Final results for the Shapeways part

And just to prove that I'm not just making up stories about repairing hundred-year-old furniture for the page hits, here's a picture of the repaired table, with the fixed caster in the center.

The table has a pedestal base that splits in the middle; each side of the base has three casters, and a central post with a caster supports the table when it's pulled out to its largest size. Here, you can see two of the original casters as well one the repaired one.

My wife mentioned this weekend that she'd always been really frustrated by the broken caster, but never had the stamina to complain to the refinisher and get him to find a replacement of the correct size. She was ecstatic that we could fix the table, and even happier that we'd learned some new techniques to make the replacement part.

Once again, thanks to Shapeways for the replacement part, and Makerbot for making me trust that I could make a correctly-sized replacement part!

...Wherein the Author Repairs a Hundred-Year-Old Caster...

Man, that Shapeways stainless steel is pretty.

I got my second delivery from Shapeways today, and it had the two metal parts: the screen door catch cast in stainless steel, and the dining room table caster printed in stainless but with a bronze finish. Both are well-finished, precise, and smooth - although the photo might make it look like the parts have print lines, the parts actually feel smooth. The larger latch piece is 1.5 inches long, so that'll give you a sense of how fine the metal 3d printing can be.

First step: reassemble the table caster. The part's pretty much perfectly finished straight from the box, but I sized the axle hole a bit small so I could drill it to the diameter of the axle. I was a bit worried about this; stainless steel is usually a horrible material to drill or machine because the metal at the surface hardens as it's cut; if I cut the stainless steel evenly removing a bit each time the drill spins, it'll drill fine, but if I don't advance the drill bit far enough, the metal will harden because of the pressure of the tool, the tool will stop cutting, and I'll have to push lots harder to drill through the casting. Traditionally, this is described as a weird "ka-thunk!" with the drill not cutting, then suddenly grabbing on the metal when enough pressure's added to cut the metal.

Luckily, the Shapeways sintered metal cut drilled smoothly and easily, so getting the axle hole square and the right diameter wasn't hard.

The original axle (only a hundred years old!) was bent, so I knew I needed a new one. The old one had both ends of the rod hammered wide to hold the wheels on, meaning that the caster couldn't be disassembled without cutting the axle. Instead, I wanted to make sure I could remove the axle if I needed to disassemble the caster. Because the wooden wheels are 100 years old (and already have flat spots), I suspect I'll need to replace those some day. For the replacement axle, I borrowed a 3/16" diameter stainless steel pin intended for trailer hitches; one end already had a wide head, and I shortened the pin with my trusty hack saw and cut a slot with my lathe for an E-ring to secure the wheels on the axle using a parting tool. I then drilled out the axle hole just larger than the axle. The axle got reassembled - pin, wheel, printed part, wheel, and E-ring - and out broken hundred year old caster is ready to go back onto the table.

Pics of the fixed table later.

The caster broke when we refinished the dining room table a few years ago. The guy doing the work was apologetic, and searched for a matching caster without much luck. I remember thinking at the time that I could probably machine a new part, but I knew it would be a challenge to get the dimensions right and get the work suitably precise. I'd even thought at the time I ought to learn about bronze casting, but really didn't need an excuse for another hobby.

Fast forward three years, and access to the Makerbot and Shapeways made replacing the part easy. Without the Makerbot, I wouldn't have been able to experiment with shapes and test that the printed piece would fit and function correctly. Without Shapeways, I wouldn't have been able to print the new part in a material strong enough to hold up the table. Without both - a handy, personal 3d printer, and a way to print in exotic materials, we'd still have a broken table.

Thanks, Makerbot and Shapeways!

First Shapeways Part!

I got a delivery from Shapeways today - a nice box shipped all the way from the Netherlands to California and containing only the tiny screen door latch. The metal parts must still be in process.
The new part is coated in a matte black finish and feels very light; with the rough finish, it feels almost like dense foam. It's a bit lighter than the original injection-molded piece and Makerbot-made piece. I worried a bit about the part's strength (I printed it in Shapeways's "black, strong, flexible" plastic), but it seems robust enough to be handled when locking the screen door. The bit of white visible on the part shows the underlying plastic color; I used a countersink to cut the depression for the flathead screw attaching it to the door.

The great news is that the Shapeways part exactly matches the original, broken latch in overall dimensions. Although a bit pricey at $6.88, it required none of the sanding and finishing I've done for my Makerbot-printed parts.

