Project Leonard – Bar Clamp Chaos

Well this week I had some time off and was finally able to devote some overdue attention to some of my many on-going projects. First in line was my 3D printer build Leonard. It’s been a while since my last Leonard posting, what with work, Koothrappali and Project Wave (coming soon) poor old Leonard has just sat in a corner collecting dust and er…. breaking!!!

To my surprise over the approximately 3 month period since I last did some development, nearly all of the printed bar clamp parts have failed and those that are still in one piece, are showing serious signs of fatigue.

The obvious conclusion (looking at the above picture) would be that I had overtightened the frame around these parts, however I strongly refute this to be the cause and/or case and lay the blame on a combination of  both material properties and poor part design.

The nuts were only tightened just enough for the parts to grip both bars and function properly and in some cases no pressure was really applied at all, such as those used to support the Z axis rods (bellow).

Fig 1: Broken clamp

I believe that over time the pressure of the frames construct, and in some cases also applied (within reason required by application) by the adjacent nuts, has caused the parts to fail. Anyway I could point fingers an justify all day but what is important is that this was a major setback that needed resolving. My solution was to design a new clamp that is more robust and also affords capability to apply grip.

What I came up with was a two half design that utilises pressure applied by the surrounding nuts to gift grip in one axis (the base). A solid body is used to relieve compression in this instance.

A 1mm gap between the top edge of the two halves above the bar and a bolt hole gifted so that just enough clamping force can be applied to the second bar if needed. The use of a maximum of 0.5mm worth of bend on each of the halves ensures that the flex is below that which would cause the plastic to snap and/or tear, however only time will tell.

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Fig 2: Clamp comparison, two halves seperate

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Fig 3: Clamp comparison, two halves together

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The above images show both the new and old clamp designs. As you can see both are pretty much equivalent in material volume. The only real impact in terms of cost is the requirement of a few additional nuts and bolts, however so far I have only needed two sets to apply a little additional pressure on the Z axis crossbar as shown in the image below.

Fig 4: New clamp bolted in place

This means that apart from these two instances, no additional pressure is applied to any of the clamps thus further reducing the potential for part failure.

The main impact caused by this issue was the need to completely reassemble most of the frame. This has severely delayed progress, however as you will be able to see in some of the images I have finally began to wire up the electronics. So far I have managed to wire up the steppers and attach the cabling to the frame. I have also mocked up the end-stops and will finalise these when I have worked out placements, and the required lengths of wire etc.

As always I’ll finish with a image showing the current state of play.

Fig 5: Current state of play

Tomorrow I hope to attach the Y axis belt, finish installing the end-stops and finally run the motors for the first time. This will leave just the install of the extruder head and the base-plate.

Project Leonard – Bar Clamp Chaos
Posted on April 14, 2012 in Projects by batts