Does anyone have an idea how much this part might cost to make
professionally? Just a one off?
115 mm diameter brass disc, 5 mm thick, 4.75 mm bore with boss and
grubscrew, no gear teeth, 30 x 5 mm holes drilled equi-spaced in a 95 mm
diameter circle. 1 x 5 mm diameter hole on a 70 mm diameter circle,
aligned with one of the 30 holes. 0.1 mm tolerance on hole positions.
0.01 mm tolerance on bore.
This is a part for an electronic master clock. I need to do a drawing to
get official quotes, and I'm not keen to go to the effort if I know it's
going to cost a fortune. Any idea what a one off would cost? £50? £100?
£250? I'm just looking for a rough idea...
Depending on how long the boss needs to be you're machining away a lot of
brass for no good reason. If the speed and loads on the part are low I might
suggest making the boss separately and using a press fit and/or a screwed
attachment to the disk. Then you only need a bit of brass plate and an
offcut of small diameter bar instead of a big chunk of 115mm bar. You could
probably get both of those free from someone's scrapbin. However you also
have more machining operations to do so the extra time would cancel out the
savings on material cost if you can't do the machining yourself.
In volume you'd probably start such an item from a casting that was close to
final shape and just needed a light cut all over.
Also try and make things to standard sizes that people are going to have
tooling for already. A 4.75mm bore to a tight tolerance means buying a
specially made reamer which will never get used again and you're going to
have to absorb the whole cost of. A 5mm bore would be a much easier
proposition. Most places will already have a 5mm reamer and if not they're
on the shelf at tool merchants for little cost.
Designing things to just fit other things is one skill. Designing things
that other people can actually make easily/cheaply is another. A friend used
to be a CNC turner at Xtrac who make gearboxes for F1 cars and other race
cars. His life used to be plagued by snotty nosed kids fresh out of uni who
sat at a CAD computer all day designing clever things which no bugger could
make because they'd never actually done any machining themselves. One of
them came up with a gearbox part that had a blind bore that started at 25mm
diameter and then had to widen out further in through a 90 degree turn to
He had to be taken to one side, given a good slap round the head with a half
brick and have it patiently explained to him that if the biggest boring tool
you can get down a 25mm hole is ummm 25mm, and half of that is going to be
shank, how the hell do you machine a 25mm undercut with it? He was then
despatched back to his little designing cubicle to design something the
workshop could actually produce.
Anyway, as a one off I'd guess about £125 to £150 from a CNC engineers.
Maybe a bit less if someone with a manual lathe and mill and dividing head
was desperate for work.
4.75 mm = 0.1870" = 3/16" - 0.0005"
4.75 + 0.01 mm = 0.1874 = 3/16" - 0.0001"
A 3/16" reamer will give a hole of 0.1875" = 4.763 mm to 0.1880" = 4.775 mm
the larger size being the most probable.
I presume that the shaft is actually 3/16" nominal!
I'm interested to know what was going to live down this hole he designed, anyway
? ISTM there would be the same problems getting whatever it was down the hole
as there would be in making it in the first place ?
Thanks very much for the suggestions and offers. Sorry I've taken a few
days to follow this up; I've had a busy weekend.
Martin, yes, your drawing is roughly what the part would look like. My
idea was to make a small master clock which would be a visually
appealing (hence the large brass disc with visible light shining through
the holes) way to supply timekeeping pulses to European slave clocks
(the kind which need an alternating polarity pulse once per minute).
Output would be via a transistor flip flop to reverse the polarity of
Brian, yes, I think the shaft is probably 3/16" as the motor is
American. I only measured it quickly with vernier calipers graduated to
0.02 mm the other night. I also have a number of brass gears which fit
the motor shaft, so one could be turned down and soldered or riveted to
the brass disc.
I need to weigh up if the master clock I'll get will be worth the effort
and expenditure. I'm reluctant to use a "black box" electronic pulse
generator, as I like to see the mechanics inside a clock, but I might be
better to buy a cheap pulse generator for the moment (39 Euros plus
postage) and save up for a genuine mechanical master clock in the
future. I also unexpectedly (and rather excitingly) acquired an air raid
siren on Friday, so I think that'll be taking up some workshop time this
I'll weigh up my options. Thanks very much for the advice.