Bandsaw Blade Guide theory!

[deema]

My good friend and ‘partner in crime’ Sideways started a thread about the tear down and overhaul of a SCM S45 bandsaw which is presently ongoing, and has led us both down rabbit holes of learning. It’s been a fantastic project that has challenged a number of our preconceived perspectives. If you want to know about spring tension / blade tension theory as well as how the bandsaw is built to name a couple of highlights in the thread you can find it here:

https://www.ukworkshop.co.uk/threads/scm-minimax-s45-bandsaw-teardown-overhaul.135069/
What became apparent, is that there are lots of theories, rumour and speculation about what bandsaw blade guides are for, how they should be set, and what are ‘best’. After a lot of research, I’m going to say, that much of it is……well…….how can I say this tactfully…….OK, as a Tyke where a spade is a shovel……just complete tosh! There I’ve said it. Now, the disclaimer, this is my perspective of the theory that I’ve read, coming into this, I thought I knew what they were used for and how to set them, but I was wrong! I’m sure there are far more knowledgeable experts out there who I hope will pitch in. It would be really useful if any pertinent published theory that anyone is aware of could be added / summarised.

Most of the theory / published research I’ve come across is based upon large resaws rather than small ‘thin’ blade bandsaws most of us use. There is also good resources in metal cutting bandsaws, much of which is relevant to wood cutting, as most of the issues transpose between the two.

This will be a glossary high level summary, there are lots of influences on the performance of a bandsaw.

First off, the UK based HSE have published a good guide on the safe use of bandsaws, which if your new to the machine is well worth a read.
https://www.hse.gov.uk/pubns/wis31.pdf

 

[deema]

If you take a look on Google and type in bandsaw blade guides you will find a lot of articles and threads about which is the best bandsaw blade guides, but little if any on the theory which may explain why one might be better than another. However, one thing all agree upon for the ‘normal’ blade guides, which includes all of the bandsaw blade guide manufactures I’m aware of, is that they should not touch the blade. There is an exception, and these are ceramic blade guides. For now, we will not consider these, but will return to them later.

The point about them not touching the blade is important. In theory, bandsaw blade guides are completely useless! They arnt required, they will do nothing at all. So what have them? Well, firstly, let’s consider a bandsaw which has no blade guides. The blade is fully tensioned and tracking properly for the tyre / wheel configuration of the saw. The blade is properly tracked to a fence. The blade is band new, properly sharpened and has an equal set on the teeth on either side of the blade, so that the kerf width is even either side of the blade. We are running the saw at the correct speed, we have the right tooth pitch (TPI) and tooth geometry for the material, we are cutting. Lastly we start feeding the material, at the correct speed into the blade. What will happen? The material will be cut by the blade perfectly, it will be a straight cut without any incident or issues. This applies to all material thicknesses.

 

[deema]

Bandsaw blade guides serve two purposes, firstly, which I don’t believe is applicable to bandsaws but is highly relevant to resaws, is that they are used to damp down and help prevent blade harmonic oscillations. This is an issue with large resaws where harmonic oscillation can cause lots of problems including poor cut quality, as well as short blade life. Now, there doesn’t seem to be much published research on bandsaws, however, from the literature the conditions that creates blade harmonics does not appear to be very relevant to the relatively small cutting depths of the typical bandsaw. Good, reference sources on resaws for those having difficulty sleeping are:

https://open.library.ubc.ca/media/download/pdf/831/1.0096927/2
https://wfs.swst.org/index.php/wfs/article/download/558/558/0
The other main function of the blade guides is to stop the blade breaking / causing an accident when any of the perfect conditions is not met. This is the main function and reason for them on the typical bandsaw. When the perfect conditions are not achieved, the wood will cause the blade to defect from its natural path, if this isn’t stopped the blade will either come off the wheels, break or jam up solid in the wood if the motor isn’t very powerful. All of which are best avoided.

 

[deema]

The most important influence on getting a bandsaw blade to work / cut properly is blade tension. I think that most bandsaws struggle to achieve the correct blade tension for the full range of blade widths they specify they are able to accommodate. Firstly, they will in all probability be based on carbon steel based blades, rather than anything more exotic. This isn’t only my view, it’s one shared in many published articles, for example:

https://www.finewoodworking.com/2012/11/09/setting-bandsaw-blade-tension
Now, it’s best to not follow the authors views on cranking down the tension on the spring, compression springs should only be compressed to circa 25% of their relaxed length, cranking down as suggested until the coils are nearly touching will in effect destroy the spring. I am not aware of any bandsaw with bump stops to prevent over tensioning of the spring.

