There are 3
adjustment that determine whether a blade will track on any bandsaw:
1.?? Blade tension
2.?? Driven wheel tilt
3.?? Drive wheel position on its axle
?
It helps to
understand why these things matter:
Bimetal
bandsaw blades are tensioned to 25-30,000psi stress, flexback carbon steel blades to 15-20,000psi.? That means a ?¡± wide bimetal blade, has about 300lb
tension in it when tensioned properly. With a blade run on both sides of the
wheels, that¡¯s 600lb force pulling the bandwheels together.? The sawframe casting flexes, any slack in
bearings is pulled out, and axles and axle supports bend.? You can¡¯t set the tracking properly until you¡¯ve
got the right tension in the blade.??It
takes more than you think; on a 4x6 its about as much as a normal person can wind
the 2?¡± diameter tension knob one handed, without any aids. It makes a real
twang when plucked, not a dull thok.? Don¡¯t
know what it will take on your 712, but if it¡¯s got a ?¡° blade, the tension
will be half as much again as a ?¡± blade. ?You¡¯ll need to decide where full tension is,
before you start tracking (see comments below on how to mark it)
?
The drive
lands on the wheels of metal cutting bandsaws are a (very short) parallel cylinder,
not crowned like a wood bandsaw.? The
blade is supposed to run not quite touching both rims. The rims are just
there to make the blade easier to put on, not to track the blade. The drive and
driven wheels and bearings are manufactured to be parallel to each other when there
is no tilt in the driven wheel.
?
However,
when there is tension in the blade, the tension pulls the wheels out of parallel
and the blade runs off. To counter this the driven wheel is tilted in at the
top, which makes the blade track toward the rim on both wheels, more strongly
at the driven wheel than on the drive wheel.
?
Broadly
speaking, you adjust the tilt so that the back of the blade runs about 1/32¡±
(1mm) away from the rim of the driven wheel and allow the sawband to find its
own place on the bottom rim, then move the bottom rim on its shaft in or out
until the back edge of the blade just doesn¡¯t touch the rim on the drive wheel
when the blade is at full tension.
However
there are a number of things to consider while doing this:
1. ??Tracking
is tension dependent; the greater the tension the more the back edge of the
blade moves away from the rim.
2.?? You can¡¯t change the tilt in the driven
wheel without releasing the tension.
3.?? So, you must be able to re-tension the blade
to the same value each time ¨C make a scratch with a scriber on one half of the
tension mechanism and align the top edge of a piece of sticky tape with the
scratch, stuck to the other half of the mechanism. It'll be different when you change blades as each is a slightly different loop length.
4.?? The roller blade guides do affect tracking a
bit (will make it move in about 1/32¡± at top and 1/8¡± on bottom wheel, but its
easier to track the blade without them in place ¨C so remove until you¡¯ve adjusted
the blade to within 1/16¡± at top and 5/32¡± at bottom
5.?? The tracking is very sensitive to movement
of the tilting set-screw. 1/6th turn changes it a lot near the
correct point, less sensitive when just starting to tilt.
6.?? Start with set screw at ¡®no tilt¡¯ position (release
blade tension, undo both bolts, back setscrew out, then tighten both bolts again,
then turn setscrew back in until it touches).
7.?? It¡¯ll need at least ? turn in on set screw
to start with so re-set the tilt with the set-screw half a turn in.
8.?? Tension blade so that the slack is just out
of the blade, (i.e. no-where near full tension), and turn on/off quickly to
see how the blade moves.? If it sits ok,
then keep it running and slowly increase the blade tension and notice how it
starts to move away from the rims.
9.?? If it still wants to move away from the driven
wheel rim at less than full tension, increase the tilt another ? turn of the
set screw each time, then 1/6th turn when you get near full tension,
until the blade tracks ~1/16¡± from the rim of the driven wheel at full tension.?
10.? ?Don't worry about the drive wheel, unless it still derails even when the driven wheel tracks properly, that shows its way too far in on its shaft (N.B. see comment below about gearbox bearing failure)
11.?? Now move the driven wheel on its shaft
until the back edge of the blade is ~5/32¡± from the rim
12.?? Re-install the roller blade guides and see if
the blade tracks just clear (1/32¡±) of the rims.? Will be ok if it looks like its touching, but
you can¡¯t hear any graunching noises (back edge of blade scraping hard on the
rim).
?
That¡¯s about
it, though the blade must be welded squarely (watch for uneven gap from back of
blade to rim as the join passes over the driven wheel) and the gearbox output
shaft bearing must be ok (if it¡¯s failed the output shaft will bend as the
tension is increased, so the blade derails from the bottom wheel only, not the top
one).? You have to keep the blade on
before you can judge either of these, so try to make it track as above first.?
?
Don¡¯t be
scared to move the drive wheel out, if it keeps derailing from the bottom when
the top seems OK , though you do need to check that the gearbox output bearing
hasn¡¯t failed? (they do it often enough
and it could be the reason why it was sold). I see your drive wheel is pinned,
4x6¡¯s use a key and a set screw, you could use 243 Loctite until you get the
position right, then re-pin it. ?243 Loctite
is weak enough to be able to change the position of the drive wheel with heat
and a gear puller to set the tracking, but even high strength Loctite won¡¯t be
strong enough to drive when cutting metal.
Let us know how you get on - jv
