Rigid construction.

Earlier on I was debating with MKSA on the subject of sturdy construction but between the improbably slow server on this forum and other things that I had to do it has taken me a bit of time to get back to this discussion - which I have moved here from the improbably long thread.

Firstly I disagree with MKSA when he says that a bed mounted on adjusting screws with springs will not be sturdy - there may be some movement in the XY plane but I will come to that later. With the spring in compression at say 1kg of force on each spring then there will be no movement in the up/down (Z) direction due to the spring until the preload has been overcome at the point where the adjusting nut is pushed out of contact with the carriage.

Secondly, the eternal debate about the desirability or otherwise of cable ties to hold linear bearings. In the above diagram the X carriage on a Cartesian printer has each of the adjusting screws pulling down a screw and opposed by the mentioned 1kg of preload on the springs. The downward force is transmitted straight from the bearings through the carriage to the chamfered shoulders that the LM8UU bearing sits on. The cable ties are tightened to about 3kg of force and are quite elastic so two ties each pulling on two halves will hold the bearing down with about 12kg (about 26.5 lbs) of force - acceleration and vibrational forces will be much less than this. By comparison, bearing blocks offer no additional rigidity and add a mass of up to 42 grams for an SC8 block or 20 grams for a SCV8UU block.



In many Cartesian printers there is something occasionally called a "squashed frog" which feeds the forces from a build stage through four sprung adjusters through to the carriage and then the bearings. The drawing below is intended to illustrate that four point mounting has problems but I use it here to show that the the legs of the frog are free to flex. The movement that I got with a simple load for a force less than the preload on the spring is 80% of the movement in is the frogs leg. Incidentally, even this is less than the movement in the carriage due to the flexing of the 8mm shafts.



Before we get too snotty about the rights and wrongs of this practice or that, I would point out the the genuine Prusa i3 is very highly though of and produces splendid prints but has four point adjustment, bearings tied with cable ties and other compromises.

On the XY movement that I said I would come back to. I use three sliding joints which can move in Z but not in X or Y. Using these does make a very small improvement to the print quality but only visible on a side by side comparison.

Mike

I have limited experience in the construction of these types of printer, but here is my 2p worth:

1. If the bed is sufficiently rigid (which is desirable anyway) then 3-point support and adjustment should be better than 4-point and is the preferred solution. If the bed isn't sufficiently rigid - which is perhaps a bigger problem with very large beds - or has to carry a lot of weight, then 4-point support may be better. I once saw a prototype of a large industrial 3D printer with a steel bed frame, which from what I saw of how it flexed needed 4-point support.

2. A factor that is often overlooked in Cartesian printers with beds moving in the Y direction is Y axis twist. This typically occurs when the Y axis is supported on 2 smooth rods, and the height difference between the two rods is not quite the same at both ends. This twist causes the bed to rotate about the Y axis as it moves in the Y direction, causing the nozzle height to vary with X position by an amount that depends on Y. 4-point bed level adjustment should never be used to correct for Y axis twist. Preferably, an adjusting screw should be provided to adjust the Y axis twist, or the construction should be such that there can't be any significant Y axis twist in the first place (which is probably hard to achieve). In budget printers that have a Z probe, compensating for Y axis twist using 4-point or mesh bed compensation is a reasonable approach.

3. I don't see any problem in using cable ties to attach smooth rod bearings to bed plates with square cutouts in them, at least in budget printers. But my Cartesian printer (an Ormerod 1) uses printed clamping blocks and screws instead.

4. There are at least 4 approaches to levelling the bed:

- 3 or 4 manual adjustment screws, adjusted by manual measurement. Tedious to do, but adequate if the printer is sufficiently stable that it rarely needs to be done.

- 3 or 4 manual adjustment screws, with firmware that uses the Z probe to measure the height differences and suggest corrections. I use this on my Cartesian and SCARA printers. AFAIK this facility is only provided by RepRapFirmware. Requires a good Z probe.

- 3 or 4 leadscrews driven by separate motors and stepper drivers, adjusted in firmware using the Z probe to measure the height differences. A good option for builds that use multiple Z motors. RepRapFirmware provides this facility and I have heard that Repetier does too. Needs a good Z probe and 1 stepper driver per Z motor, so more expensive in electronics.

