SCM Minimax S45 Bandsaw Teardown & Overhaul

[deema]

I wouldn’t replace those guides, with the right material they will be excellent, they were originally designed to be used with lignum, a good source is old bowling balls, or these days you can get ceramic inserts that may also fit, having the guides just under the table with minimum clearance is actually better than having them inside the cabinet. I will post some photos of how the lower guide is located on this saw shortly. Ive posted a thread on bandsaw blade guides you might find useful here:
https://www.ukworkshop.co.uk/threads/bandsaw-blade-guide-theory.135481/

 

[baldkev]

Good job as always
@deema on my mobile, if i press and hold over the like button, a few different emojis and a thank you sign appear.....

Interestingly my new ( to me ) startrite has a tension guage but im still guessing what is 'right'. Although i havent used it much yet, mostly put back together

 

[deema]

Sideways and I are now considering all aspects of the design we have come up with. One area that needs consideration is the capacity of the two wheel bearings. We needed to verify that the proposed worst case loads created by a M42 20mm blade can actually be accommodated by the bearings that are used. This is one reason we always use high quality bearings such as SKF, *** etc and never in rotating load bearing applications cheap generic bearings.

The Bandsaw is designed to use two 6202 bearings in each wheel mounted on an axle. This will set the limit on what loads we can reach by the springs tensioning the blade without damaging the bearings.

Bearings have many design factors specified in their data sheets, two of which are important for our application. The static and dynamic load capacity of the bearings.

The dynamic capacity is when the bearing is turning, and is virtually always a much higher value than that for the static or not turning. The reason is that when turning, the loads that the race see are lower due to there being a rotating element, this reduces the effective racial load on the race: the faster a bearing turns the higher load it can withstand. To understand this better look up force diagrams.

The bearing can handle just over 8KN of dynamic load but only 3.75KN of static load. This is one of the major reasons that you should release the tension on the blade when the bandsaw is not being used.

 

[Andrewy]

Sideways and I are now considering all aspects of the design we have come up with. One area that needs consideration is the capacity of the two wheel bearings. We needed to verify that the proposed worst case loads created by a M42 20mm blade can actually be accommodated by the bearings that are used. This is one reason we always use high quality bearings such as SKF, *** etc and never in rotating load bearing applications cheap generic bearings.

The Bandsaw is designed to use two 6202 bearings in each wheel mounted on an axle. This will set the limit on what loads we can reach by the springs tensioning the blade without damaging the bearings.

Bearings have many design factors specified in their data sheets, two of which are important for our application. The static and dynamic load capacity of the bearings.
View attachment 129822

The dynamic capacity is when the bearing is turning, and is virtually always a much higher value than that for the static or not turning. The reason is that when turning, the loads that the race see are lower when rotating due to there being a rotating element, this reduces the effective racial load on the race: the faster a bearing turns the higher load it can withstand.

The bearing can handle just over 8KN of dynamic load but only 3.75KN of static load. This is one of the major reasons that you should release the tension on the blade when the bandsaw is not being used.

 

[Andrewy]

Sorry.. is off topic, but the morticer in your signature pic looks very like the old one I have in the workshop .

 

[deema]

As we have two bearings to ensure they have a good life expectancy we checked the actual load that they can achieve and find an estimate for their life expectancy. I use the SKF bearing calculator tool that can be found on SKFs web site. We used SKF bearings to replace the wheel bearings.

First this is the data that is used to calculate the parameters

The results are:

 

[deema]

The bearings are limited by their static load capacity, so the maximum load that can safely be applied and left on the machine is 4KN. The L10 numbers represent, basically that 90% of all bearings will survive for at least this length of time. It’s the number usually used for determine the suitability of the bearings / application. For anyone interested further the SKF web site gives a good explanation.
https://www.skf.com/uk/products/rol...tion-based-on-rating-life/bearing-rating-life
This is a worst case scenario, with the bandsaw loaded to to its maximum tension, and running continuously. It would be expected to survive at least 181 days running 8 hours a day continuously. This type of bandsaw wouldn’t be expected to spend its entire life doing this type of operation, if this was required a larger bandsaw would be appropriate.

Typically this saw will generally be using a smaller blade, typically around 12mm or 1/2”. To tension a M42 12mm blade requires a lower tension. So plugging that data in to see what the elected life is:

The L10 is now 4750hrs or around 2.6 years of being run 8hrs a day (assuming 220 working days in a year). Again, if the saw was to be run continuously in what would be a production environment it wouldn’t be the right choice, a saw with larger bearings would be a better choice. However, in either scenario, for a typical professional small joiners / cabinet makers shop, this saw will probably last over 20+ years without any issues from the bearings. (That’s using it 1 hour every day with different blades)

 

[deema]

I always find this exercise interesting, it highlights the level of value engineering that has been applied to a machine. For instance, it wouldn’t have cost that much more to have moved to a slightly larger bearing for the wheels which would have increased the load capacity of the bearings and provided a longer life expectancy. But it all costs, and typically every £1 of extra material costs the end customer £8 by the time it’s gone through the distribution chain, so saving material cost it’s important to hit a marketing price point.

