click to returnCUSTOM MADE ALT-AZ MOUNT
Pictures at bottom of page

I was contacted about making an alt-az mount for a 4" refractor. I looked at many pictures of alt-az mounts, and decided what I liked and disliked about each. I have an alt-az mount for a 3" refractor, but it too had features I did not wish to reproduce. I ended up designing a mount of my own, made of 1/2" thick brass. The tripod top and the center pin are steel; the hardware is Stainless Steel. I sent the person my plans and when he said he liked the idea, I started collecting the materials. 1/2" thick brass is fairly heavy, and obviously I could have used much thinner material, but I have always found that the heavier the mount is, the less vibration - assuming all other things equal.

 

Pre-construction note

A model was constructed from wood to make sure everything fit and looked like I planned.

Construction

I first turned a piece of 3.75" I.D. x 5.5" O.D. brass to size (4" x 5") which left a nice 1/2" thick wall. I then milled 4 slots on one side and 3 slots on the other side, to ensure the two halves could not be put together wrong. I then drilled and tapped the slots for 10-32 hex head capscrews. I spent a lot of time planning and executing the next step - drilling the two holes for the screws on the side arms. The holes had to be exactly opposite each other, or the bolts would not fit properly. I didn't want to have to ream the holes bigger to make it work; I wanted it to be right. After center drilling and then drilling the two holes, I tapped them for 1/2"-20 threads. I then made two brass bolts with knurled knobs. They have a step in them to go through the side arms, which keeps the thread portion just short of going through the tube ring. After a trial fit, I cut the tube ring in 2 pieces. I then turned a piece of Cedar to 4" diameter. We used polyurethane to coat it. It is wrapped in a piece of felt, and it takes the place of the telescope tube to make sure everything fits properly, and keeps things from moving around during shipping.

I turned a piece of steel plate to about 6" diameter, and welded three pieces of rectangular tubing and a piece of 2" diameter stock to the underside. I then bored a 1" hole in the base plate and the 2" stock, drilled the holes to hold the tripod legs, and primed and painted it. I went to my favorite sawmill and got some more Red Oak. (I previously made a tripod for my 6" Brashear, and knew the lumber was from the same batch.) I cut it to length, using the 4" Clark tripod as a height gauge, and routed a corner on all the pieces. I made a jig to hold the lumber at the right angle, and made a test cut on two scrap pieces. They fit just like I wanted, so I cut the 6 legs. My wife assisted me by doing the staining, and applying the polyurethane. We sanded between coats and they turned out beautiful. I cut the two side arms from 1/2" thick brass plate. I drilled and countersunk the proper holes for #10-32 hex head capscrews. The base plate is also 1/2" thick brass, so I then drilled and threaded holes to match the bolts on the arms. 

I turned a piece of steel to about 4" O.D. x 1/2" thick top with a 1" bolt on the bottom. (It looks rather like a large flat head bolt.) I threaded the end for 1" threads and obtained a 1" locknut. I made nylon washers for the various mating surfaces. 

I put it all together and it works like a charm and looks very nice. It took a little longer than I expected to make it, but I wanted it to look and work like it should. 

To tighten the altitude, I put a nylon washer between one of the brass knobs and the tube ring. This keeps the tube from swinging, but you can adjust it to allow smooth movement. For the azimuth, I put a brass 1/4" bolt with a knob through the 2" stock under the base. This allows the friction to be easily controlled.

I wrote a set of instructions and maintenance manual for the new owner.


Turning brass tube to 4" ID x 5" OD


Boring 2" stock and base plate for 1" shaft


Jig to cut taper on tripod legs - it had to be long 
or the end of the leg would hit the table guide.

Making second knurled knob

Underside of tripod base

Threading 1" piece

Test fit for the 1" locknut

Milling slots in side of tube ring

Cutting tube ring in half

Center drilling for holes in arms

Boring holes in arms

boring hole in base plate for 1" shaft

Finished mount on tripod with piece of 4" Cedar (from tree on my property)

Underside of finished mount showing azimuth lock

tripod leg showing taper cut on legs

Complete alt-az mount

Problems I ran into making this mount:

I broke a 10-32 tap while I was tapping the hole in the brass tube ring. I have tap extractors, but they would not loosen the broken piece. I looked around on the Internet and found a solution - heat water until it is just boiling, and add alum. This is a powder you can find in the spice department at almost any grocery store; it is used for making dill pickles, I think. Keep adding alum until it seems it won't dissolve any more. The piece I had to immerse was 4" long and 5" diameter. You must use aluminum, glass, or a ceramic container. I used a crock pot, and I used about 9 or 10 of the small containers. My piece cooked for almost 24 hours, but I found out toward the end that if you keep it stirred it goes faster. What happens: the alum actually eats the carbon. It did make the brass a little dingy, but the deposit comes right off.

Another problem was locating material. No single supplier had all the brass I needed - most do not carry 1/2" thick brass to begin with. I ended up getting material from 3 different places - 5, counting the Oak and the Stainless Steel hardware.