Thursday, February 9, 2012

Makerbot Upgrades Part 4

The office Makerbot has been printing nearly constantly for nearly a month now.  It's already more than paid for itself with parts we'd otherwise have to send out for manufacture.  The modifications I've described so far have made it run reliably enough that I can leave it printing unattended in a back room for hours.

I have noticed that the printer is getting noticeably noisier over time.  There's a creaking in certain spots of the X stage travel, a horrible screech when the Y axis moves quickly, and the whole machine shudders and rattles when the Z axis moves downwards at the start of a print.  I've also noticed that it takes a lot of force to manually move the X and Y stages, even when the stepper motors are completely unpowered.  I've decided that my next modification will be to redesign the X and Y stage slides, replacing the oil-impregnated bronze bushings with roller bearings.

First to be rebuilt is the X stage. As shipped the X stage rides on two 9.5mm precision steel rods, each passing through two bronze bushings pressed into ribs in the X stage structure.  This design is over constrained.  The print bed heats the entire X stage, which attempts to expand and pushes sideways on the two steel rods, anchored at their ends in the Y stage.  That's probably the cause of the creaking sound and some of the resistance in the X stage movement.

I ordered a few dozen miniature ball bearings, and then designed printable parts to hold them.  I designed two arrangements of three bearings each, to replace the two bushings on one of the rods.  On the other rod I placed only two bearings, so that the stage is physically located by only one of the rods, with the second only providing vertical support, so that the X stage could expand without pressing sideways on the rods.  The support structures for the bearings now took up most of the space under the X stage, so I joined them together, replacing two of the wooden ribs under the stage.

As I did I also replaced the clamping mechanism that clamped the belt to the stage.  On the version of the X stage I built the belt is clamped by two little wooden sticks that are only secured with one bolt each, and easily work loose over time.  I replaced them with a single large piece of plastic, held in place with two screws, to spread the clamping force out and not as easily vibrate loose.

There's really not much room to work with under the X stage.  To get the bearings to fit I had to cut several holes in the X stage mounting plate, and carve a long groove in the Y stage structure to clear the lowermost bearing.  I did manage to get everything to fit, and when I finally reassembled everything and ran the machine there was no creaking as the X stage moved.

The Y stage had been making a terrible screeching sound when moving quickly, so I set about replacing the brass bushings on that next.  Unlike the X stage, the stock ThingOMatic Y stage isn't over constrained.  On one steel rod the Y stage slides on two brass bushings, but on the other it already rides on ball bearings, so the Y stage can float and expand or contract independently.  There's also a lot more space under the Y stage to work with, so I had an easier time designing the bearings in.

I still had to cut holes on the Y stage bottom plate to fit these in.  Here I replaced each of the brass bushings with a single piece of plastic holding three bearings.  Three screws into the face of each anchor them to the Y stage structure.  It's not quite obvious from looking at it from the outside, but I'm also using the bearing blocks to clamp the belt to the Y stage, replacing the two original little wooden clamps.

After I reassembled the machine and ran it, I found that the noise from the Y stage was completely unchanged!  Moving the Y stage rapidly still resulted in terrible screeching sounds.  On closer inspection I traced the sound to the Y stage belt idler pulley.  Both the Y and X belts in the ThingOMatic have idler pulleys on 4mm bolts, without even a brass bushing let alone bearings.  Over time the bare plastic-on-metal contact gets noisy.

I had previously installed Thingiverse item 12528 on the Y stage idler, so the pulley wouldn't be hanging supported only on one side by the plastic baseplate.  Based on that design, I made a new part to support the pulley with ball bearings on each side.

I'm using the same 4mm bolt as an axle, but not instead of the plastic pulley turning on a stationary bolt, the pulley and bolt now spin on two bearings, that are then supported by the plastic bit anchored to the printer structure.  This completely eliminated the Y stage noise.  The Y stage now moves so easily, I have to hold it in place with one hand while trying to remove printed parts from the bed to keep it from sliding around.

For completeness, I've also upgraded the X stage pulley with bearing supports.  This was a much trickier job, again due to the limited space available.  There's barely room in the ends of the Y stage for the pulley as it is.  To get the bearings in there I had to replace the entire upper wooden structural cap on that side with a printed plastic part, and cut a hole in the lower wooden part to fit the support for the lower bearing.

The poor Y stage on this machine has quite a few new holes cut in it now, but both stages are really, really smooth.  The only noise as the machine prints is the singing of the stepper drivers.  It's almost like some kind of music, especially when it prints circles.

The default printing speed set in the ReplicatorG software is 30mm/second.  I had trouble getting the machine to print complex shapes accurately early on, so I dropped the speed to 20mm/second for a while.  Since upgrading the X and Y stages with bearings I've increased the speed back to 30mm/second without noticing any drop in quality.  I even printed a few parts experimentally at 50mm/second.  Some of the surfaces were a little wavy and there were a few small gaps, so I don't think I'll be printing many parts at that speed, but it's nice to see the machine still works fairly well even at that speed.

The next step would be to replace the Z stage bearings, but as the Z stage moves slowly and infrequently, I'm not sure it's worth replacing the bushings there with bearings.

Tuesday, February 7, 2012

Recipe: Mini-Shepherd's Pie-Muffins

Apologies in advance as this post has nothing to do with robots or Makerbot upgrades.  Rather, this is something I've made for a few parties and had enough people request the recipe that I've put together these visual instructions to share.

Drew's Mini Shepherd's Pie-Muffins

3 pounds of Yukon Gold or similar yellow potatoes
1 cup lamb stock or beef broth
1 stick unsalted butter
1 egg
About half a cup of parsley
1 cup shredded cheddar cheese
A few cloves of garlic
1 medium-sized white onion
1 stick of celery
2 carrots
(Substitutions can be made to the vegetables, see below)
2 pounds ground lamb or beef
A bit of all-purpose flour - about a quarter cup as needed
2 teaspoons Worcestershire sauce
3 cans of Pillsbury Grands biscuits (the kind that are eight to a can, so you have 24 biscuits total)
Some olive oil
Cooking spray
Salt, Pepper, Nutmeg, and Paprika as needed

This recipe began as a Shepherd's Pie recipe I've been making for years at home, as a good, filling, yet reasonably inexpensive way to make dinners and lunches for several days fairly inexpensively.  Recently I was invited to a potluck event, where the requested theme was for Irish food.  Shepherd's Pie was a reasonable fit, but I wanted to make something that could be hand-held and eaten without needing a bowl or utensils.  My wife suggested making individual hand-held pie-muffins, and so this recipe was born.

This recipe takes about 3 hours to do and makes 24 pies.

Step 1:  Mashed Potato topping

Wash the potatoes.  Peel them and chop them into chunks.  I prefer to use Yukon Gold potatoes for this, but any large yellow potato will work.  Make sure you peel the potatoes completely, any remaining peel in the topping will have an unpleasant texture.  I once experimented with multicolor fingerling potatoes, which gave the pie an interesting blend of colors but were very difficult to peel properly.

Put the chopped potatoes in a sufficiently large pot.  Pour in the beef or lamb broth.  I used to use  Scotch Broth soup for this, as it contained lamb broth plus various chopped vegetables all in one, but the brand of soup I used has since been discontinued.  For this batch I'm using the last of a batch of lamb stock that my wife prepared some months ago from some lamb shanks, but canned broth will work just as well.  The one difference is that canned broth has a lot of salt added for some reason, so keep in mind that home-made stock may require salt added to match the same taste.

After adding the broth, add enough water to cover the potatoes.  Add salt if needed, and add pepper to taste.

Cover the pot.  Simmer over medium heat 15 minutes or until potatoes are soft.

Remove from heat.  Drain the liquid through a strainer.  Save about half a cup of the liquid for later.

Melt one stick of butter.  Wash and chop the parsley.  Add the butter, the parsley, one egg, a pinch of nutmeg, and half a cup of the shredded cheddar cheese to the potatoes.

Now mash the potatoes and other ingredients well until you have a smooth mixture.

This will make up the topping layer on the mini-muffins.  Set this aside for the assembly step later.

Step 2:  Meat and Vegetable Filling

We start with the vegetables.  In addition to the potato topping, a shepherd's pie should contain a mixture of various cheap vegetables - onions, garlic, celery, carrots, parsnips, or whatever else you have spare.  As this is a recipe meant for using up leftovers and scraps, the exact mixture of vegetables can very depending on what you have on hand.

Unfortunately, the only appropriate vegetables I had on hand when making this recipe was some garlic.  My wife and I don't normally eat much of this kind of vegetable - we're more into dark green leafy vegetables these days.  I could have bought them all separately, but it seemed a bit silly to me to buy an entire pack of carrots for just one, or an entire bundle of celery for only one or two stalks.  So I cheated.

There we go.  Carrots, Celery, Onion, Parsnips, Turnips, Leeks, Parsley and Dill.  All the required vegetables in a single package, in just about the right quantities.  Feel free to call me a heathen and prepare your pies from fresh vegetables instead.

 I just needed to chop them further and add the garlic.  You don't really want large chunks of vegetable in the filling for this recipe.

You should have about 2 cups of chopped vegetables when done.

Add some olive oil to a large saucepan over medium heat. Add the vegetables and cook until soft.

Remove the vegetables from the pan and set aside.

Add a bit more olive oil to the pan, then add the ground meat.  Traditionally, a Shepherd's pie should be made with ground lamb.  I couldn't find any ground lamb locally when I made this recipe.  As the idea of this kind of recipe is to stretch cheap meat with potatoes and other vegetables, I decided it was perfectly in the spirit of the recipe to use cheap fatty ground beef instead.  Technically that makes this a Cottage Pie instead.

To the beef add the Worcestershire sauce and about half a cup of the liquid from cooking the potatoes.  Traditional recipes usually call for sage or rosemary at this point, but my wife is deathly allergic to those spices so I don't use them.  Feel free to add them if you like.  Brown the meat, mixing it to ensure even cooking.

