Wednesday, September 1, 2010

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.)


  1. The Shapeways SS is *really* strong stuff. Try pushing some triangular shapes from the inside-surface out (but not all the way thru) to shave off material while leaving a strong internal support lattice.

  2. Wait, you mean Shapeways round up to the nearest CC when pricing for stainless steel? That's really useful to know!

  3. Oops, scratch that - I haven't tried changing the size of the object, and so I don't know if Shapeways rounds up or not. For such a small (4.06cc) object, cutting even a small bit off could make a difference in price.

    However, if they're basing the price on overall volume (from outside measurements), then it'll be harder to trim the piece down.

  4. Wow, it must've been really late when I posted that for me to be so wrong.

    Shapeways measures my object out at 1.7cm x 3.1 cm x 2.7 cm, for a total overall volume of 14.2 cc. They measure the volume of the object (for pricing) as 4.06 cc. They price it out at $40.60 in stainless, which matches the $10/cc price.

    So (at least for stainless steel):
    1) Shapeways rounds to two decimal points on pricing.
    2) Their price must be based on material in the object, not overall volume of the object.