So, I’ll put out some of the design parameters, and see what people think.
Okay, first off, most of this is in some embarrasingly bad CAD drawings so far, so I won’t have much in the way of pictures, unless I decide to complete it. Since a new ZenXY is in development, I’ll probably just go with that.
First off, the idea was to make a sand table that would address some of the issues that I have with my current ZenXY. Keeping it cheap is a design goal, but cheap is relative, because of stuff that I already have that I can throw at it. Some of this may not really qualify as cheap…
So some of the issues that I wanted to resolve:
Fully constrain the axes. The current design is a little loose on the Y axis, and the X axis depends on no movement at all in the bearing holder bolts. If the Y axis chatters, the bolts holding my X axis have driven their threads into the printed plastic, creating slop in that axis. This then compounds into more chatter in the Y and it’s a downward spiral from there. I want to be less dependent on single ended bolts as a part of this.
Provide for fine adjustment of the magnet, and allow for a stack of thinner magnets. Well, this is because what I could get in 1/2" round magnets were 1/8" thick discs, and not a “rod” The shipping from the source listed is prohibitive, and I could get the discs locally. In my current build, the discs are heat-shrink wrapped together, which works, but the top one isn’t constrained on the upper surface, and has managed to pop out of the tube once. I have to drop the center shuttle to put it back in which is a lot of work.
Reduce the possibilities for the endstop switches to go awry. I currently do not have a functioning X stop switch, because the block managed to get under it again, and pop the lever off of the microswitch. This makes the table unusable if I need to send a home command. This is one of the trickiest parts of the design for me, and one that I haven’t got a satisfactory answer to. I do have an answer, but it’s … not ideal.
So to start, I changed the orientation of the motors. The motor shaft is vertical, with the motors projecting upwards past the mount surface towards the top. Pancake motors might fit without needing the extra height, but I designed for this to project into the table volume. I’m using the Ikea LACK coffee table as a basis, which allows me about 50mm that I can go into the table bottom before I have any visible effect on the top surface. Since I’m designing for 48mm motors, and the end is 10mm below the mounting surface, there’s plenty of room to spare. This does make the mount a little larger than I might otherwise like, but for the table and the glass size I use, there is no conflict.
The belt runs along the inside of the tube, and through the middle of the tube for the unattached side. This eliminates what I saw as the primary opportunity for chatter in the original ZenXY design. (Ryan came to a similar conclusion for his new design, I think.) It also makes for one less place that little fingers can get into it.
For the trucks… Fully constraining it to the rail took me a long time to decide on what I was going to do. I have some small bearings (10mm OD, 3MM axle) that I thought I might use which would allow a fully captured rail. Another component that I wanted to have was to allow a little float on one rail, to account for slight bending in the tube used which also seemed to cause some problems with my current table The result was monstrous. So, I decided to simplify. I started looking at linear bearings, like the ones used with 8mm rod in 3D printers. It seems that the Chinese LMxUU bearings are being pushed out by Delrin bearings. Hmm. Hey, I have a CNC router, can I buy Delrin in sheet form? Well, yes, I can, but it’s expensive. UMHW is a little cheaper, but nothing in stock. HDPE? The price for that isn’t bad on Amazon, but it’s usability? Well, someone on CNCzone was asking about it, and while the response was that Delrin is really what you want for any load, HDPE will certainly do the job. Then they go completely off topic. Anyway, I think that the HDPE will certainly handle the very light load that a sand table will put on it, given that the bearings themselves will be the vast majority of the load. I redesigned the trucks to hold a couple of key shaped chunks of HDPE, screwed in place on the X=0 size, but left able to float a couple of mm on the X=MAX side. This allows for a slight bend in the rails not to take the whole machine off track.
Okay, so if there is some drift in the X axis, this also affects belt tension, right? I also needed to figure out a way to get the belts past each other in the Y=MAX side, since both rails are operating on the same plane, and the belt needs to go from the inside track to the outside track while the one going the other way needs to do the same. I decided to make a spring-loaded contraption to pull the belts out of the way of each other, and allow them to float just a little on the springs. (Also, if I ever need to pull the shuttle down, it will be nice not to have to unstring the belts.)
Undecided if I want to put a half twist in the belts going through the tubes. This would allow the smooth side to be in contact with the idler pulleys at the end, which is a plus for noise control, so I’m giving it serious consideration. It probably makes no difference to the part design.
The shuttle will use the same HDPE bearing “keys” and I will probably still allow one side to float, though I certainly hope it won’t need to. The magnet will be encased in a sleeve, so that I can use a stack of disc magnets. The bottom will be threaded for a 5/8" bolt. I will 3D print the 5/8" bolt (And finish the threads with a tap and die) so that it’s a non-magnetic solution. This means that I should be able to remove and replace the magnet without dropping the shuttle, and adjusting the gap from magnet to table will be a simple job.
Because of the HDPE “key” bearings, I will be able to pull the tubes up to about 1/4" from the table bottom surface.
I might shorten the key a bit from here, but this is the idea. Cut these from the 3/4" HDPE sheet and run them over 1/2" pipe (5/8" OD)