Building a race car frame jig -- or chassis jig, as it is referred to by those in the industry -- requires accurate measurements and great detail. The detail needed to build a chassis jig is important so the jig will produce superior race cars every time. For your chassis jig to be functional, you must build it out of strong material that is bolted together, not welded. The only parts you can weld are the bolt-on parts. The jig must be level, square and produce true and accurate measurements. To make welding the chassis more convenient, you should raise it off the floor .
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Things you need
- Safety glasses
- Drill motor
- Drill press
- Tapping oil
- Machine square
- 7.5 mm (5/16 inch) drill bit
- Hand grinder
- Wrench set
- 9 mm (3/8 inch) coarse thread tap
- 2 lengths of 9 mm (3/8 inch) wall tubing, 7.5 by 12.5 cm by 5.4 m (3 by 5 inch by 18 feet)
- 6 Steel plates, 12.5 cm (5 inch) square by 1.2 cm (1/2 inch) thick
- Prick punch
- 7.5 mm (5/16 inch) centre punch
- 5 bolts, 7.5 mm by 2.5 cm (5/16 inch by 1 inch)
- 40 bolts, 9 mm by 1.8 cm (3/8 inch by 3/4 inch)
Drill twenty-five 7.5 cm (5/16 inch) holes into the 1.2 cm (1/2 inch) thick, 12.5 cm (5 inch) square plate. The plate requires five rows of five holes. Accurately scribe a line 1.2 cm (1/2 inch) inch from the edge of all four sides of the plate. Scribe three more lines, 2.5 cm (1 inch) from the previously scribed lines both horizontally and vertically to form a grid with a 1.2 cm (1/2 inch) frame around it. Prick punch all intersecting lines accurately. Drill all punched marks with a 1/8-inch drill bit in a drill press. Once complete, redrill all 25 holes using a 7.5 mm (5/16 inch) drill bit in a drill press. Mark this plate as the template. Also make a top mark and a left side mark on the template. You will use this template as a pattern to reproduce and drill more plates.
Place the drilled template on top of another 12.5 cm (5 inch) square plate. Be sure the plates are flush on all sides, clamp them together, and drill all 25 holes into the plate using the template as a guide. Repeat this until all plates are drilled.
Stand the drilled plate up on top of another 12.5 cm (5 inch) square plate forming an angle. Use a machine square to check the square of the angle formed from the two plates. Weld two small 6 mm (1/4 inch) beads on the outside of the plates near the ends, and weld two more beads on the inside of the joined plates, near the ends. These two plates should form an accurate and perfectly square angle after welding.
Position the two 18-foot rectangular jig tubes with the 12.5 cm (5 inch) side facing up. Place the newly formed 12.5 cm (5 inch) square angle or template onto the tube, with the drilled plate squarely covering the 12.5 cm (5 inch) side of the jig rail tube. Tap the template angle into place. Be sure template is flush with the end, top and bottom of the jig tube. Clamp the template to the tube with a C-clamp.
Drill all 25 holes into the chassis jig tube, using the angle template as a guide. The hand drill you use to drill into the tube should have a sharp, high-speed 7.5 mm (5/16 inch) drill bit in it. Once you have drilled all 25 holes, slide the template further up the tube. Align the last row of drilled holes with the first row of holes in the template. Insert five 7.5 mm by 2.5 cm (5/16 inch by 1 inch) bolts into the holes like pins to assure the template is aligned, then clamp the template to the tube. This time drill only 20 holes into the jig tube, because five holes are filled with bolts for alignment. Repeat this procedure until both 5.4 m (18 foot) jig tubes are completely drilled from end to end on the outside and inside.
Tap all of the holes drilled in the main chassis tubes with a 9 mm (3/8 inch) coarse thread tap. There are a lot of holes to tap, but they only need to be tapped once and it makes the chassis jig easy to fabricate the needed bolt-on brackets. When a new bracket is required, bolt on a new plate and weld the fabricated bracket to the plate only.
Line the two 5.4 cm (18 foot) jig rails up parallel to each other. Enlarge the 25 holes in four of the square plates using a 9 mm (3/8 inch) drill bit. Bolt two steel plates, one on the inside end of each tube. Use ten 9 mm (3/8 inch) bolts in the two vertical rows nearest to the two vertical edges of each of the four plates.
Stand the two 5.4 m (18 foot) jig tubes on edge with the bolted plates facing inward toward each other. Slide the two 5.4 m (18 foot) jig tubes apart and insert the 67.5 cm (27 inch) tubes on edge centred between the plates at both ends to form a large rectangle chassis jig. Square the tubes with the 67.5 cm (27 inch) cross braces and be sure they are flush on top. Tack weld the 67.5 cm (27 inch) tubes to the steel plates bolted to the inside of the jig rails. Re-check the 67.5 cm (27 inch) tubes for square and flush and weld them solid to the plates.These inside braces must be perfectly square and flush after welding.
Grind the welds off the template angle and separate the two plates. Keep the template as a pattern to fabricate as many 12.5 cm (5 inch) square plates as needed to build bolt-on chassis brackets. When building the original race car, clamp the race car chassis tube to the jig rails and bolt on the needed plates for brackets to hold roll cage uprights and other chassis components in place.
Tips and warnings
- Mark a bottom and top on the drilled template.
- Mark both 5.4 m (18 foot) tubes with a top, bottom, an inside and outside. Keep the tube's seam on the inside and/or on the bottom of the jig tubes.
- Chassis plates can be made out of 6 mm (1/4 inch) steel plates for less stressful brackets.
- Be sure the template is drilled with 25 symmetric holes in it.
- It would be completely acceptable to have all the plates drilled by a machine shop to assure perfect accuracy.
- Use the two 5.4 m (18 foot) jig rails for the mainframe of the chassis to be fabricated. Make and bolt on wings or extensions as needed by welding only to the plates, never to the jig rails.
- Use more bolt-on plates to weld four legs on the jig to raise it off the floor.
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