The fatigue life of a bearing, where it starts to skid and tear up the race is finite, so, when refurbishing a machine we always change every single bearing with high quality bearings that have been tested and verified to have a know life expectancy. It resets the clock, and in comparison to the new owner both frustrated that a refurbished machine has broken down and the cost of getting the bearings out and replacing them, we believe it’s a very small investment.

For us, there are three important things concerning the bearings to look for when buying a machine.
1. High quality branded bearings are used throughout. If buying refurbished check what’s been used.
2. All bearings have been been replaced, including the motor bearings.
3. Look for machines with the biggest bearings possible. The bigger they are the longer they will last.

 

[deema]

The exercise has highlighted to us that the machine can safely be run with a 20mm carbon steel blade and left under tension for period of not being used. It can also safely be used with 16mm M42 blades without any concerns. However, the bearings aren’t really suitable for tensioning up a 20mm M42 blade due to the static load capacity. If left under tension and not in use the bearings will quickly fail. This is something that may change what we had planned to do. The spring arrangement we had devised can create a tension that is easily capable of tensioning up a 20mm M42 blade and not fatigue the springs, however the bearings are not really suitable for this.

 

[deema]

I often hear / see comments where people have cranked up the tension on a bandsaw to the point where the springs are touching and advocate that their saw can therefore take a much larger blade than is specified. Those who do this in all likelihood have not considered and done the engineering exercise to see if the springs will fatigue / fail due to being compressed by more than 25% of their relaxed length and that the static capacity of the bearings probably is sufficient for the loads, if not they are shortening their saws life considerably.

Ian at Tuffsaws usually advocates using a M42 blade at least one size lower than the stated capacity of the bandsaw, this exercise has demonstrated that Ian’s wisdom should be followed. I would go further, if it’s not at least a commercial grade of bandsaw, which the SCM saws are, then only use M42 blades that are around half the capacity of the saw. Never try to use blades larger than that specified by the manufacturer, and again, for none commercial grade saws use as a maximum carbon steel blades one size down from the maximum size in the specification to have a saw with a good life expectancy.

 

[deema]

@Andrewy it’s one of the few pictures I have of my great grandfather using an ancient Morticer in the family business. My grandfather and father were all apprenticed either master joiners or cabinet makers. I think it goes back further generations…..but no photos of that!

 

[Sideways]

As a point of reassurance while exploring spring options and blade tension, we kept an eye on the flex of the bandsaw frame by simply measuring how much the open mouth of the frame is pulled together by a blade under tension.

The good news is that the S45 lives up to it's reputation for a strong frame.
With a 3/4" carbon steel blade fully tensioned, there is just 0.5mm of movement across an aperture of over 250mm.

Nothing to be concerned about and well within the elastic limit of steel. The deflection returns to zero when the tension is released as the physics say it should.

 

[Jitter]

Allot of useful advice there. It makes me feel much more confident about the S45’s capabilities and of course the upgraded bearings lifespan within the correct limitations is great to know also. I guess there must be many chippies who are tempted to push their machines to the limits or over without realising the potential damage to the machine doing so. Possibly may not even realise that a number of issues will gradually arise and cause problems and cost money along the way. Thanks for all the interesting and important information.

 

[deema]

To finalise what we are going to do with the spring(s) we needed to ascertain how much of the spring force was actually translated into tension on the blade itself. There is a lot of friction in a bandsaw tensioning system, all of which requires force to overcome. The SCM like many others has a carriage upon which the wheel upper wheel is located that is attached via a pin to the carriage holder through a pair of slots that allow the carriage to move up and down and tension the blade. The tracking is achieved simply by the end of a threaded rod pushing against the carriage with the pin acting as a pivot. None of these have ‘slippy’ surfaces and when under pressure caused by the blade being under tension generate a lot of friction. It’s for this reason, the losses through friction that most bandsaw blade tension gauges are next to useless.

Sideways sourced a suitable spring to do tests with, this was a spring that had a sufficient spring tension and compressible length to determine how much force was being input into the system. A Lenox Blade tension gauge was used to measure directly the tension that the blade was seeing.

At this point I have to say a huge thank you to Ian at Tuffsaws. I contact Ian to ask technical questions for this project, and he very kindly offered to lend me his Lenox Blade Gauge for the testing.

The photo shows the parts of the tension system.

 

[deema]

The spring with a known spring constant was mounted in the normal place and replaced the original spring, it didn’t fit in the sling holder which we temporarily removed.

The Tension meter reads directly the tension on the blade by converting the amount the blade stretches, elongates when under tension. It also allowed us to verify that the blade was operating in its elastic region.

We slowly cranked up the tension checking the compression of the spring and the tension we were able to achieve on the blade. We also checked the amount of ‘nod’ or elastic deformation generated in the frame of the saw by placing a clock from the table to close to where the blade comes out of the top wheel.

There was a significant loss of force from that created by the spring to that generated on the blade. This will be accommodated within the revamping of the tensioning system.

