@Jarred_Baines Looks good and should print well on it’s end. Only comment is that nut trap doesn’t have enough plastic above it. You’ll push the nut through (or crack) the plastic if you overtighten.
BTW If I sound like I’m going on about those M3 nut traps it’s only because I’ve learned through bitter experience to avoid them.
Yep… I noticed it was too thin but left it for the purpose of the mockup, the counterbores for the cap screws need more meat also 
revision 0.0.1
now with more nut traps… http://i.imgur.com/Bo2cI4W.jpg
This conversation hasn’t yet touched on this idealized configuration being in competition with the goal of maximizing build volume within the machine envelope. Stacking the rods has a proven track record with Tantillus and now Tim’s Ingentis. Is there a problem here that needs to be solved by giving up 20-25 mm of X/Y travel?
What I see in the Tantillus design (gray) is the drag of the bushing being added to the larger torque that exists in both designs caused by the line attachment being offset from the crossbeam’s load. It seems what we want is the line to attach at the same elevation as the crossbeam. WIth the rods stacked, it should attach slightly below the crossbeam to somewhat balance the inertial loads with the drag from the bushing. Edit: Xmaker has this, but gives up a lot of build volume ot do it.
I briefly mentioned the loss of build volume caused by extending the part.
While the iterations being posted here are getting better, I still don’t think that any design could be called ideal in which the two rods don’t cross. Some part of the inner rod, not a piece of plastic attached to it, extends outside the inner rods.
If the rods don’t cross, the attachment point of the belt/line should be on the same side of the outer rod as the end of the inner rod, not the opposite side.
Sorry, I missed your mention of the volume reduction. I really don’t disagree with your assessment.
@Whosa_whatsis this is where we get back into looking at the whole system. Anchoring the line or belt on the inside of the xy end will reduce the xy build volume because of where you will have to locate the pulleys/Spectra winding. Of course, this I one of the dangers of designing a part in isolation.
Actually, that brings up a point I was intending to make about system design and how your initial requirements define the level of deviation from an ideal configuration for each component. I’ll do that in another post to avoid hijacking this one further.
If you use 8mm rods you can use 688zz bearings and shove them as close as possible into the corners. The travel range can be restored back to original.
Build volume, in my opinion (and I understand not everyone will feel this way) comes second to quality. I would always focus on quality first, then if the build volume was not adequate, scale the design to suit… This is why I haven’t even CONSIDERED that I’m cutting into the build envelope at this point, that is a trivial thing to overcome later.
I was under the impression that driving the axis from the furthest point from the center gives the best allowance for belts stretching / mechanics flexing - for example:
If the 2 drive points are only 5mm from the COG and when in motion your left effector takes .1mm more movement to ‘move’ due to the overall backlash between the left and right pulleys, this magnifies toward the outside, with 450mm rods you will be twisting the axis by 4.5mm in this extreme case…
Point being; the further outboard your effector is, the less you are effected by backlash and rigidity issues… Although @Whosa_whatsis is probably right in that the more rigid point is close to the rod instead of my original design…
What I was saying is, if you have a pencil on a table and with 2 fingers spaced only 10mm apart you push the pencil, if one finger pushes .1mm MORE than the other finger, the ends of the pencil will magnify this and you will have (if your pencil is 450mm long) around 4.5mm difference in how far the ENDS have moved…
If you push the pencil from its putter edges and you have this .1mm error, your total error is pretty much .1mm instead, as it won’t be magnified
I like to use extreme cases to show points I’m trying to make, if it actually moved by 4.5mm you’d have an EXTREME problem! But extreme cases are simply magnified versions of less extreme cases… By pushing that pencil at all the different points you would find the further toward the outside you push, the more accurate and smooth the movement… It could end up being a factor effecting speed and accuracy or introducing vibrations etc.
@Jarred_Baines good point. I hadn’t been thinking in terms of a machine that can have that kind of error. This spectra thing is still really weird to me, and I always just think of belts.
On the other hand, you have to think about how this type of error will interact with the (slop or lack of slop) that there might be in the bushings that there might be in the bushings and in the attachment point between the rod and the bushings. Will the bushings angle themselves to stay perpendicular to the rods, with the rod ends flex in their mounts to the bushings? Will they resist both and either stretch or break the spectra, or make it slip where it’s wrapper around the other rod? What’s the failure mode?
Yeah, I’m going to use belts, I just prefer them.
Sorry, what do you mean what is the failure mode?
I don’t see it as being TOO different from the original design, it too had the centre bar going into a linearly constrained (by the outer bushing / linear rail) plastic X/Y end. In my mockup it is also has the same constraint, doesn’t it? It should behave similarly regarding slop etc?
I consider the extreme cases to find the potential flaws in a design to find the ideal solution too, but you can’t just say “If this was really loose, that would be really bad”. You have to think these things through to their logical conclusions and figure out how they will fail. No design is completely without compromise, but you have to choose which compromises to make based not only on how difficult the failures are to avoid, but also on how acceptable the failure mode is.
To take an extreme example, if you’re choosing between design A that has a 10% chance of failing, and the failure would result in a little Z-wobble, and design B with only a 1% chance of failure, but the failure would result in causing the sun to prematurely go supernova and wipe out the entire Earth, clearly you have to choose option A.
Have we talked about printability yet? One of things I tried to do with the original Ingentis design, aside from minimising the risk of triggering any catastrophic stellar explosions, was to ensure everything could be ‘easily’ printed. This often has the most significant impact on the design choices for a part ime.
@Tim_Rastall Good point, you have to consider the failure modes in the production of the parts as well as their operation.
Oh man I don’t literally lol often, but I love those 2 comments guys, thanks for putting the universe first and printability second @Tim_Rastall
The mockup I made, although a few things need more meat / teardrop holes, is quite printable with the bushing hole pointing upwards.
And I didn’t consider what might happen if the rods do bind because as much as I keep looking at it trying to logically conclude something, surely it would be the same risks as the original design but with reduced likelihood of failure?