I am guessing that you used brass insert fittings for the side knobs that tighten into the end grain?
Anyone with a good knock-down Dob base?
Posted 10 December 2014 - 10:57 AM
J.T. asked that I post the dimensions of my Z mount. It's all 3/4" Baltic Birch plywood.
Base diameter 25-1/4"". The flat on the back is 2" from the back edge.
Zhumells and their clones are built over-tall. I shortened my sides, my friend Jims' is stock height. Sides are 16-3/8" wide. Stock height is 24-7/8". Stock center of alt bearing is around 23-3/8". My personal mount is 24-1/4" tall, 22-5/8" alt bearing height. My tube JUST misses the center bolt's head.
The front panel is 16-3/8" wide and 13-5/8" high.
Side gussets are 2-3/4" x 12-1/2".
Posted 10 December 2014 - 03:53 PM
Kfiscus, thank you. I hadn't noticed your most recent post on this thread when I responded to you just now on the PM thread. Please disregard those dimensional questions.
Everyone else, a few of us are having a sidebar conversation with CAD sketches regarding woodscavenger's scope to avoid bogging down this thread with minutae on the dimensions of this particular scope and proposed rocker box designs.
I'm close to posting on this thread a proposal for woodscavenger that will be close to the dimensions of the kfiscus base, but will fold flat or assemble within seconds--or at least within fifteen or twenty seconds.
This first proposal will not have hollow side panels, since woodscavenger is more interested in compactness than maximizing stiffness or minimizing weight, but I'll follow-up with a few ideas of lighter and stiffer (albeit slightly more complicated to build and slightly thicker) alternatives.
Please share other ideas and thoughts on knock-down bases in general, especially when they have to be fairly tall for high center of gravity dobs. This has been fun to noodle on!
Edited by jtsenghas, 11 December 2014 - 08:02 PM.
Posted 10 December 2014 - 04:47 PM
Sometimes "cross nuts", also called "cross screw nuts" fit the bill for knock-down assemblies:
... but I'm not suggesting these in the case of the OP's request for a design for a base for his Z12.
I realize that sometimes bolts (with or without knobs on the heads) have to engage in edges of panels due to access or clearance issues. If you know they will see a lot of stress and don't want wood screws or threaded inserts to strip out of edges of panels, these work very well when inserted further from the edge--1 to 1 1/2" from the edge is typical.
These usually come in round dimensions like 1/2" or 5/8", so all you need to do is drill two carefuly intersecting holes, making sure the bolt hole is deep enough that the bolt will never bottom out. Their length should match the panel thickness. Headed versions are also available.
For knock-down assemblies be sure they are tight, preferably glued so they don't fall out when the bolt is absent. In the case of formica laminated panels, such a nut can be epoxied into the plywood or particle board (but preferably good plywood) before the laminate is applied. In that case they could be made entirely invisible on both sides, and CAN'T fall out. Just be sure they are aligned with the bolt hole and tightly glued in place before the laminate is applied.
Posted 12 December 2014 - 03:08 PM
Okay here's my entry!
This is my first pass at a possible design of a Z12 knock down base for woodscavenger similar in style and dimensions used by kfiscus. Since woodscavenger reports he has a router and a router table, I've substituted 1/4" x 1 1/2" x 3/4" long hardwood floating tenons for the biscuit locators. Biscuits would work, but they would be better if they were of the hardwood rather than the plywood variety. These tenons are even more robust. Since woodscavenger wants his scope so very low, just clearing the center knob on the base, the short rear board has a circular arc for clearance. It secures to the side panels with smaller knurled thumbscrews (rather than bar knobs) for the only two fasteners. The 2" removed from one edge of the base has been moved from the rear to the front, so that the base can be stored on edge. Assembly time, 20 seconds in the dark? I took a few liberties with the ellipses and the front "Z" on the kfiscus design. I decided to use piano hinges approximately 16" long in three places (48" long is a standard length).
It doesn't have to be quite so complex, and the colors are for clarity only...
To break it down, you loosen the two knurled thumbscrews (which don't have to be very tight anyway) to disengage two 4" long sections of threaded rods from angled t-nuts:
...then swing the side panels in to disengage them from the tenons...
