CNR D-1: Bring out the big guns

CNR D-1 and the air eraser
(Progress on the shells: Knocking down the ridges on one of the trailers)

As mentioned yesterday, I’ve decided to make some progress on the long-stalled CNR D-1 project.

CNR D1 - Grit blasting at Ryan's

A big stumbling block was how to deal with the ridges that are a characteristic of 3D Printed items. Such items are built up in layers and there’s often a ridge where the layers are bonded together. This stratification was very much an issue on the 3D Printed shell for D-1 and its two trailers.

Sanding and surface primer is the usual approach to addressing this problem, but there’s a lot of shell to cover here, and the sheer magnitude of the project made it easy for me to say, “Hmm… I wonder what’s on TV?” I needed a better answer. And that answer came in the form of a big red box fitted with cocktail-length rubber gloves…

Ryan and the air eraser
Ryan sets me up for a day of grit blasting

I visited my friend Ryan Mendell yesterday for an afternoon of hobby fun. (Stephen Gardiner, who designed the D-1, joined us too.) Ryan has been building patterns for resin casting and recently started his own hobby business, National Scale Car. He makes many of his masters using 3D Printing and was looking for a better way to deal with the ridges – and found the answer in the form of a grit blaster. I was curious about how effective it was, so I arranged a visit.

I worked on the three bodies for a couple of hours and I’m really happy with the results. The grit blaster (also known as an air eraser or media blaster) did a terrific job – especially in areas where it would be difficult to sand by hand, such as the recessed doors. In fact, I realized that if I focussed on those difficult areas, I could do the large flat sections of the shells with sanding sticks – or, even better, Ryan’s Tight Spot Sanders.

Ryan has a Paasche Air Eraser and a Blast Cabinet by Central Pneumatic (obtained from Harbor Freight). For the D1, I was shooting 220 aluminum oxide at 80 psi.

The before and after photos below show a definite improvement in the curved nose of the power unit:

CNR D-1 Texture - Before

CNR D-1 Texture After

The translucent nature of the 3D Print medium used makes it difficult to see the improvement, but running a thumbnail over the surface tells me the ridges are much less pronounced. I will finish sanding this shell using my Tight Spot Sanders then give it another application of Surface Primer and see how it looks. I expect this will be a “repeat as necessary until satisfied” operation, but I now have a strategy for tackling the project, which is the important thing.

Would I add a grit blaster to my workshop? Well, I do like tools, so the answer is “probably”. I don’t have the space right now – there are other things in the shop that must find their way to the curb – but I do have a suitable air compressor to power a blaster, and I’ve already thought of where I would hang the booth once I clear space for it. I would want to do something about muffling the noise of the air compressor, but a sound-insulated cabinet could take care of that.

Meantime, I envision another trip or two to Ryan’s before this project is finished. Thanks for the help, Ryan – the next beer is on me!

4 thoughts on “CNR D-1: Bring out the big guns

  1. I always dip my 3D prints in Future floor polish before making a mold. Future is self leveling and seems to do a really good job of smoothing the print layers on the 3D print.

    Combine this with your blasting and you might get a really nice service to prime from.


  2. Thanks for sharing Trevor. This is indeed the big caveat of 3D printing, made worse by the fact we use this technology to tackle complex volumes and shapes.

    As a matter of fact, what pressure and grit did you use to blast the model?

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