Friday, 30 December 2011

Mendel90 axes

With the stainless steel bars that I use I found that PLA bushings only last a few hundred hours before they wear out. I tried Igus plastic bushings and they only lasted about the same length of time. I think you need ground rods rather than rolled to get a smooth enough surface for bushings. Possibly the lithium grease that I used was not suitable for plastic as I am sure other people must have got better life out of bushings.

The ball bearings on my Mendel have proved very durable but they do wear flats on the rods after about a year of continuous use. This wouldn't be a problem except that the rods wear more in the middle, which leads to inconsistent Z height eventually. You can turn the rods to put the flats underneath and get many more years life.

I have run some LM10UU bearings for over a year non-stop and they have not worn the rods noticeably. I did have an LM8UU bearing suddenly decide it only wanted to go one way on my Prusa's X-axis. It just needed some oil to make it work again. I think the X-axis tends to dry out because it runs over the heated bed.

I made the Mendel90 prototype with 10mm rods because I had noticed the 8mm rods sag a little on my Mendel, that has a heavier bed and extruder though. 10mm rods cost quite a lot more than 8mm and the plastic parts get bigger so I intend to make an 8mm version and see how it compares.

The X axis is similar to the Prusa but I have changed a few things: -

Note the axis is shortened in this picture, the belt has a twist not shown and a loop round the tensioning screw.

I lowered the idler and the motor to be in line with the bars because I noticed on my Prusa that the belt tension tended to bow the bars upwards slightly at the ends. It does mean the belt is a bit closer to the heated bed but I haven't noticed any ill effects.

I swapped the positions of the Z bars and Z leadscrews so that the bearing holders face inwards. That means the belt tension tends to push the bearings into their holders rather than pulling them out. That allowed me to get rid of the cable ties.

There are clamps for the X-bars so they don't have to be exactly the right length. They can be adjusted a few mm lengthwise and then locked in place. The holes are open ended at the idler end to allow the bars to be removed without removing the Z-bars first.

The motor housing is a box shape to keep it rigid while still having only relatively thin walls. The hole in the top is for the wires and lets any heat out.

I didn't use a 608 skate bearing for the idler. They might be cheap and available world wide but I found they didn't work on my Prusa, whereas the 624 bearings used on the Sell's Mendel do work. Ball bearings have a chamfered edge, the bigger the bearing the bigger the chamfer and M8 washers are thicker than M4 washers. With 8mm bearings that leaves a gap big enough for the belt to ride down and bind, whereas with 4mm bearings the gap is much smaller so the belt simply brushes against the penny washer, rather than jamming.

I prefer a bearing to a printed pulley with flanges or a crown pulley because if I am using a metal drive pulley for accuracy it does not make sense to have a printed idler.

I haven't added it to the model yet, but there is a half twist in the long return path of the belt so that the smooth side goes over the idler, not the teeth, to avoid any cogging. The twist in the belt doesn't seem to cause any problems, if it did I could revert to the technique here:

The belt tensioning is as Greg Frost's design: The ends of the belt are locked in place by clamps with mating teeth. A screw tightens a Nyloc nut against a loop of the belt.

The carriage is the full size of the extruder with the bearings optimally placed in a triangle and the belt attached at the ends. It does mean the carriage is a bit bigger than most but it makes best use of the space to achieve stability. I.e. the travel is limited by the extruder, so there is no point making the carriage smaller, other than reducing print time.

The carriage follows the rod on the two bearing side and only needs to be prevented from rotating around it by the third bearing. In order not to be over constrained the third bearing is suspended by thin but tall struts. That allows it to float horizontally but it is constrained vertically. This prevents binding in the event of the rods being slightly miss-aligned.

The underside of the carriage is shelled and ribbed to save print time but keep it rigid. That has been my philosophy on the design, strength through complexity of shape rather than chunkiness. Whereas other people have tried to reduce the printed parts to a minimum I have tried to put functionally first.

I found that I could not make bearing clamps in the horizontal direction with enough grip so I use cable ties as well on these. The bearings rest at each, end so a single tie in the middle is sufficient to keep them stable.

The Y axis sits on a flat sheet ensuring the bars lie in the same plane. Only three bearings are needed so the rod on one side can be shorter as it no longer needs to attach at the very front and back. The X-axis also uses three bearings and Z four, making the total ten, which is convenient as they tend to be sold in packs of ten. The belt is also shorter because the motor and idler can be brought inside the axis travel.

