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snowygrouch

Throw away your short belts - direct drive.

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Am not Medusa, but here's my guess: I find that some parts like to lift off my heated base plate, typically things like the bottom corner of a box when I've built-up a couple of centimetres of wall height; the top layers contract, levering up the bottom. Using a raft can leave a large mess to be cleaned off large area like the bottom of my box, so instead what I often do is extend the bottom of the box by say 10mm with a thin plate, typically .4mm thick, all the way round to form a flange. Then after I've printed maybe 2mm of the box, and well before any lifting starts, I will put something like a steel ruler on top of the flange while the printing continues, and add some small plastic G-clamps or whatever, so that I really clamp the flange down onto the HBP and there's no way it's going to lift up. These clamps usually stick outside the frame of my printer. Maybe meduza's doing the same?

It's because the cooling fan would hit a side panel that is flush.

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Peter, rather a late reply, sorry, I don't come here often! Reason for not going straight through the side was general damage/weakening to the frame. If I'd discarded the 8mm slider rods and reused those holes, the spiral rods would repalce the 8mm shafts, and the plastic nuts would have gone where the wooden blocks clamp to the 6mm rods. There are several problems with this idea! 1) the nuts are bulky compared to wooden blocks, so I might lose some traverse distance. 2) would have to make up new blocks to clamp to nuts. 3) Any adjustment (backlash-removal etc) of the nuts would alter their axial height. 4) the spiral shafts are rolled, not ground. My experience of rolled shafts is you cannot guarantee they're straight, so I'd expect some up-and down & side-to-side movement as nut traverses the shaft. I'd rather trust the 8mm ground shafts to be straight.

 

So I decided to retain the existing arrangement of shafts as much as poss. In the pics above I have in fact removed some, not all, of the 8mm shafts, and suspended the blocks from the helical shafts. This worked to reduce friction, but made levelling HBP harder, as any height-error in the helicals meant effectively the HBP looked twisted to it!

 

Since then, I've modified it further. See the pic below. All the 8mm shafts have gone back in, all the wooden blocks + hacky angle-aluminium joiners have gone, replaced by printed ABS blocks with built-in 1mm-thick "blades" connecting to the spiral nuts. The spiral nuts now each have a short 3mm bolt sticking out radially, just drilled & tapped in by eye, and each pair of these back-to-back nuts has a short tension spring hooked round the bolts. So this is my backlash-removal system; the nuts are automatically tightened against each other, but not too much, and there are 3 long 3mm bolts which then go through the pair of nuts and through my new ABS blocks. This clamps everything tight & with luck the nuts will wear so slowly that I don't need to slacken & re-tighten too often. These ABS blocks are printed in opposite halves, and also bolted together inside the frame, that gives me the clamping force to hold the 6mm shafts tight, and also means I can actually assemble this stuff! All the ABS blocks have been given a 70C Acetone vapour bath for about 30 secs, which helps strengthen my prints by dissolving the outer faces of the layers together.

 

 Especially important, the oilites have gone & been replaced by 8mm linear-ball-bearing bushes, which have much less friction. Result is a far better print now than I've ever had. Am just about to improve on the ABS printed blocks by doubling-up on the linear bearings. This will help me hold the central 6mm shafts more rigidly and squarer, hopefully improving my accuracy. I'll just have to be careful not to get these pairs of bearings mis-aligned and re-introduce friction. The series of tests below are with the current setup of just one 8mm linear-ball-bearing in each of the 4 blocks on the four 8mm shafts.

UltimakerOriginal_Dec2017.thumb.jpg.59560900905b46cceb9cea2c7282b0fc.jpg

 

Here's the Negative-Space-Tolerance-Test from Make Magazine's 2015 test suite. The STL is on https://www.thingiverse.com/thing:533472

It's come out really well; for the first time I can actually press-out all 5 cylinders with my fingers easily, and there's zero interference between the cylinders & the containing wall, even with the 0.2mm gap one.

 

This uses Cura 3.1.0 printing at 60 mm/sec, travel at 15 mm/sec, 0.1mm layer height, ABS, 15% Grid infill, 1.6mm walls, accelerations max 750 mm/sec**2, Jerk max 10 mm/s (surely Jerk's the 3rd deriv of displacement, should be mm/sec**3? Maybe they're using Jerk to describe a delta change in speed, who knows...).

