New Equatorial Platform Project
Posted 30 July 2011 - 07:04 PM
If you imagine this platform without the big semi-circular cut-outs that give it something of a vague "aeroplane" shape, It would probably look much more familiar. The other departure from a Tom O Compact model is more obvious - the lightening holes in the north sectors. Those familiar with vintage aviation may begin to detect clues to my background. Both these simple "modifications" remove quite a bit of weight from the finished product while making no difference regarding structural integrity.
I know my way around a shop pretty well, but am a certified dumb*ss when it comes to electronics. So for this project, I let my hair down and ordered a motor and controller from the master himself: Tom O. It ain't a cheap option, but should be perfect for a guy like me.
Posted 30 July 2011 - 07:17 PM
Oh, and here's the scope it's going under: a 16" f/5. See more about it at the URL at my sig line.
Posted 30 July 2011 - 07:27 PM
When you coated with epoxy did you add glass too?
No, didn't see a need for it. Glass can be added as a structural component, or for abrasion armor, but neither was thought necessary here. The baltic birch plywood is plenty strong, but needs a little help weathering the elements over time. Surprisingly, the end grain seems to fare exceptionally well - it's the veneers that fail after a while in use.
Posted 30 July 2011 - 07:33 PM
so you drilled the holes to lighten it [in the rear ] before you cut the arc, so you had to fill the last two holes?
the front pivot is the bottom angle alum upside down, one leg shorter?
Posted 30 July 2011 - 08:17 PM
GOOD EYE, Danny, LOL! The smallest lightening holes didn't quite work out, as you detect, and were plugged with dowel rod and thickened epoxy to fill in the round-overs. The next couple holes have left marginal "meat" to make me sweat a bit. Pretty dumb to install the holes before jigging and cutting....
Matter of fact, I piled so many eff-ups on that I got to thinking of this as a prototype, and when I got 'er working, to make the finished model and do it up slick as a whistle. I may or may not carry through with that, just depends on how interested I remain once I have a working platform under my telescope.
Another option was to use filler and paint to put the goofs into the history bin. We'll just have to see what happens.
Yes, the south bearing support bracket is made from aluminum angle 2 x 2 x 1/4, and cut on the table saw to provide my 30 degree tilt for the rollers. Could have shimmed one leg up to achieve same, as many do. I drew this up on a CAD program (Dassault DraftSight), but it's all in sketch form. Don't like sitting in front of the computer enough to pretty it up for general consumption, is there anything in particular you want to see a drawing of?
Posted 30 July 2011 - 08:48 PM
Posted 30 July 2011 - 11:20 PM
Here's a bit of a drawing.....
The sector/roller centerlines ought to pass directly under the Teflon pads, but I tucked 'em in closer together to keep the platform width limited to 24", as the rocker box is 23 1/2". I never figured out how to calculate run time so I just made 11" long sectors and got on with it.
Posted 31 July 2011 - 07:49 AM
Also, does the drive mechanism simply operate off friction? Or is there an engaging mechanism?
Posted 31 July 2011 - 11:03 AM
Here's a pic of the platform getting its first test drive. Remember to close your eyes when the flash goes off.
Yes, this sort of drive is called a roller drive, operates off friction. Nothing engages mechanically, and resets are accomplished by simply dragging the top across the bottom back to the start point. It seems impossibly crude, but has worked well for hundreds of platforms. I didn't like the way it felt too much last night, but I bet I get used to it pretty quickly. As I said earlier, I bought ready-made stuff from Tom O because I'm hopeless with electronics and he's the best.
Posted 31 July 2011 - 11:05 AM
So where do you get stainless steel strips? I'm using worn out shop rulers. They seem to be just the thing, and I'm really going to enjoy my new easy-to-read rulers too.
Incidentally, I have yet to install travel stops, still trying to decide which scheme I want to go with as there are several options. Anyone who uses a platform and has an opinion is more than welcome to chime in.....