The Y motor bracket is a lot more rigid than the Prusa version due to its boxy shape and being screwed to the base instead of hung from bars. The bar clamps are also hollow boxes.

The bearing holders are the same as the ones on the carriage using tie wraps .

Alignment is easy, all the bar clamps and bearing clamps have slotted screw holes allowing a little side to side movement. Initially all the screws are left loose. The long bar is set at right angles to the gantry using a set square and then the bar clamp screws are tightened. The bearing clamps on that side are then tightened. The y-carriage can then be moved backwards and forwards to pull the second bar into alignment before those are tightened. On my todo list is to float the third bearing like I have done on the carriage.

Again the belt has a half twist in the lower return path, not shown on the model. Belt tensioning is easy because the idler has a slot to allow it to be adjusted. The single mounting hole also allows the angle to be adjusted to centre the belt. I plan to move it to the front and put the motor at the back as it makes the wiring shorter and the idler adjustment more accessible. I used two 624 bearings side by side to allow the belt to wander a bit without binding. I seemed to need that on Y but not X. I may move to two on the X-axis as well to give a completely frictionless arrangement.

If you are wondering what the two large holes in the base are, they are there so that dual shaft motors can be used.

I moved the motors to the bottom to eliminate the possibility of the couplers slipping off. I made the couplers as skinny as possible to get the bar close to the lead screw. That makes the X ends smaller and allows the Z bar to rest on top of the motor giving a metal connection from the base to the top limit switch minimising the effect of the wood shrinking and expanding. For normal Reprap software it probably needs an adjustable bottom limit switch instead.

Note the axis is shortened in this picture.

The Z bars are automatically parallel to the gantry because the distance at the top and the bottom is set by printed parts. The bar clamps at each end of the rods are identical allowing the axis to be made vertical with a set square. This is done at the left hand side and the other side is made parallel by moving the axis up and down before tightening the screws.

I kept the facility for anti-backlash nuts and springs but the only machine I needed to fit them on was my Prusa. I am not sure why, but even the weight of an extra motor was not enough to overcome the backlash with gravity. I think it must have either been due to binding or perhaps the grease I used was thick enough to need some force to squeeze it out of the way. I needed stiff springs and I had to turn up the z-motor current after fitting them. The advantage of not fitting them is it gives some protection against a head crash as the maximum force you can apply downwards is the weight of the X-axis and extruder.

I considered using a single motor and linking the screws with bevel gears and a drive shaft. That would be cheaper than a second motor or a belt but I stuck with two motors for simplicity at the moment.

I have previously used 6mm aluminium tooling plate with  aluminium clad power resistors for my heated beds. These work well but they are heavy. The Prusa PCB heater with a 2mm glass sheet on the top makes a much lighter solution. The picture above shows it clamped down with penny washers but bulldog paper clips work better.

I use 3mm Dibond for the Y-carriage because it is light, stiff and stable. I tried 6mm MDF on my Prusa but it warped due to the heat and the bed never stayed level for long. I don't know how other people manage to use it.

The best bed mounting solution I have tried so far is 20mm brass hex pillars. I tap the carriage holes M3 and screw the pillars into it. I can then level the bed by adjusting them and use the screw in the top to lock the position. I don't like to use springs because they let the bed wobble.

To level the bed I put M3 washers under the back two pillars and screw them tight and lock them. I then twist the Z motors by hand to make both sides level at the back relative to the nozzle. I then adjust the front two pillars to get the bed level front to back.

The process is easy but tedious because all the adjustments interact to some extent, so you have to keep going round them. It would be better if the bed had a single mounting hole at the front in the middle, as you only need one adjustment to get the bed level from front to back. I need to make a smaller version of my Z-probe so I can auto level the bed.

I like to use an air gap under the bed for insulation so that I can cool it rapidly with a fan at the end of the build to make the parts release easier. The air gap provides enough insulation but the Dibond below still gets to around 50°C. I added a heat shield made from corrugated cardboard covered in aluminium foil tape and the Dibond no longer gets warm at all.

The slots are to clear the screw heads. I stuck it down with double sided tape but that did not hold so I added bulldog clips. If I was making another I would bolt it down.