 

Side-on pic. Pins have all been pressed out, and put back loosely so tops line up unevenly. Tiny amount of ringing on the lettering. Virtually no ringing at the ends either.

NegSpaceTolTest_1.thumb.jpg.06c9969c3550bd2eac7f9fb17460e97b.jpg

 

 

Top view. There's shadow cast by the light, and the top-surface skin iso v thin so there's porosity leaving gaps into the underlying 20%-infilled cavities. Top & Bottom layer thicknesses set to 0.8mm so no idea why the porosity. Most of my prints are done at 100% infill, so maybe something else is set a bit wrong.

NegSpaceTolTest_2.thumb.jpg.ff876cad902db000c5984adae2b48c5b.jpg

 

Underside of the 0.2mm gap end. shiny bit is HBP layer, I specified Build Plate Adhesion = None as didn't want any raft/brim stuff messing this test up. You can see the 4 walls at 0.4mm dia each where a bit of the surface layer peeled off.

NegSpaceTolTest_3.thumb.jpg.02852447626b9e44d44d571f478a4228.jpg

 

Closeup of the 0.2mm (smallest) gap pin.Right in the middle is the seam - I set the Z Seam Alignment to "Sharpest Corner". This is the worst-quality part of the surface. Couldn't see any bridging anywhere between the pins & the cylinders. Result!

NegSpaceTolTest_4.thumb.jpg.195fac228b1c38d6360fd69a57579188.jpg

 

I'm now doing the Bridging Test. I usually use supports if I have to bridge, often they're my own 0.4mm-thick walls I add in, but this test is one the Ultimakers seem to do badly on. I had problems with the thingiverse-supplied STL. Cura accepted it, but printing froze immediately after printing the bottom layer. So I made my own replica object.

 

Front view. Every span has dangly extrusion. The bottom bridge spans 18mm, the top one spans 55 mm. You can see the pillar at LHS has warped a little during build, so when the spans gets printed it overhangs a tiny amount. Looking at the underside of the print, it has come away from the Kapton tape 105C HBP, thanks to the 2mm thick base-of-pillar-area shrinking as the ABS cools. ABS is a bit prone to distorting like this! If this was a real model of mine, I'd compensate by extending the base area all round the base of the piller, or thinning it or adding support-fins or something. I's also specify supports in Cura to avoid this bridging problem.

BridgingTest_front_1.thumb.jpg.fd5d44161f04f20f6a542ba81eb0bf9b.jpg

 

Back view.

BridgingTest_back_1.thumb.jpg.eb64e01023fa4062a26c833b7130cc8b.jpg

 

Close-up of corner of highest&longest bridge.

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After that rather sorry attempt at bridging, here's the XYResonanceTest. There are 2 STL files for this, one has, I think, 0.4mm thick walls? My m/c couldn't take those, dunno if it's a Cura-thing, or the machine itself. I just selected the 2nd STL file which has 1mm thick walls. As my nozzle is 0.4 dia, and I've told Cura in the "Shell" section, Fill_Gaps_Between_Walls = Nowhere, Print_Thin_Walls = Yes (ticked). BuildPlateAdhesion section is set to a Brim of 1mm around the outside only, so it's given me 1.2mm = 3 LineCount. This print is untouched after coming off the HBP, so brim, hairy bits are all there. Pretty-much everything is perfect, except the lower half of corner 1. Maybe this is the vertical seam where Z increases, I don't know, but it's strange that the poor quality bit stops, when the path round the outside starts doing the 1mm indentation! This dodgy corner isn't on that face either!! Weird or what.

On the 3 "plain" walls, I can slide a piece of paper between the inner & outer walls all the way down, and right to the ends. There's no interference/bridging between them. One the wall with the notch, the paper goes all the way down each side of the notch. So I'm very happy so far with the repeatability of positioning.

 

Top view. Note the gap between 0.4mm thick walls.

 

XYResTest_TopView_1.thumb.jpg.a7d904a8ad518bc3dc0763a375511b91.jpg

 

6 views of the notch in top half of print.

