Breaking bandsaw blades

This is a vertical bandsaw mainly for woodworking but the setup applies to many bandsaws worth watching.

Could it be that the blade is too course for the job ?

The standard or common pitch for these saws over here seems to be 14tpi and this works well on my Chinese cheapy.

I find that Bi Metal blades perform much better for the extra cost. And yes, I keep the blade tensioned all the time.

I seem to remember hearing somewhere that the biggest cause of breaking blades was over-tensioning them.

Ians comment about over tensioning reminds me that my old style, UK built, 14" throat Startrite vertical bandsaw requires a noticeably lower tension on 1/2" blades than the Alpine 6" capacity horizontal / vertical saw. The Startrite uses a spring to apply tension via a big knob on a screw with a nifty viewing window showing the size of blade that tension is appropriate for. Much nicer than the "heave it up and hope" system used on the Alpine.

Was told that the small horizontal vertical saws had to have the blade over-tensioned to work correctly due to the combination of small, relatively closely spaced wheels and savage twist between wheel and guide. Allegedly you want wheel diameters approaching 30 times blade size on centre spacing approaching 45 to 50 times blade size if a smallish two wheel saw its to work really well. I imagine it all gets easier once you have enough wheel size and spacing so larger saws have different rules.

Wish I'd known about Brians wire trick 30 odd years back. Would have made setting mine up so much easier. As I recall matters the killer wit mine was one wheel twisting a bit when fully tensioned so the flange kicked the blade out of line as it came off the wheel. Which took a deal of sorting to generate appropriate clearances and blade support once I'd figured the problem.

Clive

Edited By Clive Foster on 03/12/2019 13:26:34

Guys, thanks for all the advice.

Brian, stretching wire around wheels is genius. My rear guide was well out of alignment, probably 4 or 5mm. I had not realised the shafts holding my guide bearings are eccentric but this adjustment was nothing like enough on the rear. But I was able to move the guide holder over as it has a vertical slot to allow vertical adjustment and its considerably larger than the bolts holding it.

Why is wire better than a blade for alignment? The blade gets turned through roughly 45degrees from the wheels to make it run vertical. This makes it hard to check alignment. Using wire it sits against the flange on the wheels and you can easily line up the bearings. Great trick.

I'll see how long this new blade lasts but will definitely be looking to source new ones from suppliers suggested.

Thanks again everyone

Dave

Edited By Oxymoron on 03/12/2019 14:49:30

Edited By Oxymoron on 03/12/2019 14:50:40

Brian Wood

I, too, would welcome elaboration of the stretched wire method.

Hello Doug and ega,

I'd be happy to. The wire is fitted against the wheel flanges and when pulled taut it provides a true reference for the track a blade should be allowed to take without being twisted about or otherwise misaligned when running through between the guide blocks. It is a great boon to be able to adjust the blade guides correctly to just touch it where that is needed

That sets the lower support in the right place for the blade, the first important setting. Now, by looking down the path it takes between the guide blocks you can easily see if the track is correct and is not twisted or otherwise misaligned to throw the blade off the rim of the wheel at the point when it runs out of the guide.

Twisting by the second guide is especially testing since that block is close to the wheel that collects the blade and that sort of action on the side of the blade will fatigue any joint, especially in the vulnerable heat affected zone either side of a blade weld

The guide blocks on my Axminster bandsaw were very badly made, just cast surfaces bolted together which were so easily moved. I had to take all that guidance system to pieces and mill proper mating surfaces into them so that they could then be trusted to guide the blade rather than try to force it through like a chicane on a motor circuit.

After those improvements I have not had one blade break since in perhaps 8 years now

I hope that explains what I did in enough detail to be helpful

Regards Brian

Another edit. Well spotted oxymoron, the wire is not influenced by the side forces that a blade is subjected to so all those competing effects are eliminated. 

Edited By Brian Wood on 03/12/2019 15:18:09

Edited By Brian Wood on 03/12/2019 15:21:58

Blades breaking can be attributed from my experience with the type of machines under discussion to the following.

Blade alignment, which is very well covered above.

Failure to anneal the blades after welding. The easiest way to resolve this is to anneal any new blade. This being done after placing on the machine but without any tension. The small hand held butane torch is ideal for this. Trying to do this off the machine usually allows the blade to kink.

Getting the correct tension, this I have found greatly extends the blade life. I have to thank Jacques Maurel for this. He wrote an article in EiM in June 2016 on making a bandsaw tension gauge. Before using this device I used to get about 12 months usage out of each blade, if I was lucky. The current blade has been on the machine 2 years. Although now that I have said that Sod's Law dictates it will break at the next usage.

Hope this helps.

Regards

Gray,

Graham, annealing the blade on the saw is another good idea. Will do that with my new blade later.