I haven't made any measurements yet but I think the difference in temperature between the middle and the edges is bigger than my aluminium beds. I Intend to try adding printed baffles at the front and the back to stop the movement of the bed pushing cold air under it.

I think I can improve the temperature distribution by changing the PCB pattern. The problem at the moment is that if the middle runs a bit hotter then the tracks local to it will have a higher resistance than those at the edges, which are connected in series with it. That means the middle will get more voltage and become even hotter relative to the edges, positive feedback. A better arrangement would be to have concentric rings of tracks running through areas that are likely to be the same temperature, wired in parallel. That way if the middle got hotter it would only have tracks near the middle in its circuit, so the increase in resistance would lower the current and give some negative feedback.

Another thing I would change would be to remove the silk screen from the top layer as it has some thickness that will reduce the thermal contact with the glass.

Larger CNC machines use cable chains to enforce a minimum bend radius on moving cables to stop them breaking. There have been several printable versions on Thingiverse but I feel they would give more friction than desirable for a small machine like this. Ribbon cables are very flexible in one direction and are surprisingly rated for 300V, 1.4A and 105°C.

For the heated bed I use ten wires in each direction plus 2 for the thermistor. I clamp it at both ends with a thin strip of polypropylene about 0.5mm thick. That forms the equivalent of a miniature cable chain but with very low friction. Here is the one under the bed: -

This one feeds the X motor and the extruder: -

The rest of the wiring is done on the back of the gantry with printed cable clips : -

The fan on the left is a powerful 80 CFM fan that I use to cool the bed from 110°C to 30°C in about 6 minutes.

The only down side of ribbon cable is that you get some inductive cross talk from the motor signals to the endstops. That doesn't affect my firmware as I only read the endstops during homing and a simple retry loop sorts that out. For firmwares that constantly monitor the endstops a simple RC filter on the inputs should fix it.

This version of the machine I call the Sturdy model. It uses 10mm rods, M4 fasteners and has a build area slightly bigger than a Mendel: 214 x 214 x 150mm. The next version I try will use 8mm rods, M3 fasteners and have an acrylic frame. I will reduce the build area to 200 x 200 x 140mm, same as Mendel so it will be more of an equivalent. I will also make a Huxley equivalent with NEMA14 motors and 6mm rods. The Mendel sized variant will cost a bit less but I doubt the Huxley will be any cheaper.


  1. Awesome write-up, thank you!

  2. Nice work. It is obvious that you have thought this out fairly well. I am looking forward to the plans for the lighter version with the 8mm rod.

  3. Thank you for the detailed documentation: I have some question concerning the IGUS bushings because I am interested in using this and would thus be very glad to get some further informations:
    a) what type of igludur polymer did you use
    b) did you apply any grease
    c) how is the specification for the rods (roughness)
    d) could it be that the rods were used with metal bearings before and thus had any "scarfs"
    e) any change to get a picture of a weared out igus bushing?
    sorry for asking that much and thank you very much for documenting your results

  4. I'm really liking this new version of yours :)

    If you're getting crosstalk problems on the endstops, it might be enough (or at least helpful) to decrease the input impedance on the electronics.
    How small are your pullups? I wouldn't expect the steppers to interfere with 1Kohm, and the 5mA closed current isn't a high price to pay.

    In any case an RC would help with switch bouncing also (you probably know all this anyways :)


  5. Wow, you've put a lot of brilliant ideas into this design!

    I expect many of them will make their way into prusa's repository, your floating bearing mounts on the X carriage for a start!

    Thanks for all your numerous contributions to the reprap community, I am absolutely convinced that without them we wouldn't be getting the quality that we're seeing these days. Would love to see you back on the dev list!

  6. Do you have the stl's available somewhere for the Mendel90?

  7. @Anonymous,
    a) Igludur G, GFM-0810-05.
    b) Yes NLGI No. 2 lithium based multipurpose.
    c) Not specified, just 304 stainless steel.
    d) They had only been used with PLA before.
    e) I have added a picture of them in the holders I used. I don't think you can see it but they are now slightly oval and the y-axis developed slop.

    I use 2K2 pullups but I am sure 1K would not fix it. The switch is normally closed so we are talking about inductive coupling in a short circuit, which has a very low impedance. I.e. I believe the two wires running next to each other act as a half turn transformer.