..then shove all three panels a bit forward to disengage them from the tenons on the front rail...
..and then stack it!
Obviously, assembly is the reverse process. The three hardwood rails screwed to the base are 1" tall, scarcely taller than the center knob, and the three panels fold flat 2 1/4" thick.
Here is a rear detail, which has been verified for tube swing clearances. A slight chamfer on its lower corners and forward side edges would facilitate assembly:
I have a number of recommended tips and tricks for a precise assembly that I won't get into here. That 1 1/2" wide yellow vertical member on the right front corner must be securely attached to the front panel. I'd recommend biscuits as woodscavenger suggested and I described on my earlier post. The slots in the side panels must be a little longer than the tenons are wide to accommodate a little build variation and the angled assembly direction, especially at the fronts of the side panels, but the only function of these tenons is to hold the sides DOWN. Angling the two bolts on the rear panel actually tightens things more against the base--kind of like toenailing fasteners.
Thoughts? Other ideas?
Edited by jtsenghas, 12 December 2014 - 03:58 PM.
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Posted 13 December 2014 - 02:24 AM
Having watched this morph with many side conversations all I can say is
Posted 30 December 2014 - 02:06 PM
Now I'm planning to build a collapsible Z12 mount, the likes of which no one has ever seen...
Posted 30 December 2014 - 05:00 PM
I don't know. JT has so many ideas floating around in his CAD brain i will be suprised if he doesn't already have a drawing of it in there somewhere. Give us a sketch or a clue.....
Posted 30 December 2014 - 07:57 PM
Posted 31 December 2014 - 10:51 AM
Well, Woodscavenger let me know by PM that he is unlikely to make a knock-down base for his Z12 since improving the structure of the strut telescope (with bicycle cables) that shares the optics with his original Z12, but he might use some of these concepts for a future 8" scope.
I don't mind in the least that he doesn't need the specific dimensions that I worked out because I enjoyed the exercise and it helps me think through my own projects, but for anyone who might want to use these ideas on that specific scope here are the cut patterns and side panel dimensions of the above 3-d views:
If you can avoid miscuts, this can be squeezed out of a single piece of good 4' x4' plywood. (Note: to avoid surface tearout make an initial cut only about 1/16" deep and then just over full thickness at each fence setting). Assumed are 1/8" wide table saw kerfs. Sequence of operations would be to make all table saw cuts first, then one side panel carefully, and then duplicate the first side's curves (preferably with a router equipped with a pattern bit).
And the side panel dimensions:
Note that the foci of the ellipses are shown for layout with the string method. For the outer profile the string could be adjusted for length to just reach the top edge of the side panel when looped around the foci. (I think I'll also post some suggestions about that on this thread also).
The difference between the LH and RH panel is shown for 3/4" thick material. For other material thicknesses make sure one panel is twice the material thickness narrower than the other and the corner piece matches the difference between the two. That corner piece could be squeezed out of that same sheet of material, but hardwood would be preferable. A single 3/4" x 1 1/2" x 6' piece of hardwood would suffice for the rails and that corner piece. If birch plywood is used, maple might be preferable to oak for color matching, but I do recommend hardwood.
As a final note, I really do recommend the side panels and front panel be about 16" tall at the hinged corners to help with stiffness even though more of a concave curve is required on the top edge of the front panel to allow the tube to clear it when it is pointed low. Increasing the length an unsupported board extends by only 20% approximately doubles the amount it will deflect under load. In the case of the Z12 the altitude bearings capture both sides of the side panels in that "yoke" so this particular scope isn't quite as sensitive as most would be in that regard, though. In the plans shown the curves on the front panel are concentric with the axis of the tube when it is horizontal for appearances.
Edited by jtsenghas, 31 December 2014 - 05:07 PM.
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Posted 31 December 2014 - 11:52 AM
Because an ellipse is the set of points whose total distance to the two foci is a constant, ellipses can be drawn by stretching a string tied to nails at each focus. Here are a few tips I like to use that represent my methods of using that technique.
Tip 1: Use a framing square to mark the minor and major radii desired, simply half the desired length and width as shown.
Points a,b, and c represent the major radius (semi-major axis), minor radius (semi-minor axis) and center respectively. Mark a sharp dot at each of these points and draw a clean thin line through c and a that extends in both directions for finding the foci.
