Ratchet and Pawl

Dave started it …
ratchet_pawl_On30IMABack in October of 2012 Dave Mason – aka mabloodhound posted to the “Post Shapeways, other 3D printer finds here” thread on the MRRfourms.com/kitfourms.com website. Dave produces On30 kits under his On30 IMA brand. He was showing his Ratchet & Pawl assemblies he had Shapeways print for his Hopper Kit. From that link – his ‘Tips & Instructions for Hopper’ he has a photo showing drilling (#53 drill) and reaming (#51 drill) of the prints.

Dave states – “I know Ed and others were asking about some minimums. My bolt heads are 7/8″ in full size, reduced to 1:48 here. The connecting pieces between each assembly measures .0185 (3/4″) thick by .042 wide. The base plate is .022, meant to simulate 1″ thickness in full size.

The connectors worked fine and the plates separate easily and trim up fine. For reference, the plates are 14” by 18″ in full size.”

Dimensions
Dave sent me his Sketchup file (more on that in a bit). I converted the file to what I call “Pesudo mm”. Sketchup was originally created as a 3D CAD program for Architects I understand. That may be why there can be problems when trying to work with the small sizes we do in our Model Railroad scales. If you attempt to draw a circle smaller than 0.018″ radius .. it won’t. You get an error .. Sketchup will fall to the ground and kick it’s little feet and refuse. That is a problem for those of us working in say .. O, HO, S and so on. I covered this more in-depth (some one say TO EXCESS) in “Prepping for 3D printing“.

Basically though what I do is this. Let’s say you have a cube that measures 1ft on a side .. 12x12x12 in. What you want to do is draw it full size and save that in Sketchup as cube_full-size.skp .. this is the file you will scale down for whatever modeling scale you are working in. Let’s use O scale/1:48 as an example since it is easy to see the relationships. In 1:48, 1 ft is 1/4 in. .. 0.25 in. That is also 6.35mm. So .. I scale the cube to 6.35 in. When I export the file to shapeways as an .STL file everything will work since in a .STL file this will be sotred as 6.35 UNITS. Upload the file to Shapeways and tell them it’s Millimeters. Works.

From O to HO
If you divide 48/87 you find that HO is 55% of O scale. Thing is .. you can’t just re-scale a model and expect it to print successfully. The problem is that there are minimums for FUD = Minimum Detail is 0.1mm/9.0039 in. Minimum Wall is 0.3mm/.0118 in.

SW_Ratchet_and_Pawl_groupLet’s take a look at the O scale version that Dave created. He says that “For reference, the plates are 14″ by 18″ in full size“.

For O scale, 18 in. = 0.375 in. .. or .. 9.525 mm. That’s the 9.513 dimension shown in the drawing. You can ignore the 0.012 mm difference .. approx 0.0005 in. is simply a rounding error probably in part due to only using three decimal places etc.

Some things are obvious where a problem may occur from reducing from O to HO .. other places a ‘maybe’. The pawl that engages the ratchet is 9.545 mm wide. Shapeways’ software will see that as a ‘wall’. That would probably pass .. 55% of 0.545 is 0.29975 which SHOULD round up to the .3 minimum wall thickness. The bracket it fits through though is 0.457 mm wide. That will scale to 0.251 mm .. under the minimum. Easy enough fix .. simply widen it to .3 mm. The bolt heads are 0.231 mm tall and would scale to 0.127 mm which would meet the minimum detail .. but I would bump that up to .2 mm to make them visible. The teeth on the ratchet itself .. donno .. I can’t predict what the software checks will make with that. I would just ‘go for it’ and see what happens.

Finished product
swrenderThere are 48 left-hand and 48 right-hand assemblies – enough to finish 24 cars.

HO scale ratchet pawl group


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