    I will be posting the files soon.

  8. > @Shane,
    > I will be posting the files soon.

    That is super, Im interested in your carriage (I do want to modify it to my own needs, and yours seems the best start)

    And also in your X axis. (because Im redesigning my Mendelmax variation)

    > Another thing I would change would be to
    > remove the silk screen from the top layer as
    > it has some thickness that will reduce the
    > thermal contact with the glass.

    Why you dont simply flip the pcb?


  9. Because the underside of the PCB is even less flat because it has the tracks on it.

    The middle of the PCB domes upwards when it gets hot so you mount it that way up if you have glass on the top and then only need to fasten it at the edges. If you mount it track side up you need something underneath to support the middle.

    I think the reason it curls that way is because the solid sheet of copper on the top expands more than the tracks do.

  10. Ah indeed, if you've already got 2K2, 1K won't fix much... that's some hefty crosstalk!
    I was thinking you had NO switches and were getting induced voltage on the floating disconnected wire; but I imagine NC is safer in case anything gets unplugged.

    You're probably right about the [switch return + ground] loop acting as a transformer then.

    I'm not sure but maybe if you split the switch or the ground over 2 wires, one either side of the other signal, the loops in opposite directions will cancel out somewhat?
    It might be more practical to change the order of the wires half way along the run of ribbon cable, to simulate a twisted pair.

    Neither will probably be sufficient but it might help.

    I'm no expert either, so I might be completely wrong :)
    Best of luck!

  11. This is a nice new design, but not really directly derived from a Mendel. Can it get a different name to help reduce the already confusing Mendel family?

  12. Any suggestion?

    Although it does not inherit any source code it is an evolution from Mendel to Prusa Mendel to this.

  13. This design looks very solid. I was looking at the Mendlemax design, but this "Mendel90" looks preferable. And, possibly less expensive as well.

    I'll be very interested in this version when you have the files ready. Great design work.

  14. Is there a slight chance to get printed parts from you (maybe via ebay like the previous Mendelparts?)

    This looks a lot better than the steel cage type Mendel!

  15. Looks like a great idea. It seems that the z axis is way overbuilt compared to the rest of the device. 2 motors and 4 bearing for the slowest axis. Could you use the z axis as the bearings?

  16. Yes I will sell the printed parts on eBay.

    Not sure what you mean by "use the z axis as the bearings".

    Two motors are the simplest solution as they provide the bearings as well. If you use separate lead screws with 8mm bearings, like the original Mendel, it difficult to centre the screws in the bearings as they are a bit smaller than 8mm and nuts tend to not be flat when tightened. Also a belt plus 2 bearings is about the same price as a motor.

  17. Nophead,

    Thanks for the great new machine. I will be waiting on the parts being for sale on the bay.

    If you are looking for pre order to finance I may be able to help you out. I would pay plus be a beta tester. Post an answer and we can get in touch.


  18. By Z Axis I meant..., Actually I'd really like to build one of these before I try an improve it. I made a cnc mill but have not made a printer. I've been amazed at how sophisticated these designs have become.

  19. I agree with the Mendel family tree getting confusing. May I suggest you name it a (RepRap) "Galton" after Francis Galton?

  20. Hmm, I wonder how may people have heard of him?

    How does changing the name to something that hides any connection to Mendel reduce confusion? Calling it Galton gives no glue to how it relates to previous machines. I.e. it has nothing in common with Darwin but is similar in form to a Prusa Mendel apart from using sheets instead of rods.

  21. I don't know, I just picked the name from this timeline and it seemed appropriate since he was close to Mendel and Darwin. Other than that, not my field at all, but I thought Galton had a good ring to it.

    Most Mendels have the typical triangluar side frame, so why not start a new line with your design? Could perhaps make it easier for beginners to tell them apart, that's all. All future RepRap will in some part be based on Mendel, but we can't stick with that name forever.

    I can see this getting popular, and making a "brand-name" statement wouldn't hurt, in my humble opinion. But still, only a brain storm suggestion of course, your design, you're the boss. ;)

  22. I agree with Nudel. This is a novel enough design that it merits its own name IMHO. It's time to move beyond Mendel/Prusa.