NotchDetail_5.thumb.jpg.521f89528195ab66002fade116cc8a05.jpgNotchDetail_6.thumb.jpg.ed8cc472ff8ea99810694f2d17d3049b.jpg

NotchDetail_3.thumb.jpg.945b326fd36be73b371d16a3a2d8f1fa.jpgNotchDetail_2.thumb.jpg.05daf9498e6079479342365a6361c50f.jpgNotchDetail_1.thumb.jpg.5bd3f835a7357a399f69df58539cb2f1.jpgNotchDetail_4.jpg.ebe721bcdb94740a2b2ae9569aa88ea4.jpg

 

 

For comparison, here are 2 pics of the same object, same layer height, printed when I was using toothed-belt drive & oilites. Pretty-much the standard m/c, maybe had my improved extruder on. This one was printed on a raft. At that time I don't think Cura had the option to not fill-in small gaps, so it's had a go at filling the 0.2mm gap between the inner & outer wall. Maybe I was over-extruding as well?

 

ANCIENT M/c - WARNING - you may be horrified !!! :O:'(

5a3d0b2133537_A002-20171222_133324.thumb.jpg.68e31a1c883af4bdf6de53087e9c2b6c.jpg5a3d0b2025073_A004-20171222_133526.thumb.jpg.f29212c30edc7c9e6c9284795e74944f.jpg

 

WARNING over - back to the latest M/c. :)

 

 

 

Detail of lettering on inside bottom surface.XYResTest_Letters_1.thumb.jpg.033c0d373ac517fa3c07a3af3d01d5b7.jpg

XYResTest_Letters_2.thumb.jpg.abd2be9e9e49353658168a09a2c5c45f.jpgXYResTest_Letters_3.thumb.jpg.39ca092779832c9e5be3782a3ab292ec.jpg

 

For comparison, here's 1 pic of the same object, same layer height, printed when I was using toothed-belt drive & oilites. Pretty-much the standard m/c, maybe had my improved extruder on. This one was printed on a raft.

 

ANCIENT M/c - WARNING - you may be horrified !!! :O

5a3d0b8b81706_A005-20171222_133823.thumb.jpg.65169396b3585c5c76f4773b8b5fb9c9.jpg

 

WARNING over - back to the latest M/c. :)

 

 

Corner 1 detail - the "bad" corner.

Corner_1_Detail2.thumb.jpg.c3e99100d67aff06f1c1431a75d384bb.jpgCorner_1_Detail1.thumb.jpg.80943371a7b6d32db8602f0a600f50aa.jpgCorner_1_Detail3.thumb.jpg.9d4b3016a087fadd388c66fc6cb8840c.jpg

 

Corner 2 detail

Corner_2_Detail1.thumb.jpg.4c9f237f9fe8bcb09309f03c3c5c59f2.jpgCorner_2_Detail2.thumb.jpg.9cfdfbecb80313116389f511e838a0a9.jpg

 

Corner 3 detail

Corner_3_Detail2.thumb.jpg.57ebfe5d9e6e9e7c691e43fd04a04cad.jpgCorner_3_Detail1.thumb.jpg.c55450581f3ae9888b52e1f0440ca666.jpgCorner_3_Detail3.thumb.jpg.1c99f8f381751fbf9aff1c6083814b5f.jpg

 

Corner 4 detail

Corner_4_Detail1.thumb.jpg.eb16f7f63a93340cadf686cf171424c2.jpgCorner_4_Detail2.thumb.jpg.755698fdabeb40f4f66d1762991c2fae.jpg

Edited by andywalter
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latest update. I've now repaired the temp controller, turned out that the AD597A chip was giving out incorrect voltage, telling the Arduino that the head was hotter than it was. I've also re-made the fan-shroud so the cooling air in now directed more downwards than before, hopefully it's not cooling the brass 0.4mm nozzle at all. I'm still not happy with the way my laminations stick together, so am experimenting with 260C extrusion, and may have to slow down to less than 50mm/sec.

