RepRap: Blog

Having assembled the current V0.04 I tested the X axis using my "Bogan Calibration Slide" - a slide with a piece of wire of known diameter on it. I noticed that some "50μm" steps were smaller than others, and after hunting around nailed this down to flexing of the X Axis Driver Bracket, despite having driven a couple of long M3 screws down it. So, the Axis Motor Diver Assy has picked up a couple of pair of mounting holes to anchor that a bit more firmly:

With the aid of a 2-hole Metriccano Square Strip (and another on the bottom), the Driver Assy and the Bracket are now firmly clamped together. Only one screw in each pair, dammit, but that's what I can do at the moment until I redo the Bracket with a captive nut cavity.

This was debugged basically by shoving Metriccano and business cards into gaps until the flexing observed through the microscope went away.

Much of last evening was spent battling with Flatpak, the app packaging system used for PrusaSlicer 2.9.2 which in my opinion is a step backwards from using AppImage. Flatpak mounts itself in its own little box, and to access filesystems outside of that box you have to specify to Flatpak (not Prusaslicer) that these should be accessible.

For instance, if I want to access the /mnt directory on Linux, where you manually mount disk drives and USB storage devices in fixed locations, I needed to run:

 flatpak override --user --filesystem=/mnt

or /mnt and all the drives mounted in it just will not appear in PrusaSlicer's file manager! This is not in the install docs.

Just to be clear, this is a Flatpak problem, not I Prusa problem. Though I would encourage Prusa to reconsider their use of Flatpak because I'm hunting down a couple more bugs in it that cause problems in PrusaSlicer.

Managed to get the X Axis connected in. The flexures all seem to reach, and moving the Y Axis manually doesn't seem to cause any deflection viewed through the cheap microscope. New Z Tower is a drop-in replacement. Next stop, Y Axis. If I can get that done and tested in the next few days I can leave the git/wiki/Printables work to do on rainy days in hotels over the following week.

V0.04 Under construction. Black parts come from V0.03 and there's not many left...

Work progresses on the Pantograph Driver. It's a bit unwieldy as-is, so I'm thinking of shortening the arms and lowering the mechanical advantage to 2:1. This should still be adequate for micron work, and I consider anything over +/-5mm movement as meeting the V0.05 design goals at this point. I'm trying to figure out ways of putting the complementary flexures to constrain the Drive Screw into either the motor bracket or the driver's frame. V0.03/4 Axis Drivers are prone to wiggle if not carefully assembled, and I want to eliminate that.

Another problem with long beams is that they require a long frame to support them, and that frame can flex. So the shorter I keep the beams, the more rigid the framework can be kept.

In other news, my Prusa XL is now operational, but it's a bit of a beast and my existing work table is clearly not up to it. I'm afraid I'll have to spend some time making a new one. This will give me multi-material printing though, which will enable a few more interesting experiments with flexures and so forth (though I do not want to make multi-material prints a requirement for the standard design).

Assembling the μRepRap V0.04 revealed a minor issue in that the X Stage Flexure now only moves 4mm before colliding with the Z tower. As moving 4mm is "pushing it" with the current Axis Driver, that's not a huge problem and existing builds won't need to fix it, but I've changed the Z Tower design anyway:

I've trebled up those vertical supports to help add rigidity.

Moving on to V0.05 that's all going to need a rehash anyway. The Axis Driver is going to change shape, so I'm not concerned about the bodged nature of this fix.

Current thoughts are to  add constraining flexures limiting lateral movement of the pantograph flexure, and only allowing it to move up and down in line with the drive screw. Haven't figured out how to nicely design that yet.

I've been nailing inconsistencies in the behaviour of several Axis Drivers and noticed that the 50mm long screws I use to hold things together and brace the motor pillars are mebbe too long. When tightened, these can force the motor mount back off the motor. So V0.04 will have fixes for that in the form of holes in the plate underneath that'll let the screws over-extend a bit.

On the green part you can see the slope I've added for the limit switch to lay in. The red part is actually 1mm shorter overall, because I had one axis where the Drive Screw couldn't reach the Bearing Block at the top. I thought "If I have this problem, someone else will" so I shortened the pillars by 1mm. That's when I drove the 50mm screw into the motor through the holes in the top of those pillars, and noticed that another Axis Driver had the same problem. Fixed dat.

