Tuesday, March 30, 2010

Making a Conoco Gas Truck, Part IV

While I was waiting for my replacement cutter to arrive, I did some cleaning up in my workshop. I should do this more often because I found some cutters I didn’t realize I had, including another 1/32” long-reach cutter! After quite a few hours, I finally finished milling both halves of the mold.

Now for the moment of truth—injecting a part. And disaster! Well, maybe not disaster, but certainly not what I was hoping for. I shot plastic into the mold and then I couldn’t get the part out at all. Stuck solid. Grump. In looking at my mold design I discovered I only had a 2 degree draft on the inside walls and no draft angle on the outside walls. That’s way to little draft, especially for such deep walls. So, I’ve adjust the draft angle on the inside walls to be 8 degrees. Fortunately, this is just removing more metal, so I don’t have to start all over. And I can mill out the plastic that’s stuck in the mold while I’m at it. This, of course, will take a few more evenings…

Sunday, March 28, 2010

Making a Conoco Gas Truck, Part III

I started working on the molds for the tank end, which looks like this:

Tank End_300 Tank End Back_300

The back side of the part presents some issues in terms of both making a mold and extracting the part from the mold. Making the mold requires that I cut some fairly deep slots into the aluminum mold halves. These slots need to about about 1/4” deep, and they’re about .047” wide at the bottom.

After I’ve designed a mold, I need to create tool paths that my computer will send to my CNC milling machine. For this I use a program called VisualMill. It takes as input a “solid” model of the mold. I then tell it what size cutter to use, how deep to make each cut, and so on. After a few minutes, it returns a tool path like this:

Tank End Core Toolpath

This particular tool path was for a 1/16” diameter cutter, which I used for the first pass. However, to get all the way to the bottom of the slot, I created another tool path using a long-reach 1/32” diameter cutter. Long reach in this case mean the thin part of the cutter is about .350” long. But that makes the cutter both somewhat fragile, and expensive (about $30 each).

I only had one of these long-reach cutters left, having broken a few others while making mold for the top of the tank. So I put my last long-reach cutter in the mill and started cutting. Unfortunately, I was a little impatient, so I set the cutting speeds too high and broke the cutter. Doohh! I’ve ordered some more cutters, and they should be here on Monday, so I’ll be able to continue working on this mold. But the next time, I’m going to cut slower so I don’t break this cutter.

By speed, I mean how fast the mill moves while it’s cutting. I had it set to move down at 1 inch per minute, and cut at 3 inches per minute. I thought that was slow enough, but apparently not. The other factor here is that I was using a flat-end cutter for removing most of the material, but I think a ball-nose cutter would be a better choice. A flat-end cutter experiences some shock when it first touches and bites into the metal. A ball-nose cutter experiences a less sudden shock because the tip is round.

Sunday, March 21, 2010

Making a Conoco Gas Truck, Part II

I continued working on the core for the tank supports and side troughs. In my previous post, I showed the inserts I was planning on using for the hollow part of the troughs. Here you can see me milling out the pockets for the two inserts.


Once finished, I flipped this mold over and milled recessed holes for screws to hold the inserts in place. Then I used a 4-40 tap to tap the holes in the inserts. Or at least, that was the idea—I broke the tap. Darn. So instead I put the inserts into the pocket and then banged on them with a mallet to firmly seat them, hoping I wouldn’t need the screws to hold the inserts in place.

The next photo shows me starting the process of milling away the unwanted, extra material from the inserts. This will take about two hours, and then I have to make several other passes with smaller cutters.


Quite a few hours later, I finally had the core finished. For this I used both flat and bull-nose cutters, varying from 1/8” down to .015” in diameter. Here is what it looks like when all finished:


Now for the moment of truth. Will I be able to fill this completely with styrene, or will I have to adjust the gates and runners so I can fill the cavity? And the answer is:


There’s a little bit of flash in some places that I think I can probably eliminate or at least reduce with some changes to the gating, but the answer is most certainly yes! So being the person I am who likes instant gratification, I threw together the parts I’ve made so far (without doing a very good job of cleaning up the flash). Here’s what they look like mounted on a PBL truck:


Not bad, eh? I like it. But I just realized I forgot to make cavities for the tank lids. Woops. At this point I need to create the mold for the end of the truck and the tank lids, and then it will be ready to go.

Friday, March 19, 2010

Making a Conoco Gas Truck, Part I

Truck 4_400I’ve been making injection molds and parts for Ragg’s…To Riches? for a little over a year now. I’m pretty new to all of this, being self-taught. So there’s been lots of trial and error. But I’m slowly getting better and trying more difficult molds.

Right now I’m working on a new set of molds for an S-scale gas tank that will fit on the back of a PBL 1934 Ford Pickup. The PBL kit is very close in appearance to the truck shown in the Rio Grande Southern Story, Volume VII on page 123. And I wanted to design a tank that would look much like the one in this photo. It’s very hard to find prototype information about these trucks, so I used some conjecture to create a “close” match.

My first step is to use a program called SolidWorks that allows me to build 3-dimensional models, like the one on the right. I like to use a different color for each part to help me remember how everything will fit together when it’s all done.

Making the Mold

Tank Core

Once I have the model, I then have to design a mold, again in SolidWorks. Molds consist of two parts—a core and a cavity. Generally, but not always, the core is the mold half like the one on the right where there is metal projecting out of the mold. The design at the right is a core that I can use for both the top and bottom half of the gas tank itself.

Making a mold like this is a little tricky, as you’ll see in the sequence below. I could have started with a thick piece of metal and then cut away all the parts I didn’t need. But that’s a lot of work, so I used another approach—I created an insert, as I’ll describe and show below. Inserts were a technique I’d never tried before.

Tanks Support and Trough Molds

I’m going to switch now to using photographs of actual aluminum parts to show you how I’m making the mold for the tank supports (the green parts in the first photo) and the troughs (the red part in the first photo) that are on either side of the tank. First I milled the cavity out of a block of 1/2 inch thick aluminum:


Next I milled a pair of inserts for the core side of the mold. To create the inserts, I needed fairly small blocks of aluminum (these are 0.5 by 0.7 by 2.5 inches) and I had a long 1/2 by 2.5 inch bar. In the past I would have cut this with a hack saw, but that’s not much fun at all. So I broke down and purchased a metal-cutting band saw. Boy, am I glad I did—it makes quick work of cutting the smaller bars, which I then finished off in the milling machine.