Thread: Elliptical Pattern on end of Bandsawed Hex Stock - What Causes This?

The attached pic shows the end of a piece of 1-1/2" 316 hex stock that was cut in a horiz bandsaw. What's the cause of the elliptical pattern?

Cold rolled, is it?

Hi
you get the same thing in a cut in a round bar .
I think it is something to do with the resonant frequency of a vibration of the blade within the bar as it is cutting .
The resonant frequency is changing as the cut in the bar gets wider and there is more of the blade within the bar
and more teeth doing the cutting.
So the elipse gets wider as the blade gets deeper onto the bar, and it gets narrower after the blade has passed through the widest part of the bar , and the metal being cut gets narrower.
But what do I know ?
Just made that up as I was typing !

Davycrocket

I have added a photo of the effect on cutting a round bar.
In fact there are multiple curves on each side . Must be harmonics of the resonance .
Tilted the piece to show the curves on the right side.
On the actual piece, the curves are just the same on both sides but Left hand side does
not get the reflections for the camera . .

Davycrocket
.

Dave,
That's how it was basically explained to me by the Lenox rep. They call it "mooning". No, not that kind!

MR Crocket

Made up or not you pretty much hit it. For max blade life you would have to modify speed, feed and blade tension.... when it's spot on much of that artifact will go away.

K.

Originally Posted by davycrocket

...I think it is something to do with the resonant frequency of a vibration of the blade within the bar as it is cutting ....

Thanks for the input. That makes sense. Sort of...

Resonance requires a 'forcing function'. (A bell is resonant, but needs to be hit in order to ring) Are you thinking the forcing function here is the action of each individual tooth alternately forcing the blade left and right as the tooth enters the work?

Originally Posted by Jim McIntyre

Thanks for the input. That makes sense. Sort of...

Resonance requires a 'forcing function'. (A bell is resonant, but needs to be hit in order to ring) Are you thinking the forcing function here is the action of each individual tooth alternately forcing the blade left and right as the tooth enters the work?

Yes the action of the tooth would be the bell clapper and the blade is tensioned so that it does indeed have a primary and many secondary resonant frequencies.

Denis

Its one of the reasons vari pitch blades are so common. It still happens, but the variable pitch constantly changes the resonance. Its also one of the reasons i like blade guides with carbide pads, not just ball races, its a area of contact and can help dampen these sorts of things, unlike the line point of contact which can just crate the stationary part of a sine wave if the blade vibrates at the right frequency!

wow, this thread has really opened my eyes, i love new info! I have noticed this effect as well on parts that come out of the band saw but had literally no clue what caused it.
thanks gents!

Now for the important question. How much does this effect affect blade life?

Tom

Massively if it gets bad enough. The teeth chip just like any other cutter if there vibrating hard enough. More to the point the noise real drives you up the wall after a few hours!

I read blogs for educational purposes. I have read an article about varying pitches on blades. I guess the information would best fit for this thread. Variable Pitch Band Saw Blades - When is it used? | BandSawBlogBandSawBlog
I hope this helps!