Thursday, May 27, 2021

Turning Down a Piece of Stock Too Large for Your Lathe Chuck

Why would you do this?  My wife and I were moving the 5" refractor to the new telescope garage.  It has ScopeRollers on it, but they really are not intended for rolling across uneven lawns.  I wanted to take the telescope off the mount, then the mount off the tripod, and move each individually.  My wife disagreed and you all know what happens when you are right and your wife is wrong.  Even if you are right, the consequences of insisting is worse than what usually happens.

So the telescope tipped over and my finger ended up somewhere that a few fractions of an inch further would have broken the finger under the mount. The mount and telescope suffered no damage (except to their dignity), but the cast aluminum finderscope mount broke.  They were two clean breaks so I was able to epoxy it back together and it is close enough to the original to work.  (The epoxy filled in one of threaded holes for the adjustment screws well enough that I had to buy an m3x0.5 tap to clean up the threads; turning a steel screw did not quite clean the epoxy out.  Epoxy is harder than I thought.)   This is an orphan scope, so no replacement finderscope brackets are available.  Maybe sitting on a shelf somewhere in China....

Still, it looks ugly and I found myself thinking, "I can machine a replacement."  Of course, that means two rings (2.61" OD and 2.20" ID), in which the finderscope rides.  The obvious choice is aluminum.  It is a bit slow to machine on the Sherline, so I making itb in acetal.  

But acetal tubing in small quantities is pretty expensive.  I decided to buy a piece of round acetal rod, turn it down to 2.61", then use a Forstner bit to bore a 2 1/8" hole and a boring bit for the remaining .02".

I had access to a 3.25" piece of round acetal rod very cheap and without shipping time.  (From the scrap bin at Interstate Plastics in Boise.)  My biggest chuck accepts 3" OD pieces, which I had forgotten when I bought this.  (Yes, I have a 6" 4-jaw lathe chuck that will go on the Sherline, but it is a nuisance to set up for making one piece: riser blocks for the headstock, tailstock, and toolholder.) While looking for a jig to hold this piece of 3.25" round stock in position on the chop saw, I found my forgotten box of acetal which included a 2.75" piece of acetal to fit in the 3" chuck.

So what follows is a suggestion for anyone in the same position who does not find that smaller workpiece.  It should work.  If you are a machinist who has been led here by a search engine, tell me in the comments how well it worked.  

1. Start out with a center finder to put a pilot hile in the center.  

2. Use a drill press to put a hole smaller than your target ID through the workpiece.  (Probably a lot smaller because concentricity and runout are going to make the resulting hole less than perfectly centered.)  

3. Now mount the workpiece on the interior jaws of the chuck.  

4. Turn the outside diameter to roughly your target diameter.  Now your workpiece fits in the exterior jaw configuration.  Bore that center hole to get a properly centered and correct ID.

1 comment:

  1. If you need to use a screw as a thread cleaner, cut a slot across the threads....2 is better....alternatively, mill some very slight flats on 3 sides so you have somewhere for the chips to go.