O Scale HSS – II

Making it operational
When I was talking to Andrew Gillette about this he was more interested in it as a scenic object – possibly operating when the switch was operated. Me being as deranged as I am .. wanted to see if I could make it so it could replace a caboose ground throw.

I’m going to be talking about some ideas to do that .. and during that point out some of the constraints imposed by 3D printing and fat fingers.

The Caboose 202S


We want to throw the lever on a ground throw .. and have it move the points on a switch. This requires that the operating lever be strong enough to to this without being oversize.

The Caboose HO 202S Groud Throw shown to the left has a handle that has a shaft that measures approximately 0.045″ x 0.085″. Scaled to full-size this would be 2.16″ x 4.08″. The ground throw is made from Delrin – a stiff, strong acetal plastic.

Pivot Pocket
section_2On the previous page I showed the pockets that are formed by the two halves of the stand. Each is 1mm/0.040″ DIA and 0.7mm/0.028″ deep. These are to accept and contain the swivel or pivot. There is a 1.7mm/0.067″ gap between these pockets. This would allow either a piece of wire for the pivot or a small pivot assembly. I’m going to look at the second, the pivot assembly.
Pivot Assembly
Sintered Nylon
When trying to come up with something like this that will be 3D printed you have to always keep the constraints imposed on you by the material being used. I want to use WS&F (White Strong & Flexible) for this project (referenced in the link). This is a fairly strong material, a laser sintered Nylon. This creates a material that is similar in some respects to a Delrin plastic. Not the surface .. Derlin is smooth and WSF is rough .. but a similar .. ummm .. ‘toughness’ I suppose. I use it for the Harp body and seems it could also be used for the pivot.
Music Wire
swivelMy thought was that a length of music wire would work – at least mechanically – for an operating handle being stiff and strong .. and readily available in various diameters.

The question was .. how to add a pivot point to that music wire? It seems reasonable that if I could print a small shaft that the music wire would slide into, it could be made to pivot between the Harp sides.

If you look at the drawing to the left, the little swivel isn’t just made up ‘out of the air’ but the smallest I could keeping within those constraints I mentioned earlier. The body has a 0.7mm wall thickness .. then a hole .. then another 0.7mm wall. The hole . 0.3mm is about as small as I think would print with any chance of not closing. I wanted something that would act as a guide-hole for a drill. The little studs of course would fit into the pockets in each harp body side.

The shoulders of that 0.040″ dia stud that fits into the pockets .. it is only 0.65mm/0.026″ long. Will a stud/pin 0.040 x 0.025 ” be strong enough? Donno. I suspect that most of the stress would be on the handle/lever through the connecting rod with very little on the swivel itself. If though it was stressed it might be a good reason to go to a handle made from brass with a brass pin.

section_3Looking back at the top of the stand we can see that there is a gap 0.8mm/0.032″ wide.

The question then is .. how big a wire could be used? Obviously .. maximum dia would be that 0.8mm/0.032″ .. in O scale that would scale up to about 1-1/2″ .. but looks aside .. the swivel is 1.7mm/0.067″ wide. A wire of a size just mentioned would leave a wall thickness after drilling out the swivel .. 0.45mm/0.018″ .. which would PROBABLY work. Using just a slight smaller music wire such as K&N #500 which is .025″ dia would give a wall thickness of about 0.021″. That 0.025″ music wire would scale up to 1.2″ full-size.

spruedOf course .. all this is fine and dandy .. but ASSUMING this would work .. how do you set the parts up to be printed? There is a minimum size of individual parts based on the size of human fingers .. the parts have to be picked out of the powder, handled etc.

If I were to do this .. ok .. this is how I would do it. These little swivels would have to be attached to a sprue to print .. much like you get with a plastic model. Shapeways sees things like a sprue as an ‘unsupported wire’.

Something like this. The minimum size of an object is 5x5x5 mm .. this little arrayed swivels measure 9.32mm on the widest.

Point of this is simply to explain some of the design considerations when trying to manipulate stuff to be printed. You can have an idea .. but you gotta push back against the constraints.

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