View single post by oztrainz
 Posted: Sat Feb 6th, 2016 03:53 am
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Joined: Wed Apr 10th, 2013
Location: Unanderra, Australia
Posts: 916
Hi all again,
OK so where was we? That's right, we were talking coal skips :bg:

oztrainz wrote:
Hi all again
Now to the saga of the skips. It has taken six years of on-and-off development to get coal skips...

The Mark 4 skips were the last fully styrene fabricated skip design. From the Mark 5 skip design things get interesting, but that can wait for the next post. Until the next time,

So here we go - resuming with a photo of a Mark 4 skip from styrene with 8mm diameter wheels, but without couplings

The Mark 5 coal skips. The Mark 5 skips were the only attempt we made at a full 3D print. For comparison a 3D-printed Mark 5 followed by the all-styrene Mark 4 skip.

The Mark 5 skips were done with a fully detailed underframe with correct bolt and bearing details and with a correctly dimensioned coal box with all metalwork details 3D printed onto the interior and exterior of the coal box.

and under

The bearings on this version were like the prototype - they only extended to 1/2 of the axle diameter. We had problems keeping the axles in the bearings.So that needed to be modified.
There were only 3 of these produced. All 3 exhibited the striations up the side. This was disappointing quality wise and it was more obvious once the skips were painted. At over $20 each for a small skip and possibly over 70 of them required, a cheaper option was required.

The cheaper option was to simply print the underframe and use other techniques to produce the coal bin part of the skip. This resulted in Mark 6 underfram and resin cast (by us) coal box bodies. The Mark 6 underfame design was modified to have a retaining ring added to help hold the magnet. The Mark 6 underframe simply extended the depth of the sides on the bearing to help retain the wheels.

A couple of other problems emerged with the Mark 6 that prevented a "dead-accurate" underframe from the Mark 5 being used. Boltheads at each end under the underframe prevented the couplings from sitting flat. So they had to go. And the small size of the bearing sides needed beefing up. Which led to the final underframe design - the Mark 7. For comparison purposes the next 2 photos are the Mark 6 underframe followed by the final Mark 7 design.

The key differences with the Mark 7 are the hollowed out magnet retainer and the removal of the central frame timber (to save on 3D printing costs) and beefed-up bearings with a 0.5mm printed hole for the axle retaining wires. A look under a fully assembled Mark 7 skip underframe

The axle retaining wires only have to support the weight of the the wheels and axles when the skip is lifted off the rails. A few puffs of graphite powder into the open bottom of the bearings ensure that these skips roll very well.

A roster shot of some Mark 7 skips in the build process

See, I told you we needed a "few" coal skips. This will be about 1/2 of the final roster. This photo was in November 2013. Wagons behind the green locomotive are on wheels.

and to close out - a photo of a couple of Mark 7 skips loaded with coal, this time, right way up. :bg:

That ought to do for coal skip designing and building. Next up - track laying on the incline.

John Garaty
Unanderra in oz
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