Monday, May 28, 2012

Building the printrbot...

One of the interesting aspects of building something is that I find myself looking at how someone else designed something and instantly come up with reasons I don't like their solution and start coming up with other ways of doing the same thing... but once I start building I begin to realize why certain decisions were made and why the many ways I would have thought were better would not be in reality.

In the printrbot, Brook attaches the linear bearings for the y-axis with zip ties... I really wouldn't have thought it was a good design, but the zip ties provide enough slack so two bearings can be attached to teach base on the same hardened steel rod.  Since almost all the force is directed downwards in the design this works quite well in practice.  It wouldn't if there were significant lateral forces or large mass differentials across the printbed, but since those are controlled it's a rather cost effective and tolerant design choice.

Four threaded rods are used to connect the two base pieces together.  Straight forward design choice and very adjustable/reconfigurable for design modifications.  Probably would be a good idea to shift the upper rods down a bit since the inside nut will hit the zip tie.  A better choice may be to widen the bases a bit and then there will be no collisions.  The bases work, but are really a bit too narrow for ideal stability of the finished printrbot.

The two z-axis stepper motors are mounted to the outside edges of the bases with two m3 bolts.  Here you can see the y-axis rods through the linear bearings bearings before the zip ties were tightened.  Very smooth movement before and after tightening them down.  The weakness with the z-axis mounting is that there is no support underneath and they bear the weight of the carriage.  I ened up putting another m3 bolt betweent he bottom of the steppers and the base (that works very very well to stop twisting for the mass of the carriage).

The motor for the y-axis is mounted to the inside of one of the bases in the same way.  The belt tensioner (two skate bearings) are attached directly to the stepper (I really like this design choice).

The belts are connected to the steppes with a printed gear.  Having an internal captured nut is a great idea.  I didn't flat my shafts, may have to go back and do that later if needed.

The z-axis uses a two piece linkage for connecting the threaded rods to the steppers.  I decided to use 20mm m3 bolts rather than the supplied screws here.  The limit switch for the z-axis connects directly to one of the base sides.

The x-axis limit switch also attaches directly to one of the bases.   The print bed is attached to the y-axis rods with four clamps, one of which has a hole for the screw that trips this switch.  Interestingly I put it in the wrong place and had to move both the limit switch and the special clamp later.  In the upper left you can see the belt gear on the y-axis stepper.

Bottom side of the y-axis stepper with the belt installed.  A couple of washers lift the bearings to the right height for the belt.

The x-axis carriage rides on two smooth rods.Here the bearings are held directly inside the mount and compressed with a bolt between the bearings.  The screw to trigger the limit switch comes out one side and the extruder mounts on the other.

The extruder is an interesting design. I ended up cracking a part of it (easily fixed with epoxy) while adjusting the hobbed bolt.  The min piece holds the stepper and two bearings for the hobbed bolt.The power is transferred with herringbone gears (in practice it's works really well - I had trouble getting the large gear properly attached to the hobbed bolt (tightening the first time got it in place correctly, but I went to far an cracked part of the arm holding the front bearing.  Tension between an additional bearing and the groves in the hobbed bolt is done with the piece on the left - getting the tension correct the first time was not fun - but when it's right it seems rather stable.

I forgot to take pictures from most of the assembly process.  Mostly straighforward.  One hint I will give you is not to worry too much about having everything tightened down in the beginning, it's not a problem to do later and getting the alignment right before all the parts are together isn't easy.  I don't plan on really locking things down until I have a few prints done and adjusted.

Next time electronics and printing...