Wednesday, 31 August 2011

Lower Fork Legs and Lasers

What with the sudden change to fantastic weather and an unplanned encounter with a surgeon's knife, I've spent relatively little time in the shed this week. However I can report on some progress

Last week I mentioned that I would start to look at the lower fork legs. These start at the same elliptical profile as the upper fork legs and taper to a round profile at the other end. Initially I had thought about starting with a round tube, tapering it and then squishing it to the ellipse. Companies exist that specialise in tapering tubing using dies that cold work the tube to the required dimensions, however there are none in NZ that have the dies for the dimensions I need. With the high NZ dollar, it should be possible to have this done overseas and shipped here. I contacted several companies who upon discovering that I didn't require several thousand units seemed to lose my email address.

Plan B is to start with a round tube, squish it in my rollers to an ellipse and then split and weld the join. By simple calculations, it is easy to work out how much material to remove so that when the split is closed the small end can be formed into a circle of the correct dimension.

Some years ago I took a welding course at night school and thoroughly enjoyed it, however I'm out of practise and don't currently have any gear so I'll get somebody else to weld the splits for me. 4130 chromoly tubing can be either gas welded or TIGed using the appropiate filler rod. There are a couple of places in Christchurch that make racing car roll cages out of 4130 so I'll approach these when I get back on my feet next week.

The bearing housings that connect the upper and lower fork legs are machined from 0.7" steel plate. To speed up the process I'm going to get the blanks laser cut from 20mm plate. Fortunately We have a local company that is still operating after the earthquakes that has the capability to do this. They require 2D .dwg or .dxf files to drive the laser so I've spent a ridiculous amount of time this week playing with free CAD packages from the internet. There is a large range to choose from, some are really good and some are not. A lot of the good ones are locked down to only output parts and assemblies in their own specific format and a lot of the ones that do output the standard formats are a bit crap. Many years ago I worked for PTC (ProEngineer) and then later SolidWorks writing their 3D CAD software so I have half an idea about this stuff and I could get up to speed quite quickly. You wouldn't believe how few product are able to produce a fillet between a straight edge and a circular one. For 3D modelling I've been using AutoDesk 123D and for the 2D drawing I've been impressed with DraftSight from Dassault Systemes. I think that with the time I've spent playing with CAD this week I could probably have cut the parts from solid using a nail file. It was fun to relearn some CAD skills though.

In other news, I've finished the month with 1001km cycled which I'm quite pleased with since I lost a week due to snow and ice. I'm now going to be off the bike for weeks which I'm really not looking forward to. Cycling keeps me sane.

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