SCM Minimax S45 Bandsaw Teardown & Overhaul

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deema

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We just ran out of time, there is a thrust bearing on order that we need to finalise the assembly of the system. That’s due any day now. Sideways will write up with photos what has been done.
 

Sideways

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Teaser indeed!

I am eager to hear and see how your new mechanism works (unless you're waiting to submit a patent first!).
It does involve surgery !
The old internal brackets need to be cut and ground out. New ones milled tapped and welded in place. Not a job for the faint hearted :)
But some of the methods are just like you would apply with a wood saw, just using an angle grinder and cutting discs....
20220221_142253.jpg
 

Sideways

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We had three objectives for our redesign of the S45's tension mechanism.
1. To achieve the tension recommended for the widest (20mm) blade that the saw is specified for when using M42 blades, nor just carbon steel.
2. To be robust, easy to use and to add in stops to prevent over compression of the springs and the damage that results from this.
3. To keep as close as we could to the original look and feel of the saw.

It's time to let you all decide if we succeeded :)

As a reminder, here is the two part pressed steel assembly that carries the upper wheel, tilts it for tracking and allows 30 to 40mm of vertical movement. In this image, the two parts have been opened like a book so that you can see the inside. Here the narrower fixed part is on the Left, the moving front part on the R.
20220125_151501.jpg


Here's the modified assembly for the upper wheel laid out like an exploded diagram:
IMG-20220226-WA0003.jpg


We have left to right:
The original tension knob
A new alloy disc with a groove cut and black filled for reading the tension
A new clear tube on which the tension scale will be applied
The original steel base of the tension scale
A new 10mm threaded rod and nuts
A new ball bearing thrust race to reduce friction when tightening and slackening the tension (boxed)
A new floating plate with guides for the outer pair of springs
A brass sleeve for the centre spring
Three new springs. These are part G25-089 from Berger tools. 25mm OD, 12mm ID, 89mm long and the G represents a specific family of heavy duty springs made from rectangular section wire something like 4×5mm cross section. These springs are conveniently colour coded. There are a few families of less strong springs and just one heavier grade but the yellow ones were a nice fit for the job. They cost less than £5 apiece. 👍
Modified original backplate.

Here are the new springs installed in the back plate (this time on the right), and the modified front plate (left).
IMG-20220226-WA0008.jpg

On the L, the welded bracket was originally higher up. It has been cut out, ground back and a new bracket milled, tapped and welded in place lower down.

On the R, three springs sit on a new welded in shelf.
A floating bridge piece rests on top of them.
Both of these plates have an oversize hole in the centre for the tension rod to pass through.
A brass sleeve protects the middle spring from the threads.
Guide rods keep the two outer springs in position. These springs are 89mm long so to limit compression of the springs to 25% max, the guide rods project 89-22=67mm.

Two nuts are locked together to form a fixed stop on the tension rod. When the springs are compressed, the bridge piece pushes upwards, force passes through the thrust bearing onto the stop and pushes the rod upwards. When all is assembled, it's this upward force on the rod that pulls on the upper wheel and tensions the blade.

IMG-20220226-WA0004.jpg



To assemble the bracket, the upper wheel is first refitted to the front part of the bracket, this is then placed over the back part and the tension rod screwed into the lifting tab to take it's weight.
A cross pin fits through two slots that allow about 40mm of vertical adjustment
This is secured with washers and circlips at each end.

At this point the upper wheel assembly can be bolted into the saw.

To finish off, the components for the tension indicator are threaded and screwed onto the tension rod.
IMG-20220226-WA0005.jpg


The black line provides a definite index mark for reading the scale. Previously there was just a thick washer and no information about whether you should take the top, middle or bottom edge of the washer as the "pointer". SCM told us that we should use the bottom edge of the washer to read the tension and avaoid over compressing the original single spring, but that isn't explained in the instructions ...

As we have changed the springs, the old scale isn't relevant anymore. I'll draw up a new one based on the combined spring rate and this will be printed on clear film and applied around the clear cup.
By keeping to a 10mm tension rod, the original knob screws neatly on top :)

A comment on the finish. Obviously grinding and welding on the metalwork affects the plating applied to these parts in manufacture. We can't practically replicate that but we do want to protect the metal afterwards. The brackets here have been degreased, given two coats of high zinc content cold galvanising spray and sealed with silver aluminium.
 
