Why not a wider "double X-axis"?

discussion of design changes / improvements / suggestions
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lutorm
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Why not a wider "double X-axis"?

Post by lutorm » Sun Apr 21, 2013 8:00 pm

After making a few parts, it's clear that I need to stiffen up the X and Z axes. Those plastic wheels don't seem to handle sideways loads very well, and in stock configuration moving the Y-axis puts twisting loads on both of them. I've seen people double up the X-axis, which clearly is the way to go, but all examples I've seen are with butting them up together, with the spindle still cantilevered out on one side.

It seems better to put the "extra" X-axis on the opposite side of the spindle, so the spindle is in between the two rails. That way both X-rails help lift the weight of the spindle, as opposed to the farthest away from the spindle being loaded upwards and the nearest one then having to take both the weight of the spindle plus the downwards force applied by the other. The drawback is of course that you'll lose much more range in Y because the gantry plates will be much wider, but I'm going to increase the size anyway so this won't be much of a problem.

This approach would also make it possible to add a second Z-axis rail and clamp the spindle in the middle. That would also take any torsional load off the Z-rails. Essentially, the machine would be totally symmetric around the spindle.

Does what I'm saying here make sense? I need to start learning to use some suitable modeling program so I can just sketch out what I mean. Apart from the lost range, are there any drawbacks I haven't thought of?

Any suggestions are appreciated.

/Patrik

WillAdams
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Re: Why not a wider "double X-axis"?

Post by WillAdams » Sun Apr 21, 2013 8:40 pm

This has been suggested / discussed previously:

http://www.shapeoko.com/forum/viewtopic.php?f=11&t=922

and implemented by incognico.
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lutorm
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Re: Why not a wider "double X-axis"?

Post by lutorm » Mon Apr 22, 2013 12:03 am

Ah, nice! Though what I mind was more what wlanfox was thinking about in that thread, with two Z-rails as well. Having a threaded rod on both of them would be the ultimate, I think.

cvoinescu
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Re: Why not a wider "double X-axis"?

Post by cvoinescu » Mon Apr 22, 2013 12:12 am

lutorm wrote:Having a threaded rod on both of them would be the ultimate, I think.
I think having two carriages on each X rail, with a Z rail each (a total of four) would be even ultimater. 8-)

That said, the reasons people don't go for a wider double X axis are, IMO, that it eats more into the working area; it's harder to make a rigid carriage for it (if the carriage isn't rigid enough, the benefit is lost; there's also the issue of weight); and the incremental gain compared to a back-to-back dual MakerSlide X is small (especially if not doing anything about the Z axis).
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northbear
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Re: Why not a wider "double X-axis"?

Post by northbear » Mon Apr 22, 2013 2:47 am

cvoinescu wrote:That said, the reasons people don't go for a wider double X axis are, IMO, that it eats more into the working area; it's harder to make a rigid carriage for it (if the carriage isn't rigid enough, the benefit is lost; there's also the issue of weight); and the incremental gain compared to a back-to-back dual MakerSlide X is small (especially if not doing anything about the Z axis).
Another possible reason may be the difficulty for changing bits and or changing to a different spindle. At least that is what I thought of when pondering this type of design.
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danimal
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Re: Why not a wider "double X-axis"?

Post by danimal » Tue Apr 23, 2013 5:15 am

I just went through all the different possibilities for stiffening up the z axis and the main problem that I came up for splitting the x rails is that it eats up a lot of work area. Putting them close together and utilizing the stainless v-wheels (the Next thing that I am waiting on) on the z axis is about the most cost effective way to go, and really it does a great job tightening everything up. I see a lot of people make blocks to space everything out, but I just bought some M5 Nylock nuts and used them to space the v wheels. I would say that 90% of the wobble has been taken out of the z axis and I can still easily change tool bits and tools as need be.

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xtonyx
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Re: Why not a wider "double X-axis"?

Post by xtonyx » Thu Jun 20, 2013 4:45 am

The design of the carriage bearings isn't the best for anything heavy. No reason to even use V groove bearings, or steel. Delrin is quiet and aluminum is nice and light.

I use a wide format flatbed uv printer with a 73" print swath at my job. It could be a bit of inspiration for a different style carriage assembly. I think the print head carriage weighs about 12-15lbs. It rides on just 5 bearings with delrin wheels. It travels about 60ips. I've attached a picture of the trolley that rides on the aluminum extrusion. Four bearings at the top, two at 45* and two at 90, the fifth is on the bottom in the middle. The rail is aluminum extrusion with a 45 degree lip on the top facing towards the carriage. The delrin wheels have only failed once in over 7 years of daily operation.

A similar version of this system is used on my cutting plotters but with a symmetrical bearing arrangement top and bottom because it has up and down forces like a router would use.

I know there are other ways to build these kinds of things, but I like the simple bearings. Actually I'm impressed by the use of two bearings sandwiching the v-wheel.. The delrin bearings I use have a groove machined around the diameter and the delrin wheel has a matching groove inside of it so when it's press fit on it stays in place... But a PITA to reproduce.

After I finish this first build I plan on trying to put something together using a big nasty plunge router spindle I have in my garage.. And It will probably need something heavier than the maker slide.

Sorry for all the word vomit.. And the picture is a bit confusing, it was from a disassembly tutorial I made for another forum.
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WillAdams
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Re: Why not a wider "double X-axis"?

Post by WillAdams » Thu Jun 20, 2013 10:48 am

I believe Improbable Construct increased the size of his mount plates when he cut new ones of carbon fiber.

There was one other upgrade which modified and lowered the motor mounts, but I'm not finding it now.
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