Saturday, September 26, 2009

Tech Tip Tuesday

This is a post that I hesitated to put up. For two reasons.

First, I think it has limited interest. It mostly applies to people who need to make multiples of the same thing. (If I was really interested in mass production, I'd be sending things out to someone with a CNC router to make multiple copies.)

Second, because it applies to people who make multiples of the same thing, it can lead to people making copies of something that requires permission to make multiple copies. In this case, a canoe design. When you buy a set of plans or a book, you are generally getting permission to make a single boat. To make others you have to send the designer a royalty. Don't forget, this is how the designer makes a living! I deal with this issue by having my students buy a copy of the book with the design in it - from this information, the could build the boat. Please remember to give the designer what is due to them - don't pirate design work!

When my students prepare their forms to build their canoes on, we try to compress time by not lofting each and every boat that we build. Before class starts I prepare what we refer to as "master patterns". These master patterns are created from 1/2" MDF sheet and are made by tracing the form outlines that are glued to a sheet of cardboard and cut out with scissors. The outlines are either from a set of plans that I've lofted in my computer and printed out, or full size patterns that were purchased.

MDF Master Pattern

We then clamp the MDF master patterns on the plywood the student will cut their forms from with the "bottom edge"of the master pattern on the plywood's factory edge and trace the outline. We also put the centerline on the plywood form (very important for setting the forms on the strongback...) and label the forms so we know which is which.

Pattern Before Cutting (Note labels, outline and centerline.)

We then use a bandsaw to the forms from the plywood leaving the line and up to 1/8" of material outside the line. We then clamp the master pattern on top of the roughly cut plywood form and use a router with a pattern bit to get an exact copy.

Roughing a Pattern on the Bandsaw

The MDF is nice as it is inexpensive and has no voids in it. MDF is the abbreviation for Medium Density Fiberboard and is made from very fine wood fibers bonded with a heat-cured resin. The downside to the MDF is that it has a very good finish on the faces making it slippery. We've had problems with the cutter "kicking" the plywood so that the two pieces are not longer in alignment, even with the clamps, ruining the form being prepared.

To avoid this issue, I'm trying something new. I've purchased T-Nuts to use as drill bushings. I drill two holes in the master pattern where they will be far enough away from the edge not to interfere with the router. (You need two so that the pattern will not pivot around a single fixed point.) I then drove the T-Nuts into the holes and drilled out the thread. I then purchased some bolts that were not fully threaded and cut the threaded section off with a hacksaw and finished them as can be seen here:

Bolt "Pins" and Uninstalled T-Nuts

The process went as follows: The bottom edge of the master pattern was aligned with the factory edge on the plywood and clamped into place. Then, using a drill, holes were made in the plywood, using the T-Nuts as bushings. After the holes were drilled, the pins were installed and the patterns traced onto the plywood. The pins were removed and the centerline was drawn. Then, the plywood pattern was rough cut on the bandsaw to within about 1/8" to 1/16" of the line. The master pattern was then stacked on top of the rough-cut plywood and pins were re-installed to align them. The assembly was then clamped to a 2x6 that overhangs from the edge of a table to prevent the router's cutter from hitting the table. A router with a top-follower pattern bit then traces around the master pattern and makes an exact copy on the plywood pattern.

It's a relatively simple system, but we get uniform patterns in a much shorter period of time. Well worth the effort if you have a lot of pieces to make and no CNC router!

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