Wednesday, December 12, 2012

The Cheapskate in me Comes Out!

Some of you saw my new telescope mounting a few weeks ago and found something definitely non-standard about it!  The photo at left shows it.  While I got a good deal on the mount, the owner kept his pier, counterweights and saddle plate for his new mount - I was on my own to replace them.   Fortunately, Dean Koenig of Starizona knew a machinist to make me 2 piers (!), one for my C-14, a tall version, and a shorter version for a Newtonian telescope.   I decided to go to Astro-Physics for the saddle plate.  But for the counterweights, I drew the line.  AP wanted big bucks for stainless steel weights - and really, they are just weights!  You could hang a toolbox off the end of the bar, as long as it balanced the tube assembly across the right ascension axis...  I tried to find a less-expensive alternative.  The weight by Roger's left hip is the result!

After looking at a couple options, I decided to take a Mirror Lab engineer's advice and get some regular steel water jet cut to rough shape, so that it took minimal machining to clean up.  And while the AP stainless weights were pricey, I copied their design by lining the hole with brass so it wouldn't scratch the stainless declination axis on which they mount.  There would also be a brass pin that is pushed by a screw to tighten it on the shaft - which also wouldn't scratch the shaft. 

While I don't consider myself a machinist, I can use most of the equipment in our little machine shop at work in a pinch.  There wasn't anything complicated about anything here, and it adds a little to the satisfaction I'll get when I use it.  The lathe is a little wimpy - it has a loose drive belt and won't take much of a load, slowing down the process some, but that is ok. 

Here are the steps I used.  First, I had some 6.125" circles water jet cut out of some 1" and 2" thick steel plate at A-1 Fabrication here in Tucson.  That step also included cutting a 1.875" hole in the center.  I figured that would come out to about 90 pounds of counterweight, which should balance about  120 pound load on the mount - well within its capacity, and more than all the telescopes I currently own.  The rough-cut disks are shown at left.  The first step is to grab it by the center hole and turn the outside diameter.  The machine marks cleaned up enough for me to stop at about 6.050" OD.

Next was to grab it by the OD and open up the OD to 2.00", which matched the OD of a piece of brass tubing I bought to line the ID.  Given that I'm new to machining steel, the surface roughness and accuracy of the lathe prevented a super-accurate fit, so after machining to a few thousandths of an inch, I cut the brass tube to the right length.
Since I couldn't do a true "pressed fit", I did the next best thing and glued the brass insert in place using J-B Weld to permanently hold it in place.  After an overnight cure, and machining the excess brass down to the steel dimensions, it was time to mount the locking screw and pin.  First step was to bore the right size hole.
The hole size is the correct hole for tapping a 3/8X16 threaded hole for the locking screw.  It helps to get the tap started using the drill press so it is perfectly aligned.  Then you loosen and back off the chuck and finish the job by hand.
Then it is time to trim down the brass rod I bought to the right diameter to fit the tap hole that i drilled in the weight.  Cut it to the correct length so it barely extends into the hole with the screw tightened down and you are done!  I can process a single weight in about 1.5 to 2 hours of after-hours activity, and am currently about half through my pile of weights.  Since these are regular steel that will rust, I'll likely sandblast them, prime and paint them before considering them finished.
So how much did I save?  Well, AP doesn't make 7 or 15 pound weights, but interpolating from the 5, 10 and 18 pound weight they do make, my weights would have run about $1150.  For this project, my water jet steel cost $550 for material and cutting, and my brass tubing and rod cost another $50 for a total of $600 - so about half price from the commercial version.  Of course, I should add something for my time, but as a hobbyist, it is hard to justify doing that, plus I had a little fun and some learning along the way.  Given what I've learned about how long it has taken for me to do the work, I'd think harder about repeating the experiment, but since I saved some money, I'm glad I did it!

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