Band Saw Question

About a year ago I bought a Wilton 7" x 13" horizontal bandsaw from Boeing Surplus for $200. It was kind of a strange deal because 1) it was so cheap, 2) it was missing the motor, motor mount, gimbal for the "top" wheel and cord, 3) it looked brand new.

The saw was <5 years old and lists for $3000. Wilton sold me the mount and gimbal for $109. Add a 1/2 hp motor and off we go.

Or not quite. This thing had a new 12 tooth blade on it, that wouldn't stay on for more than 20 seconds. It was flat across the back and sharp as a scalpal. I checked everything I could think of. Even replaced the driven wheel bushing. (Almost unbelievably Witon GAVE it to me. They said writing up the invoice would cost them more than the price of the bushing) The wheels were parallel in and the offset was nil. No adjustment would keep the blade from comingh off.

In desparation I got a new blade. 3/4" x 101"x .032" x 18T. (I already had the blade for another saw and had it shortened)

Viola, it stays on and the caw cuts. It will cut multiple slivers off of a hunk of 3/8" x 1 1/2" flat bar .025" thick with less than .002" variation from end to end!

It still sometimes slips the band off of the drive wheel, but this seems to be related to swarf packing up on the wheel AND vibration. I know that 18 teeth on 1 1/2" stock is not optimal, but no chance of getting an appropriate band thil Tuesday. The vibration does not seem to be coming from the area of the cut and varying the pressure of the cut seem not to affect anything. Either by increasing the feed, or putting more pressure on the back of the blade. It does not start immediately and it builds up gradually when it does. I suspect some kind of harmonic vibration. Because of lack of the factory step pulley on the motor I can't easily change blade speed. The bearings in the gearbox are tight, but there is some play in the drive wheel. I can move it 3/16" back and forth at the rim. Any Ideas? I think I will go remount the motor so I can try a higher speed. Clearly this was a problem child for Boeing and they finally gave up on it. The fact that they had the gimble off leads me to believe that it was taken out of service because of the band throwing problems.

I believe that the 3/16" deflection is the source of the problem.

Imagine how much that is when the blade is tensioned up a hundred ponunds or so.

I may be all wet, compare the deflection of the two wheels and they should be about the same.

Also, inline with what I just said, most bandsaw wheels have a "tracking" adjustment, usually on the upper, non-gearbox wheel, independent from the blade tensioner. This adjustment allows you to essentially tilt the axis of the upper wheel, to tune the wheels so the blade stays on.

The bearings required by the gearbox on the lower end preclude this, and so its axis is fixed and non-adjustable.

So your tracking adjustment may be loose, may be missing, or may be simply mis-adjusted. It might be a setscrew also.

I'd try poking around there first.

-Matt

After ensuring the wheels are co-planer the tracking and blade tension play the biggest roles in keeping the blade on.

I find that blade tension will effect where the band will ride on my saw. If I try to tension the blade too much the band will ride out on the tires. When overly tensioned and no adjustment is made to the tracking, the band will pop off within a couple of revolutions. Adjusting the tracking will mostly only effect the blade position on the upper wheel and the band will still stay on the outer half of the lower wheel. In this condition the upper wheel will not be co-planer with the lower: it's toed out at the bottom trying to pull the band in. Returning the tracking to a co-planer condition and backing off some on the blade tension will allow the band to ride completely on the tires. So on my saw, blade tension cannot be irrespectively cranked down. It's a matter of finding the saws happy place.

I've often heard and read how much tension should exist on a bandsaw blade. Apparently my saw can't read or hear and so it doesn't like that much tension, and I don't like blades jumping off.

In my simple mind I wouldn't think that play in the wheel/gearbox would have much influence. Any lash or play will be taken up when the saw is running and resistance in the guides should keep the gears loaded.

Determine which end pops off first and check things over there.

The saws I service have all metal wheels, no rubber tires. Both wheels need to be tracking properly. On the transmission end there is usually some provision for adjusting, some times a threaded bushing were the mounting bolts pass through or some sort of jack screws. Ideally you want the blade to track so the back of the blade does not touch the flange. Look for aproximatly 0.010 clearance between the blade and flange. Always use a new blade when adjusting. Used blades may have been stressed so they run amuck and if you adjust it with a stressed out blade, a new blade is gonna run badly. I had a customer with an automatic F W Wells unit that was breaking a $100.00 blade a day. Both wheels were worn so badly from misadjustment that I had to replace them. New wheels tracking properly, now replace worn out blade every 3 weeks, and that is in a heavy production enviroment cutting steel tube 2 shifts a day. A sign of a blade being under undue stress is the back side of the blade starting to crack. That may be from way to much feed pressure or improper tracking. As far as tension goes, most saws have some kind of indicator on the tension adjustment or are tensioned hydralicaly. If the frame of the saw is weak, it will cause the tracking to move around depending on the tension. On a cheap saw with a simple screw adjustment you can achieve the same tension by making the screw longer and placeing a die spring under the screw. When the slack is takin up, measure the spring length, adjust for proper tension. Measure again. Now you know how much to tighten it for next time. Also you want to keep your guide rollers in top shape. There should only be about 0.001 clearance betwwen the rollers with the blade installed. Example, 0.035 blade, set the clearance to 0.036 with feeler gauges. With proper maintanence your blades will last soooooo much longer and your cuts will be much nicer. Even an el cheapo saw can give decent performance if it is tune correctly. Hope this helps somebody out.
Bill

The 3/16" play is in the gearbox, not the bearings. The blade tracks well and only pops off the wheel when the vibration gets bad. It is always the bottom wheel and it is always packed with swarf. I have been taking 1/32" slices off a piece of bar stock, so I am confident in the tracking and guide adjustments. T moved the pulley to get to the next higher speed and this greatly increased the time it took before the saw started to vibrate, but had no other effect. I may open up the gearbox tomorrow and see if it has any way to take up the slack in the gear train. The irritating thing is that I can't locate the source of the vibration. I can lay my finger on the back of the blade over the area that is cutting with no vibration felt. The same is true of the drive wheel. Maybe it is coming from the gearbox.

How about installing some sort of brush to help remove the metal shavings from the saw blade before it reaches the wheel? I've done this with old toothbrushes on woodworking bandsaws.

Does this saw have a tensioning spring? Is it by chance collapsed when you have it tensioned where you want it? A collapsed tension spring could lead to vibration, especially when swarf gets under the blade on the wheel, essentially making the wheel larger and increasing tension by stressing the frame of the saw if the spring cannot compensate. The traveling portion of the non-driven wheel holder binding in it's guides would cause the same thing.

Tools

They is no tension spring on this saw, just a screw adjuster. You may be correct about the blade tension going up as swarf collects on the wheel. It already has a rotating wheel type wire brush to clean out the teeth. I already have a scaper/brush in the works.