Calibrating bent heat bed  

Re: Calibrating bent heat bed

Hi Aleksander

Try mounting the glass upside down. You can also put a strip of blue tape (or similar) under the glass along the front and back edges - see my photo here: http://shop.prusa3d.com/forum/prusa-i3-kit-building-calibrating-first-print-main-f6/y-axis-shift-t557.html#p4043

Peter

Re: Calibrating bent heat bed

I have tried to flip the glass, but the glass itself is perfectly flat, it is the heated bed that pushes it up. The blue tape trick sounds good, thanks!

Re: Calibrating bent heat bed

Hi Aleksandar

You could also try loosening the four nuts underneath the heater to relieve the strain of fitting into the chassis. Insert the heated bed into the chassis and tighten the nuts up.

The holes where the crews go through the heater have a bot of play and the heater expands when warm. If the nuts are tight when the screws are inserted into the chassis holes the heater could warp upwards when warming.

Peter

Re: Calibrating bent heat bed

I found that the alignment of the holes on the frame are not perfectly matched to the holes in the heater bed. The bed should just drop onto the frame with the screws not binding and no pushing required. If the screws bind, they can force the heater bed to flex. I suggest that you open up the holes on the frame until your heater assembly just falls into place and see if that makes a difference. I had to ream out the holes in the frame just slightly for my heater plate to lie flat.

Re: Calibrating bent heat bed

Under no circumstances enlarge the holes in the Y chassis.

This is a mistake I made with my first printer and now, even though the enlargement was only 0.5mm, the XY axis has suffered with slightly offset layers in both directions.

Once you enlarge the holes, there is no going back and no possibility of great looking prints.

The holes should be well-enough aligned if not, then there is a problem and replacement parts are required. The heated bed holes do have some play in them to allow the screws to be moved slightly; loosening the nuts will allow alignment.

Peter

Re: Calibrating bent heat bed

The bed holes in my Y-carriage are too small to allow the M bolts from the heat-bed to fall in. There is perhaps 0.5-1.5 mm that the bolts slide in at most. It's more like the M bolts "sit" on top of the Y-carriage.

This has caused me issues with being able to properly level the heat-bed (only having about 1mm of room to move the bed up/down at each corner). While I can get it level within that 1 mm, that leaves room for the heat-bed to slide as the Y-table is moved during a print (there is also slight warping as the heat-bed heats up. I've used small elastic bands to keep the heat-bed pressed against the Y-carriage.

I had thoughts of taping the holes in the Y-carriage to allow the heat-bed M bolts to be screwed in and using springs between it and the heat-bed to hold the heat-bed up.

Re: Calibrating bent heat bed

Hi Endaf

I guess there is a possibility that the aluminium coating is slightly too thick. If that's the case, you should get in touch with support.

Have you tried getting the screws in the chassis holes without the heated bed? Are the screws bent?

Problem with a spring-loaded bed is that the adjusters have to go under the chassis and they would foul the main frame with Y axis movement, unless the Y axis was modified.

Peter

Re: Calibrating bent heat bed

Have you tried getting the screws in the chassis holes without the heated bed? Are the screws bent?
Yup, they are not bent, they just do not fit into the hole in the Y-carriage. I thought this was normal, that the M3 bolts just "ride" the top of the hole, with only a fraction of a mm of the bolt going in.

According to step 14 comments section (at the bottom) of the assembly manual, others have the same issue.
http://manual.prusa3d.com/Guide/7.+PSU+and+Heatbed/45

Your's is different? The M3 bolt slides in?

Problem with a spring-loaded bed is that the adjusters have to go under the chassis and they would foul the main frame with Y axis movement, unless the Y axis was modified.
I saw that you can't add anything underneath the Y-carriage, there is interference with the vertical frame. The idea was to not use the supplied thumb adjusters and just use the allen key on top to rotate the M3 bolt when needed. Only the newly created threads in the Y-carriage would be needed to hold the M3 bolt in. The springs are only there to push the heat-bed up away from the Y-carriage. So long as the M3 bolt doesn't exit the bottom of the Y-carriage, there should be no interference with the vertical frame.

Words perhaps don't explain what I'm thinking. I will try and mock up the assembly tonight with photos.

Re: Calibrating bent heat bed

Understand exactly what you are suggesting. It should work as long as the screws are cut to length (if necessary).

Peter

Re: Calibrating bent heat bed

Here's what I'm thinking.

Unfortunately, there's not enough meat left in the Y-Carriage metal to allow taping the existing hole and using the original M3 bolt. I'd have to go slightly larger bolt (probably the next size up from M3 in imperial measurements).

Re: Calibrating bent heat bed

I agree that making the holes larger on the frame is not a good idea. I used a reamer and maybe changed the size by a few hundreths of a mm. After reading the discussions, I thought that making the hole in the corners of the heatbed slightly larger might be a solution. The nut that hold the heatbed to the screw head could be tightened after the screws are inserted in the frame and the heatbed is at temperature. This would ensure that that the screws are not causing the heatbed to flex and also would not cause y axis problems as the the heatbed would still be tightly fastened to the frame.

Re: Calibrating bent heat bed

James

That's exactly what I have done, but without having to increase the size of holes in the heated bed.

I still get a slight upward bow when hot, but with the glass high side down and a bit of blue tape under the front and rear edges of the glass I have a very level bed to print on. I even managed a print with a 50 micron first layer.

Peter

Re: Calibrating bent heat bed

Just to let you all know, loosening and re tightening heatbed screws helped, thanks for the tip!