BikeCAD is an excellent program to use for frame design. One of the primary benefits to the hobby builder is the ability to export a full scale drawing of the frame design. The picture below shows a 36″ x 48″ print of my new frame design. I’ve also used BikeCAD to generate an 11″ x 17″ color print with the key dimensions.
For a hobby builder who isn’t planning to build frames with production efficiency, a full size print can be used with a flat surface as a template for frame construction. A number of flat surfaces can be used as the datum. Typical industry frame tolerances are on the order of 1.5 mm or about 0.060″. That means you can have a frame that isn’t straight by about a 16th of an inch before it will become perceptible for enough consumers to cause a problem.
In the realm of flat surfaces there are a few places to start shopping. Large granite surface plates often pop up on Craigslist at very reasonable prices. I personally missed a locally sold 36″ x 48″ plate with stand and cover for $200. In addition, Enco has an imported black granite plate in a 36″ x 48″ size for $309.95. The Enco plate is grade A and flat to 0.0004″. That is really flat, so much so that the tolerance of the paper thickness will likely be greater. A large granite plate will also require some straps and engine hoist to move around. A steel stand will be necessary to support its weight.
If a more versatile option is required, a 3/4″ thick steel plate can be prepared to use as a datum surface. It will be necessary to stress relieve it at a heat treating facility to properly prepare a steel plate. Then the surface is blanchard ground at a grinding shop to about 0.005″ flat across the surface. The nice part about using a steel plate is its dual purpose nature. After grinding both sides will be nice and flat. Use one side for general purpose fabrication and the other for layout.
In the realm of inexpensive options a large slab of black granite counter top material can be acquired cheaply. Its likely that a big enough granite plate can be located at a home supply store. Granite counter tops are flat enough to use for bicycle frame construction, perhaps on the order of 0.030″. I would recommend looking for one with a small grain size like those in the black color. These are also light enough to be moved by hand with a friend, rather than by engine hoist.
Once a flat surface has been located it will be necessary to get a few v-blocks and squares. Grizzly has v-blocks at very economical prices. Here is a link to their selection of blocks. A set of two cast iron 3″ v-blocks can be had for $8.95. A set of machinist’s squares to line up the edge of the tube with the edge of the print on the paper will also be necessary. Grizzly has a set of 4 squares for $17.75. Here is a link to their selection of squares. The square is used such that it sits flat on the surface plate on one side an touches the tube on the other. Then line up the tube on v-blocks with the square to the edge printed on the drawing.
Couple this technology with a grinder to notch the tubes and a TIG welder and it will be fairly easy to get going on a frame project. If a TIG welder isn’t available it won’t break any rules to glue the tubes together with a MIG, brazing, or lugs. It may also be necessary to source some flat spacer blocks to get all the tubes and bottom bracket spaced at the right distances to ensure they have the same centerline despite differences in diameter. The rear triangle can be a bit tricky, but this problem is often solved by using a true rear wheel to align the left and right sides before tacking them in place.
Even if you’re using a frame jig the full size print is useful. It has all the dimensions you’ll need to setup the jig and serve as a sanity check while in construction.
Thanks for reading!