Blog: Well, This Is Embarrassing By: John Heisz

Off and on over the last year or so I’ve been thinking about a design for a drill press table. More accurately, I’ve been focusing on one particular part, the lift mechanism, to try to find an efficient and low cost way of building it. I see the basic operation of the table being lifted by four threaded rods that turn at the same time, but have been hung up on how to drive them all from a single point. The common way is to use a bicycle chain and sprockets and while that is easy and reliable, it can be costly and difficult to source the parts – I prefer to avoid specific parts like that when designing a project I’ll make plans for.

Another way that I kind of settled on was to use gears, a series of seven to link the four threaded rods. Since the gears can be made from plywood, this would solve the problem mentioned above, but does make the build considerably more complex. It would be interesting, though.

Gears are around 4″ in diameter spacing the rods in a 8″ grid

But recently another method occurred to me, and that was to drive the rods with rope, taking advantage of the same principles that make capstans work – the friction between the round spool and the rope that allow it to grip without slipping.

One of the spools, or capstans made from two layers of 1/2″ plywood

However… for some reason I can’t even grasp, I failed to see the fatal flaw with this arrangement: as the rope winds around the capstan, it will eventually wind itself right off of the spool. So, the friction that make it effective for the transfer of power works against me in this regard.

Unfortunately, I didn’t see this until after I had the mechanism mocked up, which is odd – I must be losing my edge.
I did have a feeling beforehand that something was going to go wrong with this, and hesitated devoting the time (just a couple of hours, actually) to building the test. There were three main concerns: how well the rope would grip; would there be any latency issues between the spools; and whether the rope could be joined seamlessly and not break. And as it turned out, each of these were answered positively – the rope’s grip was beyond what was needed, there was no discernible lag between the spools (although they weren’t loaded during the test, and that may have changed the result), and I was able to “weld” the nylon rope and that join is more than strong enough.

Anyway, lesson learned, and I haven’t completely given up on using rope. I can instead use it to make individual belts that drive pairs of spools and avoid the unwinding problem. And now that I have the mechanism mocked up, I can easily test that.

I also haven’t completely ruled out the geared linkage, but mainly because that would add more novelty to the project.

UPDATE: One thing leads to another and I did a bit more thinking about the drive, testing out guides to try to keep the rope from climbing (unsuccessfully) and came up with another approach that I talk about and demonstrate in this Instagram post: