Model Engineering
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Power Feed for the Milling Machine
updated Sept 06
The new milling machine
is now in
constant daily use, and it would be very nice to have a power feed on the 'X'
axis. A quick look at the commercial units available presents several problems,
not least of all the considerable cost they command. The units are generally
designed for 110 volt supply and thus require a transformer which adds to the
cost bringing the total to in excess of 280 ukp! Whilst these units seem to be
very compact and offer a variable speed and safety features, I would find it
very difficult to justify this expenditure for hobby use. The only solution
remaining is to build a unit to my own design.
There are several considerations regarding the design of a power feed unit.
To emulate the commercial 'Bridgeport' type which attach as a complete unit on
the table end is difficult using parts from the scrap box, the major problem
being the motor. Bearing this in mind, and the fact that at present I do not
have an ideal electric motor available, I decided to ensure the design was
independent of the motor. When a suitable motor was eventually found then it
could easily be incorporated at any time without recourse to any changes of the
feed drive unit. Another consideration was the glass scales for the DRO which
precluded the fitting of any drive mechanism close to the feed nut plus this
type of design requires the feed screw to be slotted which I did not want to
do.
The basic concept was now starting to shape up. The motor would be remote
from the feed unit which would be attached to the end of the table (like the
commercial type) with the drive from the motor being supplied using a couple of
universal joints and a shaft which could vary in length. I had decided that the
direction of feed would be mechanical on the feed unit and so the motor need
only have to work in one direction only. I also wanted to be able
to quickly disengage the feed unit to
revert back to hand feeding with the minimum of fuss (no spanners, keys or
screws). With the design requirements now firmly in mind it was time to rummage
through the scrap box for suitable bits. This revealed some old gears (from
electric drills) and a bevel gear and pinion. In order to make room for the
drive unit the feed shaft would need extending by a couple of inches and so I
decided this would hold the main bevel wheel and be keyed to the feed shaft to
form the drive using the pinion. The pinion would need to be able to change
direction and this was accomplished by a tumbler gear mechanism using the old
drill gears. The pinion complete with gears would then have to engage and
disengage with the bevel wheel.
This was done by providing a tilting of the
pinion assembly about a hinged point so that in the closed position the pinion
was in gear with the bevel. A spring catch and lever was used to lock the
assembly in the engaged position so that operation of the lever automatically
released the engagement. Finally, the universal joints were made and a length
of steel tubing plus a shaft which was slotted and keyed to slide in the tube
provided the flexible drive connection to the motor. After some cutting and
trying, the drive unit finally took shape and a temporary motor drive (ex
windscreen wiper 12v motor) was coupled to the telescoping drive. The test
proved to be very successful and covered all table movements and configurations
without complaint. The hardest part then followed, which was the attachment of
a cover to protect the gears from swarf and provide a generally pleasing look
to the unit. In use, the tumbler gear mechanism was ideal as it enabled a
change of instant direction without having to stop and start the motor.
Disengagement of the feed unit to enable hand feeding was done by lifting the
lever (vertical lever at top LH corner). I also added an extra spring to
encourage the unit to 'snap' out of engagement and ensure it could not
accidentally re-mesh with the main bevel gear wheel. With the advent of the
variable controller, a speed control was fitted (front).
Whilst the windscreen motor does the job it is a bit underpowered. The speed
reduction problem was solved by raiding the 'old discarded tools bin' which
contain bits from projects long forgotten. Inside was an old dividing head
mechanism which I think was based on a design by L Sparey which used Myford
change wheels. As I had a selection of these it was a simple task to modify and
build the reduction drive with the added advantage that if the resultant speed
needed to be modified it was merely a case of changing a gear.
Eventually, an old wheel chair controller turned up together with a 12 volt
DC motor from an ex car radiator cooling fan provided the ideal solution to
power the feed and enable the speed to be varied.
All electrical switches on the
feed unit are at 12volt and operate a relay for switching the feed motor. The
feed unit has an ON/OFF switch and warning flashing LED on the main control
panel. The red LED flashes if the feed unit is set as engaged thus giving
visual warning that if the feed power switch is ON the feed will operate. The
mechanism is basically very simple, light weight and robust. A safety overload
shear pin is fitted from the drive and the motor is automatically switched on
or off either when the mechanism is set in gear or released. Not wanting to put
the shear pin to its ultimate test should the travel reach its limit I
incorporated a simple
proximity switch using a micro switch and embedded this into the central limit
stop. A spring loaded plunger activates the micro switch when the limit is
reached. I was not completely convinced such an arrangement would work reliably
but on testing it stopped the feed consistently within 0.003" of the set
limit distance. Incidentally, total cost is less than 5ukp which was attributed
to the use of phosphor bronze for the bearings.
updateThe Power Feed has been
operating now for some time. Whilst the feed speed range is ideal for machining
it is too slow for a rapid feed when you wish to get from one end of the table
to the other. The solution to this problem is very easy to fix. A small collar
with a slot is attached permanently to the LH end of the feed handle. To
operate in rapid mode a battery powered drill with a mating part which engages
in the slot is used. As the drill has a variable speed the table can be moved
in either direction at any speed. A great time saver!
Power Feed Development pictures
updated Sept 07
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