Posted 31 July 2011 - 11:23 AM
Posted 01 August 2011 - 02:03 AM
sufficient friction is a must for this type of drive system. Steel on steel usually has a relatively low coefficient of friction, though it depends largely on the precise type of steel. I have made the experience that aluminum on steel and much more aluminum on aluminum has a higher coefficient, depending again on the precise alloy.
I have no experience with the broad sectors that you use (mine are usually made of 5mm aluminum). Do you achieve full contact over the entire width of the sector?
Posted 01 August 2011 - 10:54 AM
I'm new to direct roller drives, but feel confident saying there's no shortage of friction in my system. Reset is a grinding, dragging affair requiring authoritative effort, and I have had no hint of slip even though I slew my scopes enthusiastically in specific attempts to induce it as part of initial testing. If anything, I could do with a little less friction! And from what I understand, the best materials to use depends on lots of variables such as what the drive roller is made of, the width of the roller, width of sector, etc., etc. My drive roller is finely machined knurled stainless steel, and just tears up aluminum sector facing. Stainless steel, roughed up on the belt sander, seems to mate perfectly in this case.
I started out making .5" sectors, but was advised to make 'em 1.25". I also noted that Tom O tilts his north rollers back a little, for general security and stability, so I again played follow-the-leader. I used my jig as often as needed to get things exactly right, probably jigged and touched up my platform half a dozen times before I was satisfied .... and had a lot of fun doing it.
The jig is a two-part affair: the platform holder, and the tool holder. I took the time necessary to make a good tool holder that adjusts easily and positively using scrap 3/4" plywood, closet rod, large hose clamp, and c-clamp to lock in angle. Sectors were initially cut with a router, then finished with the sander as shown. The holder worked well with both tools.
Final mating of roller to sector is dead on.
Posted 01 August 2011 - 07:01 PM
From bottom to top: My first, a cylindrical bearing type built in mid-90's, used for 10 or 15 years, weighs well over 50 lbs, works superbly! Could probably put a Volkswagen on it and make it track accurately. But for whatever reason, it was never happy with my 13 incher, or with the redesigned/rebuilt 16. Beats me, I guess it must like lots and lots of weight to work with. Anyway.......
The next platform, also a cylindrical bearing type, was made for my prized Wilkinson-optics 13.1" f/4.5, in 1998, I think. It caused me to suddenly need a ladder, so I traded it off ( http://www.robertree...com/cluster.htm ) and eventually got it back, which is what started this new platform project.
So this new one not only seems to get along well with with both scopes, but allows me to use the 13 with my feet firmly on the ground. Also seen in the pic are the rewind cranks on the old platforms. The instant reset of the new platform is something I have wanted for a long time, but that isn't to say I won't miss the cranks just a little bit. I always liked seeing them go slowly 'round and 'round, something about it was reassuring in a vague way.
Both the old platforms in the photo have had their motors, electronics, and tangent arm assemblies removed for safekeeping, incidentally. I have no idea what I'm going to do with them. In a perfect world I'd find some noobies who are as funds-challenged but enthusiasm-rich as I once was, to give them to.
Finally, here's a pic of the 13 getting a test drive on the platform last night. The old 13 was my first real ATM project, and I still like it so much I can't get in a hurry to redesign and rebuild it even with all the improvements I can so easily envision. Not that I'm so sentimental, it just really works that well!
Posted 07 August 2011 - 08:04 PM
I started to plan for my EQ platform and looking to borrow the lightness of your designs. Question I have for you, how much science and how much art is there in placing the holes? Seems like I can kind of eye it? True or is their a special trick to it?
Thanks for sharing as much as you have already!
Posted 07 August 2011 - 10:52 PM
I located the lightening holes by eye, but made the mistake of doing so before the sectors had been jig-cut. I ended up having to plug the smallest holes, and the next smallest ones are awful close, didn't leave much meat to work with, but I believe it won't be a problem. So don't make the same mistake -- get your sectors cut to shape on the jig first, and THEN locate and drill your holes!
Posted 08 August 2011 - 01:41 AM