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article 04, issue 01
Numerically Controlled Routing
September 21, 2004
A method for producing plastic parts for the construction of a bicycle is presented. It uses a router (milling machine) controlled by a computer file.
IntroductionNumerically Controlled Routing (NC) is a form of milling intended for working large sheets of plastic, foam, timber or other soft material. The material being cut is held down onto the mill's working platform by a vacuum while a rotating cutter, or mill, cuts shapes according to a pattern programmed into the machine. The cutter moves in three dimensions, but generally only cuts shapes in one horizontal plane at a time.
As an engineer I've designed NC routed parts for storage boxes, airplane engines, telephones, military applications, and hydraulic and medical equipment.
As with laser cutting, programs or drawings can be transferred to the cutting machine via a computer file (DXF format) generated using a Computer Assisted Design (CAD) program. Unlike most other forms of plastics processing, there is no specific tooling required, so relatively small jobs can be done economically.
Types of Cuts
There are several types of cuts that make up an NC routed part:
Depth cuts and Hatching
The mill drills down to a certain depth, then proceeds to cut out a shape. The shape can be a single pass of the cutter or a hatched out area. Hatched out areas will have a minimum corner radius equal to the tool radius. It is simplest to make depth cuts in only one side of the part: routing can be done on both sides, but the part has to be turned over and secured in exactly the right position for this to occur.
Dowel pin holes
If a part is needed that's thicker than the raw material you're cutting, the part can be built up in layers of material. A common pattern of dowel pin holes placed in each successive layer of material makes it easy to join the parts accurately once all layers have been completed.
The mill skims along the top of the vacuum board, cutting a part free from its parent material. Through cuts are usually done last, so the part only breaks free from the vacuum board at the last moment.
Bicycle parts produced with an NC router
Examples of bike parts I've designed using NC routing include bike rack ends, recumbent seat supports and stoppers for tube ends.
[Figure 1] Stopper for tube ends
[Figure 2] Bike rack parts
[Figure 3] Author's bicycle. NC parts are bright white.
Another application for NC could be making a male plug mould for a streamliner from foam layers. If you wanted to make a very accurate plug mold for producing a fairing, the next step up would be to use a surfboard shaping machine which is nothing more than a fancy type of NC router. Look up "John Gillis surfboard" [ref 1] on the net and you will find links to a machine that makes surfboards automatically with input from CAD files, but imagine plug moulds instead of surfboards!
Recently, I found an NC routed wooden lowracer bicycle frame on the internet [ref 2].
If a plastic bike part were to be produced in very large quantities (1000's), an injection-molding tool would make these sort of parts cheaper and lighter than NC. But the cost for the tooling would be huge. Until the day you need thousands of plastic bits made to our designs, NC routing is the way to go.
Firms who have NC routing machines include caravan, furniture, cabinet and kitchen - makers. These sort of small firms will often give you a good price to custom produce a few, small NC routed items.
1. Sweedler, Dave. 1999. Shaping Machines Magic Boards on Maui. Available from URL: http://mauitime.com/v03iss02/surfspt.html
2. Lovell, Roger. 2003. 2 x 4 Lowracer. Available from URL :http://2x4.xntrick.co.uk/
example CAD file of bicycle parts: routing.dxf (153 kB)
technical drawing of bicycle parts: routing.pdf (116 kB)
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