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Sideways

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Deema's rather pleased with his work here. Very neat :)
The index ring screws on the tension rod, is secured with a grubscrew and a brass plug to avoid damage to the thread. It is sized such that the stem of the tension knob sits neatly within the clear tube and doesn't go to far down as you tighten up.

IMG-20220224-WA0001.jpg
 

Sideways

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In case you DO want to try this at home, here are the important dimensions.
This assembly is actually more complex than it looks because
1. The assembly allows tilt of the bandsaw wheel.
2. The tension rod is not parallel with the brackets because of this and it's angle changes depending both on the amount the springs are compressed and the tracking.

These mean that the threaded hole in the front bracket can't be drilled on centre, and the clearance holes in the rear bracket need to be oversize.

IMG-20220226-WA0009.jpg
IMG-20220226-WA0011.jpg
IMG-20220226-WA0010.jpg
IMG-20220226-WA0007.jpg


Just a note here that the bright zinc plated thread above turns out not to be good enough for the task. As the thread is turned under load to adjust tension, the zinc plate isn't robust enough and gets rubbed off the threads. Our local engineers merchant helped out with some better quality, self colour (aka black) threaded rod. This meets the same specification as steel nuts and bolts stamped with the 8.8 tensile strength code.
In later posts, if you look at the tension knob, you'll notice the thread has changed to black.

IMG-20220226-WA0013.jpg
IMG-20220226-WA0012.jpg
IMG-20220226-WA0015.jpg
IMG-20220226-WA0014.jpg
 
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Sideways

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This is the Dunlop thrust bearing we added. It makes it far easier to turn the tension knob. Essential given that the saw can now properly tension a 20mm M42 blade.
A ball bearing type is better than needle rollers for this application.

IMG-20220224-WA0002.jpg


Note the static load rating is 14kN. Our triple spring design maxes out at 22mm compression x 110N/mm x 3 springs = 7,260N so the bearing is more than up to the job.

Similarly, as we are re engineering the mechanism, we need to check that we are safe staying with M10 for the threaded tension rod. Preferable so that we can reuse the top knob and other parts.
One way to judge this is using the recommended tightening torque
IMG-20220225-WA0001.jpg


For am M10 thread in standard 8.8 grade steel. Max torque tightening up a nut or bolt is 57Nm.
I have no idea what a Newton feels like but close enough the weight of a 1Kg bag of sugar = 10Newtons and I can picture that.
So 5.7Kg (force) at 1 metre is about 28Kg at the end of a 200mm long spanner. 25Kg is a very heavy bag when you have to heave it onto the scales at airline check in ! Hand tightening a 100mm diameter knob on top of a bandsaw, this is equivalent to over 50Kg of force. I would judge this sufficient for any reasonable expected use.

For a final check, the yield strength of 8.8 grade threaded fasteners in the 10mm range is given as 640 MPa
10mm rod: assume a worst case 2mm deep thread, giving a minimum cross sectional area of Pi x 6mm^2 / 4 = 2.8 x 10^-5 square metres
x 640 × 10^6 Pa (1 Pa = 1 Newton / square metre)
= 17,920 N More than double the maximum force that the springs can produce :)
 
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Sideways

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Does it work ?
Thanks to the loan of Ian's tension gauge, we can see the modifications pay off in a real, usable blade tension .
Here is a new, premium carbon blade bought for the saw tensioned to just under 19,000 psi with plenty of headroom left in the springs

IMG-20220226-WA0006.jpg
 

deema

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The thrust bearing chosen has a static and dynamic load bearing capacity of at least double the force that will be exerted by the springs. The number of rotations it will do tightening and slackening the springs means that it should have a life of multiple generations of users.

The threaded rod was replaced with one that is grade 8.8, again to ensure that the thread loading is well within its capacity. If too lower grade of threaded rod is used there is a danger that the threads will strip / gaul up and destroy themselves. Grade 8.8 has a proof strength of 580KN more than adequate!

