Being an EE, and not an ME, I cannot calculate a safety factor for your stand. But I have worked closely with many ME's over the years. I well remember one quoting an ME professor as saying 'Safety factors should really be called "ignorance factors". If we actually knew exactly how a given material would behave, there would be no reason for a "safety factor."'algale wrote:With the 92 pound DW735 planer sitting atop the stand for the last several days, I measured the sag at the center of the front edge of the top as 0.019 inches. I measured the sag by placing a straight edge across the sides and inserting a feeler gauge in into the gap in the center.
This was actually very close to what the "Sagulator" suggested it would be. For those not familiar, the Sagulator is an on-line program that purports to calculate how much sag you will get in a beam or shelf based on the materials used, their dimensions, and the load. Here's the site: http://www.woodbin.com/calcs/sagulator.htm Based on my dimensions, material and load, the Sagulator calculated the sag would be 0.02 inches.
As an experiment I tried JPG's suggestion of laying some temporary fir strips left over from another project across the top (left to right) which were long enough to over hang the sides. What I had on hand measured about 1.22 x 1.50 so they were a little lighter weight than what JPG suggested (2x?). With the strips in place, I measured no change in the sag between the bottom of the front strip and the center of the top, which is a very counter intuitive result.
I am still undecided whether to convert this stand into storage (as Ed has suggested) or keep it as a planer stand. I have run a few boards through it, the largest being a six foot length of fir 2 x 10, and the stand seems sturdy in spite of its relatively light-weight construction. When I lock down the casters and put boards through the planer there is no observable wracking or wobbling of the stand. Nor does the stand wrack or wobble when I roll it over the shop floor with the planer sitting on top (although I haven't run the stand over or into any obstacles, which I think is what Ed feared). Maybe I'llg put a similar weight (couple bags of sand) on top of the stand and push it across over the threshold between the finished and unfinished parts of the basement and see what happens.
Decisions, decisions.
Al
So maybe your observations account for unexpected anomalies and circumstances. Or maybe they don't. Perhaps the accumulated experience of the folks that advised beefier construction are over-conservative. Or perhaps they are not. Personally, I plead ignorance, and thus I would tend to go with the accumulated wisdom.
If it is electrical, and I thoroughly understand every aspect of the physics involved, I am comfortable with a 20% safety factor. If it is mechanical, on the other hand, and I do not have an ME to analyze the design (mostly for hobby stuff), I go with a conservative gut feeling, and then make it at least four times as strong as that. And then stress-test it to that safety factor or higher. All of which makes it inexpensive to design, but would also make it prohibitively expensive to manufacture in any volume.
Hmmm. Risk analysis.
The key input variable being the price of that nice DW735. 
Can't rule out, however, that there wasn't some amount of cupping in the ply before construction began.