 

On the rigidity side of things, I'm going to replace the original UMO printhead with a new design which will try to get the nozzle much closer to the same level as the present rods. I looked at getting hollow 8mm shaft, all I could find was Misumi, and as far as I can see (in UK anyway) they only sell to companies, not individuals. So I'm going to do it very differently. I've ordered 4 steel rulers, 300mm x 13mm x 0.58mm thick, from Kinexmeasuring.com. I hope these are hardened steel - the company looks decent quality judging by what they sell. 2 of these will replace one circular 6mm rod; one will be laid "flat", and one "on edge" or vertically. So it's a sort-of L-shaped angle-iron going across the UMO. The ends will be clamped to the plastic helical-nuts on my direct-drive shafts. Small ball-bearings will be wheels & guides, so the ruler mounted edge-on will provide stiffness in the vertical plane, keeping the height of the extruder nozzle correct. The ruler mounted "flat" will have similar bearings, and this one will keep the horizontal movement of the extruder-head the same as the 2 helical nuts.

 

Here are the calculations about stiffness & weight:

Stiffness (I) of 6mm dia Rod is Pi * (r**4) /4 = 3.14 * (3**4) /4 = 63.6

Stiffness (I) of 13 * 0.58 Bar is b * (h**3)/12 = 0.58 * (13**3)/12 = 106  so it's nearly twice as stiff as the rod (but only in 1 plane).

Mass of Rod is proportional to X-sectional area = Pi * (r**2) = 28.27

Mass of Bar is proportional to X-sectional area = b * h = 0.58 * 13 = 7.54

but I need 2 Bars to replace 1 Rod, so total Rod X-sectional area = 7.54 * 2 = 15.1

 

So my stiffness increases from 64 to 106, nearly double, and my mass reduces from 28 to 15, almost half.

There's some added weight from the bearings, but these will be as small as I can find, something like 4mm i.d. x 7mm o.d. flanged, probably need 7 or 8 per ruler. I'll save weight by removing the 6mm linear bearings in the original head, so I think bearing masses will cancel out.

 

Now my head is reaching proper temperatures again, I'll re-do the box-tests above. I've discovered, as Torgeir said, that the "dodgy corner" is where Cura makes a 180-degree U-turn and goes back the opposite way it approached the corner; I had a minimum 5 seconds/layer time, and the gcode had a time-delay inserted at this corner, so I think that was messing my prints up. I've reduced this to about 2 secs/layer, so the delay has gone, and the corners now look much better. Pictures later when I've got the UMO as good as I can.

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Yes. Almost completed the initial layout design, here's a piccy. Uses 4 Ruler-guide-mechanisms, these are T shaped and fairly thin.

Top Ruler-Guide is the yellow plates + orangy slider + 6 grey  bearings + 6 M3 capscrews+nuts&washers. (Just spotted 2 nuts need raising!). It's the X-axis direction guide controlling the Y-axis motion.

 

Ruler-Guides have pairs of flanged bearings, 3mm thick, 7mm dia on the running face, 8mm dia at the flanges.  Two flanged bearings go side-by-side, with flanges reversed to give me a sort of double-flanged bearing. 3 of these pairs per ruler. The central pair is mounted on a slider which has a short tension spring. So will need to measure strength of that spring & calculate what acceleration it will take without introducing backlash-due-to-acceleration-forces. So zero backlash.

 

Big yellow rod is M10 stainless steel shaft, drilled 6 dia, filled with PTFE plug then drill that 3mm dia to make my thermal block.

Locknut at very top stops rotation, and the top aluminium square chassis has 6mm thick threaded hole for the locknut to clamp.

Bottom end is simply screwed very tight into the heater block. Might put a clamp as well.

Orangy thing at top is the slider plate, with bent thin yellow strip with hole in for tension spring (cylinder). Haven't put in retainer screw at far end of spring yet. Yellow plates at back are rigidly bolted to the pale cyan aluminium top chassis plate.

 

The other 3 T-assemplies are brown at left, the UMO Y axis direction vertical-load slider.

Blue is the X axis direction vertical load, Green is the Y axis horizontal motion controller.

 

Pale pink square is visual guide, 50mm square so same area as standard UMO.