  23. I'd not thought about shelled/ribbed designs saving time over sparse-infill designs, only that both save plastic over solid designs.

    Knowing you, you have quantitative measurements or calculations on this -- care to share?

  24. Hi Keith,
    No I don't have any figures and I am not a mechanical engineer so I wouldn't know where to start. I do know that boxes and tubes are pretty strong shapes and so are laminates like Dibond that concentrate the strong material at the outside.

    My resoning is this: A tube is nearly as strong as a bar so the material in the middle is not doing much for you, especially if it is sparse infill. Better to concentrate it at the outside. If you use the same amount of plastic, it will take the roughly same print time, but if it is concentrated at the outside it will be a stronger shape. Hence why bones are hollow.

  25. Name it after yourself the Palmer 90, or P90 for short. Also apropriate as thia is a fucking paradigm shift! Youll sell loads of thees parts!


  26. Lots of ppl will give this a go as the design is way less freigtening that the roded mendel!

    Also, thank you so much for the time and effort you put into the community!


  27. Fab - u - lous to quote that infamous judge.

    I'm guessing you used 3off 600x600x20mm MDF.
    (24x24x¾ inch)

    I imagine audio noise will be an issue too, given that the Y & Z motors are mounted directly onto the board, which will act as a sound board. However, your 'box' design will be easy to insulate with 25mm polystyrene sheet. This will have the additional benefit of a thermal enclosure as well as cutting down on the noise.

    I seem to remember that you have already explored thermal enclosures?

    I'll be in the shed Saturday machining the PR parts from plywood!!

  28. I used a single sheet of 1220 x 607 x 12mm MDF for £12.48 from B&Q. They cut it for free.

    Yes it is a bit noisier than a Prusa.

    I have since boxed it in with 6mm MDF but I think that makes it a bit louder as the sound is conducted to the box. I plan to try 6mm twin skin polycarbonate for a box as I think that will be a better insulator and lighter. I feel it may also be quieter.

  29. For a name, how about RepBot90? It gets 0 hits on Google (probably one now) so that is a good start.

  30. Fair enough, althu i still think you should name it after yourself!

  31. Let me suggest a couple:
    - Mendel Evo
    - Woodchuck Mendel
    - Sheet Mendel
    - Mendel90 (nothing wrong with that...)
    - Palmer Mendel

    - Reprap Hybrid

    RepBot90 just sounds wrong, too much like
    - Reprap Frankenstein

  32. Just don't call it Titanic, it might sink!
    How about following a tradition established by Ford and copied by others -- Capri, Granada, Ibiza, ... names of islands. Drop 'Mendel' altogether ... there is no similarity.
    Capri RepRap

    anyway this is WAY off topic!!

  33. How about Denelm.. it's Mendel mixed up..

    You've a history of using anagrams to name stuff.. Cnut for example ;-)

  34. Go with repbot90

    Or mdfbot90


  35. Great design. Do you think that the uprights contribute a lot of rigidity compared to more of a free standing approach like printout or this one

  36. Yes an almost infinite amount more rigidity in the direction of the X-axis. Even a thin sheet material is very stiff in the direction of its plane.

    It doesn't shake at all, whereas a Mendel does, Prusa slightly more and I expect a Printrbot a lot more.

  37. Thank you very much for answering the questions so thoroughly! I'm very sorry that my response to the IGUS bearing problem is so late. I tried to find the problem using various datasheets of IGUS, but didn't succeed:

    Stainless Steel is the material with highest wear,
    the temperature might be around 60°C and
    the motion is clearly oscillating (speed > 32 mm/s), the load is minimal and
    IGUS states that some grease (was unable to find any information what kind of grease) enlarges the valid range of velocity
    Everything seems to be well in the spec such that the wear should be in the range of 1 um per km. This means about 60 um for 500 hours @32mm/s.

    My explanation is, that the IGUS bearings are quite sensible to incorrect fittings. The wear is strongly correlated to the pressure. Perhaps a PLA bush is not sufficient to provide the structural integrity necessary to prevent local deformations.

  38. PLA is quite hard so I don't think it deforms much and I reamed to hole to 10mm, which gave a nice press fit for the bearings and they didn't need any other fixing( I had intended to fasten the flange with a small self tapping screw).

    The were used underneath the y-carriage of my Prusa, so not much above room temperature.

