Another band saw blade snapped – Have I got the tension wrong.

Hello All

I keep snapping band saw blades like there is no tomorrow – I’m worried I’m not getting the tension on the blade correct. I would be grateful if those in the know would take a quick look at the you tube video below and let me know how far off the mark I am.

Thanks

David

Might be a bit loose. I was given the advice 'you won't be able to turn that knob enough by hand to make it too tight',

My blade definitely makes a higher note than that when twanged.

Advice is "Pluck return side of blade. Tension blade so it will give a clear musical note. (no buzz)"

You have a PM.

Neil

Edited By Neil Wyatt on 01/11/2015 20:39:28

Thanks Neil

As in tends to snap so often I also happened to be videoing it when it happened – I notice each time the blade weld runs past 'something' the band saw gives a little jump. Although I’m not sure what that tells me!

David

Thanks Neil

As in tends to snap so often I also happened to be videoing it when it happened – I notice each time the blade weld runs past 'something' the band saw gives a little jump. Although I’m not sure what that tells me!

David

Could be you have some blades with poor joints, my experience has been with the same type of saw that if the blade cut square and didn't slip it must have enough tension.

Emgee

Just read your last post and the jump is a bad joint in the band, 

Edited By Emgee on 01/11/2015 20:54:13

Hi

If it jumps at the weld it suggests the blades you are using are poor quality. The weld should be as smooth as the rest of the blade.

If the blade breaks at the weld,most suppliers will give a free replacement

Regards

Keith

Leeds UK

David,

I have the same bandsaw.....I tension the blade as Neil suggests....hard!

As for blade breakage, I have bought from suppliers and even bought longer ones to shorten.Those that I have shortened, I have silver soldered....of the bought ones and those I've altered, none have snapped at the joint.

The breakage always occurs either side...this suggests to me that failure is due to fatigue....my thoughts for whatever they're worth....

I always tension my bandsaw so much I'm frightened of the frame breaking but good quality bi metal bands certainly last well

as said if it jumps its a bad or missing tooth

In 25 years with the saw i've only broken 2 blades at the weld and none at elsewhere...just worn out perhaps 200 blades in all

I think the worst job is cutting sheet material it seems to catch at the blade join try and keep 3 teeth in the job at any one time

Bigger stuff cuts well right up to the 4inch capacity with my prefered starrett blades i can cut 100mm MS in perhaps 2 minutes

Thanks all – I’ll get a new blade and try with considerably more tension.

David

I'd start by going over the blade with a micrometer at a few points and compare with the joint. If the joint is significantly larger, I'd try adjust the pinch bearings so its not too tight on the joint, but not too loose everywhere else, failing that, I'd rub it down a bit. Ideally (in my mind anyway) the bearings should *just* kiss the blade, enough to eliminate excessive movement but not tight by any means.

I'd cut the same piece of bar (but with a bit less blade tension, only a bit less so the blade slips rather than jams) and when it starts jumping in the cut turn off the machine, make it safe ,then by hand work out exactly where that snag is. I'd personally start by running the blade joint through both guides and checking the bearings are all able to just about slip round and don't have any tight spots.

I'd also check the left side 3rd bearing, I thought I could see (although not every the time) the blade moving up/down when it jumps and that 3rd bearing should stop that. (it could be that its bouncing off the other way)

If you view the video in 1/4 speed you can see the machine force the blade into the material, this is a sudden vicious cycle, the more it jams because of that, the more it forces the blade in... and so on. It pivots about the 3rd bearing on the right side nearest the drive wheel and snaps it at the point. - Each time the weld is already through the material when the jump happens so perhaps its the vicious cycle rather than a one off force.

And before starting that, it could be worth checking with nothing in the machine, that the blade is perpendicular to the base of its vice (I has some real issues adjusting a friends saw from axminster as the blade guides were not aligned, they had some freeplay but both were at their opposite extremes just to get it somewhere near. 

Apologies if I'm telling you how to suck eggs.

Edited By Russ B on 02/11/2015 00:27:48

I had a similar problem but maybe not as bad as yours and i was getting about twenty cuts to a blade on 50x50x2RHS with the standard type blade - starrett brand , approx double this with a bi- metal blade from a local manufacturer.

I stripped the saw down and gave it a good going through and it has now been about five years and i have replaced the bi-metal blade once due to it eventually going blunt after how many cuts i know not but it would be over a 100 for certain.

So what did i find was wrong with the saw during the rebuild ?

1: the idler wheel did not run true on its axle radially or axially and the bush was a very loose fit on the axle

2: both wheels were tapered where the blade runs

3: the idler wheel was not on the same plane as the drive wheel -it actually sat high by around a millimeter .

