I recently picked up a used SCMI Uno 36" wide belt, single drum with platen. After replacing a couple switches it's running fine. I have made up a 3' wide slab using 3/4" melamine with a 80 grit section of belt and have trued up the rubber drum. The platens were wore down pretty bad so I replaced the felt with new from McMaster Carr and new graphite. I'm having trouble getting the rebuilt platen to leave me a smooth final pass when running cabinet doors through. The initial passes using just the drum I do at 100 grit. I am trying to use both 120 and 150 with the platen, light passes, but I'm getting slight ridges/valleys. Nothing huge but more than there should be. Is there such a thing as trueing up the graphite on the platen? Also, I'm not that happy with the belts I've been using, don't seem to last long at all. I think these were from Klingspor. What does everyone else use and like? I've heard 3M makes good ones but would like some input before investing in many more. I'm a small, one man shop so not a huge volume of material goes through.
You might want to take a look at the felt on your platen. If that's not flat, don't expect the graphite cloth to fix it. I've seen sanders with abused platens give the same results you're describing. You could probably pull the platen and run it through the sander to level out the felt, then recover with graphite.
I have run the felt through to flatten it up and seems good. The graphite seems to be somewhat rough so wasn't sure if it needed any break in time or smoothing up. Are there different grades of graphite rolls?
I don't know if there are different grades of the graphite cloth, but I wouldn't be suprised if there was. I do know from experience that getting the cloth stretched smoothly and tight over the felt is the hardest part. You definitely don't want any slack or wrinkles.
Thanks guys, I think I will try 3M's belts. After changing to a new belt I checked the ridge pattern on a sample wood block and it's different than the first belt. It seems like there's a little inconsistency in the belts, not as noticeable if it were a coarser grit but at 150 it needs to be better. My platen may not be the main culprit although I may try and get that smoother too.
There is always some inconsistency in sanding belts, especially in cloth. The substraight in cloth is not as flat as paper so the height of the grains is not as accurate as paper.
Graphite cloth often has lumps in it that need to be smoothed out before use. It really depends on the grade used. 303 graphite is the best for consistent finish. The heavy duty is the worst. I often sand it down a bit to smooth out imperfections in the cloth.
If the ridges are running with the direction of feed and not against it, then these two factors will be the most prevelant causes.
When you set your platen to the drum you want to start by lightly sanding a piece of material with the platen retracted. Without moving the machine height you want to stop everything and put the part back underneath the platen. Lower the platen until it very lightly touches the part so you can feel a bit of tension between the part and the sanding belt. Your goal is for the platen to only take a couple thousandths of an inch.
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