Hi Peter:
Any center
drill will help you start straighter than just the drill. I
might suggest drill the hole first, mount the blank on a
mandrill and then cut the teeth. That way if you bust the drill
in the blank, you don't loose all the gear cutting time.
This is one
way to improve hole accuracy: drill the hole and then grind the
sides to size. I did a punch this way, two 1mm die holes in a
2.5x10mm punch (it made a part like a chain link for a watch
crystal base). It made nice parts, but in the end the whole
project was scraped.
Carl.
On 2/3/2025 2:45 PM, Peter ashby via
groups.io wrote:
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I had to make a new worm gear for an N gauge loco, 5mm dia 1mm
pitch in brass, the only issue was it needed a 0.8mm (32 thou)
hole for the motor shaft.
the first one i made I threaded the outside of some 5mm brass
rod, then tried to drill using a standard 0.8mm drill bit. now
centering the drill is the main issue, and though it looked
good, when mounted on the motor shaft and taken to full speed,
there was vibration.
so i had another go today, this time using a 0.8mm carbide
PCB drill, these have a 3mm shank and a drill section about 1/2"
long, being very careful when starting the drill, barely
touching and just very gently started the drill- now these
drills will break if you breath on them wrong, but i managed to
get the drill centred with no apparent wobble, then pushing my
luck I drilled to a depth of about 10mm (3/8") this went on the
motor nicely, but was a little loose, so I tried again with a
0.7mm PCB drill and that one was too tight (maybe a press fit, i
am not sure.
needless to say, the n gauge loco is now running smoothly.
I do love the precision i can get from my unimat 3.
does anyone else have any techniques for drilling similar
small bore centre holes?
Peter
Carl