Tip 2: Use the square to find the foci by measuring from point b the distance to that line in both directions corresponding to measurement a:
Mark a fine intersecting line through that major axis on either side of the center. If the ellipse has a very thin width compared to its length (has a large eccentricity) and the angle is so small that it is difficult to determine that intersection precisely, calculate the distance instead from the center using the Pythagorean Theorem. The hypotenuse would be "a", so this distance "f" from "c" would be the square root of (a2 - b2).
Tip 3: Use push pins, dental floss, and a mechanical pencil for layout tools. Push pins have small smooth diameters and heads to keep the string from rising up significantly, dental floss is nearly inextensible (very non-stretchy), and the mechanical pencil has a fine metal tip to engage in the string. Put push pins in both foci ("f" locations) and at the minor radius as shown. Tie a very tight loop around these three push pins. It helps to knot a loop at one end and to cinch the other free end through it to get good tension.
Tip 4: Remove the push pin at the minor radius, engage the tip of the mechanical pencil in the loop of the knot of the dental floss and draw the ellipse by dragging the pencil with moderate tension on both lines to the push pins completely around the ellipse. Using a closed loop of string around both pins rather than fixing each end of the string allows you to trace the ellipse continuously. Sliding the string rather than the pencil within the string avoids bumps you would otherwise encounter as you pass the knots. If you find that your particular string or dental floss has too much friction to slide easily around both pins, you can slide the pencil within it, pausing to spin the knots past the pencil for a smooth arc. If you are sliding the pencil within the string, it helps to have one of those mechanical pencils with a steel tip around the graphite "lead" to avoid having the string cutting into its tip.
Voila! Not a new idea in general by any means, but the devil is in the details of the methods used!
Edited by jtsenghas, 31 December 2014 - 10:08 PM.
Posted 02 January 2015 - 11:44 PM
Some thoughts on hollow panel construction:
In some cases I think it would be worth making knock down bases like we are discussing with hollow panels to reduce weight by up to half and to increase stiffness. This does slightly complicate the build process and increases the thickness and the costs a bit, but in some cases the payoffs could be well worth it. Just think of how stiff hollow panel doors are for their weight. I thought I'd post here a few ideas and descriptions of techniques to give some food for thought.
A table saw method
In this method the cross section of the edges would look like this:
The blue cross-section represents a solid edge. It may have to be made to extend into the panel on the edges that hinges or other hardware are fastened, but in most areas would only have to extend about 3/4" past the edges of the outer panels if a good adhesive is used. It could be deliberately made to match or contrast with the thin plywood panels. It could be made of a softwood if large weight savings are desired, or hardwood for appearances and dent resistance. The center green cross-hatched area could be an air gap, or have just a few thin ribs on edge across the larger expanses or, especially if the outer panels are especially thin, polystyrene foam designed for house insulation. Such foam is remarkably cheap and available in inch dimensions. 2014 prices in big box stores for 4' x 8' sheets of 3/4" foam are about $15 per sheet and 1" sheets are only a few dollars more. (Leftover scraps have a lot of uses, too.) They can be easily sliced into manageable pieces right at the store with a long utility knife. Urethane glues such as Gorilla Glue bond very well to both the foam and the wood, especially if slightly moistened with water, although squeeze-out does tend to foam a bit and need to be scraped off. This construction technique is a lot easier when straight edges are used. The optional quarter round corner radii are best put on with a router as a final step after glue-up and finish sanding.
One possibility for using this technique for side panels on a very tall rocker box is illustrated in the next two pictures:
Note that in the first picture there are a couple of 60 degree and 30 degree angles. The ends of the edge pieces would have to be mitered at 30 and 15 degrees respectively to match. Note also that a couple of internal blocks are used where needed, such as for the altitude bearing arc (which would be made after assembly through all layers), or the bottom edge, for subsequent assembly to the base. In this case it is easiest to design and construct it from the inside out. For example, in the case shown, I would first make the two thin panels, and then cut the edge pieces to suit, possibly sanding them for length for a tight fit. If I wanted the inside gap to be 1" thick for 1" foam, I would make the edges thicker than that foam by two panels thicknesses. (Shown is 3/16"). The edge pieces could all be rough cut a little long, then ripped precisely for width (1 3/8" plus a tad for sanding in this case), and then cut precisely on both sides to receive the panels (5/8" x just over 3/16" in this case to allow for sanding with the center precisely 1" wide). Only after that would I miter the ends to fit tightly around the thin panels. If I didn't have a really sharp table saw blade I'd make them about ¼" long and finesse them with a disk sander. The filler blocks could also be ripped to match for thickness or purchased 1" thick. So-called 5/4" boards are 1" thick precisely and are available in small pieces in such stores as Menard's in the U.S.