DC78EE6C-A6AF-4DD5-B1DC-7B2BC4342977.jpeg
 

Sideways

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We decided that the top needed to be tidied up rather than just reusing original parts:
20220227_155253.jpg


Two tuffsaws premium carbon blades were fitted, tensioned and tracked.
17,000 psi is a a good high tension for carbon blades. On our scale, these two blades (12mm and 20mm) reached tension at 8mm and 15mm.
20220227_161726.jpg


12mm blade
20220227_161143.jpg


20mm blade
20220227_161739.jpg
 

Ttrees

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@Sideways thanks for making the video of the ping test.

It sounds like the blade is hitting off a guide or something, certainly not as musical as I was hoping for, can you clarify?
i.e is there a point in which the blade stops ringing out and makes it sound as such?

Cheers

Tom
 
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Jitter

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This setup looks awesome guys! I’d presume (knowing nothing about engineering) that the incredible tension on the 10mm threaded bar would stop any movement/slip on the thread with vibrations while the saw is running now that the upgraded thrust bearing is making it easier to dial? Other than this, I can’t see any possibility why it would not work really well.
 

deema

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The test cuts were done with the blade guides fully retracted. From end to end the blade cut straight to within 0.1mm when measured at either end. Not only does this highlight its setup correctly but also that the blade guides do not when the saw is correctly set play any part in the cutting, they are only there to stop problems due to either poor setup, bad practice or a dull blade. This is not to say that you don’t need to set your blade guides, you should they are an important safety feature.

@Jitter There Is still sufficient friction in the system to ensure that the threaded rod won’t walk out.
 
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Sideways

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@Sideways thanks for making the video of the ping test.

It sounds like the blade is hitting off a guide or something, certainly not as musical as I was hoping for, can you clarify?
i.e is there a point in which the blade stops ringing out and makes it sound as such?

Cheers

Tom
Hi Tom, yes the blade was definitely hitting something when plucked, hence the harsh metallic vibration you noticed. I'm not sure exactly what, part of the guide mechanism I think.

Before we were fortunate enough to get the proper blade tension gauge, we talked about ways to measure tension and it's not easy.
If you have new springs, properly specified and well made, it is easy to calculate tension because you know the newtons/mm compression, can measure the spring length and the blade cross section.
Friction losses can introduce a sizeable error though.

Another cheap method is use of mobile phone apps for tuning instruments, bandsaws etc. These all rely on the idea of a plucked string, known length and detect the frequency to calculate back to tension. Deema bought one and we couldn't really make it work. The fundamental frequency for this saw at proper tension seems to be around 180 Hz. That's pretty low. Compared to many musical instruments it's a slack string and you have to pluck very gently, not with your fingernails, to get a clean note that low. The phone struggles to hear it.

Of course the audio method depends on the length of the "string" so all else apart, smaller bandsaws will make a higher note than bigger ones when both are correctly set to the same tension.

The lenox tension gauge has been very useful to us in this project. The concept is very simple. It just measures stretch in the blade directly and assumes a fixed average value for stress / strain to display the meaasured stretch as tension in PSI. We'll be making our own version of this to use with the workshop dial indicators :)
 

FlatlandsF7a

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Really fantastic work guys.

I'd love to do this to my saw, but I don't think I've the skills to pull it off! Ingenious design, and lovely to see it come together so well. Congrats on a great build, and thanks for showing us your working step by step, this is an invaluable thread to any S45 owner!
 

squib

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What a superb thread and a huge thank you for spending the time and effort to share this.
I have only just read it, which is a shame as I have exactly this model, except mine is 240v, and could have helped with measurements and I also have a manual etc .
Mine had been caned cutting up waste in a uPVC factory and I had to have the bottom wheel sleeved for new bearing. Actually twice as the first engineering guys made it worse!
Also I used urethane blue tyres which seem ok but the tracking is a bit tricky on some blades, having said that I don't use the Bandsaw that often and tend to use a different blade for different jobs which means I maybe don't spend enough time on setup.
I look forward to hearing about your tension meter.....I have a spare dial guage...
 
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