Building the Platform

The previous post covered construction of the platform's circular segment. This one covers the rest of the physical platform, excluding the drive and electronics.

In the descriptions below I refer to various parts by name, some of which may be more obvious than others. This photo labels the parts:

The first step is to cut the south bearing riser, which mounts to the tilting platform, and the south bearing base, which mounts to the base. The south bearing is located between these two pieces.

The bearing riser is 100mm wide. The bearing base is 180mm wide. This extra width is not strictly necessary but it allows the rear (south) leveling foot to nest in between the support blocks, thereby keeping the platform a bit shorter. The lengths of the bearing supports is not important at this point; cut them extra long and trim them later. Bevel the bottom edge of the bearing base using the same blade angle you used for the circular segment (which is the complement of your inclination angle).

The second step is to construct some triangular support blocks to mount the circular segment and the south bearing supports. I made these out of the same 4/4 maple stock as the circular segment but anything sturdy would work. I did not record dimensions for these but they are not critical and it's easy to work out about what they need to be.

O Tannenbaum? Actually, support blocks. From top to bottom: circular segment blocks, south bearing riser supports, south bearing base supports 

Glue the small support blocks to the front face of the circular segment blocks, equidistant from the center, such that the hypotenuse is on the face and the short edge is flush with the bevel. 

Now it's time to build the tilting platform. Again, refer to the design spreadsheet and accompanying diagrams.

For the tilting platform, I used 12mm (1/2") Baltic birch. Since all the load is at the three corners, which are supported from below, this is plenty stout. Measure the chord of your circular segment. It should be close to the width between adjacent feet of your telescope. The larger of these two measurements determine the width of the platform. The length needs to be somewhat more than the sum of dimensions (x)+(y)+(b-a) from the spreadsheet. Initially make the platform about 10-15 cm longer than this; you can trim it down later. Cut the platform as a rectangle with these dimensions from the plywood.

If you have not done so already, cut a bevel on one end of the platform using the same blade angle as before. Mark the top and bottom, and mark a centerline from north to south on both sides.

On the bottom side, measure distance (x) from the beveled edge. This is the position of the Azimuth Axis, upon which the telescope is centered. From that point, measure the distance (b-a) and mark a line across the platform. It's more important for the line to be parallel to the back (beveled) edge than it is to be square to the sides. This is the mounting position of the front face of the circular segment.

Laying out the AA and circular segment. Although it's hard to see, the right edge is beveled. (x) is 270mm. (b-a) is 123mm.

Transfer the (b-a) line to the top of the platform. If your foot displacement (fd) is zero, the front feet of the telescope will be on this line. If not, draw another line offset by (fd).

Mark the top of platform for your telescope's feet. I removed the feet from the telescope's base and inserted a cut-off nail into the screw holes. Then I inverted the base and located it so that the front nails were on the line and the rear one on the centerline. Then I lightly tapped the platform to mark the feet locations. This way, the feet holes are assured of being in the right place. Alternatively, you can measure carefully to locate them.

After using nails in the telescope base to mark the feet. Small circles indicate the nail indentations.

I made shallow mortises for the feet using a 1" Forstner bit centered on the nail holes and drilling about 2/3 of the way (8mm) though the platform.

Next, mount the circular segment to the bottom of the platform. Align the front edge with the line you made earlier. For this first mounting, I only used screws. When I did the final assembly, I glued it.

Positioning the circular segment. The scrap board underneath (with the orange clamps) just helps keep it in position while the screw holes are drilled

# 6 screws driven into the segment and the support blocks underneath

The next step is to mount the blocks that support the south bearing riser to the underside of the platform. The short side goes against the bottom of the platform and the hypotenuse is flush with the bevel at the back of the platform. The riser is 100mm wide so the outside of the blocks should be about 95mm apart to account for the eventual rounded edges of the riser. Clamping these securely so they stay in place while you drill the screw holes is tricky. I used a square block clamped to the platform and then clamped the blocks to it. I then drove #6 screws through the top of the platform into the blocks. Again, no glue yet.

Mark the lengthwise centerline of the riser on both sides. Position the riser so that it extends just past the blocks and mark the location of the bottom of the platform. Extend this line across the riser, square to the edges. Mark the bearing location at distance (z) from this line, on the centerline.

The horizontal line is at the bottom edge of the platform (see below). The dot above is the bearing location.

Next, drill a hole for the bearing spindle. My bearing is a 12mm steel shaft held by flange coupling on the back of the riser which rotates in a pillow bearing mounted on the front of the base. (Admittedly, it's overkill to have a ball bearing here. Many other designs use simple friction bearings; that would be perfectly OK. Regardless, the bearing should be positioned so its center axis is on the mark.) I used a 12mm Forstner bit centered on the marked location to drill through the riser.

