It was all going so well... At the end of Part 2 I had created 1 leg of the gantry support. It looked the part, had a nice fit and slid fairly nicely along the rail. In the Easter weekend I built the second gantry leg and screwed the Y axis between the two.
At this point I realised I needed a hole in each leg to take the threaded rod that drives the Y axis. No problem, some careful meassuring and drilling and presto. But now I realise my previous tightening of the bearings in place on the Y axis had left the central hole not quite central.
So I slackened off all the bearings, threaded the rod through and realised I should have done this in the first place. As now when I tighten the bearings I know the platform is being held centrally by the threaded rod.
So finally x and y are assembled togther and looking the part
However there is a problem.
If you read part 2 you may recall I intentionally left a few mm gap to the sides of the runners on the gantry support legs. The bearings run tight to the top and bottom, but the sides I didn't want binding against the brackets. Well what I failed to realise was that this means the whole gantry can twist a little left and right, or slide a few mm one way or the other. When it does this some of the bearings basically fall off the flat top of the square section. This is not good at all!
I spent a while pondering a few solutions. One idea was to make side boards with a plastic surface that would pull up tight to the outside of each leg, and restrain it to running straight. But this would add a bunch more friction and get in the way. At one point I thought I may need to go back to the drawing board and start again.
After a break in the sunshine, I hit upon the ide of just narrowing the gap at the front and back of the leg, to fit snugly around the square section. I have a piece of plastic chopping board that has been my go-to source of machinable plastic, and I figured it's shiny, waxy surface would limit the friction whilst still being firm enough to guide the whole thing.
I quickly fitted 4 pieces, 2 at the back and front of one leg. And it was so effective I decided it wasn't necessary on the other leg. Which also meant avoiding adding extra friction if it wasn't necessary.
Having now got things sliding smoothly, and stably back and forth I turned my attention to the drive for the x-axis. I spent some time looking on-line, and realising that I could probably make my life easier had I followed one of the more standard designs. The main examples of diy machines seemed to use a chain drive, with the motor housed in the gantry. This isn't an option at this point, and the ability to drive both legs at the same time seems potentially more complicated for the path I have chosen. So for the moment I settled on sticking with threaded rod for the drive. And I figured I'd try it on just one leg and see how I get on.
With this setup I just used my cordless drill/driver to grip the end of the rod and drive it. The gantry had no problem sliding back and forth driven this way, so I hope a proper stepper motor will have no problems either. There was a little torquing on the gantry at start of motion, and time will tell how bad that is. For the moment I'm going to carry on and hope that it's good enough. It may turn out the first thing I attempt to make with this device is v2. Perhaps based on some of the well established designs I should have used in the first place. However I'm having fun on this learning experience.
The next things to do are to re-adjust the z-axis with a proper piece of threaded rod now I've got enough. Then tighten up the Y axis bearings again, set up the z axis, and make a mount plate for my little rotary tool (which I'll test with before taking the plunge on a decent router)
Then I need to figure out how long it will be before I can justify the high cost of motors and drivers and power supplies and controllers. Anyone with good advice on mostly diy options here, feel free to get in touch!