- No adjustment at all, instead rely on a Z probe and bed compensation. This might be considered acceptable if the construction of the printer ensures that any errors in bed tilt or Y axis twist are very small; but I suspect that this is rarely if ever the case.

Edited 1 time(s). Last edit at 01/28/2018 01:37PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I think the problem is whether the cable tie type of solution is restricted to budget printers. My take on it is that the bearing block is actually undesirable as it contributes extra mass and adds no rigidity at the forces involved in a 3D printer - an exception being made only for those where the forces are very large. I recently checked some SCV8UU bearing blocks with LM8UU bearings and found in each case that the bearings could be pushed in with a moderate thumb pressure which is typical of a fit of about 0.0005" or about 12 microns loose.

On the number of leveling points, my personal guess is that the squashed frog has to be flexible or using 4 points would twist the bed - something that has no up-side. From memory it wasn't too onerous but I am much happier with three point leveling.

Mike

Cable ties proved to be a completely adequate method of fastening the bronze bushings i use. Compared to linear bearings with blocks my setup weighs a fraction. One bushing for an 8mm rod plus two cable ties weighs 5.5g. I don't know of any lighter way to connect a moving part to a rod.
As for three or four mounting points, yes in theory three points should have an advantage, but in reality the disadvantages of a four point connection are barely noticeable. It might take a very few minutes longer to level out the bed, but since i do that about once every year it isn't to much hassle.
Reducing moving mass and simplifying construction are two core goals when designing a new printer, cable ties and four point mount fit the bill.

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Pillow blocks to hold linear bearings do have a couple advantages over zip ties. They allow some adjustability of the position/alignment of the bearing, and with some bearings, allow adjustment of preload. Over time, nylon zip ties absorb moisture from the air, are affected by heat and exposure to UV light, and become brittle and eventually break. Sure, they're cheap and easy to replace, but if it happens in the middle of a 20 hour print, it isn't cheap at all.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Quote
the_digital_dentist
Pillow blocks to hold linear bearings do have a couple advantages over zip ties. They allow some adjustability of the position/alignment of the bearing, and with some bearings, allow adjustment of preload. Over time, nylon zip ties absorb moisture from the air, are affected by heat and exposure to UV light, and become brittle and eventually break. Sure, they're cheap and easy to replace, but if it happens in the middle of a 20 hour print, it isn't cheap at all.

Tie wraps (as the way bearings are mounted in general) are popular because it is simple, cheap and gives the required flexibility to compensate for misalignment, overconstrained system, thermal expansion not taken into account.

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

deleted.

Edited 1 time(s). Last edit at 01/29/2018 04:13AM by MKSA.

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

I'll throw my lot in. I'm not a fan of zip ties for much other than cable management. Yes they are 'good enough' for some things but they have a tendency to stretch a bit and loosen, that and it's just too easy to print a bearing holder or something, with so many parts of some 3D printers printed, why not just integrate a bearing holder in there somewhere? This is more a pick at how some prusa x carriages are than the y carriage, but still. I think the weight advantage is grasping at straws, if you wanted to minimize weight don't move the printbed. At that point it becomes even easier to integrate a bearing holder into where the bearing will go.

As for 3 vs 4 point leveling, I much prefer three point. On a reasonably sized bed (i.e. around 200x200), or anything without significant sagging or flexing, there is no point in having more than three points. I never thought much of it, with my first printer (a prusa clone), I'd send the nozzle for a few laps around the print bed and level each corner. I say a few laps because I had to go back to each corner and readjust after the last round. It wasn't a big deal, and it didn't particularly bother me, but when I got a printer with a three point leveling system leveling was much quicker. Just one pass would do. Both work, I find one marginally better, but there is no doubt 4 corner leveling prusa clone beds are the easiest to source.

On one point I have to capitulate, the _digital_dentist has a very good point about adjustability: Mounting linear bearings in a slot machined in an aluminium or glass fiber carriage does need to be accurate as miss-alignment can easily lift the bearings off their bed - pillow blocks will tell you about the problem by tightening up at parts of the travel. On the life of cable ties though, good ones made by Thomas & Betts or 3M should take even heated 3D printer environments quite well but possibly not UV. Cheap unbranded ones are much less good.

Mike