Raising Y Rods on Prusa Mendel

I have the RepRapPro version of a Prusa Mendel and I have been struggling with bed leveling since last August. I now believe my problem is that the Y rods are too low at the front of the machine and need to be raised by about 1mm. I'm not sure how this is possible but the bed is tilted down towards the front of the machine for reasons unknown.

Here is what the Y rod arrangement looks at the front:

How can I raise the front ends of the Y rods up slightly?

Thanks, Andy

PS. note that the bed uses a three screw arrangement so I don't see a way of raising only the front edge using just the screws.

Edited 2 time(s). Last edit at 04/29/2013 06:10AM by ajayre.

Hi Andy,

You might want to consider adjusting the triangular frame. shorten slightly the front (almost vertical) threaded rods or lengthen the back ones or a bit of both.
This will offset the top assembly towards the front ever so slightly as well as bringing it closer to the front.

With three point levelling you should be able to tilt the bed in any direction. Place the nozzle as close as you can to each of the mount points and adjust them to get it equidistant with some form of feeler gauge which could be anything that is flat.

I am struggling to understand how changing the length of the bars will make any difference. It will alter the angle between the Z axis and the Y axis but I don't think it can affect the height of the nozzle above the bed as it moves in the Y direction. The bars suspend the X axis at some point in space but no matter where that is the bed needs to be level with respect to the plane that the Y axis moves in, which is determined by the Y bars. I.e. to keep a constant distance from the nozzle the bed has to be exactly parallel to the Y bars.

The Y bars should be perpendicular to Z axis but if they are not you get slanted objects, but the nozzle can still be a fixed distance relative to the bed.

What does matter for bed levelling is that the two Y rods are parallel to each other. If one end of one bar is say too low then as the bed moves along the Y axis it rotates around the Y axis. That means with a perfectly flat bed you can level three corners but not the fourth making it appear as if the bed is not flat. I encountered this on my Acrylic Mendel90 and fixed it by putting a paper shim under one Y bar clamp. I could then get it level in all four corners.

The alignment of the the Y bars isn't easy to change in the classic Mendel shape. I think you would need to put a shim under one of the feet.

Edited 3 time(s). Last edit at 04/29/2013 10:00AM by nophead.

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If you're only out by 1mm, perhaps it is a combination of not perfectly round smooth rods, and not entirely equal accuracy in the printed Y rod clamps. a half millimeter discrepency in the printed parts and the rods could add up to a 1mm difference depending upon how they are put together. Your smooth rods appear to be ground a bit where they are in the clamps, if so, that would make a difference. Also, if your smooth rods have been installed with any kind of a bow to them, that will cause this discrepency as your bushings do not travel completely to each end of the rods. Try rotating the smooth rods 180 degrees along their axis (spin them).

Regardless, 1mm difference front to back is not important. Shim it up as Nophead suggested, if it concerns you, but the real soltution is to correctly square and level the frame, and then adjust the bed using the levelling screws when the machine is actually assembled and ready to use. Trying to achieve perfection with the frame when you haven't even finished building the machine is really looking for problems that don't matter. I see from the photo that you still have a way to go before you need to consider this.

Thanks for all the replies and suggestions!

To clear some things up: I don't think making the rods longer will help - only raising them.

The picture is from the build instructions - my machine has been operating since last August but I've only been able to use the center of the bed.

I've put some tin foil under one corner to raise it by about 1mm and that has improved things somewhat but it still feels like a nasty hack solution as I'm sure it will affect heating of that corner.

I'll try some of the other suggestions such as rotating the rods, etc.

Andy

my theory behind my suggestion was that the bed is "running" away from printing head.
this would be due to the Y rods being on a slope in relation to a flat table.
Ajayre, can you check the level of these Y smooth rods within the accuracy that you need?

When I put together my prusa i2 I had an enormous problem with the y bars being out of plane with the x axis... even though i made a gig and made sure all triangles were the same length. What I came up with is to completely eliminate the y bar clamps on the front of the machine and replace them with zip ties holding the threaded rods and smooth rods together. I then tightened the nuts/washers on each side of the zip tie clamp to raise to where i needed it. After much research I found that the issue was actually the printed clamps that I used to build the machine, the relation between the y rod centerline and threaded bar was slight different between all clamps (+- .05 to +- .2) I took 2 of the closest clamps in relation to that ratio and placed them at the back of the machine, and the zip tie clamps in front so i could adjust the actual level on the front. I know it is a bit of a hack but it did work well and has continued to do so.