Joseph Gwinn wrote:
I recently acquired a bandsaw that will cut wood or metal (aluminium
at 3000 fpm and steel at ~100 fpm). It always cut at an angle with
the original cheapo blade. Tightened and adjusted. No dice. Gave
it a 5/8 inch wide 10-12 tpi bimetal blade. Much better, but still
crooked.

The bandsaw books in effect say that this is to be expected, and
describe various ways to cope with the tendency, but I could not see
why.

The mitre guage that came with the saw is pretty flimsy, but even that
didn't seem to explain the degree of crookedness. Now, on table saws,
the greatest accuracy is obtained by use of a sled that carries the
stock past the blade.

So, I made a sled out of a 3/8 by 3/4 by 12 inch cold rolled mild
steel bar (that slides in the mitre slot in the table), a piece of
1/4 inch 6061 T6 aluminium plate about 6x8 inches that I had laying
around, and a piece of 3/8 by 3/4 inch aluminium bar (set vertical
and accurately perpendicular to the steel bar and thus mitre slot).
The pieces are held together with some large flat head socket cap
screws. The sled reaches to within 1/16th inch of the blade, so the
stock is well supported.

In use, the stock is held in the sled against the aluminium bar, and
the sled is pushed past the blade. On wood, this works quite well,
but on metal it still drifts a bit, the stock being held in the sled
by hand. So I drilled another hole in the sled to accept a 1/4-20
flathead socket cap screw, and attached a small strap clamp. This
allows the metal to be clamped firmly to the sled. This works: the
cuts are now perpendicular.

So, the lesson seems to be that a significant cause of crooked cuts is
the stock drifting sideways while being cut. I don't know the source
of the sideways force, but it cannot be that large. It may be
nothing more than the vibration of the saw causing imperceptible
walking.

Joe Gwinn

Apparantly it is not 'el-cheapo blades or a cheap saw that generates
off-angle cuts.
(or at least I assume that *this* outfit uses quality equipment)
I recently got some half inch thick 6061-T6 al. plate from Online Metals.
One side of a six inch square of this material has a milled and square
surface. The other three sides were off by as much as 1/32" over the HALF
INCH!
Moral?
If you really need a six inch square +?/-0 order at least 6 1/4.
Also - don't assume any band saw cuts straight.

Ken.