Issue FWW #95 has an article about putting on band saw tires and creating a crown on both wheel tires. I’ve never seen crowns on both before…have you????
My 6 X 48 belt sander also has a single crowned drum.
Why does the crown help keep the band saw blade and/or sanding belt tracking better than with no crown?
Replies
Back in the days of belt driven machinery, the pulley wheels were crowned to keep the belts on the pulleys. I can't explain the physics, it almost seems counterintuitive, but it nevertheless works.
Yes, and those machines that were out of line a little always had the belt clime to or come off the high side of the pulley. Have you ever lined a tractor up with a threshing machine with the belt riding the pulleys exactly on the first try.
Tink (if you don't mind), I must say I've never lined a thresher up with anything, but I have set up a few belt driven shop machines. Indoors you have the starting advantage of level +- floors and fixed machines, probably much easier. But as you said, the belt runs off the high side, so the crown acts as the high side and tends to keep the belt centered, more or less.
Being quite a bit older than the average poster here I suppose not too many of us have worked with threshers. In fact, they were pretty well out of the picture when I got old enough to align one up.
Having worked with crowned pulleys, though, I am surprised how well a flat pulley works. My MM16 seems to track a blade just fine and as near as I can tell, the pulley is flat.
The main benefit is that having a crown allows you to easily adjust the tracking, simply by tilting the crowned wheel. If the blade wants to move too far to one side, you can tilt the wheel so that it applies a force that tends to push the blade back in the opposite direction. By having the tilting wheel crowned, you create a sort of "sweet spot" where the blade wants to settle in on. If the wheels are uncrowned (but perfectly aligned), then there is no sweet spot, and the blade can easily wander back and forth because there is no centering force. If the wheels are uncrowned and also misaligned, then the blade will immediately move right off the wheels in the "downhill" direction.
Older rubber tires needed to be crowned by hand, but modern polyurethane tires have a built-in crown molded into them, so you don't have to do anything to crown them--they come that way.
-Steve
Thanks for the reply Thumbnailed and Schafer.
I would like to invite any Finewoodworking staff to post in regard to both band saw wheels being crowned. Mr. Robert M. Vaughan in Issue #95,page 50, advocates this procedure.
My question is.....what manufacturer offers a band saw with both wheels crowned? I'll bet you.. there is none. I'm not saying Mr Vaughan is incorrect, I just want to understand the physics of a crown and why it works. Perhaps Mr Vaughan can explain in more detail about the benefits of both wheels being crowned.
Obviously, single crown works, but why? I, like others, have experience with crowns and basically understand the "sweet spot". I just don't know why/physics it works.
By the way, Vega Enterprises of Decatur,Illinois used to offer a belt sander that did not have a crown drum. Their sander,instead, did the opposite and had the moving belt pass over a curved graphite support that made the belt do the opposite and become convex instead of concave. It seems both seen to work better than flat. Why? Why? What is the physics?
Don't be afraid to post your speculation/hypothesis either. This is a fun topic for us all. Perhaps we can even suggest other "tracking" options.
DonC
Don,
You ask "Obviously, single crown works, but why? I, like others, have experience with crowns and basically understand the "sweet spot". I just don't know why/physics it works."
The answer is that a belt or band is going to centre itself over a crown because the diameter of the wheel or pulley at the crown is larger and therefore the surface speed is higher, which results in the belt or band being pulled onto the centre of said crown. The same would apply to the Vega belt sander you mention.
I have not seen a bandsaw with both wheels crowned, but see no reason why it should not work-as long as the crowns don't fight each other (they will if not aligned ).
The bandsaw pictures had no tires when I got it -the band ran on the flat iron wheels which are 2 inches wide. I glued on FLAT "tires" I made from conveyor belting so that I could run very narrow band saws without killing the set. The tilt of the top wheel sets the blade to run wherever I want and I never worried about crowning.
Yet the stroke sander I made only tracked a belt nicely after I crowned both the tracking/tensioner wheel and the driven wheel. The third (driving) wheel remains uncrowned.
The options are to not get fussed. One gets it to work then one gets happy....Philip Marcou
"My question is.....what manufacturer offers a band saw with both wheels crowned? I'll bet you.. there is none."
You lose!
http://www.lhup.edu/~dsimanek/scenario/crowning.htm
http://www.suffolkmachinery.com/did_you_know.asp
Modern band saw wheels have a crown machined across the face of the metal wheel. This crown can be as little as .010 thousandths, for tracking 3/4" and narrower bands, or as much as .030 thousandths for tracking 2" bands. This crown causes the effect of self-tracking.
I've been in the woodworking field for 35 years and working on machinery for 25 years and all bandsaws I've seen have crowns on both wheels. You don't need Bob Vaughn to answer questions.
Edited 7/16/2008 7:52 am ET by RickL
Edited 7/16/2008 9:52 am ET by RickL
"I've been in the woodworking field for 35 years and working on machinery for 25 years and all bandsaws I've seen have crowns on both wheels. You don't need Bob Vaughan to answer questions." That is a really low shot at an author that has done more to add to the collective knowledge about machines and equipment than most of the others involved with the publishing business. I would venture to say the FWW hasn't published any articles on machine repair and operation of any value since Bob stopped writing them.Maybe next month my tools and shops issue will come and I can read how to clean and align my tablesaw. For the 101st time. And maybe there will be some new alignment must haves that I haven't seen yet. Can't wait.