The three-latch picture shows how the part compares to the original latch and to my Makerbot-printed part. One surprising difference is in wall thickness. I'd designed the original model to print on the Makerbot, and to simplify printing, I sized all the walls 0.74 mm (or the thickness of two extrusions.) By doing so, the Makerbot didn't try filling between the extrusions and fouling on extra plastic. The Makerbot-printed walls end up at about 1.4 mm, close to the thickness of the original parts. By contrast, the Shapeways part walls came out exactly 0.7 mm thick - half as thick as the original. It looks strong enough, but the part seems surprisingly thin.

Now, I just need to wait for the metal parts to arrive!

Differences between Models for Makerbot and Shapeways

I just sent my first parts off to Shapeways - a couple replacement pieces for our sliding doors, and the replacement casting for the dining room table caster. As productive as the Makerbot is, using an outside printing service works for these items. Two of the parts (the caster and a catch for a screen door) need to be printed in metal for strength. The third, a latch for our screen door, will be printed in black plastic. I'm curious how the latch prints; I've been able to print lots of these latches on my Makerbot, but the part requires a lot of sanding to make it as smooth and shiny as the original. I'm looking forward to seeing how the Shapeways's final product looks.

I'll post details once the parts come in.

I was a bit surprised at the changes needed to get a part ready for Shapeways. Take, for example, the latch. When I'd originally modelled this part, I'd drawn it in SketchUp as a solid object. To print it on the Makerbot, I run it through Skeinforge to generate the gcode file which describes exactly where plastic gets extruded. One of Skeinforge's neat tricks is to let the user decide how solid the object should be. Should it be 100% plastic all the way through, or just contain a cross-hatch which provides enough support for the object, but only fills 20 or 30% of the model with plastic? That decision both speeds up printing and cuts the amount of plastic being used. That cost isn't a big deal because the Makerbot plastic costs around $50 for five pounds, or around 10 cents a cubic centimeter.

[Four versions of the sliding door latch. Clockwise from left: original Arcadia screen door latch, injection molded in black plastic. Original model, drawn as a solid. New model, designed to be hollow for cheaper printing. Partially sanded latch almost ready to be used.]

Shapeways, by contrast, assumes that you want the object completely solid unless you design in hollow areas. Their price is also higher because of the better quality printing, the cost of the machines used, and postage. For plastic models, the cost of an object varies from $1.50 to $3.00 per cubic centimeter for plastic and nylon materials, and $10 per cubic centimeter for the sintered stainless steel models. So when I uploaded my original Makerbot-friendly design to Shapeways and asked for a quote for the object, I got a price of $5.63 for the 2.57 cc object, printed in the "White, Strong and Flexible" material.

Ok, time for a bit of thought. I'm a bit cheap, and almost $6 for the latch is on the edge of my tolerance. All that solid material didn't seem essential for the part - were there ways I could cut down on the plastic used? I looked at the original part, and saw I'd made my model a bit too large, so I cut down the shape a bit to match the original, and brought the price down to $4.79 for 2.19 cc of material.

But could I do better? The original injection-molded plastic casting, although cracked, was still around, and I saw that it was actually hollow. Injection-molded pieces pretty much have to be hollow, from what I understand. The plastic needs to cool as quickly as possible after being placed in the mold, and needs to cool evenly to avoid warping. I tried hollowing out my original piece, but found matching the non-right-angles with SketchUp's extrusion tool worked really poorly.

Instead, I started my design from scratch again, this time planning on hollow walls that were 0.7mm or 1.4 mm thick. Those numbers were chosen so I could make test prints on the Makerbot easily; 0.7 mm walls get built as a double shell of plastic filament, and 1.4 mm walls get built as four-thread shells. I extruded side-to-side to get the non-right-angle on the base, then filled in as appropriate to get support for the metal tab that actually moved the latch. It took me a few more hours, but I finally got the piece down to 1.62 cc and a final price of $3.93. I printed a couple on the Makerbot; the pieces are a bit rough, but definitely usable. I'll be interested to see how the final Shapeways parts work.

My other big lesson was why I like the idea of 3d printing complex shapes. The third model, the screen door catch, is a relatively simple design that can be fabricated with machining tools pretty quickly. The version pictured took me a good three hours of careful work (including tweaking the design when it didn't actually fit the existing screw holes). Unfortunately, one wrong turn could have ruined the piece. That three hours of work was also fine for a single model, but I'd hate to have to make three or four of those catches. Being able to make a single model and then print a bunch of parts is much more appealing appealing.

So, my key lessons for the last week:

* It's fun to have access to Shapeways to print final models. I'm looking forward to seeing the final parts.

* Different designs are needed for the cheap-and-Skeinforgeable models for the Makerbot, and the expensive-and-solid models for Shapeways. Sometimes the same model can be used in both places, but plan on a bit of tuning.

