Big fan of "the ideal" and "getting as close to perfection as possible:

Big fan of “the ideal” and "getting as close to perfection as possible: I have a question which I think we could ALL benefit from knowing the answer to:

The Ingentis ( / UI-T-Slot / Eustathios ) is worlds ahead of my current Mendel machine. Comparatively, it is much more ideal, using the x/y gantry design and being driven simutaneously from both sides which disallows the ‘twisting’ action when the drive belts are only attached on one side - Reducing vibration, allowing higher speeds etc.

Who knows / cares to help speculate, which of these pictured setups is better for the X/Y ends?

I am of the understanding that the position of the belts in both the pictured cases is slightly inferior to the Eustathios, where the belts are on the OUTSIDE of the linear rails. The belts on that machine are also (look like) driving at the same Z-height as the linear rails - In the two designs pictured the belts attach ABOVE the X/Y ends, which means when the belt ‘pulls’ the x/y end, it starts to “tip it over” a little, then the end catches up and moves. The amount it ‘tips over’ by is MICROSCOPIC… but - it happens nonetheless… and if it can be avoided, ideal conditions can be met and the highest possible speed and accuracy are achieved! (yay!)

I need more minds to weigh in on this!

@D_Rob @Whosa_whatsis @Tim_Rastall @Jason_Smith @Eclsnowman @ThantiK @Thomas_Sanladerer + Anyone else who might know!

Have a look at the diagram and description of this CNC machine:
http://www.alfametalmachinery.com/en/select_by_brand/you%20ji/cnc%20horizontal%20machining%20center/HMC-Ram%20Spindle/162/0/#structure

It shows how, being driven from 2 ‘ends’ rather than 1 end affects vibration, stability (and therefor max speed and accuracy).

In the first pictured X/Y end I’ve just posted, the first and second linear rails are also on the same PLANE… I THINK this is more ideal than the second type in which the bar is offset from the plane of motion, but, I’m not SURE!

Let’s discuss and determine the ideal setup, then I, Tim or anyone else can come up with a design that will perform the closest to ‘ideal’.

The blue version requires more of a length tolerance for your shafts - but as all the motion occurs on that plane, I can see an advantage in high acceleration situations - especially if you can get the c.g. of your effector on the same plane as well!

Blue is better, as you say… no tipping which would encourage binding. Also I would think it would help square the side rails on setup.

That’s what I am thinking @Anthony_White - the length tolerance on the bar WOULD need to be accurate, but also - the cross rail will support the outside linear rail from flexing inward - another plus for stability! Placing the drive (effector) directly opposite from that cross-bar (on the outside of the x/y end, same z level as the bars) seems like the best of all 3 designs (sorry no Eustathios picture, refer to “tinyg” video a few posts back :wink:

The blue version is using straight bronze bushings like the Ultimaker. My version on the right uses self aligning bushings. I actually like the blue one but I don’t see now you secure the spectra. Hmmm, actually it won’t work as the spectra that spans the 2 shafts will fowl against the plastic. This is why there is a curved notch on the outer edge of the one on the left. I originally started with Sublime xy ends and only made minor mods as I didn’t see the point of reinventing the wheel. There are better ways to capture spectra line than Sublime method imo.

The blue is better as is does not apply moment on the plastic part when moving. In terms of length tolerances, the blue design does not need to be tight at all. Simply make the sockets/ferrules deeper than they need to be and as a screw and nested nut to clamp the rod in place. You just need to get the blue parts into the correct position by moving the stages back and forth while gradually tightening all the clamps

The Blue one is very unforgiving of imprecise bearing rod lengths. I dunno if that’s an issue or not, but the Grey one allows some overhang.

The consensus seems to be that the blue is better, so I’ll concentrate on the potential issues with that design.

First as others have stated, the length of the rod is crucial with the blue design whereas the other is tolerant of length differences. Even if the length is perfect, any flexing of the outer rods could cause it to become loose or even fall out. This is made much easier by any looseness in the part surrounding the end of the rod, and such looseness would allow the blue piece to pivot around the outer rod (resulting in the end of the inner rod moving up and down. This also means that any up/down force could cause it to loosen over time.

Two ways to strengthen it against this type of loosening are to lengthen the part so that encloses more of the inner rod (this will of course reduce the travel space available to the effector) and to make a teardrop shape (or a polygonal hole) with a corner that the rod can be pressed into with a set screw so that it is pressed between five points (two pairs of points colinear, parallel to the axis of the inner rod and the fifth point, the set screw, on the opposite side, centered between the other four).

This version also makes it harder to put the attachment of the drive belt/line as close as possible to the attachment point of the rod.

I would go with a design closer to the grey one, but extended to wrap around more of the end of the rod to fight any flexing in that mounting point. I would also carefully design the shape (possibly printing in multiple parts) to ensure that both the bushing and the rod are held firmly at right angles to one another, with the tight clamping of each keeping it from rotating around the other.

Of course, if the outer rod could actually pass through the inner one so that there is no joint between the two to potentially flex, all of these problems would go away, and the ideal attachment point of the belt/line would be through the center of the inner rod, outside the outer rod. This means that the inner rod would have to be a larger diameter, which would normally mean more mass (a bad thing), but if you used a hollow tube, I think this would get you pretty close to “the ideal”.

Hm, I noticed on the Ingentis that only one end of the x/y rods is clamped down to keep the rod from binding. The blue design doesn’t have any clamping, only press fit(unforgiving of loose tolerances for movement reasons), and as long as the rod is the right length, it’ll be constrained and still be free to move slightly if it needs to. Is there a flaw with this idea?

You can also just tie the spectra right onto the clamping screws.

@Nazar_Cem If the rod is loose enough to slide in and out of the hole, it will also have room to wobble (and more, because of flexing plastic), making the inner rod unstable in the Z direction.

@Whosa_whatsis Ok, so the solution is to use a screw to clamp it as you said? Would you need one on both ends or only one?

like this? http://imgur.com/HGF6OZf

@Nazar_Cem yep, that would do it.

No, that’s not good because you’re pressing it into a flat side. You want to press it into a corner to get a solid connection, so you should use a teardrop shape or a polygonal hole.

Also, the spectra needs to be very tight so you have to keep it under tension then tighten the screws to clamp it in place. I would make the gap small enough to allow it to completely close once tightened. I might even introduce a toothed/ridged profile to the inner surface to improve grip on the line.

Hmmm. @Nazar_Cem on second thoughts that sort of nut trap screw clamping thing is prone to splitting the plastic if you over tighten. Id go for a clamping solution similar to the way you are clamping the bushings.

@Whosa_whatsis Oh I see what you mean. http://imgur.com/a/9V2M2

@Tim_Rastall To tighten the line, could you tie it to one or both of the screws, wrap it around the shafts, wrap it around the screw(s) on its way back, and pull on it lawnmower style and make a knot?

Come to think of it, what I said earlier about hollow tubes being ideal would be perfect for use with this: https://plus.google.com/116889746506579771100/posts/8ZmKAXfWphz

@Nazar_Cem Your first picture there is what I was talking about, yes. Your second picture will not work, as the hole will be tight at the opening but looser at the back, so the mouth of the hole will be a pivot point for exactly the type of play I was warning about.


These are pics of the mockup I just did, the effector is on the same axis as both rods and since there is an effector on both sides, it is driven from the centre of gravity :slight_smile:

The hole in mine is not teardrop shaped, but it should be, other than that… What do you guys think?