My next layout is to be based on Fortrose station in the highlands. The station had a 50ft Turntable, my intention was to get one of the Midland Railway Centres 50ft kits, unfortunately the business has closed, so are currently unavailable. Although the 50ft Turntable would have been correct, my HR Clan locomotive would have been too big (it being 59ft over the buffers), so a 55ft would be better. A few searches and I came across the drawings for a 55ft GER turntable, £1.80 later and I have a set of drawings. A few issues to sort on alignment, but now loaded into Fusion and calibrated as a Canvas. Whoa ....... lets put the brakes on this and define the project a wee bit rather plowing into this at 100 mph like a bull in a china shop. Limitations Max build plate size for printing horizontally is 298mm - 54ft 9 1/4 in = 384mm (the total size of the rail on the turntable). Now this may be able to be printed at a 45 degree angle, like Mossy's Coach roofs - it would be tight and take over 27 hours to print. So is likely to have to be printed in sections. Same limitations apply to the well, I don't have a wood lathe to turn the base, so would have to print it in slices like a pizza. I do have routers so could machine some of the well and frames in MDF / Ply, and 3D print the cosmetic slopes. Drive My intention is to use either a Nema 17 or 23 stepper motor. As I currently use Nema 17's on the traversers, it should cope easily with a turntable. I already have a motor controller (TB6600), and will probably use an Arduino for the logic control. The motor will be mounted to the side of the turntable and not directly to the spindle and use a belt drive. Several reasons, reduces the overall height, quieter and if something jammed the belt will jump or snap, better than the model. GT2 toothed belts will be used for the drive as these have minimal backlash and both belts and gears ate readily available (read as fairly cheap). A little context here for the movement, Nema 17 stepper motors come in two main varieties 200 steps per rev (1.8 degrees per step) and 400 steps per rev (0.9 degrees per step). We've established above that the track bed is 384mm in dia, giving a circumference of 1206mm. For a 200 step motor the rotational movement per step = 1206/200 = 6.03mm. For a 400 step motor the rotational movement per step = 1206/400 = 3.02mm. The rail is only 1.7mm wide so acurate alignment is gonna be almost impossible. Using gearing on the motor can reduce this, my maximum print width is 160mm (M3Max print bed is 298 x 160) and I would print the gear directly on the print bed for flatness and surface finish. A 160T GT2 gear would be approx 101mm in dia, combined with a standard 20T motor pinion would give me an 8:1 reduction, now my resolution is 200 step motor 0.75mm per step & 400 step motor 0.38mm per step. We're getting closer the 200 step motor is acurate to half a rail width, and the 400 step motor is acurate to a quarter rail width. Stepper motors, or more importantly the motor controllers have another trick up thier sleeves. Micro stepping, by energising more than one coil at a time the motor can sub-divde the step or take micro steps. Normally 1/2, 1/4, 1/8, 1/16 & 1/32 steps. The TB6600 can do upto 1/32 steps. So with a 200 step motor @ 384 dia with a 1:8 gear reduction and 1/8th micro-stepping enabled the movement per step is 0.09mm or approximately 1/19th of the rail thickness. Putting it another way, I have 19 steps of the motor to get the alignment correct. It does mean a full rotation will need 12,800 steps to complete. Right back main subject and playing with fusion 360 A quick sketch of the well was created Then revolved through 360 to create the well This will not be the final well shape as it will be modified to suit building in layers cut by a router. And there's more Paul
And then there was the table The Girder is being built up in quarters, which will get mirrored to form the full width front girder, then mirrored again to form the rear girder. Finally the internal cross braces extended to the full width. But and it's a big BUT, the table is too big for me to print in piece, so I need to divide it up. In two halves is doable, but would have a join in the middle which isn't on the prototype. Although Mossy's experments with resin gluing show this can be hidden. Or do I print in 3 sections and make the join at the girder section joint This would allow the center section with all the bearings, drives etc to be made up as one assembly. Paul
Having noted our other discussion on this else where, I see better why you say 3 parts. What I would maybe suggest is the vertical girder edge is fully on one part and the other half just a flat end. I would then guess a tab for the other part to align to and aid stregnth on the hidden inside under the deck.. if that makes sense ?
Hi Andy Yes that makes sense, looking at the above drawing, there is a plate connecting the underside of the girders to each other, and there are internal bulkheads which will stop anyone seeing between the girders. That would allow Tabs to be bigger / longer and maybe even like a dovetail. I do worry about the weight that the turntable will need to support, so in the bits that can't be seen there maybe hidden beams added Those beams could be printed as a single piece and added into the structure from the top, secured using screws to the girder assembly, and also support the rails. A cosmetic decking applied after. Paul
The girder supports the rails But lets think about this ..... Oh no ..... Print the turntable bridge in three parts, and between the internal braces (the dotted lines) that cannot be seen on the outside, create supports to take a piece of Ally angle on each side. The angle is secured with screws and locks the bridge together before the cosmetic covers are installed. The ally angle provides the structural support. Paul
Just off a tangent for a moment… is there any support in the outside edge? Wheel or such (like the Peco OO turntable?) or is it just omitted in the diagram above? Surely it’s not all supported on that central bearer?
Hi Andy As this turntable was powered by Shank's Pony, the central pivot was actually a sphere / ball to allow self levelling, and each end of the table has a pair of wheels running on a support rail - much like the Peco model. Electrically for me the rail will be continuous and permanently connected to one rail, the other rail will be connected to the central pivot. I have an Auto Reverser unit to maintain the correct polarity. Paul
G`day Paul, Quite impressive and looking forward to seeing how it finishes up.. One thing I would suggest needs careful consideration, is the accurate placement of the track on the road bed. It is critical that it is exactly the same at both ends of the table. It can be checked by lining it up with your IN track and your OUT track......then rotate the bridge 180 degrees and it should still line up perfectly.....if it doesn`t line up it needs tweaking. Gormo
Hi Brian Yep, on the traversers I used a piece of ply that was a good fit between the rails and designated the datum track. This track was then used to set the first exit point, and all other tracks set to align with this. For the traverser I will lay the A end bridge track to gauge and on center, set the entry road to correctly align using a 4 to 6 inch piece of snug fitting plyso that the track approac is square and not slightly off. Then spin the turntable round and match the B end to the entry road. If I manage to stick to the track plan then there is only one entry / exit position, although I have a feeling I may have to use the turntable to provide access to the engine shed. Cheers Paul
