Old Style Belt Sander Platten/Table Misalignment and Partial Solutions

[algale]

Disclaimer: This is kind of long and wordy and probably boring. Those obsessed with tolerances and alignment issues should probably stop reading right now. Otherwise you are going to find yourself in your shop testing the alignment of your belt sander table and possibly discovering a problem. But you may also learn a solution. Those not obsessed with alignment issues can also stop reading because you probably won't give a hoot.

I recently came into possession of an older style Shopsmith belt sander. What makes this an older style belt sander is that the platten sits on 12 washers -- two behind each of the six screws that hold the platten in place. The newer belt sanders don't use these washers at all and have bosses on the castings that raise the platten.

This is my first Shopsmith belt sander but I noticed immediately that the platten and table were not nearly parallel. After removing the belt, the problem was really obvious. By bringing the table right up to the platten as far as it would go, the table touched the platten on the right side but had a gap on the left side which, using my feeler gauge was at least five hundredths (0.05) of an inch wide. While this probably wouldn't make a difference for free hand sanding, in my opinion if makes using the miter gauge on the sander for precision work suspect

I checked the forum and didn't see anything talking about this issue. I also watched the Sawdust session on the belt sander tune up and also saw nothing that addressed this issue. Maybe I had the only bad belt sander Shopsmith ever made?

I decided to take the machine apart. In the process I did discover an iffy bearing on the drive drum (which I replaced) but nothing that explained the lack of parallelism between the platten and the table.

I reassembled everything and still had the same gap.

I took off just the platten again and, on a whim, decided to put a micrometer on the washers. What I discovered was that there were two distinct washer thicknesses. Of the 12 washers (all of which look original) six of the washers were about 9 thousandths (0.009) thicker than the others. I decided this wasn't an accident. But if the washers were ever used in a particular order to shim the platten, that order was long gone because of my previous disassembly efforts (if not because of a prior owner's disassembly).

When I reattached the platten this time I put all six of the thicker washers (two per hole) on the left side of the platten (where the gap was) and all six thin washers on the other side. When I put the table back in, there was a noticeable improvement in the parallelism of the table to the platten. Where there had been a 0.05 gap, the gap was now only 0.005. Much better!

Moreover, I discovered, quite by accident, that the order in which you tighten the set screws that hold the table posts can have a definite impact on how parallel the table is to the platten. This may even impact newer belt sanders (without washers). I discovered this because I had mounted a dial indicator jig in the table's miter slot to measure my misalignment. I noticed that tightening the left side set screw tended to pull the left table post in toward the platten by a few thousandths. Likewise, tightening the right set screw tended to push the right table post away from the platten by a few thousandths. I got best alignment by first tightening the left side, then the right side, then loosening the left side before finally retightening the left side. Doing that allowed me to get very good alignment of the table to the platten.

If after reading this you decide to test your own belt sander, I would be very interested to know if you find a parallelism error between the platten and the table.

Al

[JPG]

Disclaimer: This is kind of long and wordy and probably boring. Those obsessed with tolerances and alignment issues should probably stop reading right now. Otherwise you are going to find yourself in your shop testing the alignment of your belt sander table and possibly discovering a problem. But you may also learn a solution. Those not obsessed with alignment issues can also stop reading because you probably won't give a hoot.

I recently came into possession of an older style Shopsmith belt sander. What makes this an older style belt sander is that the platten sits on 12 washers -- two behind each of the six screws that hold the platten in place. The newer belt sanders don't use these washers at all and have bosses on the castings that raise the platten.

This is my first Shopsmith belt sander but I noticed immediately that the platten and table were not nearly parallel. After removing the belt, the problem was really obvious. By bringing the table right up to the platten as far as it would go, the table touched the platten on the right side but had a gap on the left side which, using my feeler gauge was at least five hundredths (0.05) of an inch wide. While this probably wouldn't make a difference for free hand sanding, in my opinion if makes using the miter gauge on the sander for precision work suspect

I checked the forum and didn't see anything talking about this issue. I also watched the Sawdust session on the belt sander tune up and also saw nothing that addressed this issue. Maybe I had the only bad belt sander Shopsmith ever made?

I decided to take the machine apart. In the process I did discover an iffy bearing on the drive drum (which I replaced) but nothing that explained the lack of parallelism between the platten and the table.

I reassembled everything and still had the same gap.

I took off just the platten again and, on a whim, decided to put a micrometer on the washers. What I discovered was that there were two distinct washer thicknesses. Of the 12 washers (all of which look original) six of the washers were about 9 thousandths (0.009) thicker than the others. I decided this wasn't an accident. But if the washers were ever used in a particular order to shim the platten, that order was long gone because of my previous disassembly efforts (if not because of a prior owner's disassembly).

When I reattached the platten this time I put all six of the thicker washers (two per hole) on the left side of the platten (where the gap was) and all six thin washers on the other side. When I put the table back in, there was a noticeable improvement in the parallelism of the table to the platten. Where there had been a 0.05 gap, the gap was now only 0.005. Much better!

Moreover, I discovered, quite by accident, that the order in which you tighten the set screws that hold the table posts can have a definite impact on how parallel the table is to the platten. This may even impact newer belt sanders (without washers). I discovered this because I had mounted a dial indicator jig in the table's miter slot to measure my misalignment. I noticed that tightening the left side set screw tended to pull the left table post in toward the platten by a few thousandths. Likewise, tightening the right set screw tended to push the right table post away from the platten by a few thousandths. I got best alignment by first tightening the left side, then the right side, then loosening the left side before finally retightening the left side. Doing that allowed me to get very good alignment of the table to the platten.

If after reading this you decide to test your own belt sander, I would be very interested to know if you find a parallelism error between the platten and the table.

Al

I assume what is really relevant is the alignment of the platen to the miter gauge slot, not the edge of the table.

[algale]

I assume what is really relevant is the alignment of the platen to the miter gauge slot, not the edge of the table.

Yes that's true. But the miter slot appears to be parallel to the edge of the table so that the misalignment of the table to the platten translated to misalignment of the miter gauge, which I think I have now resolved.