Tip: When browsing lumberyards for thin plywood measure the thickness of ten stacked pieces and divide by ten for a precise average thickness. If you can measure ten panels for thickness within 1/32" you can calculate the average thickness within about 0.003". So-called 1/4" plywood is sometimes as thin as 5 mm, or about 0.200", which is much closer to 3/16" than 1/4". Choose the thinnest material you can with one good face.
A router method using pattern bits
A slightly easier method, and one that allows more easily for curves is one that uses pattern bits on a router. These will give the appearance of really thick plywood, but could weigh less than half as much if a gap of 1" is used. Again, the core could be hollow, or foam, or have just a few supporting ribs.
Assemblies done this way would look something like this:
The sequence of operations I'd recommend in this case is to first make the outermost profile of one internal panel at least 3/4" thick, finessing the edges with belt sanders, disk sanders or drum sanders and being very careful to keep all edges square without rounding or beveling of edges. I would then duplicate it on another panel of the same thickness for the other side of the rocker box using a pattern bit such as that shown on my post on page 1 of this thread. Only after that would I cut out the interior holes as large as I dared for the application with a handheld jigsaw. A bandsaw could also be used if an entrance cut could be suffered in an unobtrusive area such as a bottom edge--just glue a thin piece of wood into that kerf to maintain overall dimensions. That interior work would not have to be neat or smooth. I would cut the foam to fit if I were using foam for the core next. I would then trace that pattern on the thin outermost panel material (four pieces in this case for two sides of two panels) and cut those about 1/4" oversized with a jig saw or bandsaw. I would then glue all the layers together and let the glue cure completely, cleaning up squeeze-out on the edges. I'd probably use Gorilla glue on the foam and Titebond III on the periphery. After the glue is dry I would trim the edges flush from each outermost surface, being careful to have the bearings on the router bit follow those precise middle panel contours.
The final result would look like 1 3/8" solid plywood and would be nearly as stiff as plywood that thick but would weigh about as much as 3/4" plywood if the core were 1" thick. If the core were 3/4" thick then the assembly would look like approximately 1 1/8" thick plywood and weigh about the same amount as 5/8" plywood and be about half the weight of 3/4" particle board.
I know I've been going on and on about hollow panel construction in this thread, but I know a lot of ATMs are unsatisfied with the weights of their projects and the actual assembly techniques of the above suggestions are not too difficult if thought through well. Stiffness does increase with the cube of the thickness of solid materials and nearly as much for hollow core panels and that can be a huge improvement if you don't mind things looking a bit thick and solid. Dobsonian bases for solid tube scopes with very high centers of gravity could really benefit from these techniques, and the desire to raise some of the shorter dobsonians without requiring an additional table can be accomplished by making "stretched" rocker boxes that weigh less than the originals. These techniques could be used whether or not the bases knock down--just carefully include solid thicknesses in the areas used for fasteners such as the knobs mentioned previously or the screws for hinges.
There....I've said my piece. Now go amaze yourselves and your friends.
Edited by jtsenghas, 03 January 2015 - 10:04 PM.
Posted 04 January 2015 - 03:27 AM
The "uvula", named for the flappy thing that hangs down from the top of your mouth at the back, connected at the short side (North) of the ellipse, is what connects the rockerboard to the ground board. The next post will have a picture showing the penciled-in stars (about 400) and Milky Way. They will be wood burned into the ground board's surface.
You can also spot the time marks (light pencil) at the outsides of the circle. This will allow the scope to be "dialed in" to a chosen date-time combination.
