RepRap: Blog

A couple of ways this parallelogram flexure can be used. Obviously, these are just lash-ups and the actual frames will be designed somewhat more competently with end bearings, anti-backlash, limit switches etc. They do at least demonstrate a much greater range of motion than the existing drivers.

The one above is more compact (50mm longer overall that the V0.03/4 driver), with shorter members. It'll drive the free end up and down. Nice, but somehow there is a motion stage that it needs to fit to somewhere. I like it though.

This one drives the free end horizontally, so is more likely to be out of the way of the final stage hardware. But it needs longer support structures, which will themselves flex, and is rather bulky to handle overall.

The exercise here is to get constrained linear motion and 2:1 linear reduction out of the driver, so that it can be driven by any old GRBL hardware. Either way I drive it, there is obviously one unconstrained direction. But the stages themselves are constrained, so I just have to make sure that movement in the unconstrained direction doesn't matter. Half of me wants to go "go linear and damn the reduction" but it'll be needed in later versions so I am persisting for now.

 

Notes to self: Mount points on the parallelogram centre need to be correctly spaced for 10mm centres. Driven end must not rub on support structure. Driven end for vertical driver needs to have a rest position above the drive screw, and the motor needs to be dropped by 10mm+. 

While I was away I did that panotgraph axis driver with the cranked arms. When trying to practically mount it, I realised it's a bit long. Here's a photo of the new one and a V0.03 Axis Driver:

The problem is the length. If I have to drive one end, and support the other, I need a structural member 140mm long, and that's going to have unavoidable flex to it. Ideally I'd have the supporting member no longer than the 60mm of the V0.03/4 driver. If I just shorten it up by half, the joints will have to bend beyond the limits of what I can do with PLA. There are a few options, and I'll probably take them all:

1. Fix the middle, not the end. This will work, but I'll need to change the ratio as fixing the middle on this version would give a 1:1 ratio, i.e. no advantage whatsoever. When I change that ratio I need to make the left-hand half a bit longer, but there's some benefit to he had there.

2. The arms don't have to be at 45 degree angles. It's the length of the arm that determines how much the flexures have to bend by. So if I scoot things together and sort of pre-flex the arm, I still get the mechanical advantage but there's less stress on the flexures. My model assumes 45 degrees, which is a pain, but that's fixable.

3. Make the beams narrower. They're a bit overkill at the moment. Can probably take a bit of overall length off there too.

There's probably a completely different solution involving clever linkages and complementary flexures. I remember seeing something a while ago, but to be honest couldn't figure out how it worked and can no longer find the references. May have been a fever dream or optimism. I'll give it one more go, and if I can't figure it out I'll get on with the depositing stuff again.

Putting cranks into the arms of the Axis Driver has allowed me to put the anchor points and drive nut outside of the range of movement of the flexure. This means that I can put a drive screw vertically through the centre of one end of the mechanism without poking the screw into one of the arms.

 

I took the opportunity to fix a whole load of my broken maths assumptions, and the structure appears to be behaving when I try a variety of movement reduction ratios. It's also a bit more compact, and thus more stable.

As range of motion is now a bigger deal from a prototyping and fabrication perspective, I'm going for a 2:1 reduction ratio in this prototype. In theory that should still allow movement of less than a micron, which is plenty when the smallest feature I can make is around 10 microns. Not too dissimilar a ratio from what you get out of a conventional 3D printer.

Unfortunately I'm not near my printer for a few days, so this is going to have to remain theoretical for a bit. I'll upload to github when I get back and have tested it, unless anyone hassles me in the comments below :)

Followers of μRepRap might be interested in the MicroManipulator project here. This is using some bespoke hardware, but it is all Open Source. By driving ordinary stepper motors as servos and using an array of magnetic rotation sensors and handmade bearings, he's managing to get movement resolution down to 50 nanometres.

As yet no fabrication capability, but there's no reason a μRepRap probe and deposition system couldn't be strapped onto it. There's some interesting control software there that might be useful too.

Anyway, here's the video intro https://www.youtube.com/watch?v=MgQbPdiuUTw 

Slight change to the Z tower to get rid of the spacers needed when mounting large NEMA17 motors on the Z Axis Driver. This also allows the Z Motor Pillar Assembly to be bolted to the Z Tower as well as the Z Axis Driver, leading to a more rigid structure. Moving the Z Axis at high accelerations has been shown to make it wobble a bit, and this should help. The updates have been uploaded to Github and Printables V0.04 repositories.

I've also spent a bit of time getting the Prusa XL running. While it prints multiple materials, that trick won't be needed to build μRepRaps as it would limit people's ability to reproduce the project However, the μRepRap itself is intended to be a multi-material printer, so the XL will provide an opportunity to develop the techniques for multiple material prints and create macro-scale models for demonstration purposes.

Example of internals of a multi-material print (model frog) on a Prusa	XL

 

Testing is going well (though I did sacrifice a stepper driver to the cause). Well enough to feel confident about releasing the V0.04 to the world to tinker with. The main build plates now look like this:

There are V0.04 documentation updates on the Github wiki, but they're not very polished. That'll get tidied up over the next day or so. If anyone needs anything clarified quickly, or spots a bug, please mention it here so I can fix the docs.

https://www.printables.com/model/1286343-reprapmicron-micron-accurate-3d-printer

https://github.com/VikOlliver/RepRapMicron

 

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).