Blade tension is vital as it affects two aspects of the blades ability to cut straight, firstly it affects the beam strength of the blade.

Picture taken from here

https://www.lenoxtools.com/Guides/LENOX_20Guide_20to_20Band_20Sawing.pdf
Vertically, the blade should remain straight and at right angles to the table whilst cutting. However, if the blade tension is too low, the blade will bow, the blade guide at the back of the blade will come into contact with the blade if the blade starts to bow and stops the blade being pushed off the wheels. So, when setting the rear blade guide, it should be set so that it isn’t touching the blade, but is as close to the blade as possible to stop the blade moving backwards on the wheels. There are other conditions that cause the blade to be pushed back a few of these are:

1. As the blade dulls, it won’t cut as quickly or as well, to make it cut the user pushes harder on the material to get the teeth to bite and hence pushes the blade back into the rear guide.
2. If the teeth per inch (TPI) of the blade is too high for the material being cut, the gullets of the teeth will fill up before the tooth has exited the material. If the gullet becomes full it actually pushes the material away from the tooth stopping it cutting. This in turn pushes the blade backwards as it causes the blade to bend.
3. If the blade touches a blade guide it will create friction. The friction causes the blade to heat up and as a consequence it will increase its length. This reduces the tension on the blade and causes it to bend and move backwards. This is why the rear bkade guides should not touch the blade when being set.

Beam strength is increased by using a wider blade, the thicker the material being cut, the wider the blade should be. So for example, cutting up to 50mm material can be done with say a 12mm word blade, but material of say 100mm thickness would be better cut by the widest blade the saw can take. Equally the blade guides should be set to minimise the length of blade that can be unsupported, as this reduces the amount of bow that can be induced in the blade whilst cutting.

The rear guide therefore is there to stop the blade being pushed backward. It needs to be hard wearing, solid and supportive and create as little friction as possible to stop the blade heating up.

 

[Ttrees]

I'd sooner the table be adjusted to suit the blade, and not the other way round.

My guides are a bit sloppy from welding them, and side rollers are really just a safety thing, which may have to be sorted if I ever need cut at max capacity ripping straight.
I can see the need for them when cutting deep curves, so obviously have a place on some machines with suitable tires, or if one wanted to dress or install new tires for that.


The thrust guide is necessary on all, as it stops the blade from walking off the wheels/keeps it on the grippy section of the wheel, i.e teeth off the wheels on flat tires,
so it will return sooner to that position rather than the blade deflecting
and possibly getting damaged in the process.

Carbide tipped blades are becoming more and more affordable, and available in all sizes and guages from the far East, fair bit cheaper than the M42 blades for instance,
if you know where to look.
The performance of these might make it possible to actually make use of the side rollers
when ripping/resawing, rather than a blade which is on the way out, and needing to be changed.

Type in "Chinese carbide bandsaw blades" on sawmill creek, and you may find reference to a fella called Tai Fu mentioning Taobao and Ali express.
You can get a good look at them, even if you're not a "creeker" can still see some pics.
Took me ages to find that, but I wanted the link for again, and glad I found it.


Tom

 

[deema]

When the blade isn’t tensioned up properly it’s ability to resist deflecting sideways is reduced, a slack blade has the ability to twist far easier than a properly tensioned blade. All of the conditions that cause the blade to move back into the rear blade guide can also cause the blade to twist. There is also another two main causes of blade twist:

1. The blade is not properly aligned to the fence, the blade cuts either away from or into the fence. By pushing the material forward against the fence it effectively causes twist in the blade.

2. In curve work, where an arc is being cut, it’s vital to select a blade that can cut that radius of the curve that is desired.

Table from

https://www.tuffsaws.co.uk/index.php?route=information/information&information_id=13

Picture from

https://suncatcherstudio.com/woodworking/bandsaw-blades/
A blade cuts a kerf, or slot in the material that is wider than the main body of the blade. The radius a blade can cut is limited by how much the blade body can turn within the kerf. The wider the blade, the larger the minimum radius is can cut. If you try to cut a radius tighter than the blade is capable for, the back edge of the blade will come into contact with the material and start to twist the blade.