You may also be able to see the pairs of parallel lines that straddle the planisphere's cutout. These mark the positions of the 2 uprights of the rockerboard.
*Note: This is for decorative purposes only. This is in no way an object locator!*
Edited by kfiscus, 04 January 2015 - 04:12 AM.
Posted 04 January 2015 - 03:36 AM
You should be able to spot Orion quite easily.
P.S.: The 2 lower "corners" that deviate from a nice curve are from the original planisphere used as my pattern- cited in the previous post.
Edited by kfiscus, 04 January 2015 - 04:16 AM.
Posted 04 January 2015 - 04:53 AM
Interesting idea. I wonder how effectively this concept could be pushed into a serious object finder.
Posted 04 January 2015 - 02:37 PM
In a private message Kfiscus told me the planisphere he is making could serve as a display item during daylight, but the stars won't be labeled and he also described to me the clever technique he is planning to employ to make the Milky Way. I'll let him share that in his good time, but it doesn't sound like he is planning on it being a serious object finder. Being on a groundboard doesn't lend a map well for use, in my opinion. I did suggest, however, he could reverse the images of the stars so that when the groundboard is turned to put the constellations in the correct direction they could be used to identify them by a mental upward projection. When the scope is turned north the horizons would match reality and the constellations shown near the east horizon of that ellipse would in fact be rising. That would, if course, show backwards constellations on the map which some may find too bizarre and would require the groundboard to be turned during the night. I suppose an extension of this idea would be to make a fast table-top dob with a larger diameter than necessary base with a functional reversed planisphere. If the azimuth bearings were Teflon pads straddling the map layer it could be a spinning intermediate layer. Really, though, this concept is more art than navigation.
Interesting idea. I wonder how effectively this concept could be pushed into a serious object finder.
Posted 06 January 2015 - 12:48 AM
This thread OOZES awesomeness !!!
Great work guys !!!!
Posted 06 January 2015 - 01:25 AM
PS: I added 2 more line segments that I'd missed (Eri. & Oph.) after posting the photo.
Edited by kfiscus, 06 January 2015 - 02:59 AM.
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Posted 06 January 2015 - 08:45 AM
How is the burning in done..?
Well, kfiscus? Are you going to let everyone in on your technique on the Milky Way? Did you use either of the methods I suggested for masking or find a better way?
Posted 06 January 2015 - 12:02 PM
Thanks for asking about the Milky Way burning technique. Originally, I told J.T. about an idea that I got while watching "Antiques Roadshow". An appraiser was raving about a valuable antique piece of furniture with intricate inlaid wood. He mentioned that the shading was done with hot sand. I thought that would be really cool. I thought of using cold sand as a mask on the non-Milky Way parts. J.T. mentioned making a metal mask. As I looked at the problem, the solutions looked to make things too complicated and I was worried about not having control of the levels of burn.
So, I just spent 2 hours making very tiny spiralling loops with the tip of a woodburner "spatula" tip. Not fun, but I'll generally do something like this once, telling myself "You'll never do this again...".
I'll post a picture of the planisphere outline on top of the map Tuesday night. I'm pleased with how it looks.
Edited by kfiscus, 06 January 2015 - 01:14 PM.
Posted 06 January 2015 - 12:04 PM
Edited by kfiscus, 06 January 2015 - 07:00 PM.
Posted 06 January 2015 - 03:46 PM
cool for the day but how about glow-in-the-dark paint for the stars so you can appreciate it at night.
Posted 06 January 2015 - 04:52 PM
Nicely done, Kfiscus, and I think the wood-burned images will look really nice when it is finished. I think, however, you'll be a bit disappointed with how much the "uvula" covers your careful work on Ursa Minor and Draco, though.
Posted 06 January 2015 - 06:32 PM
Cepheus, U Minor, and Draco have all filed grievances...
Woodscavenger, I'm contemplating very dim and dimmable to zero white LEDs shining out of the 3/4" thick base. (No red for this application.)
Here is the promised photo of the 2 pieces stacked.
Remember, kind readers, this is for show daytime and night, and at outreach events and my Earth & Space Science classroom. The top piece will be gloss black- making the star map "pop".
Edited by kfiscus, 06 January 2015 - 06:34 PM.
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