Tuesday, May 29, 2012

Finishing up the printrbot build...

The x-axis on the printrbot is driven by a belt.  Rather than having a fixed loop, the ends are attached to the carriage directly (I'm uncertain how it was intended, but it seemed to make sense to loop the ends this way and use zip ties to secure them...  This ends up working surprisingly well (I will probably make special clamps for tensioning later).

The idler on the far end is just a skate bearing between two large washers - works perfectly.  This end (one side of the z-axis) has two linear bearings pressfit (actually works fine) to bind the hardened steel rod and a captive bolt to attach to the threaded rod -> attached itself to one of the z-axis steppers below).  I ended up putting another captive bolt with a spring inside to control backlash later.

The complimentary end holds the x-axis stepper (with a nice printed gear for the belt), another set of bearings for the vertical smooth rod and a captive nut for the threaded rod on this side.  The actual x-axis smooth rods are pressfit.

There is enough space under the threaded rods on the bottom to put the control board.  It's a good idea to only attach one side first or it becomes quite hard to get the connectors for the motors and endstops attached.

I'll look at the quality of the signals later but it's a good idea to check the current limits for the output from the drivers before everything is wired up.    My board was set to send a bit too much current (the stepped got way too hot) so I backed it off to the point where the steppers failed to turn reliably and then gave them a bit more... don't overdrive steppers.

I decided to put a zip tie unter the y-axis stepper for additional support.  this ends up working very very well, but later I will print a part designed to do it better I think...

This is my first print (Mr Jaws #2).  I had to use blue tape (well I didn't have to, but I wanted to try printing and my kapton isn't here yet).  I got a 10x12 piece of 3/8 inch glass at the hardware store (nothing special about it) and just cut a piece about 5x5.5 inches and taped it to the hotbed.

After a bit of not having anything stick at all I found out that making certain the bed is leveled (much more accurately than I expected) and having the limit switch for the z-axis exactly right (piece of paper underneath has light tension no matter where the tip is on the hotbed) -> printing/ticking first layer is 100% reliable and repeatable.

Ok, so it's not perfect but it's not bad at all I think for a first print.

Layer alignements are good, axes are good, flow rate is good.  I can see some artifacts from having the layer height not a nice multiple of the steps/mm... This I don't care about now but will refine later once I have at least replacement parts printed in case I break something.

Yes Mr Jaws is a chip bag clip...

Printing the rail clamps went well, but they weren't ideal - functionally ok but a bit of flare...

The reason for the flare I learned when I failed printing a gear (the one on the points with an x).  It puzzled me for a bit how I could get crazy flare like this... But the answer was easy... so easy (and frustrating to figure out the first time) that it's worth  making a rule...  If you ever change the extrusion temp. or readjust the tension on the extruder you must do the calibration again... the flow rate is almost certainly going to be different.  The gears on the left are correct and all that changed is the calibration for the extruder rate.

Small herringbone gear is good.  The little bot is banging stuff out now with no problems or adjustments at all.

I'm still printing at only 0.4mm layers here and will play with increasing the resolution after I have replacement parts made.  I have not flatted my shafts and I suspect that might be a requirement for higher speeds with might be needed for playing with higher resolutions.

Here's the complimentary herringbone.  Still printing away without adjustments (as long as I don't play with the extruder tension it seems to just work perfectly).

As a side note, the two times I had to rethread the filament were because I hit extrude before the temp was high enough... what happens is that the hobbed bolt shreds the filament and the grooves fill with dust so nothing grabs... Very easy to fix -> heat the extruder -> run backwards to clear the filament -> open the tensioner -> flick out the dust with a needle from the groves in the hobbed bolt -> blow the dust away -> rethread -> CALIBRATE AGAIN -> and you're all set to print.

This is really a fun little bot... once I have replacement parts printed I'll see just what kind of quality I can get out of it.

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...

Friday, May 25, 2012

Printrbot arrives...

Yay, my printrbot has arrived.  At the end of last year I came across a neat kickstarter project by Brook Drumm for a simple 3D printer.  I was ready to get a Makerbot at the time (which looks quite good and has been refined a lot over time), but there was something about the way Brook wanted to do the project that appealed to me -> so I decided to back him.