Then I inserted the shaft into the hole, slipped on the flange coupling, and mounted it with screws. A self-centering bit is handy here.

Finally, position the riser with the line positioned at the bottom edge of the platform. Make sure it's square vertically and centered on the platform. You can clamp a piece across the riser at the top edge of the platform to keep it positioned accurately (not shown). Screw the riser to the support blocks.

Positioned with the line (barely not visible) at the edge

(Note: as I write this it occurs to me that it could be easier to mount the blocks to the riser first, then mount the riser to the platform. In fact, I'm pretty sure that's what I did on the prototype. Whatever works, as long as the bearing ends up in the right place relative to the platform.)

This completes the tilting platform! Now it's on to the base.

For the base I used 18mm (~3/4") Baltic birch plywood. Start by cutting the base the same width as the platform, and 20-30cm longer.

Mount the support blocks to the south bearing base in the same way you did for the platform. The hypotenuse goes against the blocks; the long edge is flush with the bevel.

Clamping setup. The striped board is just a scrap to clamp the block in position. 

The next step is to mount the pillow bearing (in which the shaft turns) onto the bearing base. Cut a pair of boards to support the platform parallel to the base while you assemble it. The width of these boards depends on the height of the platform above the base, which depends on the width of the circular segment, the height and position of the roller bearings under the it, and the drive roller. For my platform, I used 10cm which worked out well. A platform with a different geometry might need to use a different height. One approach would be to set the platform on the supports, locate the roller bearings and drive roller under it, and adjust the height as necessary.

Temporary support boards

With the bearing shaft in place in the flange coupling on the riser, place the platform flat on the support boards. Then slide the bearing base towards the shaft. Slip the shaft into the pillow bearing and bring it all together until there is no gap. Mark the location of the bearing on the base.

Slide the bearing base (at right) towards the platform

Insert the pillow bearing, close the gap, and mark the location of the bearing on the base

Slide the pieces apart again and screw the pillow bearing to the base at the marked location.

Position the bearing base on the platform base so that it's square, centered side-to-side, and near the back edge. Mark the location and clamp it in place. Drill though the bottom of the base into the support blocks to mount it.

Now it's time for the roller bearings. Let's start by looking at the complete assembly.

The bearings are nothing exotic: 6mm ID x 19mm OD x 6mm, aka 626-2R6, which are inexpensive and readily available. The axle is a M6 x 60mm bolt. The spacers are a combination of 5/16" hex nuts, which slip onto the 6mm bolts, and 6mm washers. Eventually I removed the hex nuts and used proper bushings; you can see that in the later photos of the finished platform.

The sides of the bearing block are made from 9mm (~3/8") Baltic birch. The base is a piece of maple. The inside of the block needs to be at least as wide as the width of the bearing surface at the bottom of the circular segment. In my case the inside width of the blocks is 33mm.

To determine the height of the block, decide where they will be located under the circular segment. In the design spreadsheet, on the 'Drive Motor' tab, there is a row that gives the linear distance the platform travels for the full extent of its travel. The bearings must be at least half this distance inboard from the ends of the segment to allow the platform to rotate the full distance. Mark the segment at this point (or slightly inboard) and measure the vertical distance to the segment at that point.

The height of the axle's center in the block is the vertical distance to the segment, minus the radius of the bearing. For me this was 55mm.

Construct the blocks by cutting four identical pieces and drilling the axle holes at the specified height. Round the corners and edges.

Cut the base pieces to length and width, then glue and clamp them together. No fasteners are needed.

The shaft is just to keep the sides aligned

When the glue dries, drill holes in the bases for mounting the blocks. I suggest making the holes oversized to allow for minute repositioning (you can see these in the photos below). Insert the M6 bolts and assemble the bearings as shown above.

Now for the payoff. With the platform and base fully coupled via the south bearing, and the platform on the support boards, align the platform's sides and center with that of the base below.

Align platform and base (bearing block is not yet mounted)

Place both blocks so that the rollers just contact the bearing surface and are centered on it. The locations of the two blocks should be symmetrical, or very close. You can shimmy the platform a bit to split the difference between making the blocks symmetrical and the aligning the platform over the base. Loosely clamp the blocks in place.

Now remove the support boards. The platform should roll smoothly and evenly on the bearings. You may need to adjust the yaw angle of the blocks to align the bearings with the direction of travel of the bearing surface.

When you are satisfied with the position of the blocks, screw them to the base. If you made the holes oversized, you will need washers under the heads. I used cabinet screws with built-in washers. 

I found that when significant weight (like, say, a Dobsonian telescope) was on the platform, the bearing riser flexed a bit toward the base. So I built another small block and inserted it into the space.

If the riser flexes at all it rides on this bearing

Now you have an equatorial platform! You just need a drive. We'll cover that in subsequent posts.