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Y-Rod Alignment Helper for Prusa MK3 Bear Upgrade

by motocoder Oct 13, 2018
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Hi, just a warning, the tool is ~1 mm to short.
I'm currently converting my MK3 to the bear upgrade. When I align the rods with your tool, the space between the center of the rods is ~172 mm and not he 170 mm which it should be.
See step 8 of the bear upgrade guide for the dimensions.

I checked in Slic3r PE, the length is 102,35 mm. When I measure the printed tool it is 102,3 mm.

I like your tool, but it should be corrected to the right size.

The attached file is a screenshot of the Fusion 360 model that the Bear creator uses with two of my build helper components added to it. You can see that the rod spacing is 170.00 mm, and the build helpers are properly aligned and just touching the edge of the rods. I used this CAD model to design the rod, so that the spacing would be correct. Is it possible that the side rails of your Bear are not exactly flush with the front and back rails? It is possible to have this, but still have everything square.

I will double-check with the Bear author to make sure this model is correct.

By the way, you should NOT be using this build helper to align both rods. You should just do one rail (recommend the left one), and then use a caliper to adjust the spacing between the rods. The reason for this is that the exact spacing is important for optimal bearing life.

Thanks for your reply. Now I begin to wonder why my frame is "~ 2 mm wider" (in fact it isn't, its exactly 370 mm wide). : /

My v-slot extrusions are cut precisely and are perfectly flush and squared. I'm actually a bit finicky when it comes to assembling and constructing things. So every thing is tripple checked and aligned down to 0,5 mm pecission with calipers and a high pecision ruler.
I actually aligned everything with calipers and then checked with your helper tool, because I forgot that I had printed the tool.

The bed in my assembly runs smoother than in my original Prusa build and that was quite fine. So I'm not so worried that something is wrong with my assembly.

After looking at your CAD assembly, calculating the needed length of the tool by my self and checking against my printed tool, I agree with you that your dimensions must be correct.

I'm sorry for straight claiming that your tool must be the problem. But that seemed so obvious to me, cause I perfectly aligned everything and hadn't had problems.

Still have no idea why the tool is ~1 mm to short in my assembly. Maybe it's something with my PLA printed tool and the part which goes inside the v-slot notch is a tiny bit too large and that sums up with other things to be ~ 1 mm off in the end.

I hope I didn't made to much trouble, cause I only wanted to help. Thanks and have a merry christmas. :)

Hello @k_lab I am the bear upgrade author. I have checked the tool and dimensions looks good to me. It should not be a problem to have the whole bed shifted by 1mm.

However why do we have that difference is not totally clear to me. Are you sure the rod spacing is 170mm on back and front?

Hello @pekcitron I mearsured everything again and I can confirm that the space between the rods is 170 mm at the front and back.
The space between the rodholders is 134 mm at front and back.
all rodholders are 36 mm wide and have 82 mm space on the outside til the extrusion ends.
This sums up to 370 mm in total, which is the expected length of the extrusion.

I think the cause for my 1 mm problem with the tool relies in my PLA printed tool. I think the part which goes inside the v-slot notch is a tiny bit too large which results in the tool not sitting flush on the outside of the extrusion.

Sadly I can't test if the printer actually works, because my MK52 heatbed is loosing the magnets (they are falling out). So I have to repair that first. I wish Prusa had used more and better adhesive for the magnets, they are falling out quite easily and my printer is < 1 year old and I nerver heated the bed above 90 °C.

The profile for the portion that goes inside the V-slot was taken from the design for one of the Bear parts, so I am very confident that it is appropriately sized. Of course, it's possible that things didn't print correctly on your printer. I could add a hole so that a tee-nut could be used to secure the helper to the frame.

No apologies necessary, in fact I am very glad you posted this. I had exactly the same experience when I assembled my Bear. I chalked it up to errors I had made in assembly, but after your post I went back and measured and I don't see anything that would account for this difference. Like you, I am completely baffled by this!

I have asked the Bear designer to take a look, and he has said that he will, but it very busy with other things at the moment. Hopefully when he takes a look he will have an answer.

And Merry Christmas to you as well.

Thanks, I hate those steps that require measurements, these type of tools are great for using deduction rather than trusting some crappy measuring tool. Looking forward to trying it!

You're welcome. Just make sure you actually measure the spacing between the rods with a caliper, as that dimension is very important for the life of the bearings.

I'm hoping to use the assembled bed itself to find the correct gap, by just letting the rods settle into the loose holders, once one rod is locked into position using your tool.

Thanks for this, I will give a try soon!