Bandsaw - How Much Wobble is OK

[DLB]

A summary of my assessment of my equipment: Mounting the eccentric tubes with no SPT revealed that equal clamp pressure did not result in equal stability. Front tube would rock, left to right, and continue to do so at very high clamp pressure. Anomalous clamping in the front mounting hole appears to be caused by a high spot at the front edge, essentially directly across from the SPT clamp engagement point. Behavior is analogous to clamping a pipe in a typical C-clamp. I suspect the high spot results from abnormal wear which results from improper SPT clamp adjustment, which in turn can result from the different clamp mount depths that are a feature of some headrests, including mine.

First photo is a standard eccentric for reference. It is rotated and back-lit for clarity to show normal clamping behavior. The engagement area is 180 degrees from the clamp.

Standard eccentric mount

IMG_7942.jpg (360.62 KiB) Viewed 5427 times

The second photo shows what I eventually determined was what I wanted to do. In order to get away from the mounting hole high spot, I wanted to eliminate the contact at 180 degrees and replace it with two contact points on either side. Through an iterative process I moved the contacts out farther than I originally planned, ending up with two contact points about 120 degrees from the clamp. There were two ways to move the contact point, adding material at the desired contacts or removing material from the undesired point. I removed material because adding it didn't occur to me. I don't endorse removing material. Adding material has several benefits over removing it. So I borrowed algales's suggestion of shims and blended it with my idea of using three 120 degree surfaces instead of two 180 degree surfaces. The shim stock I have wasn't suitable, the photo captures the intent using electrical tape in lieu of a good shim. Note the clearance 180 degrees from the clamp.

Eccentric modified with electrical tape

IMG_7943.jpg (375.4 KiB) Viewed 5427 times

Removing material worked fine, but is somewhat crude. The main reason that I think the shim method would be better is that the shims will give the eccentric a radius very close to the radius of the mounting hole over the contact area. Either method will move the center of the mount to the front (removal) or rear (shim) and necessitate rotation of the eccentric to correct spacing of the SPT mount portion. The shim method would allow iteration if necessary, the removal method is unforgiving. I used a series of flats that I made with the disc sander (520 table was far superior to my old 500 table for this, no comparison) and radiused the last flat with a belt sander to blend it with the untouched portion of the tube. It is imperative that the flats are straight so that the new contact area is straight up and down in the mount. The third photo is my modified tube, note the clearance and new contact area, the other contact area is occluded by the SPT portion of the tube in the normal rotation.

Eccentric modified by material removal

IMG_7944.jpg (335.25 KiB) Viewed 5427 times

I doubt that I could think of, or list, all of the cons of this approach. I'm only including it here to enhance people's understanding. Note, for example, that the eccentric is designed for +/- 90 degree rotation off nominal for +/- 1/4 inch horizontal alignment. With the modification done like this, I'd guess anything over 15 degrees, perhaps less, would significantly degrade performance. Its one redeeming quality is that it works, significantly improving stability in this one headrest. As I stated in a previous post, improvement in a good mount was negligible. Bottom line is I do not recommend this, even though it worked. If I tried again, I would find a way to make the shim method work.

[dusty]

Very interesting. Great information. However, I have one question that I must ask.

I agree that there is some lateral (and maybe unwanted) movement when pressure is applied to the top of the bandsaw (pushing toward the headstock). My question though is what adverse result does this movement produce other than the power coupler tends to move up or down some very small amount.

It has been my experience that when the headstock and bandsaw are connected via the coupler the bandsaw is pushed to one side thus no movement. Even if it does move the relationship of the table and blade is not effected.

What real problem is being resolved?

[DLB]

Great question, Dusty. I don't think any real problem was solved by improving clamped stability from 0.6 degrees to 0.3 degrees. Especially since that figure is without the stabilizing influence of the headstock coupled in, which takes both down to ~0.1. It helped me personally because that wobble in one mount would have bothered me. The only reason I added that to the thread was because I thought it might be portable, or at least worth thinking about, to the question of tailstock slop.

But going back to my original issue, I had a pair of couplers break and I had a lot of wobble, comparable to the unclamped numbers we've thrown around. At one point I thought that the wobble caused the couplers to fail. Now I think it's more likely that they were both symptoms of the same problem. That being that my clamp was poorly adjusted and only clamping the back eccentric. (By a lot, not that it matters.) That led to an examination of everything involved and the need for a biased adjustment to counter manufacturing variance in the headrest (now believed to be somewhat common) and the clamp itself (probably manufacturing error). At that point, I'd say I was overly sensitive to the wobble when the solution did not produce the amount of improvement I expected.

So how does, or is it even possible, a clamping issue result in coupler failure? This part is hypothetical. We all understand that we adjust our SPTs for vertical and horizontal alignment. (I believe it was you, Dusty, that said you had a coupler failure that you attributed to alignment in another thread.) In addition to vertical and horizontal alignment, my theory is that there can also be pitch and yaw error where the rotational axis of the SPT is not the same as that of the headstock. Normally those errors would be small, a non-issue just like a small error in vertical or horizontal. Yaw error is almost completely design controlled and I'm sure, very small. And it has nothing to do with left/right stability. Leaving pitch.

When we move the BS left and right to measure wobble we are changing the angle (pitch axis) of the drive shaft by the same amount. When we push the BS left when coupling the headstock we are doing the same thing. We may be reducing or increasing any error, but we are changing the number. If we accept BS table angle, left to right, as a close proximity to BS drive angle and accept the way tubes as representing headstock drive angle then we have two planes that make it easy to measure. I think that this angular difference might have resulted in coupler failure through overheating. I have no alternate theory that would connect the known clamping issue with the coupler failures, nor evidence that they are related.

Problem - Now, with the wobble improved plus headstock coupled I have about 1.1 degrees difference between those two planes, way tubes and BS table measured left to right. I don't see how the original problem, one side of the clamp working, could have possibly added more than 0.4 to that. So, even if my theory is right, I didn't reduce that error to the extent that I would like, and I probably did not find the main contributor to the error.

-David