* It's often better to start the design again from scratch rather than try to tweak an existing solid Makerbot model because of the difficulty of changing faces inside the SketchUp model. I did have better luck when I switched SketchUp to show a wireframe, rather than solid, object.

* Even when I can machine a part out of metal, I don't think I'd want to make more than one or two of the same part. 3d printing in metal gives me an easy way to make duplicate parts.

It's Different When the Printer's Always There

There's always the decision of whether to buy some complex tool like a 3d printer, or just borrow time on one.  When I was thinking of getting a Makerbot, some friends in LA reminded me they sell high end 3d printers, and if I could send them an STL file, they'd be glad to print my part on the nice machine in their garage.  I've never taken them up on that offer. As I've seen when borrowing time on a metalworking lathe, having to *go* somewhere or wait for the opportunity to use a tool increases the chance I'll never do it. Having access to the machine at home makes it much easier to experiment in some spare moments, and try crazy ideas without the planning and commitment to go elsewhere to do my project.

I got reminded of the benefits of having a machine handy this week.  I've been running low on projects for the Makerbot lately.  Luckily, a friend sent a pointer about Jay Leno's  use of 3d printers to keep his classic cars running.  He'll design a part, print it, and if it fits correctly get a copy made in metal, or sometimes use the actual plastic piece when appropriate.

That reminded me of one of my lingering repair jobs.  Our dining room table dates back a few generations (and supposedly came around the Horn to California), but when we refinished it a few years back, the woodworkers broke one of the casters.  It's an odd design - finding the patent is easier than finding a modern reproduction - and the small (1" cube) bronze casting wasn't easily reproduced.  I think I could machine a replacement piece, but I know it would take a few hours work to make a piece, and I'd probably have to redo the piece a few times if it didn't fit perfectly.

Luckily, Shapeways is printing stainless steel these days at $10/cubic centimeter, so I realized if I could make an STL model of the piece on my Makerbot and test that it's the right shape, I could send it off and get a replacement piece.  Our table would then be ready for another hundred years of use.

The problem is that I wouldn't trust that I could get the design of the piece right on the first try... or tenth try, and I certainly wouldn't try making a replacement piece with a three week turnaround time and $40 charge each time. But I've got a Makerbot, and while it won't print in metal and might not be as precise as the expensive machines, I know it's available, and I can easily run through multiple design attempts in an evening. If the design works, then I might send it off to be printed.

So with a couple evenings of work in SketchUp and on the Makerbot, I'd printed four example pieces, found cases where my measurements were off, and tuned the design.

Helpful hints when trying to replace a mechanical piece:

1) The original piece was a bronze casting, and whoever made the original master had worked to make a beautiful piece: smooth curved shape, no sharp edges.  SketchUp, with a bit of work, can add such curves.  However, once the shape stops being a simple rectangle, it's really painful to adjust the position of axle holes and precise measurements.  As in machining, I learned to start by making a very simple shape, add the high-accuracy features, print a model, and test whether it fits and works mechanically.  Once the shape works, then I started playing with the design.

2) By hand, the easiest way to add curves to a SketchUp design is to draw a curve on a flat face, then extrude (or un-extrude) the shape through.  This only allows curves on one face since extruding doesn't work so well with curved faces or angled faces.  SketchUp also doesn't find intersections between curved faces automatically, so extruding (for example) a pair of intersecting holes through an object doesn't correctly handle the intersection.

The Rounded Corners SketchUp plugin might have helped here by letting me round or slope edges to better match the appearance of the original casting.  Note that the Makerbot renders 0.3mm thick levels, so having more than 3 faces on a rounded corner with radius 1mm wouldn't get me a better surface.

The Rounded Corners plugin only works well if I did all edges at the same time, and unfortunately it kept trying to round the drilled holes. I finally gave up, and used the grab tool to manipulate edges to round off the design.

3) I still need to figure out how to do the cupped socket in the casting.  I made the socket here using the "Follow Me" tool, but I'm also tempted to just get the 3d printed metal piece close, then machine the part with a ball-end mill to the precise dimensions.  Machining stainless steel isn't easy, but it might be easier than trying to get a smooth cupped surface at the correct dimension in the STL only.

Now I just need to get my courage up and ask Shapeways to print me a copy in metal. Heck, maybe one of the strong plastics would be enough to get the caster rolling again!

(BTW, Shapeways's price for this piece in stainless steel is $40 - probably worth it so I don't have to machine an equivalent piece. Interestingly, I'd thought their price was based on material used, but it looks like it's actually based on object volume. Because the casting measures in at 4.06 cc, I ought to see if I can get rid of that last 0.06 cc to drop the price $10.) (Bzzt. Ignore Robert's predictions about cost on his late night posts.)