Things I've learned...

while designing custom model train modules with Sketchup from Trimble, plus the SketchUCam plugin from PhlatBoyz and the CNC USB Controller from Planet CNC. All 3 of these products are free and have all the features I need.

I design small, portable, table top model railroad modules for T-Kits.com. These conform to the N scale, T-Trak standard. T-Trak is popular with model railroad clubs, because they are easy to set up, take down and transport to train shows. Show layouts are sometimes very large, when several clubs attend. T-Trak is one of many portable module systems.

All model train layouts start with benchwork. Benchwork might be as simple as a piece of plywood sitting on some sawhorses, or as elegant as a Mod-U-Rail system from Woodland Scenics or a .Benchwork system from Seivers. Benchwork is rarely seen because it is covered with scenery and tracks.

T-Trak modules are nothing more than open bottom boxes. The pieces have dados and rabbits to make them easy to assemble. We knew at the very beginning that the pieces would have to cut out with a CNC router.

So what did I learn?

First, from start to finish, it only takes me 2 hours. All I need are the module dimensions, the thickness of the plywood and the diameter of the router bit in the CNC router.

Second, designing all the pieces in 3 dimensions lets you check how the pieces fit together when assembled. When I first started, I spent a lot of time making things fit, Today, I can look at the flat drawing and know it will fit.

Third, once designed, it's easy to create the G-code command file. Nice work Phlatboyz.

Fourth, the spoil board on the CNC router needs to be prepared with a fly cutter, router bit, to make it as flat as possible.

Fifth, after you install the final router bit, jog the tool down until the it just kisses the spoil board. Set this to the thickness of your material in the CNC controller. For example, when using 6mm thick Russian Baltic Birch, jog down and then set the Z value to -6mm. Most CNC routers repeat to +-0.002” (+-0.05mm). The goal is to cut through the plywood, but not deeply into the spoil board...so I set the depth of cut, in SketchUCam to the thickness of the plywood, plus 0.002”.

Sixth, plywood is never as thick as it's published dimension. For example, when using 1/4" plywood, cut the dados 1/4" wide. The wood will always fit into the dado.

Seventh, if you want a tight fit when you insert the plywood into a dado...cut the dado with a very fast feed rate The resulting dado will have cupped sides. This requires a very powerful and strong CNC router.

Chuck Ely

 

Chuck, good to hear that you too enjoy the advantages of SketchUp and the SketchUcam.

About point four in your list. Why do you care if the spoiler board gets, well...spoiled, in the process? I have the spoiler board there to be spoiled. I never level it before hand. When I generate the g-code I have the setting in the parameter box set to 102% over-cut. Sometimes I have it set to 100% since, as you've mentioned, most of the time the material is thinner than it's official dimensions. That way the spoiler board gets scorched very little and I can use it many times.

 

Hi Yoram,

The reason that I don't like to cut into the spoil board is that our model train modules have some very tight tolerances. We sell kits, which our customers assemble. The assembled modules have to be square, plumb and true +-0.04" (+-1mm). The hardest thing to control is the thickness of the plywood that remains after I cut a rabbet. If I'm working with 1/4" plywood, the thickness of the material after the rabbet has to be 1/8" +-0.01" (+-0.25mm). The only way I can consistently achieve this is to start with a flat spoil board and then carefully set the tool height. BTW, I've been told over and over that it can't be done...but I did it anyway.

Currently, we cut all our parts on a commercial CNC router in the $200k range. I'm looking for a really strong, 25" x 50" CNC router with moderate speed that can cut with this accuracy. Our vendor is required to prepare the spoil board with a fly cutter, prior to starting our order. So far, we haven't had to re-surface the spoil board in the middle of an order.

Chuck

 

Chuck, recently, two guys here had built a 4x8 machines. The machines are all home made and cost a fraction of the price that you mentioned above. Maybe that's the way that you should go too. If I remember correctly, one of them mentioned truing his bed with a fly cutter.

http://www.phlatforum.com/xenforo/threads/4-x-8-router-table-build.3046/
http://www.phlatforum.com/xenforo/threads/openrail-kx-5-version.3056/

 

Hi Yoram,

I'm looking for a 25x50 because that's all the room that I have. I'm looking for a kit because I don't have a large budget. The OX at http://www.openbuilds.com/builds/openbuilds-ox-cnc-machine.341/

looks promising. I'll have to be convinced that a cog belt drive is strong enough for a 1 HP spindle cutting 1/4" of plywood. The OX with screws would be just about ideal.

Chuck

 

I think that both builds I mentioned above use a timing belt. The biggest advantage of a timing belt is that you don't need an expensive zero backlash nut. Use a belt a bit bigger than 1/4" if you have a doubt and you'll be fine.

1HP spindle is not that big, btw. It's about the size of a good size trim router. There are a few people who use a trim router on the PhaltPrinter III with great success and that printer is using a 1/4" timing belt.

 

However, you lose the mechanical advantage of a screw. I'm not against timing belts. It's just new to me...and I just need to see one in action. It appears that the OX has two belts driving the X, and 1 belt driving Y and Z. And I agree about the cost.

I haven't decided on a spindle, but don't think I need more that 1 HP.

Chuck

 

I agree. A trim router is plenty of power to cut wood. Even with a 1/4" bit. Don't forget to use the multipass to cut wood more than 1/4" thick. Better cutting shallow and fast than deep and slow.

 

Hi Yoram,

Since time is money, I really want to cut all the pieces in one pass and relatively quickly. In addition, I'd like to gain control over the order in which things are cut in SketchUCam.

I'd like a button similar to Re-Order Groups called Re-Order Cuts. With that button depressed, I would select line segments in the order that I want them to appear in the G-Code file. The beginning of the cut would be the end of the line segment that is closest to the select point. If the start of the second line segment is at the end of the first segment...there would be no Z movement. I used to do a little programming, so I started looking at the files...but I don't have the necessary skills. I would have to first, buy the Ruby for Really Old, Grouchy Dummies book.

We currently use 5-layer, 1/4" thick Russian Baltic Birch plywood. It has a lot of good qualities, but cost too much. I've been searching for a replacement and found one today at Home Depot of all places. It's a 4-layer, 1/4" thick, 2'x4' birch plywood in their project section, at roughly half the cost of Russian Baltic Birch.. Oddly, it measures 6mm thick. I don't really care, as long as it's never thicker than 6mm.

Chuck