Get real? It's not a derogatory thing on Vaughn. He's not the only service guy out there. There's plenty of other very qualified sevice techs out there. When I was doing tech work I worked with many other techs sharing info. No one can know it all. I was a generalist so to speak. I wouild work on many brands such as Delta, SCMI, Giben, Morbidelli and would talk to the specialist on their model of machine to help me out. As far as gettin an answer to the crown question there plenty of real answers you can find for yourself instead of waiting for one particular person to give and answer.
Edited 7/21/2008 8:18 am ET by RickL
"When I was doing tech work I worked with many other techs sharing info."
But you were not sharing it with he general public?
That is where FWW misses the mark every time. Its not personal to me, but Bob was the last writer for the magazine who shared with all of the woodworkers who read along. The reviews and other help articles are terrible. Cool Blocks and detentioners aren't must haves, and neither are masterplate alignment tools.
I am not a betting man. I can tell you the laguna I have , is crowned on both wheels.
Tom
Edited 7/16/2008 8:36 am ET by gofigure57
Edited 7/16/2008 8:37 am ET by gofigure57
Very interesting about both wheels crowned. I have experience with 20" Delta, 18" Parks, 12" Sears, and a Shopsmith. They all have the top wheel crowned only.
Don, must be a Euro thing. This saw is 8 years old from Italy / Laguna.
Tom
My old Makita band saw has crowned wheels.
I've never used import bandsaws but I've never seen American saws without crowns on both tires. The crown keeps the teeth of the blade off the tires and prevents them from getting chewed up. The crown hits the blade pretty much in the center. As you tip the wheel, the blade scoots to either side of the crown. I set the blade so it runs on the center, on most small blades it's right behind the gullets. I think replacement tires in polyurethane may be flat but it's hard black rubber on Delta, Kalamazoo, Startrite, Tannewitz, Rockwell and older Powermatics that I've used.Beat it to fit / Paint it to match
I use urethane tires, and have not seen any crown built into the tires themselves. The tires seem to have an even thickness.There are bandsaw wheels that have a crowned surface, which a thin urethane tire conforms to, resulting in a crown.I think that the advantage of having a crowned tire/wheel is that the response of the blade tracking position to adjustments to the wheel tilt become less "twitchy". My bandsaw has flat wheels, and until I put a crown on my tires, it seemed that the blade would move very quickly from the front of the wheel to the back of the wheel very quickly with a very small adjustment to the upper wheel tilt. After crowning the tires, this adjustment became much more predictable.If you think of a bandsaw wheel/tire with a flat profile as having an extremely shallow crown, and keep in mind that under tension and under power, the bandsaw blade "wants" to move to the highest point on the wheel, this makes a bit more sense.
Edited 7/16/2008 10:53 pm ET by wilburpan
Imo the sweet spot is found when adjusting the tracking of the upper wheel as to keep it going towards the crown ,yet not quite over it . When you adjust the upper wheel you are essentialy toeing it in or out . You state that under tension the blade wants to move to the highest spot which Imo defies physics.
Under tension the blade wants to shrink as it is stretched. Logic says it wants to find a path off . Adjusting the upper wheel allows you to fix it on the path towards the crown or dead center of the crown, but not quite over . At dead center you are relying on the thrust bearings to keep it from going over the top when feeding material into the blade. When I adjust the tracking, I try to favor the front side of the crown ever so slightly. This way when you apply force during cutting, theoretically the blade will have to stretch more to ride over the crown on the wheel . On blades 3/8" and up I then set the thrust bearing back enough to to allow for this movement while making sure not to get into the gullet area. These are my observations , hope they help.
Tom
See previous posts, the blade does want to ride to the high spot, I think someone had a plausible explanation of why. It feels wrong, but sometimes reality doesn't match our expectations. Anyone want to discuss Heisenburg's uncertainty principle, for starters ? For you handtool guys, that's quantum physics ;-)
I am not qualified to discuss quantum physics. I will try to explain my thinking. If the blade wanted to move to the high spot, then it seems that the wheels would never need to be adjusted to achieve this . They could be set at the factory to be in the same plane and the blade would always stay centered. I guess what I am thinking is that we make the blade go to the high point by tilting the top wheel, it dose not seem like it wants to .
Maybe you could help me with this question. At the point when we adjust the wheels to ride the blade on the crown are the wheels at that point coplanar? This would help me to better understand. Thanks for the reply.
Respectfully Tom
Ps; I just reread Philips post , is that the one you were referring to?
Edited 7/17/2008 6:29 pm ET by gofigure57
Tom,
I was just goofing around with the quantum physics, I'm not qualified either, but the point is still valid. The crowning works to draw a belt or blade to the high point, thus tending to center it, more or less. Phillips seems to explain it, but since I don't understand the physics, it might or might not be the valid explanation. Nevertheless, it's still true, it kept the belts on the pulley wheels for a couple hundred years of the early industrial revolution, so it's not opinion or speculation, it's just unexplained to my satisfaction. There must be someone out there with enough blue collar physics to explain, so, what about it ? Speak up, out there, give us the straight dope !
I have the small 10 1/2 Inca -- no crown. But that is unusual - most American BS have crowned wheels. "The Bandsaw Book" if I remember discusses this issue.
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