SCALE TUBE FABRICATION



A lot of people have asked me about scale chassis construction. I thought I'd put together some basic info on the subject. My experience has been with mild steel and stainless steel tubing only. As such, I have no experience with brass or aluminum construction.

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Auto brakeline tubing Stainless tubing & piano wire

I have often used mild steel automotive brake line to fabricate tube chassis pieces. This works well and is easy to work with. I have found it does vary by manufacturer as far as stiffness and outer coating. You'll want to sand off the paint or coating on the ends prior to brazing it.

As a heat source I use a micro-torch fueled by oxy-acetylene. This provides more than enough heat to do brazing at the scale I work. I use 1/16" diameter Harris Safety-Silv 56 brazing rod. This rod melts at about 1200 deg F. It is a high silver content brazing rod that flows well on carbon and stainless steels. I buy it at my local welding shop for about $12 per one ounce canister. I use the matching white Harris flux with the brazing rod. (Note: Some people have had problems with hardened residual flux left on their tubing. Make sure you are using a flux that matches the brazing rod you are using and try to avoid overheating the joint.)

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Micro torch Safety-Silv 56 1/16" brazing rod Flux for brazing

A typical tubing cutter, as shown here, is the cleanest way to make most cuts. However, you can use a Dremel or die grinder with cut-off blade, hacksaw, etc. For mating tube joints, it is best to fish-mouth the tube so the two pieces mate very close. This will also provide the greatest strength at the joint. This take a little practice, especially when mating at an angle other than perpendicular. I have cut fish-mouths using a 1/4" rat tail file, Dremel tool, abrasive cut-off blade in a table saw, and with an end mill bit.

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Tubing cutter Tube fish-mouth Dremel bit Rat tail file Mill bit

Bending steel tubing is fairly easy, it just takes a little practice and patience. I have several automotive hand tubing benders that work very well. Try a few bends and you'll get the hang of the bending dimensions. I also recommend bending hoops starting from the centerline and working each direction with the bends. You'll find that each bend will have a slight directionality and bending from the center out in each direction will keep the hoop more symmetric. I also occasionally hand bend tubing around solid round stock, as needed, for oddball shapes or radii. The form below is made from two pieces of aluminum rod and a piece of steel plate. I use it to bend stick pack cradles.

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Tubing benders Round rod form
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Starting chassis
When starting a scratch chassis, I will often build the main horizonatal structure as shown here. In this case, I will build it in two halves and splice the two halves by splinting the tube with a piece of solid rod inside.

I usually plan my main horizontal structure on paper and use it as a guide when bending the initial pieces. From there I may work from other drawings, sketches, or may just design on-the-fly.

Holding pieces in place while brazing is one of the trickiest parts of building a chassis. I usually work on a piece of sheet steel and use various methods to hold the tube in place. I use scraps of steel plate, box tube, welding magnets, small c-clamps, and bailing wire as holding aids. Your ingenuity will help here as you try different configurations.

As I work and assemble each piece, I usually try to check dimensions with a tape measure or caliper and also eye-ball the chassis for straightness and symmetry. An angle finder can be useful as well. Checking often will help you discover an error before you add more tubing to the assembly.

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Prototype chassis Fabrication aids Typical joint Tubing examples

I typically use 3/16" size tubing, but have built with 1/4" tubing and one large chassis with 5/6" tubing. For small braces I sometimes use 1/8" solid steel rod. I fish-mouth all the tube, but find it unnecessary when using the smaller rod. I will grind it at an angle for a closer mating fit. Mounts can be added by brazing sheet steel to the chassis. I have also brazed small nuts on the tube for holding body panels and such.

Thanks for reading!



Copyright (c) 2003 by Jay Kopycinski, All Rights Reserved.

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