Add flour as you cook to thicken the mixture.  You want to end up with a semi-solid paste-like texture, as these pies need to hold together once cooked.  When the meat is just about cooked return the cooked vegetables to the pan and mix well.

This will make up the filling of the mini-pies.  Remove from heat and set aside.

Step 3:  Pie Crusts

Preheat the oven to 425 degrees F.

Spray your mini muffin trays with cooking spray.  I use four disposable aluminum trays that hold six muffins each when I make this, but there's no reason you can't use non-disposable muffin trays instead.  Open a container of biscuit dough.  Remove the biscuits one at a time, and with your finger spread each one out into a disc of about twice their original diameter.  Press the dough disks into the muffin tins to make dough-cups.

Once all the biscuits are in the tins, place them all in the oven.  Cook them for 6-8 minutes, then remove from the oven.  You do NOT want them to be fully cooked at this point, they should be just starting to turn brown on the outsides but still raw in the middle.  Keep the oven hot, you'll be needing it again soon.

While cooking, the centers of the pies will have swelled up.  With a spoon (or your fingers, once the pies have cooled enough) press the centers back down to make them into cups.

Step 4:  Assembly and final baking

Fill each pie-crust just full to the edge of the crust with the meat and vegetable filling.  You should have just enough filling to fill all 24 pies.

Next, scoop enough potato topping mixture onto each pie to cover the meat.  Again, you should have just enough to evenly cover all 24 pies.

Onto the pies sprinkle a bit of paprika, and then the rest of the shredded cheese.

Now return the pies to the oven, and bake for 12 to 15 minutes, or until the cheese is melted and the dough is browned.  Remove from the oven and let cool slightly before removing them from the baking trays.

Makes 24 muffins.
These can be eaten immediately while hot, reheated at a party, or stuck in the fridge for a week and eaten cold for lunch.  So far this recipe has been a huge hit at every potluck I've brought it to, but it also works well as a way of making a week's worth of dinners and lunches in one evening.

Sunday, February 5, 2012

Makerbot Upgrades Part 3

When building our Makerbot ThingOMatic, I was surprised by how little thought was given to cable management.  After carefully detailing the mechanical assembly, the instructions for wiring the machine are little more than a hookup diagram and some vague advice on taping the wires together to keep them neat.  There seems to have been little to no thought given to how to run the wires to the moving parts of the machine.  Most of the wires can be run along the inside corners of the machine and merely given enough slack to let the moving parts move freely.  The real problem is the build platform cable.

The build platform moves along two different axis, and is in near-constant motion when the machine is printing.  The connector on the front of the platform is completely unsupported, and the cable tends to drape down onto the rails and belt and get pinched against the frame when the platform is near the front of the machine.

It's unsurprising that the HBP connector is one of the most common parts to fail on these machines, and the Thingiverse has countless files for strain reliefs and cable clips and management systems.

My first attempt to keep this cable out of trouble was to simply zip-tie it to the mounting for the Y axis endstop switch, and then try to encourage it to coil in loops off to the side of the platform, as shown above.  This really wasn't satisfactory, the cable was still scraping on the Y axis belt, and tended to get caught when the platform was all the way at the front of its travel.  Getting pinched didn't just risk damaging the cable, it interfered with the movement of the platform, which was a problem as some of the parts I needed to print nearly filled the entire print volume.

A cable guide downloaded from the Thingiverse helped to some degree.

While not a complete solution, this at least kept some strain off the HBP connector.  I also began experimenting with designing a piece to anchor the build platform cable to the side of the Y axis.  My first few attempts didn't work very well - while they were keeping the cable bundle mostly out of the way, I was still losing some travel on the build surface due to the cable getting in the way of the platform's movement near the edges of the print envelope.  This solution worked well enough for printing small parts, but I needed to come up with something better to get the full use out of the machine.

We use industrial pick and place machines for circuit board assembly at the office.  While helping to maintain them, I had noticed that they used a cable chain system to manage the wire bundles to the moving gantries.  I decided to try and build something similar, as much to see if the ThingOMatic could print well enough to make that kind of complex moving part as to try and fix the HBP cable issues.

The result is this.

Admittedly, it's a bit of overkill for what it needs to do.  It's also not at all how cable chain is normally used - the chain should really be lying flat in a channel and bending with a constant radius when the gantry moves across it, not hanging free in space like this.  This works well enough for what I needed here anyway.  The chain constrains the cable to a single plane of movement, keeping the cable in front of the build platform from rubbing against the stationary parts below, and keeping the cable off to the side from getting pinched between the corner of the build platform and the frame of the machine.

Since I installed this modification (and those mentioned in the previous posts) the machine is working well enough to sit in a back room printing objects unattended for hours.  The cable chain has held up remarkably well, considering that the moving parts are just irregular plastic surfaces rubbing against each other.  I have yet to see any sign of the HBP connector failing, but I expect it will eventually, as even with the strain relief the connector is being operated beyond its rated maximum current and temperature.