Wednesday, 14 September 2011

Weight Watching

Apparently I have fat knees. The very kind Mr. Middleton informed me of this fact after last week's post. Now, whilst I do have a baker's cyst behind my left knee that periodically causes mild swelling, I wouldn't exactly call them fat. It appears that Mr. Middleton is referring to the junctions between the upper and lower fork legs, the bearing housings, as the 'knees'. I like this description and will use it henceforth and also pretend that I invented it. There will be no royalties.

The knees have been put on a crash diet this week. Just out of interest I weighed one of the blanks before machining, it was a hefty 1lb 14 3/4oz. (~ 872g) according to my wife's kitchen scales. Which I obviously didn't just use to weigh a dirty lump of steel, darling.

The first item to make is a male 1.75" X 26 tpi blank to act as a thread checker for the female thread of the knees. It's easier to machine an accurate male thread and then machine the female thread to match it rather than vice versa. I used a 60 degree thread form rather than a 55 degree as I don't have 55 degree tooling and BSC threads are 60 degrees. I like BSC threads anyway. During this period, the early 1890's, bicycle thread standards were beginning to settle down a little. Prior to this, each cycle company would have it's own proprietary threads that, generally, were not interchangeable with any other. This usually means that when restoring old bikes to running condition, most parts have to be hand made to suit. I use the three wire method to check when I have the thread to the right depth rather than just relying on tool feed as this doesn't take into account any radius on the tool tip.
Here's one I made earlier.

Actually that's a lie because I got my kids to help me cut the thread and bore the recess. I had one watching the dial thread indicator and the other engaging the lead screw at the correct moment. They argued over whose turn it was to do each job. Watching the dial was boring but engaging the half nuts was "awesome". Apparently. In this manner I hope to instill a working knowledge of engineering stuff that they can possibly use one day. I have fond memories of being similarly instructed in my own father's shed from a young age, but mostly I just wanted to squeak the rubber rabbit that lived in the middle of the drill bit holder.

The next job is to mount the blank onto a face plate and bore the undersized hole through it. I can't remember where I read it, but a great tip is to use old bearings as parallel spacers behind the workpiece so that you can machine right through without fear of damaging the face plate. 
A boring photo
Now without changing the setup, I can thread the bore. Due to clearance issues, I had to feed in at right angles rather than the preferred 29.5 degrees.
Note the upside down tool to cut on the backside (no topical jokes please)
of the bore so I can  see what's going on

With the thread complete, the next job is to convert the lathe to a milling setup.
This is adequate for small jobs like this but I wouldn't want to remove
a lot of material like this, it isn't rigid enough.
The male blank machined earlier becomes very useful as a means to securely bolt the blank to the vertical slide. Pivoting the blank around this central bolt into the appropriate position allows the tangs that insert into the elliptical tubes to be milled to width and thickness. The upper fork leg is also angled in from the bearing to the narrower head joint. This can be accomplished by swivelling the vertical slide (thanks Dad) to the correct angle. The elliptical shape will be filed by hand afterwards. 
Progress this week has lopped 12 1/4 oz (~ 347g) from each knee
with plenty more to remove yet
In other news, now that I'm temporarily unable to ride, I'm also having to watch my weight. I usually come in from a ride and eat everything. Clearly that has had to stop whilst I'm incapacitated. I seem to be using my lathe as a surrogate bicycle and spending more time with it than is probably healthy. Last night for example, I just popped out to the man cave for a quick spot of milling and when I next became conscious of such things it was bedtime. My wife swears that the shed has Tardis like qualities, she was not impressed.

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