Wednesday, 26 October 2011

Trepanning my head - part 2

This week, I've been boring a hole in my head. Not my actual head of course, that would be silly and may effect the completion of the geared facile. Also, I've been away with the whanau for a while, you may or may not have noticed.

Now where did we get to?

A light shave
With the head still mounted in the four jaw and the fixed steady, 0.050" has been shaved from the inside and outside of the area that will become the lugs for the fork legs, this has defined the limits where the fork tubing will slide onto. Also a 0.5" hole has been drilled 2.5" into the head from the under side. Now the head can be removed, flipped around and remounted. Time to bore a deep hole.

I confess to being a little nervous about boring this hole and have asked advice from my Father, who is an engineer, and from Mr. Middleton, who is not. The hole is 4.5" deep and is shaped like a wine bottle with a narrow neck (1.0") and then slightly wider (1.080") for the remainder of the depth. The hole is effectively blind with a flat bottom and a 0.7" diameter counter bore 0.1" deep. This will all have to be machined using the dials only as there will be no visibility down the bottom of the hole with a boring bar in the way. A bit like flying on instruments only, so to speak. The neck of the bore is threaded at 20tpi for 0.75" and this thread then gradually runs out into free space at the wider part of the bore, a nice engineering solution.

The bore is actually a through hole, but is only 0.5" diameter at the lower end. The first job is to drill the remaining 2.5" from the top down to meet the first hole (depth of bore is 4.5" with 0.5" remaining at the base). Starting with small drills and in steps increasing the size and reducing the speed until the 0.5" drill meets the first hole, I was pleased that the two met perfectly concentrically.

Start small
Concentric with the hole from below. Hoorah
Because of the depth, I need to use as larger diameter boring bar as possible to maintain rigidity. I'd also like to be able to use the same bar to machine the counter bore and face the bottom of the hole. A little trig. calculated this as a 16mm bar which requires a minimum 18mm hole to start. I bought one which uses the same CCMT06 inserts as the rest of my carbide tooling just for convenience. I used Carbide NZ who provided a great fast service. Unfortunately, my wife noticed the parcel and asked awkward questions. I'll need to build up my family credits again. First job is to enlarge the 0.5" hole to 18mm to the correct depth for clearance for the boring bar.

The largest drill I have - 18mm
This required careful depth calculation as the drill tips are 118 degrees and I need a flat bottom. By engaging the lead screw and feeding the saddle along with the graduated lead screw hand wheel, this is relatively easy.

The boring bar was a huge success and the hole has come out well. The counter bore at the end proved a little tricky but as far as I can tell by looking down it is OK. I'll be able to tell when I machine out the back of the head for the neck recess. A good tip I was given for boring is to have the tool tip a whisker above centre height, this means that any slight catches or dig ins will tend to push to tool tip away from the workpiece and not into it. This becomes important in a small home workshop where I don't have the ability to flood the tool with cutting fluid to remove the chips. A whisker, by the way, is a precise measurement, it is slightly bigger than a smidgen but not as large as a tad.

For some reason I forgot to take a picture of the boring process.
As consolation here  is a  photo of all the swarf from the machining of the head.
I have a Chinese company interested in mining rights
The next job is to cut an internal 20tpi thread on the narrow section of the head.

In an Abingdon ball bearing head this thread is not used to adjust the head bearing
as in a conventional cone bearing head but is used to lock the neck in place. More later.
I'd previously made a male threaded part to screw into this new thread, this serves two purposes. First it acts as a thread checker when cutting the thread and also as a new centre for centre turning again later.

Here's one I made earlier
The last of the turning operations on the head is to taper the main section. This is reduced from 1.6" to 1.3" over 4.5" inches which works out as a 1.9 degree taper. The Myford top slide only has about 3" of travel so I had to take two bites at this and feed the saddle along. In practice this was fine.

Note the false centre in use
Next week I'll start carving the handlebar mount.

In other news, as previously mentioned, we've been away. First to Mount Cook Village with the family courtesy of my employers. I like Mount Cook and thoroughly enjoyed the trip, we had heavy snow falling on arrival and then bright sunshine for the remainder of the weekend. Perfect conditions for some beautiful walking.

Our hotel - The Hermitage - is in the centre of the photo
Then last week we all travelled to the Coromandel Peninsular where we did all the usual touristy things

On Monday morning I got up early and went for a fantastic bike ride in beautiful weather on deserted roads. Was there some big event on the Sunday night?

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