 

[deema]

For anyone serious about using a bandsaw a blade tension gauge is a vital tool to have and use. We used the gauge to check my Felder bandsaw, and that was actually applying significantly higher tension than the tension indicator on the saw suggested. So, the question is, how does the gauge work in case anyone wants to make one. A Lenox is eye watering expensive! In case you want one they are around £420!

There are a few uTube videos on building one, here is one for reference……I make no comment on woodworking practices shown!



Any material will elongate when under tension, the amount of elongation for a given force is related by the Young’s modulus. The standard equation is rearranged and what we are measuring written in:

Force / area = Youngs Modulus * change in blade length / initial blade length

Force / area is the units of PSI which the gauge reads directly, or by a conversion N/mm2

The gauge grips the blade at two points, one end of the gauge is fixed, and the other is a seesaw that creates a pivot magnifying the elongation of the blade by 3. Ie 1:3. The two points when the blade is slack are exactly 100mm apart. So, by direct measurement, 0.1mm of blade elongation was measured by the clock as 0.3mm to be 30K PSI or 206.84Nm2

If you plug those numbers into the equation you find that the gauge is calibrated to use a Youngs Modulus of 206.84! (Give or take a smidge due to measurement error)

Most HSS has a Youngs Modulus of 207 and including M42 fits within a band of c190 to 220, which although fairly broad only represents a maximum range of potential error of circa +/- 7%, which is more than accurate enough for setting blade tension. The bandsaw indicator would have been circa 60% out with the frictional losses!

 

[deema]

Last bit of ‘boring stuff’ before we get back to doing stuff. Sideways today made a good analogy. A Range Rover Sport (not that either of us have one) weights with a single driver and all fluids around 2000Kg. Each wheel therefore sees around 500KG or c4900N which is exactly the tension required for a 20mm M42 blade.

So Wheel bearings typically have a life of c100,000 miles or, c 161000KM.
A Range Rover Sport tyre has a circumference of 2.54M
So, the wheel rotates 63,385,826 times in the bearings expected total life.

The bandsaw rotates at 750RPM, so the bearings will have done the same amount of work as the car wheel bearings in 1408hrs. A little less than the L10 (which is roughly work they need to do for there to be a 10% chance they will have failed) for this bandsaws wheel bearings!

However, the car wheel bearings are significantly larger, they are around 90mm OD and have a width of 51mm. Where as the 6202 used in the bandsaw are only 35mm OD and only 11mm wide.

The bearings in a bandsaw are doing a lot of work!

 

[Andrewy]

@Andrewy it’s one of the few pictures I have of my great grandfather using an ancient Morticer in the family business. My grandfather and father were all apprenticed either master joiners or cabinet makers. I think it goes back further generations…..but no photos of that!

I haven't used it in a long time.. there was a similar one in a local industrial heritage museum therefore I wondered just how old it was. No makers name on the castings.

 

[Sideways]

Teaser post
The S45 has now been modified with a new spring tensioning mechanism.
This uses three springs in parallel, moved inside the upper wheel tension / tracking assembly so the look is very little changed.

Initial tests look great. It easily goes up to the 20,000 psi needed to start properly tensioning a 20mm m42 blade with ample headroom to spare.

Hard stops are included at 25% compression so that the springs can never be over compressed and this is equivalent to 6,600N force on the wheel hub / 3,300N N blade tension so there will be no question of being able to achive proper blade tension upto the max specification of the saw.

If you look back at Deema's posts on page you you'll see that we need 4,900N at the wheel hub to tension a 20mm M42 blade to 25,000 psi, the top end of its recommended range. We have 20% in hand to cover losses due to friction in the mechanism and any weakening of the springs over time, though this shouldn't be an issue if they can't be over compressed.

The springs are 89mm long with a rate of 110N/mm compression. That means 330N at the wheel hub and 165N tension in the blade for every 1mm on the tension gauge which will be subtly remodelled but stays on top like the original.
The feel of the adjuster with the new springs is nice

A couple more sessions in the workshop and this will be all done

 

[FlatlandsF7a]

Teaser post
The S45 has now been modified with a new spring tensioning mechanism.
This uses three springs in parallel, moved inside the upper wheel tension / tracking assembly so the look is very little changed.
View attachment 130087
Initial tests look great. It easily goes up to the 20,000 psi needed to start properly tensioning a 20mm m42 blade with ample headroom to spare.
View attachment 130088

Hard stops are included at 25% compression so that the springs can never be over compressed and this is equivalent to 6,600N force on the wheel hub / 3,300N N blade tension so there will be no question of being able to achive proper blade tension upto the max specification of the saw.

The springs are 89mm long with a rate of 110N/mm compression. That means 330N at the wheel hub and 165N tension in the blade for every 1mm on the tension gauge which will be subtly remodelled but stays on top like the original.
The feel of the adjuster with the new springs is nice

A couple more sessions in the workshop and this will be all done

Teaser indeed!

I am eager to hear and see how your new mechanism works (unless you're waiting to submit a patent first!).