 

 

Sample_pic_1.thumb.png.2d59623ee633743af0eee1b00a52741f.png

 

 

Sample_pic_4.thumb.png.4364e19b7248f57739915728480917da.png

 

Sample_pic_3.thumb.png.f70d6393537502802945cca554c3a349.pngSample_pic_2.thumb.png.0e290772b90f8ca940fcee8157d9135d.png

A bit more ro add yet, needs support for Extruder Column at the bottom to stop it shaking around. I don't think  need cooling fins on that, but there's space if I do. Not sure how to fit my cooling fan in yet!

 

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Having slept on it (I do all my best work in my sleep! :)) the above layout needs to be improved, and simplified. I tried to minimise overall height & keep it compact at all costs. Tesult is a spaghetti mess of internal support beams, and it's difficult to route the heater & thermocouple wires to where they won't touch some sliding item. Latest thoughts:

 

1) It's desirable to get the plane of the 2 X-Y horizontal-motion Guides as close as possible to the height of the tip of the nozzle; this will minimise twisting torques trying to get nozzle out of vertical.  So the above layout will be inverted, to put the flat sliders at the bottom. The lower one will be level with the top of the nozzle's thread.

 

2) Height doesn't matter so much; we have plenty of vertical space above.

 

3) Want to keep the vertical Guides close to the horizontal Guides, esecially in X-Y directions, so the attachment to the blocks running on 8mm rods isn't huge. Not a problem for these blocks to get a bit taller than now.

So I will stack vertical Guides above the related horizontal Guides. By interleaving the X-axis Guides & the Y axis Guides, this will keep the design more regular, and the 8-mm rod blocks more similar. So I will have an L-shaped design, looking from above, with one corner of the Carriage completely empty. I can then route wires up that corner, well away from the moving parts.

 

4) Want the nozzle at the front of the Carriage, I like to see what's happening & have access to remove plastic build-up with tweezers etc. So one branch of the L-shape goes at the back, the other will be at one side, but which?

 

5) My bowden tube gets a bit stretched when nozzle is at front-left of UMO, so want the nozzle as far right & back as poss to reduce stretch. Won't put at back as visibility more important, but can put it at the right side. So the Y-axis sliders will go at the left side of the new Carriage.

 

From bottom to top, I can choose whether to stack the guides X-horiz, Y-horiz, X-vert, Y-vert , or Y-horiz, X-horiz, Y-vert, X-vert.

As UMO 8mm X-rods are above the Y-rods, it makes sense to choose X-axis Guides above Y-axis. Decision made. I already have my X-axis spiral-screws above my Y-axis screws, for that same reason. It all fits. New design pics later today I hope.

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5 hours ago, andywalter said:

5) My bowden tube gets a bit stretched when nozzle is at front-left of UMO, so want the nozzle as far right & back as poss to reduce stretch. Won't put at back as visibility more important, but can put it at the right side. So the Y-axis sliders will go at the left side of the new Carriage.

 

You could relocate your material feeder to eliminate this issue. I think it was David Braam who at one point suspended the feeder inverted above the center of the machine, using a tripod and rubber bands. 

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Yes indeed, I remember someone doing that! But my extruder mechanism is built on a substantial piece of aluminium plate, it's rather large & heavy as it uses dual geared clamp wheels. Besides, I'm thinking about eliminating the bowden tube using that neat ?ZeroG? idea where the extruder motor(s) are placed in a gimbal at one corner, with splined shaft reaching across top of UMO, and all you have then is a clamp-wheel-gripper thing  on rotary mount directly above nozzle shaft. I'm going to have a strong 10mm dia threaded shaft, easily enough to take something added to the top of it. I've been impressed by the ability of Replicator 2G to retract very cleanly, thanks to having extruder wheel mounted immediately above the nozzle, not at far end of elastic rod & tube. So I'm thinking of having 2 step motors counter-rotating (maybe geared down as my present extruder is), in gimbal at one corner, 2 carbon-fibre square tubes reaching across UMO and passing through centres of my 2 grooved aluminium clamp-wheels. These wheels will have 4 small but wide roller bearings in the middle, running on the 4 faces of the tubing. Should be torsionally very stiff indeed.

