I'm wondering whats causing the marks in the photo. I can get rid of them with the platen usually, but if they occured at a coarser grit, its tough. Maybe its the belt splice? Happens with different grits (same brand belt though). Can anyone recommend a belt brand that they have had good luck with? Thanks.
That looks like the belt splice to me. Most belts will do that when they reach a certain level of wear. If you look at the belt and the splice is starting to get smooth and shiny, it's time for a new belt. I've tried a lot of different brands, but they all do that when they get worn. Put a new belt on and run that board again. I'll bet those lines disappear.
Those occurred with 150, but it happens with 120, 100, etc. That belt is pretty new, but I will try a brand new one. Maybe I just got a bad batch. Drums look OK to my eye, no major scratches or gouges.
Take a set of calipers and measure the spline of the belt and then measure the rest of the belt. The spline should be no thicker than the rest of the belt. Most cheaper belts the spline will be thicker and leave these marks. You can tell by the picture that the spline is taking off more than the rest of the belt just by looking at the wear on it. We had this same problem and went to 3M brand belts and never looked back. Yes they do cost a little more, but they last longer and save a lot of time sanding those marks out.
Thanks for all the feed back. As for stock removal, I take off .01" with each pass, occasionally running pieces through without moving the table. I measured a couple belts, and found the splice to be thicker by .002-.005". Not sure if this is an acceptable number or not. I also noticed the leading drum has a grease fitting on it. I'm almost positive I've never greased it, not sure about the previous owner. Maybe there's some vibration occurring?
Take the belt off and run the machine. That will tell you if you have a vibration. A vibration would put a much tighter pattern in the wood. It would be a mark for every revolution of the drum. .01" can be a lot for a 150 grit belt. If the splice is anything thicker than the rest of the belt, you need a new supplier and new belts!
I get chatter marks when I run stock without the platen and the splice is thicker on my belts. Never had an issue removing them with higher grits for finishing passes using the platen. I follow a strict stock removal guide showing the max stock removal per grit and minimum thickness to remove the scratch. The chart is taped to the front of the machine. Max stock removal for 150 is 0.006" leaving a 0.002" scratch, 120 is 0.01" with 0.004" scratch. Check your stock removal rates for each grit. 0.01" is a heavy cut for 150, twice the thickness I take per pass. If you're taking heavier cuts than the grits are rated for that might be part of the problem and you can significantly reduce belt life.
To isolate the origin of the marks, sand two boards. Crank conveyor speed up for the first one and slow it down for the second. If the marks are similar it's chatter from the drum. If the spacing of the marks is different, the splice is the problem.
Thanks. The reason I take off .01 is because thats the smallest amount that registers on the digital readout. Starting at say, .83", I push the button once, and it reads .82". There is a manual micro adjustment, but no way to know how much its moving. I guess I could count the revolutions of the dial. I had previously compared the marks at two conveyor speeds, and they do in fact spread out at faster speeds, tighten up at slower speeds. I think I will try some new belts.
I mentioned checking your depth of cut at other grits because when you take too deep of a cut that can exaggerate the chatter marks from the spline making it difficult to level out with the platen and finer grits. My chart has a max cut depth of 0.017" for 100 grit and 0.024" for 80 grit. I don't know what grit you're starting with and knowing your machine doesn't register thousandths try running your grit sequence close to the max cut depths I listed and see if you can reduce the chatter to a point where the platen can level the board with the finishing grits.
I think it's always money well spent buying higher quality consumables and I'm sure you'll get better results without a raised spline so I'm not saying you shouldn't upgrade the belts. By running some test boards with your current belts, staying close to or below the max cut for each grit, you give yourself a little more accurate information on how the machine and belts are performing. You may find you can get decent results with the current belts you have. Even if the problems continue it's a quick way to find out if the belt splice is the primary issue or taking too deep of a cut or both.
Keeping the stock removal to the minimum necessary to remove the previous grit's grooves will extend belt life and help to minimize washboarding. I found a good chart as well as a wealth of info in wide belt setup and operation at the Surfprep site.
You can gauge the rate of fine manual movement of the table by counting the handwheel rotations necessary to move it say .05" and doing the math.
When I last worked with a widebelt we had the least chatter with 3M belts with taped finger joint splices that were no thicker than the rest of the belt.
I am a technician for SurfPrep abrasives and I work on sanding machines for a living.
That mark is a very classic belt splice mark. This is usually most apparent with cloth belts using a thick tape splice without properly skiving the backing to produce a thinner joint.
Once the grit on the joint is worn away it is impossible to get rid of the mark entirely.
This is why we use a zero tolerance lap splice on our paper wide belts. Once we go into production on cloth belts we are going to properly skive our joints so we have zero tolerance on those too.
There are a couple things you can ask your supplier for. Grit side tape on the finer belts will help. The wavy joints are simply a way to widen the area of the joint to try to hide the marks. They can show belt splices marks the same as any other tape splice.
You will see less of the marks with the platen running. Try to sand something with the platen pulled up out of the way, then stop the head and run the piece under the head and lower the platen until it barely touches the part. This will get your platen working with your drum and hopefully hide some of the marks.
Grit Max Min. to remove
60 .035" .015"
80 .024" .008"
100 .017" .006"
120 .010" .004"
150 .006" .003"
180 .004" .001"
220 .002" .0005"
The first column is the maximum a grit can remove.
The second column is not the scratch depth, but how much thickness must be removed to touch the bottom of the valleys created by that grit. You don't want to remove more than that number with the next belt.
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