The side blade guides stop the blade twisting too much / helps to resist the blade twist. If the blade twists too much it will break, which is exciting and does happen. It’s a good reason to never stand at the side of a bandsaw, as when it occurs the blade can come out at you at a rapid rate of knots!

Again, having the blade guides adjusted to minimise the distance between them for the material being cut reduces the amount of twist that is induced into the blade / increases the resistance to twist.

Side blade guides need the same characteristics as rear blade guides for the same reasons. They should be set as close to the blade as possible without touching, they should also be set to be just behind the teeth, as that way they provide the most support to the blade when they do come into contact with it.

 

[deema]

So, although in theory blade guides are not required, in real life, as blades dull and we are all human, they stop accidents and allow for human error. If the blade is touching a guide, something isn’t right, and you need to take a look at your setup or what you are doing.

Coming back to ceramic blade guides, ceramic blade guides probably offer the best combination of properties. They have very little resistance, so don’t heat up the blade very much and therefore are often recommended to be set in contact with the blade. This provides the maximum support of the blade and makes it cutter straighter for longer than other blade guide types.

An example of such a guide is:
https://info.lagunatools.com/setting-ceramic-guides-on-the-1412-bandsaw
However, for most users, where they are not using the bandsaw for hours at a time, each and everyday, ceramic blade guides are not necessary (but nice!). Most top end bandsaws offer GL guides as well as Ceramic, ceramic being a relatively new entrant. A range of guides can be seen here.

https://www.scosarg.com/accessories...roduct_list_dir=desc&product_list_order=price
Setup of this type of guide is applicable to any guide, and a good summary is given by Panhans (Big thank you to Paul at Scott Sargeant for his input)



These are very hard wearing and by their use in professional grade machines would suggest the next best to ceramic guides.

The more occasional use bandsaw, or hobby grade tend to use ball bearing based guides, the side bearings designed to rotate when they come into contact with the blade reducing friction. However, the rear bearing often wears quickly as it’s being used in a configuration that does not suit bearings.

Old bandsaws. / resaws before modern bearing guides would typically use lignum vitae wood held in an adjustable holder as their bearing guides. Lignum vitae is now cities listed, so no longer available (apart from in old bowling balls!) was an great blade guide. A naturally slippery material it induced minimum heat. Very hard wearing and at the time cheap and easy to make new ones. These days, if anyone has this type, an old bowling ball can supply enough material for their guides to last a lifetime.

 

[deema]

So, in summary, blade guides in theory aren't necessary or needed. However, because we are all human they do play an important part in bandsaw safety.

Blade guides do not make the blade cut straight, that’s caused by other factors, what they do is limit the amount it can bend out of alignment which is created by other factors not being correct, they are gate keepers, limit switches, when everything is setup correctly they are not involved in cutting.

With the exception of ceramic blade guides, set them as close to the blade as possible, but don’t loose too much sleep over the last micron!

Set the blade guides as close to the material height as possible, this will make the cut better.

The most important influence on the blades ability to cut is blade tension and not the blade guides. Buy the stiffest bandsaw you can afford rather than investing in blade guides unless your using the bandsaw for hours every day.

Buy a high quality blade, and set it up correctly, this will again have a larger influence on the cut than blade guides.

Lastly, blade guides, all will work the key for me in choosing one machine over another would be how easy they are to adjust and stay set. Personally I would avoid ball bearing guides, only because bearings are not designed for this application. I would not upgrade the old Lignum vitae blade guides if my saw had them…..just buy an old bowling ball to get a source of the correct material.

 

[Sideways]

All this because I asked Deema to take some photos of the lower guides of our current SCM s45 project so that I could answer a couple of the questions asked !
Nice one

My experience also is that the bandsaw should be set to cut correctly with no involvement of the blade guides, If you set your saw correctly and take it easy it absolutely works. Try it with the guides pulled back and see how the blade moves backwards a little on the upper wheel as it starts to bite then settles and cuts perfectly well as long as you don't push too hard.

