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Leadscrew problems

Posted by jkeegan 
Leadscrew problems
April 18, 2010 12:22PM
Progress on my Mendel is progressing at breakneck speed. I've run into a few issues though, that I'm hoping people have already encountered and can give me a "oh yeah that's just x, go do y and you'll be fine" boost.

Issue 1: Left leadscrew hard to turn when installed (no z belt)

When I built them on their own, both leadscrews spun fine in my hand on their bearings. The right one is still smooth even when installed into the Mendel. The left one, not so much.

As I screwed the left leadscrew into the captive nut on the Vert bearing 360, the threaded rod didn't "want" to go straight through the center of the z-leadscrew-base hole.. Instead the rod tended offcenter towards one side (unfortunately now I'm not sure which side).

Not knowing if maybe this was normal (I now don't think it is), as I screwed it in further, I pushed the bearing to one side to get it to fit into the z-leadscrew-base.

Now it turns very hard (silicone grease doesn't help) compared to the other leadscrew.

At first I couldn't even lower the x axis (even without the z belt). I found that if I pulled the top of the problematic left leadscrew to the right a bit, it would turn more easily.

I've tried moving the two assembled z-bar-top-clamps on the top left of the Mendel to the left and to the right, with some improvement, but it wasn't a full solution.

Has anyone else had this problem? Do I need to disassemble everything all the way back to the Vert bearing 360? (shudder)..

Issue 2: Mendel rests on my leadscrews

Before installing the leadscrews, my Mendel rested just fine on 4 frame vertex pieces. Now, the bolts at the bottom are the lowest point, and the Mendel would rest on them if I let it. I've been putting 4 small scraps of MDF beneath the vertex pieces to keep the leadscrews suspended.

I rechecked the picture and I have exactly the right number of nuts and washers, in the right places. Is it possible that the thickness of an M8 bolt could affect this? Are people using thin bolts or something? Why isn't everyone seeing this? I know I read Adrian saying that some double stick padded tape can reduce noise, but I assume it wasn't also fixing this, was it?

The frame vertex pieces were some of the earliest pieces I printed. Did they change in actual size at some point?

Ok, those are the two issues affecting me most (aside from the drive pulley problems I'm having, which I'll address with new pulleys). Any help is appreciated!

--Jeff Keegan (Just Another RepRap blog)
Re: Leadscrew problems
April 18, 2010 04:13PM
1: Just a shot in the dark, but perhaps the rod is not perfectly straight? Or perhaps you need to make the x-axis a tiny bit shorter?

2: I read about some people having the same problem. And some of them use this: http://www.thingiverse.com/thing:2003.

Love your blog by the way. Your high resolution pictures is a very good help in my build. (Just made a test assembly of the first part of my frame today).
Re: Leadscrew problems
April 18, 2010 08:20PM
no idea with your specific issue, but I use to test push-rods for straightness by rolling them on a piece of float glass (ordinary window glass, a mirror will work too). Float glass is supposed to be flat to a few thousands of an inch.
Re: Leadscrew problems
May 11, 2010 11:16AM
I had a similar problem with one of my leadscrews. It moved fine for almost a whole revolution, then catched onto something and was difficult to move. Turned out one of the m4 screws which holds up the bearing was a bit off, so the m4 washer touched the leadscrew m8 washer every now and then. I just removed the m8 washer and now it works.
Re: Leadscrew problems
May 11, 2010 11:37AM
I have a working theory as to what the problem is.

As I said above, I'd tried moving the z-bar-top-clamps left and right to try to fix the problem, with no luck.

Now I realize that what I really need to do is move the rod (via modifying the Vert Bearing 360 assembly) forward or backward, not left or right.

My guess is that the problem lies with two poorly built x-vert-drive-side-plate-360-end_2off pieces.. I printed those with PLA, and the "top" part - the part with the half-circle hole that comes in contact with the X-bar - isn't tall enough. From what I can tell from the model, they should be around 17mm tall, and measuring them on the machine I get around 15 for one and 16 for the other.

So I think that the leadscrew is being pushed too far towards the back of the machine, because there wasn't enough plastic pushing the X-bar away on the back side..

My current plan is to disassemble the Vert-Bearing-360 assembly, add some washers between the x-vert-drive-nut-trap_4off and x-vert-drive-side-plate-360-end_2off pieces, and if possible some between the x-axis-side-plate-nut-jig_2off and x-vert-drive-side-plate-360-end_2off pieces..

If I can space it correctly that the leadscrew "wants" to be center, I hope I can get things working well enough that I can print out new and better x-vert-drive-side-plate-360-end_2off pieces (along with one of vik's extruder pieces that clamps onto PTFE with wood screws, but that's another story).

Wish me luck!

--Jeff Keegan (Just Another RepRap blog)
Re: Leadscrew problems
May 11, 2010 03:40PM
I had this problem with my Mini-Mendel build. The notch in the 360 side of the z-leadscrew-base was a little off. I'll try to explane how I fixed it. My bars and leadscrews are straight.

The 180 side was fine so used it to find what adjustments I needed for the 360 side. The rods and leadscrews all need to parallel to each other. Both side to side and front to back. Put the z-bar 360 side just above the base.

First I made sure the z-bar on the 180 side was vertical. Then ran the x-axis down as far as i could. Make sure the bar and leadscrew on 180 side are parallel.

Adjust the x-axis length so both leadscrews are parallel.

Adjust the z-bar-top-clamp so both z-bars are parallel. You might need to adjust the base front to back.

You should see an error, if there is one here, were the z-bar and base meet. I had to file my notch a little deeper. Hope this helps.

Edited 1 time(s). Last edit at 05/11/2010 03:43PM by Duaner.
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