Brian, the wire idea is so logical, when you do it and makes truing the guides so much easier. Once I'd removed my guides I could see the contact area on one side bearing showing where the misalignment was. Then replacing the guide with the wire in place was so easy. I'd just trusted the setup as the machine was delivered, stupid mistake.

So pleased I asked the question, some great tips gained.

Dave

Hello Oxymoron,

It took me over 5 years of frustration and breakage costs before I thought of that idea so I am glad to be able to pass it on. Thank you for the endorsements

Regards Brian

Brian

Thank you very much for explaining the wire method of verifying guide and wheel alignment.

From experience with my saw, admittedly an earlier one and less well made than most current suppliers offer, two further points are worth checking.

Firstly wheel flange thickness and truth relative to the surface the blade runs on. The flanges on mine are neither constant thickness nor properly true to the main wheel circumference. The combination wobble makes it remarkably difficult to see what is actually going on. Mine needs a small, but rotationally variable, gap between the back of the blade and the flange to ensure that a combined high spot doesn't launch the blade out of line once pre revolution.

Secondly all the clearances on mine are very slack allowing things to move around as tension is applied to the blade so best alignment can be lost during the tensioning process. Worse it doesn't always settle back properly after changing the blade so alignment can be lost during initial running. Fortunately such loss of alignment is fairly easy to hear and see if you make a point of looking out for it. Frustrating but fixable.

I really should bite the bullet and re-machine the thing but I use it so rarely these days as I have a Rapidor power hacksaw for stock cutting whilst the Startrite vertical handles most other things, that it hardly seems worth the effort.

Clive

Some years ago there was an article either in ME or MEW regarding making a simple angle iron jig to hold a broken band saw blade, ready for Silver soldering it together again. I made one, together with another simple home brew device/jig which ensures both bevel edges on the broken blades are ground to the same angle on both ends of the blade, prior to Silver soldering. This little gadget slides on the side of the plate of my belt sander, I push it and the blade forward and the belt forms the angle on the blade in which the Silver solder will be when heated. I find it easy to add extra bits as and when required. Don't discard those broken blades. John

I've heard the opposite

Neil

Neil and Ian, don't worry too much it's only hearsay.

Emgee

Oh dear, Neil is being pseudo provocative again.

Both under-tensioning and over-tensioning blades leads to premature breakage. Right tensioning is needed for longest life.

Depends on the design of the saw as to which is worst. Many saws won't drive well enough to break the blade if seriously under-tensioned.

The common small horizontal / vertical saw will quite happily break blades when under-tensioned because the blade has to have more than the, objectively, correct tension to work well. So under-tensioning enough to let it judder around, partially jam et al will still drive it well enough to break the blade. Especially if too fine a tooth count is used on larger jobs making the thing very prone to jam up due to filled blade gullets half way through the cut line.

The brutal blade twist between wheel and guide really doesn't help nor does the seriously sketchy blade downfeed pressure controls.

Superb value for money though they are these saws are a mass of contradictions and compromises that need carefully informed use and setting up to get the best out of. Which unfortunately they don' t get. Perfect performance out of the box is theoretically possible but who in the intended (guy or gal in a shed) market sector can afford it!

Clive

Edited By Clive Foster on 03/12/2019 19:28:11

have not bothered to read all the previous posts!

My 4 1/2 " bandsaw used to break blades with alarming frequency.

1. Made sure that everything was aligned, guide rollers, tracking etc etc.

2 Found that the driving and driven pulleys were running out. Removed and cleaned up in the lathe.

3 Saw an article in Engineering in Miniature about a Blade tensioning "meter"

The meter consists of a frame with a fixed leg which is clamped to the bandsaw blade. The other leg also clamps to the blade, but pivots on the frame of the meter, and drives a DTI. The article went into the maths of it, Blade material strength / stress etc and advised a small range of extensions to provide the correct tension.

So the DTI is Zeroed with the blade untensioned, and the tension increased until the extension lies within the "acceptable" range.

On the first trial, the saw cut a disc about 1.5mm thick whrere the thickness differed by by only 0.025mm

I am not suggesting that this is consistent level of perfromance!

But I cannot remember when I last had to replace a broken blade. They seem to last longer, and wear out.

Maybe I've been lucky!

HTH

Howard

Thanks for your explanation about the wire Brian, that all makes perfect sense so I must find a length of wire and have a go with my machine.

I was watching a youtube video about silver soldering a broken blade recently and the chap did not scarf the joint but simply cut the ends carefully then fluxed and soldered them. The result seemed to be fine but I did wonder how strong silver solder is over such a small area. I may give it a go to see if it's just as good as a scarfed joint. It would be easier to prepare the ends and might help prevent thickening of the joint after soldering.

Doug

Posted by john fletcher 1 on 03/12/2019 17:24:04:

Brian Wood:

Thanks for taking the trouble to elaborate.