This means that the first rule of tracking a blade is to wind all the guides back out of the way as they have no part in the process.
Then track the blade.
Then bring the guides up to the blade at the end.

I have also seen several bandsaw blades ruined by inexperienced users trying to cut curves that are too tight for the blade fitted. The side guides are not there to let you cut overly tight radii with the wrong blade. The table and radius chart are handy references.

Likewise many narrow blades snapped by users trying to cut thick timber that is too thick with blades that have fine tooth pitch, small gullets between teeth and have no chance of clearing the waste out of the cut. This is especially bad with the longer wood fibres created when when rip cutting.
Ripping thicker timber needs a low tooth count and large clearance between teeth. It's exactly the same as needing a ripping blade on a tablesaw. Unless you only cut thin stock, a wide 3tpi blade is one of the most useful bandsaw blades.

 

[Hornbeam]

I think it depends on what process you are doing. If you are using the bandsaw to cut say tennons and you dont set the rear guide tight to the back of the blade (after tensioning) then as you cut the tennon the blade will move back slightly. As you come to the end of the cut and you ease off the pressure, then the blade will track forwards slightly resulting in an overshoot of your stop line

 

[hlvd]

In my experience if you don’t set the back bearing correctly the blade won’t cut very well.

I’ll push the untightened bearing tight against the blade, start up the machine and let the blade push the bearing as far back as it can.
Once the bearing stops contact and spinning I’ll stop the machine and tighten up the Allen bolt.

Works perfectly every time.

 

[RobinBHM]

in my experience its best to avoid doing curved work with a blade if you want to then cut straight

 

[hlvd]

in my experience its best to avoid doing curved work with a blade if you want to then cut straight

In an ideal world yes, in a busy workshop no.

 

[Sideways]

As you come to the end of the cut and you ease off the pressure, then the blade will track forwards slightly resulting in an overshoot of your stop line

But this is totally predictable. It's how the blade naturally tracks on the tire and the harder you feed, the more it moves. To avoid oversteer on a narrowboat, you return the rudder to centre before you complete the turn. You can easily back off the feed pressure on your bandsaw before you reach the line to allow for this ?

 

[Spectric]

Try it with the guides pulled back and see how the blade moves backwards a little on the upper wheel as it starts to bite then settles and cuts perfectly well as long as you don't push too hard.

Would you not say that the upper rear guide is probably the most important because without it you could in effect slowly push the blade of the wheel, similar to how you remove a bike tyre.

 

[deema]

in my experience its best to avoid doing curved work with a blade if you want to then cut straight

That’s an interesting experience, what problems do you encounter. I have to admit, I use the same blade for both and as long as I haven’t distorted a thin blade (usually overheating it) by cutting too tight a radius and forcing it (no, nobody ever does that….right!) I’ve not noticed any issues with it cutting straight.

 

[deema]

Would you not say that the upper rear guide is probably the most important because without it you could in effect slowly push the blade of the wheel, similar to how you remove a bike tyre.

Both rear guides are vital, you can push the blade of either wheel.

 

[LJM]

The set up that you‘re recommending is exactly how I’ve always done it, having been taught to do so by my father; he’s done it like that since he was first taught to use one, decades ago. This goes to show that whilst there’s a plethora of information at our fingertips, a great deal of it is, at best, reinvention of the wheel for the sake of it, or to get views on utube! And, moreover, that you’ll probably find that the old guy who paid attention at school, may well know a little more than that enthusiastic youth with a social media profile, but no work to show for it.

 

[Peter Sefton]

in my experience its best to avoid doing curved work with a blade if you want to then cut straight

If you do a lot of bowl cutting or similar it will stress/wear one side of the teeth more and you may then struggle to cut straight lines from the fence on ripping work.

 

[Sideways]

Would you not say that the upper rear guide is probably the most important because without it you could in effect slowly push the blade of the wheel, similar to how you remove a bike tyre.

That's my impression, though it may simply be that the lower guides are out of sight, out of mind, and are more often neglected.
I think that both sets matter and when tracking a blade I loosen off both sets and reposition all 6 guides to the blade afterwards.
Hlvd's method of letting the blade push the rear guide to a natural position is super easy and I tend to do that. It also works to just take a light cut which pushes the rear guides a little further back before stopping and locking everything down.