppach blade in. It's hard to be 100% specific about this as while it there more or less all the time in deep cuts the intensity/volume gets quite a lot louder at times. Like whatever triggers/drives it comes and goes a little.

The guides are solid metal discs on plain bushings rather than ball bearings, but they are all turning smoothly and freely. I'd agree that the back ones look like candidates to kick off the vibration, but the fact that it's still there with the guides backed right out suggests that there are other triggers of whatever mode of vibration it is that the blade is getting into.

Another peculiarity. When switched off the saw has since new for 10 or fifteen seconds after it comes to a halt product a sort of bonk bonk bonk sounding noise from inside the lower casing. Whip the covers open and grab the (stationary) lower wheel in the short period while this is going on and it feels like there is some sort of low frequency decaying pulsing torsional vibration coming through the drive that matches the period of the bonking noise - although once this stops the wheels are perfectly smooth to turn.

I don't think this is mechanical, although it could be. Is it possible Bob that when the motor is switched off that there's something electrical going on - like maybe a decaying electrical oscillation involving the capacitor and motor that results in the motor seeing pulses of current that create pulses of torque?

 

[dickm]

This is what I'm referring to. I can't help feeling that the wiping action of the Basato arrangement is wrong and does cause screeching on mine.

This curious arrangement of the back bearing seems almost universal, but as RogerS says, also seems daft. Does anyone have a logical reason why the bearings are set so that they are taking a lateral thrust rather than the (apparently more sensible) radial one? And why the sideways thrust on the blade as the bearing turns doesn't always set up resonance problems?

(Of course, us Startrite 352 fans don't have these problems - the meehanite tipped rod works fine 8) )

 

[9fingers]

Another peculiarity. When switched off the saw has since new for 10 or fifteen seconds after it comes to a halt product a sort of bonk bonk bonk sounding noise from inside the lower casing. Whip the covers open and grab the (stationary) lower wheel in the short period while this is going on and it feels like there is some sort of low frequency decaying pulsing torsional vibration coming through the drive that matches the period of the bonking noise - although once this stops the wheels are perfectly smooth to turn.

I don't think this is mechanical, although it could be. Is it possible Bob that when the motor is switched off that there's something electrical going on - like maybe a decaying electrical oscillation involving the capacitor and motor that results in the motor seeing pulses of current that create pulses of torque?

My guess is that this is some sort of braking system that is possibly injecting pulses of DC into the motor. I presume this is using a timer (10-15seconds) rather than a more intelligent system that looks to see if the motor has stopped turning.

I suspect that if you stop the machine by switching off at the socket rather than the in built stop button, that this pulsing will not be there and also that the saw might run on for a bit longer than when it is stopped as designed.

Bob

 

[ondablade]

Ta Bob, i'm a bit slow. That sounds most likely. I described it but never twigged what the likely cause was. I can't confirm as all the manual says is that it's CE approved - there's no reference to braking.

I can't answer Roger's point about bearing direction ****, but a guess is that as fitted the wear is spread over the entire face of the thrust wheel/bearing. This as a result of its rotation.

A bearing lined up as a more typical cylindrical roller might develop a grove in the surface that would disturb tracking - as the blade would normally always contact the same circumferential line...

 

[paulm]

I believe Bob maybe right on the braking, my EB planer thicknesser does the same thing too after being switched off. Doubt it is related to any of the other problems, just a curiosity in itself.

Cheers, Paul