Edited by andywalter
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Hopefully this is my final layout for the 4-Ruler-Guide replacement fo 6mm rods project. Guides are now single-sided = slimmer & lighter, and by juggling the widths & exactly where in the middle I put the anti-backlash tensioner wheels, I've managed to overlap things nicely while still keeping decent clearances. It's 6mm less high than the prev design shown above, much neater layout leaving front-right corner completely clear, and the X/Y motion-control rulers are level with top of nozzle & top of heater block, so I'm really happy so far.

Various views:

 

View_1.png

View_2.png

View_3.png

View_4.png

View_from_Front.png

View_from_LHS.png

View_from_RHS.png

Bottom_View.png

Top_View.png

View_from_Back.png

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On 1/27/2018 at 2:27 PM, andywalter said:

Yes indeed, I remember someone doing that! But my extruder mechanism is built on a substantial piece of aluminium plate, it's rather large & heavy as it uses dual geared clamp wheels. Besides, I'm thinking about eliminating the bowden tube using that neat ?ZeroG? idea where the extruder motor(s) are placed in a gimbal at one corner, with splined shaft reaching across top of UMO, and all you have then is a clamp-wheel-gripper thing  on rotary mount directly above nozzle shaft. I'm going to have a strong 10mm dia threaded shaft, easily enough to take something added to the top of it. I've been impressed by the ability of Replicator 2G to retract very cleanly, thanks to having extruder wheel mounted immediately above the nozzle, not at far end of elastic rod & tube. So I'm thinking of having 2 step motors counter-rotating (maybe geared down as my present extruder is), in gimbal at one corner, 2 carbon-fibre square tubes reaching across UMO and passing through centres of my 2 grooved aluminium clamp-wheels. These wheels will have 4 small but wide roller bearings in the middle, running on the 4 faces of the tubing. Should be torsionally very stiff indeed.

You would still have the masses of your motors to contend with on just the wrong diagonal moves. With all the stiffness you are building into your XY carriage, perhaps you would be better served by keeping the motors stationary, and for each, running a splined shaft up the side (y axis) and through a bevel gear assembly mounted to the slider block, with a second splined shaft running through the head, to the opposite slider block. The additional weight of the bevel gear assembly is probably more than offset by the lack of moving motors and the simpler head.

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Bevel gears etc are a complete no-no, I'm sorry to say. The last thing I want is any more backlash, = rattle, noise & unpredictable inaccuracies. Mounting 2 step motors on gimbals in one corner means all you're doing is using a long lever to turn them a small angle, about a pivot that runs through the centre of gravity of the motors+gearbox. This is actually a tiny force that's required, far less than the force required to accelerate & decelerate the "ideally" placed MakerBot Replicator motors on top of the nozzle carriage, and probably less than the force required to accelerate the mass of the transfer shaft + 2 bevel gears that you suggest.  The gimbal vertical axis doesn't have to be close to the corner of the HBP either; you can hang it as far outside the UMO corner as you like; the further out you place it, the lower the rotational acceleration when nozzle is v close to that corner (the worst case for forces, but the best for extrusion control!). My steppers are already well outside the UMO case; I'm happy to hang the extruder steppers maybe 10 to 15 cm out from the UMO corner. My extruder gearbox alreads reaches 17 cm out from the back. You can't get much simpler than a shaft going straight from extruder motor through middle of the actual wheel driving the filament; if it needs a reduction gearbox stage, the fewer of those I have, the better. And I  would make that backlash-free by using one large dia cylinder clamped against the stepper shaft, rather than using gear teeth. The forces are low enough to do that, as the tangential force will be maybe 3x less than the tangential force where the clamp-cylinder forces the filament into the nozzle.

 

And in fact putting the gimbal out from one corner isn't ideal, it means the longest transfer shaft. Putting it out from 1/2 way along one axis make better sense, by a tiny amount. Reduces the distance to the (2) far corners opposite; you increase the angular sweep a bit, but you also reduce the max acceleration, as the mid-point of the near axis is further away from you, than it would be by putting the gimbal diagonally out from one corner. That max acceleration is the thing you want to minimise. My nozzle is at the front of my nozzle-carriage, so ideally gimbals+extruder go at back of m/c, which is already where my gearbox sticks out a long way, so this fits nicely.

