Sunday, September 19, 2010

Concrete Tie Car

My friend Dean Daughenbaugh (who founded New Rail Models with me) is always coming to me with little projects for custom loads. The most recent project is a flatcar with concrete ties. This is actually a pretty cool project. You can see some prototype photos here:

The first link is photo he sent me when he asked me to do this project, and the flatcar is a UP flatcar. The second photo provides more details and is on a BNSF flatcar, showing that these flatcars and loads are used by multiple railroads.

This project will require two molds. The first mold will be for the concrete tie loads, and the second version will be for the metal brackets that help keep the ties in place (I’m assuming).

I started by creating a 3-D model of one layer of the ties:

Tie Strip

The idea is to make these as a single injection-molded piece and then stack them, like this:

Tie Stack

I’ve started to cut the mold for these, and we’ll see how these turn out. Next up will be creating the molds for the metal brackets that will be glued to the flatcar.

O-Scale Gas Truck

We had a number of people ask for an O-scale version of the gas truck kit that I made, so I decided to start working on it. Everything was going just fine until near the end of the milling. At this point, the end mill started to wander, gouging the mold, and then the cutter broke:


You can see the problem in the center of the photo. I was using a long reach 1/32” diameter cutter to sharpen the end of the bands on the tank. The band detail was originally cut with a 1/16” diameter cutter. So I was going back with a finer cutter to make the bands sharper. Unfortunately, the long reach 1/32” cutter was much too flexible, so it got off track (it curved to the right on one pass). And then on the next pass, it curved to the left and broke. Grump. The thin part of this cutter is a little more than .4” long so it can reach down to the bottom of the band at the very right. But that long, thin shaft is not stiff enough.

I could fix this mold by removing about .010” from the top of the mold and then milling the cavity again. However I also realized I should be user a thicker master. This block of aluminum is 1/2” thick, and the cavity is .04” deep, which doesn’t leave much room on the back to resist injection pressure. So I think I’ll order some 1” thick stock and start over.

Friday, September 10, 2010

Flash be Gone!

At this year’s NNGC, Jimmy Booth suggested that I could rescue molds with the “flash” problem I’ve been having by sanding down the face of the mold, so I thought I’d give it a try. He suggested using 500 grit sandpaper and said it would take about 5 minutes.

Well, after 5 minutes with 500 grit I switched to 220 grit because I just wasn’t making much progress. I think I spent about 1/2 hour sanding away (perhaps it wasn’t really that long, but it felt like it). And here is the result:


Success! There is still a little “flash” around the ears on the top, so I’ll probably do a little more sanding. But now the parts come out of the molds essentially clean. Thanks Jimmy!

Backlash on my Taig Mill

I’ve been fighting a small problem for a while and decided to tackle it today (since I’m on vacation until Monday). Small circles weren’t coming out quite round. The reason is what’s known as backlash. When the milling machine changes direction, such as moving to the right and then changing to moving to the left, the knob turns a little bit before the table actually starts to move again. This is usually a result of the lag nut not being completely tight around the screw.

I’m using a program called Mach 3 to drive my milling machine, and it has backlash compensation. But the trick is to figure out how much backlash compensation to enter into the program. A while ago I’d set my Y axis to .003”, but recently discovered that I also had a problem with the X axis. But this time I decided to be systematic about it.

I created a toolpath that cuts small .050” diameter circles with a 1/32” diameter cutter, and then tested different backlash numbers to see what I would get. Here are the results:


None of these are a really nice, clean circle, but after staring at these under a microscope, I decided that a backlash compensation of .002” was the closest to a nice circle. It may be that I can get a better circle after some more tweaking, but I’m going for improvement rather than perfection. This is certainly better than the 0” I had before, which you see in the center of this image.

Thursday, September 9, 2010

Making of a Conoco Gas Truck, Part IX

Some of you are reading this after you’ve already purchased the final kit. I was working on finishing the kit parts up to 9:00 PM the night before my morning flight to St. Louis, so I didn’t have time to take photos and post an update before it was off to National Narrow Gauge Convention. And of course, once there, it was all fun and no time to get on the computer… The kits actually sold better than we expected. In fact, Ragg sold all the kits I made for him at the show, so I’m making more parts.

I also neglected to remind Jimmy Booth to make sure he had plenty of 1934 Ford truck kits on hand, so he sold out on the first day, and it would be another two weeks before he would have new kits. Woops.

I made a new mold for the oil cans, and they turned out rather nice. Here is a photo showing one S-scale oil can with the “flash” removed (more on that next):


And here is a close-up of the parts still on the sprue:


Notice the “flash” around these parts. Once again, the tool path gouged a path for flash into the mold. I noticed this after making one half of the mold, so I was able to eliminate the problem on the second half. But unfortunately, this means I have the “flash” on the parts. Darn!

Fortunately, now I know to look for this issue, which I believe is a bug in the program I’m using, and I now know how to get around it. Even better, Jimmy Booth gave me a suggestion on how to fix my existing molds, which I’m going to try soon.

Next up, I redesigned the mold for the tank lids and handles. The previous version was too hard to build. Here is the improved version:


The handles are much finer than before (because I got rid of the “flash” problem on this mold). And because the holes are larger, it’s much easier to glue the handles in place. I’m pretty happy with how these turned out. By the way, this is blown up way above actual size, so you can see the tool marks.

Unfortunately, between making these new molds and making a set of parts, I ran out of time to make a separate mold for the bottom of the tank. As a result, the kits have two tops. You can easily shave off the tank openings for the bottom half if it bothers you. However, you would only be able to see that they’re there if you turn the finished model upside down.

With the strong sales at the show, and quite a few requests, I’ve decided to make an O-scale version that will fit the Berkshire Valley 1934 Truck kit. This kit is the O-scale version of the PBL kit, which were both part of the Wheel Works line of kits in the yellow boxes.