The project ended up drawing a lot more interest than he originally expected...  This is a great thing when starting this type of business (more customer interest, more initial capital) but it meant that everything had to be scaled up fast.  Of course this led to delays in the kickstarter rewards, but so what... the goal is to get a business off the ground not to get rewards out fast.  It's been interesting to see how Brook and the community as a whole communicated about the changes in the design, the release of information about business decisions, design files, product revisions, assembling the printrbot, and of course the very few complainers about the delays.  I'm rather pleased with the way things  have developed and think it's likely Brook will end up with a nice little business and a happy community of customers.  There is an unofficial discussion board at printrbottalk.com that has been very helpful.

Opening the box is always interesting... Every company has a different character in how they pack things, and how the quality control is done before shipping.  I knew there would be a lot of light objects and a few heavier ones and that if the packing was wrong lots of damage could occur in transit.  Happily they did a very good job - exactly the right type and size of packing material and positioning of parts - there is no way to tell after opening if the box and actually shipped across the country or just sat on a table... perfect.  So let's see what's in the box...

There are a few lasercut pieces.  The larger one is for mounting the heated build platform (attaches to parallel rails on one axis and drive belt connectors).  The two smaller pieces are used to mount the electronics board.

A standard atx power supply.  Not exciting, but it will be fun to look at the quality of the output later.  Sometimes they can be unexpectedly good, sometimes not.  This project shouldn't come close to pushing the limits so it ought to be fine.

A big spool of ABS filament (about a pound I think).  The motors are off to the left - nicely wrapped in foam.

There are 5 motors in total.  The two smaller ones are wired together (z-axis intended to run in parallel).  All have connecters already attached - connectors look good.  I'll measure the resistances later.  All five turn nicely and the detents feel good.  (with the parallel wired little ones a cute thing is that turning one by hand will cause the other to turn as well).

The heated bed (6 inches by 6 inches) is already wired with a connector.  I'll measure the resistance later - it's a long pcb trace as a resistive heater strategy.  Board is very flat and not warped - mounting holes in each corner.  There are also 3 endstop switches and a thermistor already attached to individual connectors.  Solder joints are good but no heat shrink - not certain why but maybe there is a reason I will discover later.  A couple of extensions for the extruder finish out this bag.

This little bag has 6 bearings (they look like standard skate bearings), 25 little zip ties and 5 pairs of linear bearings for the smooth rods.  The packing list says 11, but there were only 5 pairs.  It's not a problem since there are only 10 needed in the printrbot.

In the white cardboard box we have a jumper (I think for enabling the atx powersupply), a usb cable, a spare extrusion nut, the hot end and the electronics board.  There are two connectors on the hot end - I think one for the heating element and one for the temperature sensor.

There are four 10 inch threaded rods, two 8 inch threaded rods and six 10 inch hardened steel rods.  It's always a good thing to check whether the cut threaded rods are straight (all six were good).  It's also good to run a nut down the threads to see if there are any binding areas (all six were good).

There are lots of little hardware pieces: screws, nuts, washers, strings, tubes and belts.  Everything matched the packing list.  When I have lots of parts I prefer to lay them out on paper with labels - of course as I build things get all messed up, but it helps when making certain all the parts are present.

The printed plastic parts are of a better quality than I expected.  There doesn't seem to be a lot of cleanup needed.  The side that was on the heated print bed is a bit shiny and a few have little bubbles on that surface - nothing to cause a problem. Here we have two base pieces, a mount for the extruder, a mount for the carriage, a mount for the motor (x-axis I think) and a mount for the idler.  They all seem to be both lighter and more rigid than I expected.

This group of printed parts is interesting.  The most complicated piece is the extruder (top center).  There is another piece right below it that helps tension the filament into the extruder.  There are two herringbone gears, two drive belt gears, a bearing mount for the y-axis I think, a mount for a limit switch, two belt clips, two pairs of sandwich clamps (I think for the z-axis) and four clamps (3 the same, 1 with a slightly longer segment and an extra hole, I think for mounting the bed to the rails.

Now we know what came in the box and checked everything off the included BOM (missing one unneeded bearing) -> we can start to build.