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Sarrus Linkage Mark III

by fdavies, published

Sarrus Linkage Mark III by fdavies Dec 12, 2009

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Description

This is the next step in my attempt to make a Sarrus linkage based 3D printer. The idea is to have a cartesian mechanism without those long rods and bearings.

I built three of the Mark III and mounted them in a x-y arrangement as shown. They can move over a square about 105 mm wide, and someday may carry an extruder. They are driven by DC motors taken from inkjet printers. These motors are driven in a servo arrangement using quadrature optical encoders and optical strips removed from the same printers.

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To check the strightness of movement you could put a small LED on it and take time exposure photgraphs so the LED leaves a trace behind .. needs a tripod though.
You can set a 300 gram weight on it and it still moves smoothly. I could not say how straight a line it moves in, though.

fdavies
How much weight can yours lift?

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Instructions

Print: 2 end pieces 2 armatures 3 spine pieces 6 hinge plate type a 6 hinge plate type b 1 motor plate (modify to fit your motor) 1 belt plate 1 pulley plate 1 opto plate 1 DB9 bracket

Do a lot more stuff (details available upon request) like trimming, drilling out holes, etc. Mount either a stepper motor with belt (not shown)

Make three of these things and connect them as shown.

To check the strightness of movement you could put a small LED on it and take time exposure photgraphs so the LED leaves a trace behind .. needs a tripod though.
You can set a 300 gram weight on it and it still moves smoothly. I could not say how straight a line it moves in, though.

fdavies
How much weight can yours lift?
I made each axis double ended for two reasons. Firstly, I am moving the axis with a belt that needs something (motor or pulley) at either end. Secondly, it keeps everything stiffer.

I have double horizontal hinges on each end of an axis (4 in all) because these are what holds the weight.

redsteakraw, I am not sure what you mean about it moving into the build platform. Nothing in the mechanism interferes with the u-shaped wooden platform shown in the pictures.

I basically d
esigned this by instinct, without any detailed mechanical stress analysis, so it is quite possible that there are better ways of doing it. You could remove 6 of the hinges on each axis, leaving only 2,and it should still work. I suspect that it would be looser and weaker. However, it would certai
nly be easier to make. Some one should try it. I am currently working on designing a z-axis that works as well as I would like.

fdavies
Can you get away for the Y axis to only use two Sarrus Linkages so it doesn't move into the build platform when it compresses? One end is supported by the ground and the other sides could be supported by the top / outside. I see the X axis uses only 3 linkages, I just wanted to know if there was any reason it was built like that.
My god! Awesome.
Can you weigh the plastic on just one of the linkages without any electronics please?
I was thinking about the best way to make a fully printable Cartesian bot using this and I think I have the basic system down pat. you can do 3 axis with only 3 moving bars (this will also eliminate the rotation issue, as that only exists becouse of 2 different linkages) it will need a fair amount of support structure to make work but if we print off individual blocks that snap together support isn't an issue. it could also change tool heads but sadly we would have to use live space :( on the other hand we wouldn't need any extra motors, the tool head switcher can be fully mechanical and printed.

I have a simple mock-up attached (very smiler to Mendel).
Making this thing is a bit like writing a sonnet. It is an attempt to do something within a set of somewhat artificial rules (parts printable within 100mm cube, printable on my repstrap, minimum nonprintable parts, etc). If you change the rules by allowing the full set of tools that our civilization has developed for mass production and adding cost as a limit, you would get very different results. I suspect that it would be closer in style to the cheap inkjet printers one can buy these days.

I am currently working on a Z axis, and then on certain improvements to the Mark III which should stiffen it up. I do not have good tools for quantifying the hysteresis, etc. I hope to show that it is worth something by showing the community what it can print.

fdavies
O, that someone might mass produce this.
Way impressive... am wondering if you've managed to pin down some specs on resolution and/or repeatability (hysteresis, backlash etc...)?
Looks like an ingenious method of getting things done. Although I did notice a little bit of slop on the lefthand axis, in the youtube video that was posted. I'm guessing since the righthand axis is being driven that you can't really expect the other side to stay perfectly parallel due to flex. I'd recommend duplicating the drive mechanism on all beams, so that you don't get that flex. Unless that little bit of shifting is acceptable. Perhaps you already have plans on fixing that, or maybe the flex will go away when both sides are afixed to a common backing.

Fantastic project, I can't wait for more updates 8-)
A rack and pinion setup will also be a printable alternative for the timing belt.
This is made of ABS.

You can't really see it in the video, but the central axis rotates along a vertical axis when its direction of motion changes (like angular backlash). There are several solutions to this, and one would be moving the build platform as PhantomAGN suggested. Others are motorizing both parallel axes, or putting a parallelogram arrangement in.