    I think the most likely problem is the surface quality of the bars, which I think are rolled rather than ground.

    I will try to post some pictures under a microscope.

  39. Really neat design. I think this one will be very popular. There are some neat touches here, like the ribbon cable, uneven Y axis bars and pinch bolts. It would make a nice addition to your sales on eBay.

  40. I love this design, will definitely have a go at building one.

    Regarding the name, RepRap Euclid has a nice ring to it.

  41. Nice work! So many improved aspects that I would like to incorporate into my current printer build. May I ask, where did you get the Dibond? I haven't been able to find it in the US, probably not looking in the right places.

    1. I bought it on eBay. Sign makers use a lot of it. I have been told you can sometimes get off cuts from them free, or very cheap.

      I think there are also a couple of other trade names for it.

    2. shows a $18+$15 24"x24" alternative

  42. Mendel90 is perfectly fine, though if folks want you to use a different name, Ortho-Mendel might more strongly highlight the right-ness of the angles.

  43. This comment has been removed by the author.

  44. Polypropylene is a common plastic similar to polyethylene but more flexible. It can be bent backwards and forwards many time without breaking so it is often used to make "living hinges" such as Tic Tac lids, so it is ideal for this application.

    I cut the strips from an A4 document folder, which is why they are bright purple.

    I haven't found a source in sheet form yet. I did find one in the UK, but they only sell to schools.

  45. did you finish your mendel90 acrylic? how much did you get the acrylic sheets for? the bad thing about it is that I can only find 2m x 1m sheets of acrylic here, so making 1 printer that has small sheets of diferent thickness is not cheap :S

    and acrylic is somewhat heavy, so it gets expensive to buy from a chinese supplier or something like that...

  46. I have built it, wired it up and tested the axes. I don't have time to get to grips with the nightmare that is reprap software at the moment.

    The acrylic sheets were expensive because I got them laser cut and the one off cost is high. In the UK I can buy acrylic cut to size at close to the same price as large sheets. My plan was to get a large batch made and sell them but laser cutting is not really accurate enough for tapped holes (but mine worked).

    The acrylic machine looks nice but the MDF version is much cheaper, stiffer, lighter, easier to make and more robust. Here is a video of one made from polycarbonate, which is a little more expensive than acrylic but not brittle:

    Dibond and even solid aluminium are both stiffer and lighter than acrylic, so I am looking at those.

  47. nophead what you think of this aluminum mendel frame (not the axis or anything else, just the frames)?

    it's clearly not as sturdy as the mendel90 but it might do the work and it uses just a fill of aluminum extrusions that are a lot more "plug and play" (not the best word to describe what I mean but I think you get it) than MDF (or acrylic) plates... what you think about it? for example, I see mendel90 as a great printer design but it's not as easy to a someone new to 3d printing to go and work with MDF or acrylic as it is to work with aluminum extrusions... what you think?

  48. If you have access to cheap extrusions cut very accurately to size then it makes a nice frame when done that way instead of using triangles (like MendelMax), which add no strength in the directions that matter. I expect it is stiff enough

    When you buy sheets cut to size it is not the size, but the right angles that matter. So they are very accurate. When you use extrusions the lengths and screw positions are what create the right angles so it has to be machined very accurately.

    In terms of RepRap I think rectangular sheets are far more common in the world than aluminium extrusions, and cheaper.

  49. what oil you said you use in your lm8uu? I'm trying to find out one to use in mine, they don't show any extra friction right now, just asking if I need some one day which one should I use... what about the 608 604 bearings, do they need oil as well? which one?

  50. I have only ever needed to oil one LM8UU that suddenly seized in one direction only. IIRC I used car engine oil as I figured it would handle the temperature above the bed.

    I haven't needed to oil 608 or 604 bearings. I think they come packed with grease.

  51. I recently upcycled a fairly large amount of 1/4" (6.35mm) high quality glass. I have already cut out all the print beds I need. I am seriously considering using it as a base for a Mendel90. I am not sure what the advantages would be, but it will be damn pretty. I imagine bolting m3 screws onto glass is a bad idea, perhaps epoxying the printed parts to the glass would work better. If I can figure out how to get my clear PLA (or PC maybe?) to stay clear after printing that would be best of all. Crystal Mendel :)