A Theoretical Bandsaw Question

[Bill Space]

I have an old, but well respected, 20” Davis & Wells bandsaw I am setting up as my main bandsaw. But I understand with the factory spring it maxes out at about 15~18,000 PSI on A 3/4” blade. Forgive the generality, I did check what the maximum pull the spring can put on the blade, but forget what that number is at the moment, and also forget what the gage of the blade was when I did the calculations. None of these specifics relate to this question anyway, which is generic in nature.


I put a dial indicator between the upper and lower horizontal sections of the “C” frame of this bandsaw, between the axles, and measured the deflection as I cranked the spring nearly fully compressed. I got 0.012 deflection between the axles.


Now it seems if I wanted to increase the maximum blade tension I could purchase a stronger spring. This would result in more stress on the bandsaw frame. Not sure how much frame deflection would be safe. This got me thinking...


The vertical wood shop bandsaw is not a sawmill. There is likely a practical limit to the width of material that is resawed most of the time. Does one normally resaw a 10” wide x 12” high, or larger, pieces in the home shop. I keep telling myself probably not.


So now to the question. It is likely that for a given bandsaw one could increase the size on the tension spring until the frame of the saw was damaged, while trying to reach a desired level of blade tension. At the same time, resawing is generally ( my assumption) dealing with relatively high and narrow pieces. For example, maybe 12” high (or more) and 4” wide, or thereabouts, or thinner.


So why could one not add a temporary vertical support in the C frame between the axles. Granted, this would require a hole to exist(or be made) in the saw table, to allow positioning of the support. And require the swapping of the tension spring. But the extra support would seem to negate the fear of damage to the saw from the use of a heavier spring. So higher blade tensions could be attained without fear of damaging the saw.


How hairbrained is this thought?


Bill

[Bill Dufour]

I think the problem will be attaching it to a solid point at the bottom. You do not want to overload the trunnions. How about lifting the upper casting from the top to the ceiling. that should safely add several hundred pounds of pull.
On metal working horizontal milling machines it is common to add an outboard support to the outboard arbor bearing to make it more ridgid. This would be similar to adding a top bearing and support to a shaper arbor.
Bil lD.

[Bill Space]

A hole through the table would remove the trunions from the picture I think...

Bill

[George Makra]

Maybe your over thinking this. Any time you put a load on any thing there will be deflection. You get into trouble when there isn't give.

IMHO blade selection is the most important thing on your band saw.
Most ailments can be cured with the correct blade.

I have a Laguna 18 inch saw and I resaw 10" or better old oak barn wood which is very hard. In get through 10 ft or so in a couple minutes.

[Bob Leistner]

https://www.youtube.com/watch?v=4k-r5utmU2Q

[Jared Sankovich]

I think you are fine.. I have .080" deflection @ 40,000 psi (1"x.033" blade) on my 21" import saw

[Van Huskey]

About .020" deflection is where you are beginning to push the saw and .040" is considered (postulated by FWW) to be the limit of where you want to push a saw. Operating within what I consider sane range I have rarely seen over .010" on one of my saws.

As for essentially shoring up the frame you are assuming the tensioning mechanism itself can handle a significantly higher spring rate spring.

Keep in mind the old iron (and steel) 20" saws were light to at best medium duty. A modern 20" steel saw will be heavier built in general. The D&W is a nice saw don't butcher it, if you need more saw get more saw. If you shore up the frame and don't blow the tensioning mechanism into pieces you are going to want more HP at the point when you have added a new motor and starter and reinforced the frame you have spent more time and money than just getting a bigger saw.

I understand it is theoretical but the frame is just one peiece of the puzzle.

[David Kumm]

As Van said, it is also about the design and strength of the upper wheel support, the diameter of the spindle itself, and the size, load rating, and distance between the spindle bearings. The entire machine is made of components to work within a certain range. Exceeding those with just a spring can be done but you need to understand the machine and how it was made. Checking the deflection doesn't give you all the info you need. I see the same thing with shapers. A light duty shaper with a larger diameter spindle still runs on the same strength quill and bearing set up and isn't appropriate for some of the cutters run on them. Dave

[Jared Sankovich]

About .020" deflection is where you are beginning to push the saw and .040" is considered (postulated by FWW) to be the limit of where you want to push a saw. Operating within what I consider sane range I have rarely seen over .010" on one of my saws.

As for essentially shoring up the frame you are assuming the tensioning mechanism itself can handle a significantly higher spring rate spring.

Keep in mind the old iron (and steel) 20" saws were light to at best medium duty. A modern 20" steel saw will be heavier built in general. The D&W is a nice saw don't butcher it, if you need more saw get more saw. If you shore up the frame and don't blow the tensioning mechanism into pieces you are going to want more HP at the point when you have added a new motor and starter and reinforced the frame you have spent more time and money than just getting a bigger saw.

I understand it is theoretical but the frame is just one peiece of the puzzle.

Is that distance measured at the centerline between the wheels or on the outside at the guides?

[Van Huskey]

Is that distance measured at the centerline between the wheels or on the outside at the guides?

At the guides.

[Bill Dufour]

At the guides.

I have thought long and hard about your deep quote and I do agree a compressed spring does weigh more. But it has to compressed not just a coil spring relaxed to it's rest position.
Bil lD.

[Bill Space]

At the guides.

My measurement was between the axles, which would be about half as much as what would be measured at the guides.

I don’t plan on modifying the saw in any way to gain increased blade tension over the original design, and just poised this question as a learning experience.

Sounds like the major limitatins of making a modification like I suggested would be the top wheel assembly’s ability to handle the increased stress, and maybe the wheel bearings ability to function satisfactorily with the increased load.

Bill

[Van Huskey]

I have thought long and hard about your deep quote and I do agree a compressed spring does weigh more. But it has to compressed not just a coil spring relaxed to it's rest position.
Bil lD.

Correct, it has to have energy added, E=mc^2 and all that jive.