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Repairing the Advance lathe bullgear - 2 adjacent teeth missing:
My bull-gear lost 2-teeth when a parting tool jammed with swarf & stalled
the lathe - must have been a weak spot - still, not knowing the history of the
lathe I can't tell if it was simply a manufacturing weak spot, or caused by earlier abuse of
this lightly built lathe.
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This is the set-up I used for machining. I mounted the gear on a
homemade mandrel, between centres, using the indexing
head. The faceplate drives a 'lathe dog' (or 'carrier',
locked into place with bolts & plates). Indexing is by using a 65-tooth change-gear. |
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Preparation for repair of the broken teeth - setting gear &
cutter in position. |
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Mill slot. It may not look centred, but it is - I guess it's
just the angle of the photo. |
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Mill to shape with a dovetail cutter.
n.b: yes, it did move a bit here. Accidents happen. I forgot to
fully tighten the faceplate, hence the dovetail's not quite straight -
luckily it's not important here. |
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A small wedge of cast iron was milled to fit the slot & leave
material to machine gear teeth. |
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Insert is brazed in place.
Unfortunately my hands shake a
bit nowadays, so I managed to braze adjacent tooth spaces - just
means I have to mill them out too.
Both faces & outside of the gear are turned down on the lathe to
remove excess metal.
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A piece of 3/16" square HSS was ground to match a good tooth space
on the gear, then inserted in the boring head (or a flycutter).
The gear was lined-up, depth of cut was set equal to the original tooth
spaces. Using a slow speed & fine feed, the new teeth were
reproduced in the dovetailed insert.
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A close-up of the finished milling job. One edge of a new tooth
chipped slightly on one end (which you can't see). It's not enough
to compromise the integrity of the tooth, so I'll ignore it.
All that's left to do is clean up the edges with a fine file or wire
brush, then re-install in the Advance lathe. |
p.s.: The backgear's now installed into the lathe & seems to be
working well.
Repairing the Ta Shing idler screwcutting gear - 2-teeth missing:
This has more robust gears, so I will use a different method to repair
this one.
Larger gears can be repaired by drilling a series of small holes where the
tooth was, putting a small pin in each hole (pin diameter close to the
thickness of the root of the tooth). The pins are brazed into place
& to each other with plenty of brazing. The idea is to build it up
enough metal, then new teeth can be cut into the pin/brazed area.
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Here, I have milled one of the broken teeth out (lower right), & am
about to mill out the second broken tooth.
As you see, I'm not actually using an end mill. Instead I'm
using a broken drill that has had the tip reshaped into a crude end mill -
It's not that I don't have an end mill of this size, but rather to prove
that, in many cases, a makeshift cutter will work just as well. |
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I have chosen to use 3mm dia. steel pins for this job. A row of equally spaced holes, 5mm
deep, have been
drilled to accommodate the pins. It's not critical that they are perfectly centred, but it is highly
desirable for the roots of the pins to fall completely within the root of
the new tooth (for maximum strength). |
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Pins inserted & waiting to be brazed. They are made slightly over length
& are a close, but not a tight fit into the 3mm holes, to allow the
braze to flow.
The pins are made from 3.2mm mild steel welding rod, turned down at one
end for a short length. The protruding part of the pin is polished
with coarse emery cloth to remove any surface rust.
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Brazing done - the sides & the circumference of the gear have been
machined free of excess braze & reinforcing pin. The repairs
have rings drawn around them. Here, I have just finished milling the
first pair of tooth spaces (lower circled area). Note the gears are bolted to an angle
plate, the 2 smaller gears are fixed & allow me to index to the next
tooth by correct placement of the large gear, then bolting & clamping
it.
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A mould was made to test the shape of the new teeth & spaces - It was made by pushing Araldite putty into a section of the gear, covered with a release agent - once set & cleaned up, the putty is
a near perfect impression of an area with good teeth.
The tooth spaces on the gear were machined, then the teeth were
lightly filed until this mould fitted correctly.
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Finished & ready to install - the repaired teeth are circled. |
note: If only part of a tooth is missing, it can be built up with weld,
braze or similar, then filed or machined back to the correct shape.
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