The Github Wiki has acquired a few updates on assembling the V0.04 parts - not finished yet though. I've assembled the 3 Axis Drivers and the XY Flexure Assembly. Just have to bolt them all into a solid lump and fit/smoke test everything. Will upload the V0.04 model changes then.

OK, so I broke the first pantograph flexure. Fortunately I printed another one overnight.

Slight change to the model - I got the centre lugs slightly mis-spaced for Metriccano holes. Fixing that. Also because I fragged a flexure because it was hard to poke the nuts in under the flexures, I'm extending the mounting brackets.

The +/- 8mm range of movement doesn't cause any collisions, but there is a fair bit of flexibility in the beams that allows some torsion if they're driven off-centre. Shorter, thicker beams might be needed.

Another change I might make is to stagger the  anchors the other way, which would allow the drive screw to travel between the gap in the flexures. At the moment, if I tried that, the drive screw would have to poke through a beam. Damn.

One thing I realised is that if you put two of these flexures back-to-back, they form a very stable one dimensional linear reduction flexure. It's bulky, but might be useful. 

Had a few thoughts while away (the second half of my travels have been postponed), so here's the next iteration of the parallelogram/pantograph flexure. No droopy bits on the print this time. I chickened out from moving the drive platform more than +/-7.5mm because I want to do a few other tests before I stress-test it. But that's plenty for the next major version of the Maus RepRapMicron.

 

The flexures are now only 4mm wide instead of 6, and 0.6mm thick (2.5mm long BTW). Their profile has changed too, with the footings that hold the flexures being plain vertical sides, and the top flexures that have to anchor to a 45 degree beam now have dead flat undersides. Oh, and the top beam is now level with the bottoms of the upper flexures so no crud falls out during printing.

With the narrower, cleaner flexures this is far easier to flex and the torsion on the ends is vastly reduced. I'll have to put it in some kind of test frame and give it a go.

My main concern is that if I have a horizontal drive screw the thing is going to bounce up and down because the drive screws seem to always have a little bit of eccentricity - which matters when you're using microns. So the axis being driven will have to have good constraint in (what in this illustration is) the up/down direction. If not, I'll just have to build some complementary flexures into the drive frame, but I'd rather not have to.

This flexure system should work in 2 dimensions, but that's not what I'm designing it for. Letting it flop around on two screw-driven axes unconstrained at this point is going to introduce a lot of errors. Later, maybe.

It looks a lot worse than it is, folks. Not that there aren't changes to be made. It seems to move as planned at first glance, but was an awkward print, and the 3DEA filament I was using had poor bed adhesion.

One of the Flexible Couplings I'd made from it snapped internally - source of my suspicion about the X Axis Driver - so I switched to eSun PLA+. Quality filament pays for itself in reduced reprints.

The top flexure doesn't print in a stable way, but that might be fixable with some tweaks to the top beam. Testing it shows the flexures to be a bit stiff, so altering their length/width seems like a good idea and I might fix the profile a bit (the code there is a bit bodged). There's also more torsion at the two ends than I'd expected. That might resolve with more flexible flexures, but I'll have to pay attention as to where I put the drive nut.

I'll test it more thoroughly anyway, but not today. My schedule is very packed for the next few days with a shooting competition, surgery consult in the city, and some family business. The end result being I'm probably not going to get much done for the next week or so other than pushing the odd update to github. Hasta la vista.

I've put platforms for drive screws, anchors, drive flexures etc. on a trio of parallelogram flexures. In theory the flat bits remain parallel when the flexures move in the XZ plane.

Having a little difficulty printing it as it's bloody cold here and things tend to detach from the print bed. Hopefully soon I can give it a bit of a test.

One thing on my mind is which dimension to drive it in - motion will be in the same direction as the drive. If I drive it along the Z axis, any wobble induced by the screw drive will be constrained in the Y axis. If I drive it on the X axis, wobble will be constrained on the Y axis. If I drive it on Z, both X & Y will wobble (though Y is fairly well constrained).

If I can assume that whatever I'm driving is constrained on the X axis, it makes sense to put the Z. All the framework would need a redesign, of course, and the XY Table might as well be redesigned to take advantage of the new driving arrangement. *Sigh*