Sandy Gross Puzzle Pan
Page 2
Building the Puzzle Pan
Start by setting up the Jig Block. Lay out and drill holes as necessary for a 4-3/32" wheelbase and a full 5 inch guide lead. Draw motor box alignment and main pan outlines directly on the block. The forward extent line is 1-3/8" from the axle centerline, and the motor angle is 15 degrees, measured with a protractor.
Take your time and make sure to get everything square and centered:
I do the layout on the jig block with a 6 inch machinists rule and a protractor, and draw the lines with a "Sharpie Ultra-Fine Marker" using the rear axle centerline as my reference point, but actually measuring from the front surface of the axle.
Don't forget to subtract 1/16" from you measurements to account for the axle radius, and if you are building a chassis using 3/32" axles, the radius is 3/64". Hope you paid attention in math class; accuracy counts!
Starting with some .032" (half hard, common 260 alloy) sheet brass material I bought over the Internet:
You don't have to buy a piece this big to build a Puzzle Pan chassis! In fact, a 4" x 6" piece should be more than enough. My local model railroading hobby shop carries 4" x 6" sheets of .032" brass in its K&S display, so I know it's available. Your local hobby shop either carries it, or should be able to get it fairly easily.
I blue the brass with layout dye and scribe the outline of the main pans. This is strictly old school sheet metal layout; it may seem like tedious work, but with a bit of practice it actually goes pretty quick!. Then I put on my SAFETY GLASSES and cut out the pans with my Dremel tool:
The cuts may look a bit like a hack job, but I'm using fiberglass reinforced cutoff wheels so I can go fast; this is about 20 minutes of actual cutting (and several trips to the utility sink to water-cool the brass sheet... when you go fast like this, man does it get hot!)
To keep the cutoff wheel from also cutting into my work bench top, I lay a piece of corrugated cardboard (the thicker the better) down first, and then lay the sheet brass on top. When I cut through the brass, the wheel cuts into the cardboard, but not the bench top.
The fiberglass reinforced wheels are thicker (almost .050") so the kerfs are really wide, but these cutoff wheels are absolutely bulletproof. I was going to say "you cannot shatter them", but I wouldn't want to (duh!) encourage some jackass out there to actually try. Suffice to say they are extremely tough and will not shatter under "normal" use!
To finish the pans and bring them to exact shape and dimension, I first use my small bench top 1" belt sander to get in close to the scribed lines, and then use a hobby vise and 6 inch flat file to hit the mark. Inside corners can be difficult, so I use a square (about 1/8") needle file to do the job. Take your time with the finishing work; everything else is attached to these parts and if they are not "right", nothing else can or will be either:
Now cut and finish the motor plates and the (dinky) drop arms. Don't forget there is supposed to be an .008" gap all the way around between the main pan and the motor plate so it can move around when the chassis is finished:
Moving on to the axle tubes and motor brackets, the axle tube is 1-1/16" long with a cutout as shown in the drawing. The motor brackets are shaped and dimensioned to fit precisely in the crowded endbell area, so follow the drawing to get it right.
Now the one piece motor box. Fabricating an accurate replica of a one piece motor box, or any motor box, when you cannot see or measure the original is problematic at best. If you cannot measure the original, how do you know you got it right? The best answer is that when it "looks" correct from every angle, fits into the chassis and also fits the parts (motor, gears, tires) that you are going to install, you've got it.
In this case the motor is a Mura "A" can with a Mura "Unmeltable" endbell, configured for right-side endbell drive. These motors are about 1/16" longer than motors using Mabuchi cans, so the motor box must be wide enough to fit. The motor angle is 15 degrees (measured from the photo), and the motor box is located so that the left side front corner is 1-3/8" forward of the rear axle, and the right side front corner is a shade over 3/4" from the right side of the chassis. The "knee" bend on the can (left) side of the motor box (again, from the photo) ends up a bit aft of, not aligned with, the rear edge of the main pan.
With all these parameters to meet, I'd like to say I hit the bulls-eye on the first try, but you wouldn't believe me and it wouldn't be true. It took about five tries before I had the various dimensions between the bends figured out.
Here is a description of the process I use to figure out where the bends go. You do not need to go through this process to complete this build, because I've already done it and written down the dimensions, but you should know that if you can measure and bend precisely and consistently, it works for any build:
I start with making the motor box wide enough to fit the motor I'm going to use. In this case start with roughly 1-3/4" from the free end of the piano wire to the first bend (bend one). Then, 1-29/64" from the inside of bend one (the right-side front corner of the motor box) to the point at which I make bend two (the left-side front corner of the motor box) results in a 1.555" wide outside dimension box that fits the motor and the bracket with a small amount of wiggle room, and it also closely matches the photo (important for a replica). So, right off I know where the first and second bends go. I write these dimensions down on paper.
Next is the "knee" bend (a.k.a. bend three); which has to meet two parameters: It should be a bit aft of the trailing edge of the main pan, and allow the motor box rail to clear both the motor can and the rear tire on its way to the rear axle tube. I place the unfinished motor box on the jig, center it, line it up with the motor angle and forward extent line, and mark a spot about 1/8" aft of the line on the jig block that marks the trailing edge of the main pan. I measure the dimension from the knee bend (bend three) to the mark I just made and write it down on paper, then I make the bend.
Last are the two upright bends. To best estimate the location for these bends I again place the unfinished motor box on the jig, center it, line it up with the motor angle and forward extent line and, dropping down vertically from the rear surface of the axle tube and adding about 1/64" for bend allowance, I mark the rails. I measure the distance from the first bend to the mark on one rail, and the distance from the knee bend to the mark on the other rail, and write them down, then I make the bends.
Having done all this, I have a starting point. If the motor box I just made doesn't fit exactly, and it usually doesn't, I can easily experiment by making more motor boxes, changing the dimensions between the bends as necessary (and writing down my changes) until I get it to fit exactly.
Once I get the dimensions worked out, I can make any number of motor boxes and they will all fit like this:
Remember, you don't have to re-invent this particular wheel because I've already done all that, and written down the dimensions. Just use those dimensions and the measure-bend-measure-bend process, and all should be well.
Now, tack solder the motor box to the rear axle tube. Attach the motor bracket to the motor and tack solder the assembly into the motor box. Check carefully for fit.
The knee bend is distinctly aft of the trailing edge of the main pan, as it should be:
But the motor does not fit in the motor box:
At this point you should not change the design to reposition the knee bend farther aft (it won't look right), you certainly don't want it farther forward (that will make the fit even worse), and you cannot make the motor box narrower or wider (then it won't fit properly into the rest of the chassis). Now what?
Note that's a stock motor being crammed into the motor box, and it helps to illustrate something about building replicas: You must be aware of every last detail of the original you're trying to replicate. Check the photo of the original chassis; the can on the motor Sandy actually installed in the car had been modified.
Here's a photo of the stock can and a modified can. Modifying the stock can like this to improve cooling was fairly common in 1968:
Here's the same motor box again with the modified can. It fits much better:
Now we have some accurately sized one piece motor boxes:
Putting all the pieces together, we get this:
Next up: Drop arm and motor plate assembly... (Click here)
