Questions and Corrections to McWire Cartesian Bot v1.2

First of all, thanks to Zach for the awesome walkthrough of his McWire build! I started this project not knowing anything and feel much more confident about approaching this project.

The beauty of this design is that everything doesn't have to be perfectly square or parallel. I would say the most critical things to get right is to make sure the X and Y rail that have the bearings sliding on them are perpendicular. The other rail on each axis is really only there for support and has no real effect on the stage travel. The other critical thing is to make sure the Z rail riding on bearings is perpendicular to the ground for the same reason as the X and Y stages. However, the Z rail doesn't necessarily have to be straight on with the X and Y stages because as long as it is straight up and down, you should get good results by properly setting your home position.


Anyways, I have been looking over everything and as thorough as Zach's instructions are, it seems like there was an earlier, older build and these instructions are updates to it. As such, it looks like there are some inconsistencies which I hope this thread can help to address and fix.

Here are some corrections I've made:

- For coupling the stepper motor shaft with the threaded rod, use a 3/16" ID tubing instead of the mentioned 1/4" ID. This will give a tighter fit.

- If buying the 3/4" pipe floor flange from Home Depot, the flange mount holes and distances are smaller. The ones I found fit 1/4"-20 screws very well. The original instructions use 1/2"-13 screws, which are twice the diameter. This may also be the reason why the vertical base requires a 1/2" thickness material, for the countersinking. If this is the only reason for the need for 1/2" thick material for the vertical base, then with the smaller flange from Home Depot, you can use the same 1/4" thick material as the rest of the build calls for.

- The mounting points on the X Stage for the Y Rails are too wide apart and would be safer to bring them closer together so that the Y rails ride closer to the center of the teflon bearings on the Y stage. I also brought the fixed bearings on the Y stage closer in (which brings the Y rails in a little bit more closer) so that the bearings don't protrude out the side of the Y stage.

- To be able to utilize the full range of the Y stage (10-1/16" x 7.5") as a worksurface, I think the best way to split the aluminum channel is 21" X rails, 15" Y rails, and 12" Z rails. This will allow the toolhead full access to the work surface which is the Y stage.

- I am not certain on this, but I think the 4" long extension springs may be too long, especially for the Y and Z stage bearing arms. The holes for the bearing arms are only spaced about 2.5" inches apart so I don't think the 4" spring would provide much, if any, tension to keep the bearings sliding tightly against the rails. My guess is that 3" springs would be more appropriate but I don't know if this is a correct observation.

- Also, can someone explain why the Z stage is so tall? Making the Z stage shorter would give more range of movement, wouldn't it? I'm not sure what the reasoning behind such a tall Z stage is.

If you've build a McWire Cartesian bot, please chime in confirming or correcting these assumptions and please post any other helpful tips and corrections.

Thanks!
Peter

> - For coupling the stepper motor shaft with the
> threaded rod, use a 3/16" ID tubing instead of the
> mentioned 1/4" ID. This will give a tighter fit.

I had to do the same. Also, make sure the outer diameter of your tubing is small enough that it doesn't rub on the aluminum L pieces. If it rubs enough, it can cause the stepper to slip inside the tube.

As far as the drive itself, I wrapped several layers heatshrink around the long nuts on the threaded rod. This will keep them in place in the pipe clamps better than electrical tape. I also have one axis using a larger piece of thick rubber tubing which works just as well.

> This may
> also be the reason why the vertical base requires
> a 1/2" thickness material, for the countersinking.

Different tool heads, especially heavy ones with stepper motors/dremel tools you can flex the acrylic Z stage a little. I think part of the reason to have double thickness is to cut down on this.

> - I am not certain on this, but I think the 4"
> long extension springs may be too long, especially
> for the Y and Z stage bearing arms.

You can cut the 4" springs in half for the arms, it turned out to be about the right length. I can't remember for sure, but I think I trimmed a little off all the sloppy springs just to make sure there was enough tension.


As for your other questions, maybe someone else has some info.

I've got a few McWire hacks on my blog, but I'm still in the process of documenting everything I've done over the past year.

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[www.binaryconstruct.com]

Thanks, Jeff. I checked out your blog and would love to hear more about the fixes you made to your McWire Bot.

As for the springs, just so I understand you correctly, you essentially used 2" length springs for all of the bearing arms? Are you using the 1/4" outer diameter, 0.032" wire that is in the BOM? I am wondering how much tension is needed in the springs to hold the bearings to the rail.

Thanks,
Peter

Here is the exact item from McMaster-Carr of the springs I have (I dug up an old invoice)

[www.mcmaster.com]
Type: Extension Springs
Material: Steel
Steel Type: Zinc-Plated Spring-Tempered Steel
Ends: Loop Ends
Overall Length: 4"
Outside Diameter: 1/4"
Wire Diameter: .032"



I halved the springs for the arms extending forward on the Z stage holding the plate against the U-channel aluminum (circled in red). I think I only trimmed a little bit off (1/2" or so) for the rest as these springs don't need to be in extreme tension, just enough to put a little pressure on the rails.

When you cut the spring, take a pair of pliers and bend one of the coils out 90 degrees to remake the end loop. I had a little trouble getting the springs to stay on the arms as well, it helps if you bend the nails back into the arms, locking the springs in, and close the loops on the ends of the springs.

edit-inline pic

Edited 2 time(s). Last edit at 05/07/2009 12:22PM by Jeff.

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[www.binaryconstruct.com]

I see. Thanks for the pic and clarification! So the 5" length is definitely too long.

For the self-tapping screws, what length did you get? I think the BOM calls for 1/2" length but the link to McMaster list 3/4" length.

Also, what are the little notches in the stages for? There are these thin rectangular sliver cut outs in the X, Y, and Z stages that I have no idea what they're for.

If you are talking about the notches on the edges that are opposite of each other, those are to mount the paddle or whatever you use to trip the opto endstop. If you are refering to the back part of the Z plate (the double piece), I think they were intended for a captive nut/bolt to hold the arms, but the arms were moved to the aluminum L pieces.

Here is the McMaster part I used for the self tapping screws.
[www.mcmaster.com]
Stainless Steel Type: 410 Stainless Steel
Head Style: Pan
Pan Head Style: Pan
Drive Style: Phillips
Inch Thread Size: 10-24
Length: 1/2"
Drive: #2
Thread Length: Fully Threaded
Drill Size: #17-#16

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[www.binaryconstruct.com]

Actually, here is my total order from McMaster from a little over a year ago. Most of the part numbers are still good though.

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[www.binaryconstruct.com]

Thanks again, Jeff!

I just bought a tap and used plain 10-24 screws. The tap was about $5, and it makes it much easier if and when you have to disassemble and reassemble everything, no worries about cross threading.

Edited 1 time(s). Last edit at 06/24/2009 04:41PM by Cameron MacLachlan.

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