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heat bed mounting linear bearing

Posted by davem 
heat bed mounting linear bearing
December 23, 2013 04:21AM
Hi All,

I have not seen this issue mentioned so I thought I would share this under a separate subject.

I found the mounting of the heat bed laser cut MDF rib onto the single linear bearing needs a more positive attachment. Originally the mounting method relied on an interference fit between the 2 parts but I found this is not enough. Once assembled the bed had a tendancy to lift off the single bearing. I mentioned this to someone at RRP who dismissed the problem as there were "no forces involved".

The fact is this. If the bed is not sat on the bearing properly then any Z homing will be set to some indeterminate bed height. When you start printing the weight of the print will start to push down the bed until it is sat on the bearing properly. This will have changed the 0 position bed height particularly at the farthest edge from the Z drive.

To address this I drilled a couple of holes through the MDF rib near the bearing so I could pass a cable tie through the rib and around the bearing to secure the two together.

Dave M
Re: heat bed mounting linear bearing
December 23, 2013 05:21AM
Hi DaveM

Thanks for your feedback. We haven't had the y-carriage pop off the front bearing in-house, but that doesn't mean the problem doesn't exist!

It could be that you have laser cut parts that have moved slightly during cutting. If you move the Y-carriage back and forth, does it tighten up at one end? This would indicate that the y-axis-end-plates are not a matched pair. If it's feels the same all the way along the axis, and the y-carriage is trying to pop off the bearing at all points, I'd guess the its the y-axis-rib that may not be accurately cut.

It could also be a construction problem with the y-carriage; anything that adds space between the back bearings and front bearing will change the position of the cut out in the y-axis-rib relative to the bearing. For example, if the two ribs don't fit together well, or the bearing clamps are not sufficiently tightened. Though the bed-insulator top sheet usually pulls these into place.

However, I like your workaround! Just make sure the axis isn't tightening up as it moves from end to end. It might be worth us cutting a hole for a cable tie, to provide more positive location, though generally cable ties are my least favourite kind of fastener...

If you feel you need any replacement parts, please let me know by email to support at reprappro dot com.

RepRapPro tech support
Re: heat bed mounting linear bearing
December 23, 2013 05:52AM
Hi Ian,

Thanks for the reply. I have a better solution in mind once i am printing. It just needs a bearing clamp which will slot over the bearing the MDF will sit in the in each U leg. A cap head will pass through each leg of the U and through the MDF.

Dave M
Re: heat bed mounting linear bearing
December 23, 2013 07:08AM
I also found the front bearing/bed connection was not very positive.
the carriage was running smoothly but over a number of moves the front bearing slipped sideways a bit

In my case I already had a 'spare' hole for the tie wrap as I had completely ignored the build
instructions for the carriage and glued the wood parts together then varnished it all instead of using
the captive screws, so just put a tie wrap around the front screw point and the front bearing.

The other rather weak point on the bed is the screw holes for the bed mounting at the corners are
very close to the edge of the MDF plate leaving very little material. I cut out 0.4mm stainless shims
for the 5 corners and epoxied them in place to give this more durability.

I think having the MDF plate even 2mm larger all round would make this a lot stronger, although I
may be a bit paranoid, maybe it will be fine as is in actual usage.

Re: heat bed mounting linear bearing
December 23, 2013 07:37AM
Chris, I agree with you that the holes are too close to the corners, My MDF plate has partly delaminated between one of the holes and the corner.
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