I am currently working on a Z axis hanging from the center as vik and gregr guessed. I am not planning on using a plastic screw like Eric mentioned, but that idea has some interesting aspects. I suspect that the screw would like to sit on the filament layers, and so it would naturally replicate its own layer spacing.

I have some ideas about a different sort of extruder that I will present if a prototype ever shows they are worth anything.

fdavies
I'm an amateur at this so forgive if this seems obviously wrong but for the hinges could you possibly leverage a ball and socket to avoid the need of metal pins for your hinges? . Multiple ball and socket linkages between two plates should effectively limit flexing to an acceptable amount, and if you lubed the socket hopefully friction would be able to be overcome by your stepper motors. I could be wrong, depending on the accuracy of the printing, I don't know what the tolerances would have to be to a allow a workable fit between the plates.
I hope you don't mind, but I uploaded the video to youtube to make it a little easier for everyone to watch

youtube.com/watch?v=RpCHcZrsyNU
Wow, really wonderful! This is really exciting to see it running so smoothly. Thanks for showing that this is probably a viable path!

I often lay away thinking about mechanical setups that might work. While at daytime I can dismiss most ideas, one of the stuck, which would be particularly suitable for the Z-axis. I noticed that Philipp's (Wizards23) screwable box would probably work as a medium-friction axis, as well as the stru
ctural part. It would probably need no current to stay in the same position, which is efficient for mostly 2.5D motion in the cartesian bot. Any decent stepper motor would probably be have enough pull out torque to move the axis. You only need a collar around the printed thread, just long enough to
prevent axial motion without rotation of the 'thread'. It will have no backlash as well (which is not problematic for a Z-axis, though). The bottom part could have a gear integrated into it (just a simple OpenSCAD union operation of a gear and this OpenSCAD part). The inner axis could be extended ea
sily by stacking modular parts that you either super glue together or that have a friction fit. From playing with these boxes I'm fairly confident that this will work!

Another tip: if stiffness is an issue when you have a heavy stepper based extrusion mechanism, the bowden setup will have big advan
tages. You effectively remove most of the weight from the moving axes. Even the bowden cable guide can be printed, the drag chain by Zaggo does a great job at conducting 3mm filament while adding only little friction (especially when printed from slippery HDPE).

The timing belt could also be 'print
ed', perhaps using one vitamin part (steel wire) as an axle. Except for the motors, you have effectively a fully printed RepRap!
If it's not too strong, wouldn't it be better for the linkage to carry the build platform?
Current extruders are still quite a bit heavier than most parts.
Two more of these with an extruder slung between would probably work great as the Z though.

I'm really looking forward to seeing this design take off. (hopefully, my own machine will be working by then, and I can print it!)
Logical progresssion: Presumably a head suspended from the central segment to give the Z motion ?
mattw - in reply to vik
seems to me a Z as a bottom piece might work as well.
which plastic is this printed with?
i see the future
i'm just had no words after the video

awesome work, cheers !
I was sketching out something like this about a year ago but didn't have a printer working to actually make it. You've just gone and done it! This is BRILLIANT work that you've done here!
This is really amazing!

So, I guess the next thing to do is add a Z axis?
Amazing!, Good Work! =-O
There are probably better ways of putting a video here, but I have added a MPG file to the downloads that shows the system operating.
I see you have motors on it. Can you upload a video of each axes going back and forth? I would LOVE to see it move.
I printed all this on a 'repstrap' that I built mostly from old printers (old dot matrix printers had really great hardware). It uses a pinchwheel extruder heavily inspired by the work of Nophead on reprap.org.

I did design all these parts to print in a 100 mm cube, both so that they would not be too big for a Cupcake, and so that it could print itself.

Why three units? Well, ABS plastic is only so strong, and not very stiff. With just two units at right angles, the bottom one would have to withstand
significant torque (twisting) from the weight of the extruder (when I put one on), and would let the top axis tilt out of the horizontal,

Why did i make it as three (mostly) identical subassemblies, rather than as a monolithic entity?

1. I am still not 100% sure that this will be the final conf
iguration, so I want to be able to reconfigure if necessary.
2. The warping of large objects that is plaguing the reprap community means that it is better to make several small pieces that you connect together rather than one big piece. This is why the spine and armature are broken up.

fdavies
this is so amazingly badass don't even know what to say. i wish you the best of luck sir!!! what did you print all these on?
How's the gitter? I mean can you giggle the X axis along the Y axis?
So Beautiful! :-D
Very impressive!
But why 3? If you join two at their center pieces you would have an X-Y table right?
And since we are printing why join them afterwards ;-)
Ohh wow, that is amazing! You are a true artist/engineer!
Oh my you have done it - looking at the files you loaded - I have not been able to view them yet. AOI and my laptop are not yet getting along. . . Do you have any video clips of the mechanism in action? very nice work on the progress so far. Thank you for letting us see it.
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