Drilling hardened steel

Hi, I am attempting to refurbish a very worn Lucas Automatic Timing Device for a classic bike. One of the bobweight pivots is badly worn, and I need to make a new pivot pin and socket. The socket has been worn off centre and out of round and I plan to bush it in phosphor bronze and re-drill to 3/16 bore, the pivot pin nominal size. I can mount the bobweight using a projection on the Bach Concentric with the original hole, then drill using cobalt(?) drill, but this will then be out of centre to the worn bore and liable to catch or break or wander off centre. Or, I could drill down the worn hole's centre to make it round, bush and bore, this might put less strain on drill but it still has to go in an out of round hole. Any suggestions? Thanks, Andy

You will have trouble getting a hole like that drilled using a drill press, there is too much slop in most of them. I would have difficulty using a mill and a stub length solid carbide drill. A short carbide end mill would be a viable alternative with a very slow feed. I don't think a cobalt drill would stand a chance.

Andrew

Best to bite the bullet and pay for a spark erode job.

Emgee

If you can set it up in a lathe at the right centre, maybe in a 4 jaw chuck, you can use a dremmel type grinder with a grinding wheel to true up the hole held in the toolpost.

David

+1 on David's suggestion above. Or maybe even drill it out with a short carbide end mill cutter held in the lathe tailstock.

For holding small odd-shaped jobs like that in the lathe I often use a square of scrap aluminium plate say 2" square or so and 3/8" thick, or whatever is in the scrap box. Then clamp the part to the plate by drilling and tapping holes where needed to put a piece of flat bar etc across the top of the part as a clamp. Or even just screws and washers on some parts. Then the plate is attached to the faceplate with a couple of bolts through the plate and faceplate slots. You can then move the whole plate around to get the part centred exactly where you want it.

Edited By Hopper on 07/01/2020 23:53:32

Hi Andrew

I think I can visualise what you are saying, as I guess you have the centrifugal bob weight set up for a classic British twin. The first thing I would suggest is that the bike is going to run a lot better if you fitted an electronic ignition like a Pazon or Boyer. However, if you want to keep things as they are and not spend too much then I understand you are looking for a simple fix.

I am going to be more simplistic than the previous contributor's good engineering suggestions. The bob weights are always very sloppy on the pins, and tend to go from full retard to full advance in a sudden jump anyway. Precise engineering is not essential. Why don't you fix a solid bush made from bronze or even cast iron into the bob weight hole with loctite. You could custom shape the insert by hand filing to match any worn hole shape. The loctite will fill gaps if you make the shaped insert a press fit in. Then drill a hole, in the right place, that just fits over the existing worn pin. You could dress the pin by hand if it is very mis-shapen or tapered. Use a long, thin strip of emery to work it round and smooth. Hopefully it will still be big enough to do its job of holding onto the bob weight.

Maybe nearer to bodge-engineering than precision but it is always possible to over-engineer a repair job.

Norm

Edited By norm norton on 08/01/2020 10:12:04

The other thing to look at on your bob weights is that the slots (and pins) that limit the amount of movement also wear. This results in too much movement. So if you set timing at full advance at 3,000rpm in the normal manner, when the revs drop to idle, the weights move too far and retard the spark too much. Results in poor idle and hard starting. And on Harleys with wasted spark ignition, backfiring out the carb.

Is it worth the effort ? There are spares out there at most autojumbles and some new ones from the magneto specialists Try Tony Cooper in Halesowen Birmingham.

Make a steel guide bush and clamp it in place to guide a 4 flute carbide endmill, your pillar drill will easily accomplish this.
A brass guide bush would probably be fine for a one-off.
Note that I am speaking from commercial experience here, not speculation or theory.

If you're lucky, the guide bush - end mill (or even drill bit) approach above will work just fine. However, it hasn't always worked for me, especially for slender cutters, because the off-centre forces on the milling cutter may cause it to cut sideways into the bush. After a ridiculous amount of procrastination and analysis-paralysis on one job, the solution came. Add a sleeve to the cutter (superglue), so the guide bush is protected: only the sleeve runs in the bush, the cutter becomes an end-cutter (only) and the whole setup gains rigidity as a bonus. Good luck.

Could you just drill as said & then measure how much you are off centre. Then make a offset bush to suit. Out of interest what bike are you doing. I & lots of friends are into Classic British bikes.