Monday, July 20, 2009

Using a CNC Cutter for Model Building, Part 2

(If you're directed here from TechShop's blog watch, you'll probably find my live steam locomotive experience more interesting. I was very happy to use TechShop's metal lathe to turn the wheels I couldn't make with my equipment at home.)

I've had a bit more experience with the Cricut CNC cutter and "Sure Cuts a Lot" scrapbooking software for the Mac. So far, I'm making some interesting things with the Cricut, but I think I'd still have better luck with a laser cutter.

To recap: I've seen some beautiful laser-cut model railroad structure kits, and I wanted to experiment with both designing structures flat, and hoped on speeding up my model building a little The Cricut scrapbooking cutter is a small standalone computer-controlled die cutter that can cut sheets of thin cardboard or occasional heavier materials. The Sure Cuts A Lot program is a third-party drawing tool for the Mac or PC that controls the Cricut cutter via USB.

My first experiences (in the last blog entry) showed a bunch of problems. I could draw cutting lines in a drawing program and import them into Sure Cuts A Lot, but found that it would tend to cut shapes with a continuous cut and round corners when doing so. It also wouldn't always cut pieces all the way through, so small pieces (like window openings) would need to be trimmed out by hand. None of this would be a problem if I was doing rounded shapes like letters and clip-art for scrapbooking, but won't work for small model railroad structures where I want sharp corners and need pieces fully cut out to reduce labor.

Luckily, SCAL imports vector drawings in the SVG format - an open, XML-based format, and one that's pretty easy to edit either by hand or with a simple computer program. I ended up writing a quick-and-dirty filter to take my cutting drawings, and massage them a bit so they work better on the Cricut:

* All multi-segment lines get broken into individual straight lines to force SCAL and the Cricut to only cut lines and not try to curve connecting lines.

* All lines get extended about .01 inches to try to cut all the way through at corners.
* Any lines colored red get cut twice, and get cut in opposite directions. I saw the knife didn't always bite through until it had been moving for a bit, so cutting the same line in different directions ensures the whole line length is cut through.

* Any lines colored green are considered scribing or embossing lines, and get placed at the front of the list to draw. After all the green lines are cut, a bunch of tiny lines in one corner of the drawing are added to get the cutting head off the drawing. By doing this, I can set the cutting blade (manually) at its minimum setting when I start, let it scribe all the lines that shouldn't cut through, then change the cutting blade to a deeper setting while the print head's doing the small lines.

If you try this at home, also be aware that SCAL silently fails to cut if any part of your drawing extends outside the cutting area. Make sure to keep your drawing inside the dotted boundary lines. Also make sure to measure your drawing at its largest point in your drawing program, then use SCAL's detail mode to enlarge the vector art to that size so you're cutting at exactly the size you want.

The different behavior for red and green lines is an idea taken straight from the laser cutters which already use different color to use different strengths or speeds of cutting.

By running my cutting drawing through my SVG processing program, I'm getting better results. I've actually used the cutter to scribe individual board lines in cardboard, and find the double-cuts in window openings ensures the pieces get cut through. The picture shows a building front in really thin cardboard. The board lines were scribed by the cutter, and the window openings pretty much fell right out.

The wood-frame office for the Hyde Cannery isn't the best demonstration for the cutter; it's all straight lines and minimal windows, and I know I could cut out all the pieces in an hour with a straightedge and Xacto knife.

A better sample project is this curved-front building next to the railroad tracks in Campbell. This building is visible in photos from the 1930's; the glass-block window in the front and inset tile crosses at the roof line are obvious details that are made for the cutter, either on clear plastic (for the glass block window) or scribed onto the wall itself (for the cross detail.)

I tried cutting pieces of this building out on the Cricut; the result isn't presentable yet, but it showed that the Cricut could actually help me with hobby projects. First, I made my model out of styrene, not cardboard - 0.020 inch for the walls (two thicknesses laminated together for a 0.040 thick wall) and 0.060 inch for a floor and curved form for the wall. I didn't expect the knife to cut through the plastic; I just made sure it scribed the plastic deeply, then I snapped the plastic apart along the cut lines. The curved floor piece (with a 2" diameter curve) required a pair of needlenose pliers to get the plastic to break, but generally the scribe lines from the Cricut were enough to snap the plastic apart. I also learned that the waste material I'd seen between pieces on laser-cut work wasn't needed for styrene; keeping the parts adjacent and snapping them apart saves a lot of work.

The problems with the curved-front building were mostly because of the thin material I used for the walls. The curved base cut nicely, but the 0.020 styrene for the wall didn't stay parallel when only glued to a curve at the bottom of the piece. (It didn't help that the 0.020 plastic I was using was already scribed to resemble individual boards, and the plastic cracked along one of those scribe lines.) For a second try, I'm going to need to cut a floor and ceiling piece (with supports between holding the two apart), then glue the walls to that armature so everything stays square. I'll also need to try to cut some small windows now that I'm getting cleaner cuts out of the Cricut.

So, that's my second week of experience with the Cricut. It's not a laser cutter, but it can do some interesting things. For my little one-off projects, it's not saving me a huge amount of time, but if I was making a lot of identical buildings, it could come in handy.


  1. Robert, thanks for the great blog and interesting tests on the cutter. I am a computer geek and love to try and tie in technology and this is great information.

    I also like the way you are documenting your railroad journey! I am finding that really fun and I am also enjoying documenting mine to share.


  2. Sorry bout that link..

  3. Hi, Paul, Glad you've enjoyed the blog, and hope it's inspired your model building too!

  4. It has, and I am enjoying watching your layout and building progress!