Re: CAD library

Paul Koehler



Makes perfect sense to me.  Huh?  vbg


Paul C. Koehler


From: STMFC@... [mailto:STMFC@...]
Sent: Saturday, August 16, 2014 7:12 PM
To: STMFC@...
Subject: [STMFC] Re: CAD library



Some notes from the real world…


When you get to your late 70s your list of “not in my lifetime” possibilities is considerably longer than it is for you youngsters in your 60s. That certainly colors my view of 3D printing and libraries of CAD files. So what follows is the view of someone more likely to receive a pacemaker than a 3D printed Pacemaker boxcar in the foreseeable future.


I won’t discuss 3D printers except for how they affect the design process. We had an extended thread on 3D printers in July 2013 which may be worth reviewing.


The two printing processes I use the most are stereolithography and mutltijet modeling. Ross (no last name) mentioned 3D Systems’ Viper high resolution SLA (stereolithography) machine. Shapeways’ FD and FUD (frosted detail and frosted ultra detail) parts are created on 3D Systems’ even higher resolution MJM (multijet modeling) printers. SLA builds by drawing the image of each layer on the surface of a UV curable liquid resin. It’s a vector process. In hi-res mode the laser beam is 0.002” dia, the layers are 0.002” thick, and the surfaces are the smoothest of any 3D printing process, but it can’t do parts with overhanging features. MJM printing is a raster process (think 3D inkjet printing), with resolution of 600 x 600 DPI  (and increasing) and layers less than 0.001” thick. It’s faster and less expensive than SLA, the surfaces are slightly textured and it can handle overhanging features. Surface texture is no big deal for 1:1 objects, but it can be for our miniatures.


A good designer has to be familiar with the capabilities of the manufacturing process he’s designing for. By their nature, stereolithography underbuilds features slightly, and MJM machines overbuild by a similar amount. Not a factor unless you’re designing things like ¾” dia rivet heads in HO. For SLA designs I increase the head diameter by 1/8”; for MJM designs I decrease it by the same amount. It may seem like no big deal, 1/8” in HO is just under 0.0015”. But if I use the same design file to print an uncompensated ¾” rivet head in each process,  the difference in the printed rivets (5/8” SLA vs. 7/8” MJM) is 40% and is very noticeable. Prototype rivet heads are not full hemispheres, but to get all the rivets to print properly and be visible through a couple of coats of paint, I make them full hemispheric domes and put each on a 1/8” riser. If a car uses several different sizes of rivets, and you want to represent the visual differences, it may be necessary to design the smallest ones so they print reliably, even if that makes them larger that they should be, and increase the other sizes accordingly. In HO I know if I want to represent a series of surfaces that are offset from one another, like a window frame, or a rivet batten on a car side, the layers must be separated by at least 3/8” for the layering to be noticeable. In other words, it helps if you can apply the precision touch of an engineering designer with the eye of an artist.


So there we require two different design files for a single part, depending on what process will be used to print it. For HO. But those HO rivets won’t print if reduced to N scale, and they are probably too large if printed in O scale. The differences in the level of detail required for the various scales is what tempers my enthusiasm for a CAD library. Ignoring the design audit issue, a printer will try to print everything in the design file, but parts smaller than the resolution limit of the printer will be blobs on the surface. So a highly detailed design file that prints gangbuster parts in O scale will have to be dumbed down considerably for the smaller scales. And vice-versa. Too many variables for this old mind to process.


Finally, Dennis mentioned that I “gamed” Shapeways’ process to get good parts by ganging three parts facing in different directions. It’s actually five different directions, as shown here:


Those are left- and right-handed versions of compressor boxes used on passenger cars with B&O-style York air conditioning. The plate is approximately 3” x 3” and cost $54 from Shapeways. Too expensive for parts to use as-is, but just fine for resin casting masters. Shapeways built the plate on edge and I did get two good parts. But only two.


Nothing is as simple or as straightforward as it seems.


Tom Madden


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