Modular Altitude Azimuth Telescope Mount

Discussion in 'Metal casting projects' started by CoreyM, Nov 3, 2017.

  1. CoreyM

    CoreyM Copper

    No joke, that is is my last name, guess I found the right hobby, ha.
  2. Tobho Mott

    Tobho Mott Silver Banner Member

    BA-HA-Ha-haha, I'm not actually psychic, just took a gamble this (3rd item above) was you and it paid off, is all... :D

    Thanks for the sub, now that I'm sure...

    I meant it about the pix being awesome though.

  3. ESC

    ESC Copper Banner Member

    Nice work Corey. Research elastomeric bearings to zero out your breakout forces. the drawback is their thickness and angle restriction, but they might smooth out the fine movement. Even a homebuilt silicon sandwich might work.:)
  4. Jason

    Jason Silver Banner Member

    WOW... I just knew the moon was made from swiss cheese. Cool shots.
  5. Peedee

    Peedee Copper

    I can't see the flag we left there......

    Really cool stuff.
  6. Petee716

    Petee716 Silver Banner Member

    Excellent reading on all subjects covered. Thank you gentlemen.
    Peedee, I thought the flag was in a storage locker somewhere in Hollywood. (Kidding... Really guys, just KIDDING!)

  7. Negativ3

    Negativ3 Silver Banner Member

    Kubrick took it to England :)
  8. Peedee

    Peedee Copper

    I can see footprints, I reckon some bugger has been up there and had it away......

    Would a silicone spray help with the sticky action between the plates? I'm assuming I'm over simplifying how this face to face concept works.
  9. Jason

    Jason Silver Banner Member

    Triflow in a spray can is what I would try. Doesn't attract dirt and is slippery as hell. It can be hard to find.
  10. CoreyM

    CoreyM Copper

    Over the weekend I made the proper Vixen dovtail clamp for the mount.

    Because of the overhang in the dovetail, in order to sand cast this part I would need a more complicated mold with sand cores, or I would need to post machine(file in my case) the dovetails from an over-sized and drafted casting. Neither of those things are appealing.

    So to cast this part with the overhangs in place, I used the lost foam process.

    For foam I use building supply insulation. I buy in bulk to support my R/C plane hobby, but for single sheets, dollar store foamcore poster board can be used once the paper is removed.

    The goal for this part is to interface a vixen telescope dovetail to my custom bolt hole circle. To quickly and easily ensure the parts fit I use the foam itself to create an impression of each part. I realize the solidified aluminum will shrink about 1.4%, but it usually it doesn't really matter at this size.


    To build up the dovtail clamp I had to laminate sheets of foam together to get the right thickness. hot glue could be used, but for big flat joints like laminating I prefer to use spray adhiesive, since it adds very little material.

    To match the angle of the dovetail, I cut out the impression created earlier and used it as a template to set a razor blade's angle along a straight edge.

    I used the dovetail template to mark out a base plate for the proper spacing between the angled pieces.

    I glued the pieces together using hot glue, and trimmed away the excess, completing the dovetail clamp. The impression of the mount's bolt hole circle was cut out to make an adapter plate.


    Using spay adhesive I fixed the completed dovetail clamp to the adapter plate. A feeder was then glued to the thickest part of the completed pattern.

    The completed pattern was then buried in sand(in a metal bucket), with the feeder poking out of the bottom of a funnel carved in the surface. Great care must be taken to avoid deforming the flimsy foam pattern. I usually place the pattern on thick bed of sand, and then gradually pour the rest of the sand over the top, shaking the bucket every now and then to assure the sand finds it's way into the nooks and crannies.

    For the melt, I usually over heat slightly to give it a little extra energy to vaporize the foam
    I always like pulling lost foam castings out of the sand. It's like a magical carrot, you plant a bit of foam, and yank out a metal part(somtimes).

    I drilled out the four mounting holes on the adapter plate, fortunately the casting picked up the impressions in the foam, so it was already laid out for me.

    I drilled and taped 3 1/4-20 holes for clamping the dovetail, and I made 3 corresponding knurled brass screws on the lathe.

    I used my lathe and 4 jaw chuck to face the bottom of the casting to make it flat, so it won't deform the friction plate when they are tightened together.
    That was the first time I faced an irregular part in a four jaw chuck. The sound of a low RPM interrupted cut is almost meditative.

    The surface finish at the low speed was not great, but it is flat now, overall I'm happy with it.
    I cleaned it up a little on a belt sander after that picture.

    I assembled it all and test fit the scope.

    Fits like a glove.

    This is a lot more confidence inspiring than the flimsy temporary foamed PVC bracket using the single 1/4-20 screw.

    Very happy with the outcome.
    Last edited: Dec 4, 2017
    Tobho Mott and _Jason like this.
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Nice result and application of lost foam process.

    _Jason likes this.
  12. Tobho Mott

    Tobho Mott Silver Banner Member

    Great job!

  13. garyhlucas

    garyhlucas Copper

    Really nice job! A couple of thoughts. You might talk to Igus about materials for the Rulon J ring. They now make printable bearing materials, and what you really want is a material with as little stick slip as possible. I used to use a friction clutch called a PolyClutch. When I needed bigger sizes than they made they sold me the friction material in sheets but would not tell me what it was. You have a complete ring of friction material but you might consider just 3 friction disks as that would ensure absolutely no rocking with the least amount of tension. Those PolyClutches applied load to the clutch faces using small diameter springs in hole drilled around the adjusting nut. Longer springs provide easier tension adjustment and the tension remains more constant than spring washers over temperature and wear changes.

    Also I saw your use of a large reduction string drive. If you have ever looked at the drive on a Ferris wheel that is what they do, with multiple wraps around the drive pulley. The secret is the drive pulley is grooved and they have a grooved idler pulley with its axis tilted so the cable coming off the drive pulley goes into the idler groove and is twisted slightly to line up with the next groove in the drive pulley. This works really well. I built one to pulley a gunnery target back and forth at Ft. Dix NJ and another to pull greenhouse benches very smoothly in and out when a regular capstan slipped like crazy.
  14. chris.trotter

    chris.trotter Copper

    This thread blew my mind. Thanks! (not for the telescope wonderment though - now I have another hobby for the future)
  15. PatJ

    PatJ Silver Banner Member

    The results that people are getting with the foam work is impressive.
    Great work on the mount.

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