JJ Smith Belt sander issue.

Hi

I'm a carpenter who does cabinetry work mainly, but I'm no expert when it come to mechanical matters.

I got a belt sander from JJ Smith, fixed it up, got it running (after it had been sitting for at least a year) and I have done a few jobs using it.

Unfortunately it has developed this issue where it dosen't always oscillate the sand paper belt backwards and forwards and just shreads it after traveling too far one way.

I believe I have identified the part that is the issue as shown in the photos.

It's an air based piston. The piston is pushed out when the input of air is switched and the reverse, depending on the side the paper is on.

This piston will not always make the switch now, it'll work only sometimes and only if I encourage it. Almost like it's sticking, just not making the switch inside. I can tell the input of air is changing so it is an internal issue.

Does anyone know how this can be fixed or a place where I can get a replacement part from?

Thanks
Jack

I am not sure how that one works but on my 16” sander the belt tracks back and forth and at each end it has safety switches that shut off the machine if the belt tracks too far. One common problem is low air pressure which can have several causes and another problem is from dust buildup in the switches. Check everything and clean any filters etc and keep testing until you find the problem.

M. Moore said:

I am not sure how that one works but on my 16” sander the belt tracks back and forth and at each end it has safety switches that shut off the machine if the belt tracks too far. One common problem is low air pressure which can have several causes and another problem is from dust buildup in the switches. Check everything and clean any filters etc and keep testing until you find the problem.

Click to expand...

Thanks for getting back to me.
Sadly ye both saftey switches do operate but the belt is traveling to quick one way to stop intime because the air powered piston dosent even try to send it back along the roller .
It could be an air pressuer issue I will see if I can attach it to a more powerfull compresser and see if it makes a diffrence.

Mine doesn’t need a high pressure or large volume.
Test it without the belt to see if you can get the upper roller to track back and forth. If it only moves in one direction then you have found the problem. I suspect it is just a clogged line or valve. The dust does tend to get everywhere.
Recently my on mine all of the hoses became brittle and cracked. Luckily I had some of the same size hose and was able to replace them all.

Do what M said and take the belt off, then put your hand in front of the air signal on each side and see if the piston cycles freely.
Try it with a finger directly over the port, then try it out at about the distance the belt would be running as is approaches & passes the port.

When i got my RAMCO, the piston ends had dug divots in the stops, and would stick and jam. The cylender ends were also worn oblong around the shaft and leaky. I ground the stops smooth, and replaced at least one that was too deep; and replaced the cylinder assembly. Yours does not visibly look worn like mine was, but check that the mechanical connections allow it to move effortlessly without sticking or ratcheting. You may have to rebuild it if the O-ring(s) inside are hardened, cracked, or simply worn out.

That will probably still not be the end of the story - "somewhere" on the machine will be a set of flow controls and dampers that are screw adjusted to set force and acceleration rates for the oscillation. If the belt accelerates and travels too rapidly in one or both directions, it will over-ride the signal before the cylinder can react. Being air so called logic, the controls will be essentially simple & logically laid out and designed, but essentially black magic to adjust. Also, even if you get it adjusted well for a new/average belt; old belts or a poor splice can lead to a belt running faster to one side than the other.

Try to get a manual with an air system schematic - at least it will show & label what the parts are or are supposed to do.

There are generic electric eye retro fits. I don't know much about them.

smt

PS: unless you have gone over the machine completely and adjusted it up from first principles, the above might or might not be a band-aide.

Is the contact drum true?
How about the idler rolls?
Are all the above adjusted in their bearings to have parallel axes? (Or possibly with wear, parallel surfaces?)
Does the big tension idler above slop around or is it fairly stable and the only easy movement is by the tracking cylinder?
Is the platen in use and if so, is it worn in such a way that it influences belt tracking?

I know this is not your current problem but it relates to wear on the machine, and cost & energy efficient sanding: you may already know this, the platen should not be used for calibrating (makes the belt less aggressive and less flat). It is most practical to use the platen for grits above 120 or so when smoothness takes over from dimensioning and flattening. For instance, if you are going to go to 180 grit, don't even use the platen until 150 or 180 and be sure it is not taking more than a few .001"s per pass. Seriously. If the calibrating/contact drum does not make work flat, then it needs work and that might be a factor in tracking.

smt