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That makes sense, but if I understand you correctly this means that the orientation of your driving wheel will change relative to the column of filament as the shaft sweeps. Might this twisting snap or weaken the filament? Will the entire hot end rotate with the angle change, or will the filament twist again between the driving wheels and the melt zone? (Sorry if this sounds negative. I really like the idea, and I may be overthinking it.)

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Yes the filament will rotate between the driving wheels & melt zone, and you've got a good point there. I would hope that ABS would survive this, as usually it will be molten at hot end when moving like this, so with luck the colder & solid section will rotate freely inside the PTFE section, and the hot gooey rest of it is fine. Some day I want to try out metallic filament which can be fired in a furnace (& yes I've made my own mini-furnace, 1"x1"x2" high! some years back), and I think that stuff might be brittle, so I really like your idea of a rotating hot end! It hadn't occurred to me, so thank you for it! I read that these metallic filaments don't like bowden tube feeders - maybe there's too much push & pull reversal going on? Hmmm...

 

Another advantage is it eliminates the (small) torque trying to bend the rectangular drive-rods. And if they're Carbon Fibre, with rollers running on them, I have no idea whether CF likes little wheels running up & down it, so it's best to minimise all those loads on it!

 

I have some miniature ball-races in front of me for another project, they're 15mm i.d. & 21mm o.d., 4mm high, so these would be perfect to have a pair holding the downshaft.( Planned downshaft is 10mm o.d., I could reduce this to 8mm, just, using extra-fine thread, or I could go up to 12mm o.d. with tons of metal to play with - I want minimum metal for low weight, maximum diameter for rigidity as nozzle is a few cms away from bearings, but the metal wall must not be too thin. I have space to play with these values).

 

10mm o.d. downshaft looks ok; I will use 0.75mm pitch thread, that cuts 0.46mm radially call it .5mm, so 4.5mm radius is outer radius of solid bit;  allow 1mm wall thickness leaves 3.5mm down to 1.5mm filament for the PTFE sleeve, so the PTFE sleeve has wall thickness 2mm. I think this will work nicely.

 

If the hot end rotates, you must be sure the nozzle tip is concentric with the bearings. So I would have a circular ring sitting inside the bearings, lets say 1mm i.d & 15 o.d. to fit mine, and I'd put 3 grub screws through that at 120 degrees, so by adjusting these 3 & clamping the down tube, I can adjust the eccentricity down to zero. The test would be to disengage the rectanguler drive rods, and print straight lines in X & Y directions while waggling the rotation by hand, and looking to see if any waves appear. Or simpler may just be a dial gauge on the cone of the nozzle.

 

I can clamp the downtube in the bottom bearing with 2 thin washers and locknuts, this bearing can take the main vertical forces. The top bearing can have the 0.5mm gap all round with the 3 grub-screws, and that's enough adjustment to correct for any slight errors in runout at bottom bearing & the nozzle+heater block. I really like this idea; it does mean I need to find a bit more space around the heater block to let it swing +- 30 degrees or so, but the axis of the downshaft will still be inside my 50x50mm "box" carriage, so it will be fine I'm sure. I'll see if I can add all this to the current design, should be able to.

 ==================

Update to design pics above: just realised I stacked my layers wrong - oops! The brown ruler should be mounted above the blue one, similarly the flat pair below. I want to get the end-points of these rulers as close as poss to their 8mm rods. Just a small editing job,  swap the X & Y positioning coords, and negate them.

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On 1/27/2018 at 1:47 PM, andywalter said:

Hopefully this is my final layout for the 4-Ruler-Guide replacement fo 6mm rods project. Guides are now single-sided = slimmer & lighter, and by juggling the widths & exactly where in the middle I put the anti-backlash tensioner wheels, I've managed to overlap things nicely while still keeping decent clearances. It's 6mm less high than the prev design shown above, much neater layout leaving front-right corner completely clear, and the X/Y motion-control rulers are level with top of nozzle & top of heater block, so I'm really happy so far.

Various views:

 

View_1.png

 

 

 

 

Di you do any calculations on the difference in torsional stiffness? My gut says that while the rulers will be stiffer in bending, they will deform more in torsion (trying to rotate the print head about the X or Y axis)

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