Sunday, September 07, 2008


RepStrap Build - Laser Cut Parts Kit - Z tensioner modification

I have made a modification to the Z axis tensioner that seems to solve problems that I was having when trying to get my Z axis working. I tested motion in the Z axis and was successful on the first try with the modified design without any adjustments. I don't have my Z sleeve bearings yet so I'm not running it too much but in my early tests the belt didn't slip and the motor didn't stall. The modification places the tension on the center of the belt and I think that the placement makes the Z axis load, from the perspective of the motor, appear the same in either direction. I know that the Z axis only moves in one direction during extrusion and it may not matter much but I think the load to the motor should be balanced. Some of the current and future efforts to mount different toolheads that will utilize the cartesian robot for applications other than 3D printing and may require the Z axis to behave the same in either direction of travel. In the original design, the tensioner is mounted on the diagonals and is placed offset to one side and very close to one of the Z pulleys.

Here's a diagram:

The modified design places the tensioner in the center.

Here are a couple of pictures of the modification:

I took the 2 diagonals and mounted them horizontally, 1 above and 1 below the centerline of the belt. I think that the cartesian robot is rigid enough to deal with the lack of diagonals on the tensioner side. It will still have support in the horizontal direction on the tensioner side. If more support was required, diagonals could still be mounted on this side if the horizontal rods in the Y dimension were longer such that they are long enough for both sets of diagonals. The lack of diagonals becomes a feature though because when your part is complete you can just reach in and pull your part out without the diagonals in the way.

When mounting the horizontal supports for the tensioner take care to place them such that they do not hit the Z pulley flanges. With the motor off, move the belt by hand to make sure there are no clearance issues. When everything moves without interference you can fire up the electronics and test without breaking anything. Here's a picture of the clearance I'm talking about.

Mine are not perfectly aligned because this was merely a test to see if the design worked. I am going to replace the broken acrylic parts that mount the diagonal support rods with machined delrin or aluminum. I feel that these parts need to be stronger and made from a less brittle material. I have broken more than 8 of them. Haven't thrown any tools yet though! I must be getting mellower with age...

One last note: The parts that mount the tensioner to the rods, item number 89 on the Z Sub-Assemblies drawing, were originally designed for the rods to pass through them diagonally and could be re-designed specifically for their use with the rods mounted horizontally. They seem to be sturdy enough and are working fine but they could easily be re-designed to accommodate mounting on horizontal rods if a problem comes up.

Did you check the period of the belt? I'm building a BitsFromBytes Rep(St)rap as well, and I discovered that the period of the acrylic gears is about 2.36 mm (45 mm diameter * pi / 60 teeth), while the belt I bought from McMaster has a period of 2.0 mm. That was way too much variance and resulted in a lot of slipping. I ended up using aluminum gears from McMaster rather than the acrylic gears, and now the slipping seems to be gone.
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