4: the drive shaft and the idler wheel shaft were not parrallel , this can be adjusted easily on my saw with the jacking bolts on the mounting bracket.

Idler wheel copped a set of bearings and skim off the OD, drive wheel was set up on a mandrel and the OD skimmed to true it up.

I shimmed the drive wheel and adjusted the idler wheel axle so a straight edge would contact the outer rims of both wheels , the straight edge is layed across both wheels as close to the centre as possible (basically the same way you lighn up vee pulleys or sprockets etc) .

Back off all the guide rollers and set the blade up so it just about touches the back lip on the idler and drive wheel , you just want it to have a few thousandths clearance and not trying to chug its way through it, run it under power to let the blade settle on the idler & drive wheels .

Set up your guide rollers so they just touch the blade , With the power off and the saw unplugged grab a roller between two fingers in you should be able to get it to slip and rotate with a little resistance - if they are too tight you will be swageing the blade between the side rollers which will kill both the blade and roller in short order.

The rollers that ride on the blades spine are there to stop the blade bending like a bannana so if theyre too tight they will fight against the drive& idler wheels as they control the blade tracking not the rear guide bearings , if these bearings force the blade forward it will not track correctly on the drive/ idler wheel and the usuall incorrect fix is to adjust the idler wheel tracking adjustment .

All this will do is force the blade to bow and break, so wheel tracking first then set the guide rollers up

Don't use too much down feed pressure its just more stress on the blade and i usuall find the cut will wander all over the place.

Neil is correct that pokey little knob for blade tension will probably break before you over tension the blade but if the saw is going to sit unused for a while you should back off the tension and i usually put a tag on the switch to remind myself that i have done this .

There have been a few articles in MEW about these saws with some excellent modifications - may be worth checking them out.

Ian

Edited By XD 351 on 02/11/2015 03:22:40

Hello David,

Check the blade guide block alignments, particularly the one that feeds the blade back onto the drive wheel.

Tension a wire between the two wheels and then refit the blocks to see if poor seating in their mountings puts a twist into the blade as it runs through. I had a lot of trouble with broken blades until I put that right.

Regards

Brian

The instruction manual for my saw says tighten the blade, then pluck it until it makes a musical note (on the saw frame casting there is a letter B, wonder if that's the note). My current BI metal blade has a bad weld, and has a quite bad bump each rev.

Ian S C

Hi David

I rather like these blades, in particular the welding method, Not all saw distributors will have the equipment the do electron beam welding so check first. The cost more but they last.

**LINK**

When I first fit a blade I loosen the tension a bit and set a gentle feed for the first few cuts this allows the teeth points to loose any burrs and round off and initial sharp edges.

Some saw shops are not that careful about joining, that happened to me a while back.... I used my feet.

Regards
John

what make of blades are they

Thanks for a very comprehensive set of answers everyone – hopefully I should be able to improve things. What I didn’t say in my original post was that I was using an 18TPI tooth blade, and it’s been helpfully pointed out to me by the suppler that this is a little too fine for 3 inch steel. I’ve ordered some 14 TPI and will try these as well.

If you don’t mind I won’t say who the blades are from. I think it’s more likely that fault is with either my band saw or the wrong type of blade (or me!). The blades are from a trusted and reputable source and I think it would be unfair to cast doubt upon them when they are probably not the issue.

David

Edited By David Cambridge on 02/11/2015 12:31:54

to fine there having a laugh try a better blade

If it jumps, very careful and gentle grinding of the high point can smooth out the transition and reduce the shock.

You only need a very small mis-alignment to cause a jump.

Neil

Hello again David,

10 tpi is correct for steel on these small bandsaws, certainly in that section, even 14 will be too fine and I now suspect clogging is partly to blame for your failures.

Blade speed might also play a part, I run mine at the top range of the three pulley speeds available, the 64 1/2 inch blade gets round in about 1.5 seconds I tried the lower speeds but it was all much too slow. Oddly, I found coolant only helped the blade off the wheel rims, perhaps because it helped swarf stick to the blade and get carried through onto the drive wheel rim. That however was before I re-aligned the blade guides as I described earlier.

I still cut dry though as my saw is on probation at the moment and has yet to convince me it has become a good boy.

Brian

I too had endless problems with blades for my Warco CY90 clone . Tried blades from a variety of suppliers, all with the same result . Finally I spoke to Tuffsaws ( No connection other than a very satisfied customer ).

Having supplied me with thinner hobby blades for my 3 wheel saw and solved that blade problem I measured the wheel diameter of both saws. The CY90 was larger but not by much. Thinner blades for the CY90 have solved the problem . (Thinner blade fitted in March and still going strong.)

Brian