Replicating laser-cut parts

In which your narrator confects his own idiosyncratic method of replicating laser cut parts.

The recent acquisition of the UK manufacturer of the reliable Rapman printer, BitsfromBytes {BfB}, by the American firm 3D Systems has exacerbated a long standing problem that the Rapman's users' community has had, vis, getting drawings of the laser cut acrylic parts that make up the system. While BfB uploaded drawing files of one of their early Rapman designs into the Reprap website, they have neglected to do so with version 3.0 and later models to the best of my knowledge.

That wouldn't be such a problem save for the fact that the acrylic parts in higher stress parts of the Rapman tend to develop shear cracks and spalls after hundreds of hours of operation. Rapman owners are then left to either request replacement parts from BfB or print their own spares. Interestingly, there is a healthy spares design effort underway for the Rapman, part of which is hosted on the BfB website itself.

Here the printed white ABS grips for the x-axis linear bearings replace the lower plate for the extruder carriage in a much stronger configuration that the original.  Indeed, at this moment, there is a very exciting redesign of its printable parts underway by an Australian newcomer that greatly reduces the print time required for what were finicky corner blocks.

All this aside, there are many pieces in a laser-cut printer that simply need to be printed as-is in ABS rather than redesigned.  Heretofore, I found myself carefully tracing the parts out on paper and using calipers to get the dimensions.  While that approach works well with many parts some, like the extruder z-depth stop plate are much more problematical.

This particular plate is the antithesis of rectilinear and locating the holes and pockets is tedious to put it mildly.  I'd thought that it ought to be possible to use an ordinary 2D scanner to capture this sort of information.  Today, I gave it a try and discovered that it was not as straightforward an operation as I'd imagined.  


I had an extra copy of this part that I'd bought still in the protective blue film, so I threw it in my Epson Perfection V500 Photo scanner, a very cheap and extremely high resolution machine.  Even with the blue film the image captured didn't have a lot of colour information that would let you think that you could separate the part from its background.

I pulled that one into Slice and Dice and tried to separate out the image with RGB manipulations to no avail.  I then tried putting a intense red background behind the piece figuring that the blue would override the red and tried playing with the colour mixes in both Slice and Dice and Photoshop, again to no avail.

Two things were killing me; the transparency of the part and it's depth, which you can see very clearly in this scan.  When I tried to just capture the edges by going high contrast, the depth of the object spoiled everything.

It occurred to me that I could paint the part on one side with tempra paint, which can be wiped off of slick surfaces.  Rather than drive into town and given that the part was already protected with a blue film I simply spray painted it with red enamel.  I then put it back in the scanner with a piece of dead black HDPE sheet behind it and instantly got the colour separation I needed. 


Popping the scan into Slice and Dice and filling the black with green, I had an image I could work with.  As you can see, it was a little nasty, largely due to the messy laser cutting on some features and a bit of flare here and there.

A few moments in Paint cleared that up.

Now that I had crisp colour separation, defining the part boundary in Slice and Dice was trivial.

While I was in paint I took the pixel counts across the diameter of the outer circular feature boundary and measured the same feature on the part with calipers.  I then adjusted the size of the image in Paint.


With a bit of pushing and shoving in Slice and Dice, I processed the part and created a print file.  Here you can see the print roads for the part.

With a print file, I did a trial print to check the dimensions.  The acrylic part lay precisely over the printed part.

I photographed it again with the original part slightly ajar so that you can see the holes in the print.

If you look closely at the several layers of part print that I ran before aborting it you can see that I need to increase the print flow a touch.  More importantly, with a part of this size and complexity, however, I am going to have to write a routine that makes a better job of reducing transition distance between print roads.  Transitioning was taking far too much of the machine time.  That's on my "to do" list now.


What I've demonstrated here is not a smooth operation on Slice and Dice just yet.  I was mostly trying to prove the concept.  That was a huge success.  What this means is that owners of laser cut reprap machines can readily exchange parts information with nothing more complicated than an ordinary 2D scanner and a set of calipers.  This should give us considerably more flexibility than we have at the moment.


I am certain as I finish this blog entry that someone is going to show me a simpler, faster way to do this in a few hours.  That's certainly happened before.  If not, though, we have this approach.  :-D