Re: [ATM] Upgrading a battered 10" Newt focuser

[Bob <astropabob@yahoo.com.au>]

Hello Martyn,
	
Martyn Cole wrote:

<snip>

> I need to increase the size of hole for the drawtube from 50mm (1.97")
> to 60mm (2.36").  The wall thickness of the tube is around 1.5mm (0.06")
> and, from the battered look of the tube, easily damaged.

I had to do this very job not long back. I did it without buying any
new tools since I already had the two files in my workshop.

 > I'd like some help in deciding what tools I should use to enlarge the
 > hole, bearing in mind I'm not known for my metal working skills :)

A fine "half-round" file and a fine flat file.
Also a compass (for drawing circles).

My method:
Push the compass point into a piece of stiff card and draw a circle the
same size as the original hole in the scope tube. Tape the card inside
the tube aligning the circle with the hole in the tube.
This gives an accurite centre for the original hole.
Draw a 60mm circle on the tube with your compass and remove the
card as it's no longer needed. (I used a fine line felt tipped pen in the
compass to draw on the steel tube.) Use the corner of the flat file to
make "shark's teeth" like cuts _almost_ down to the line all the way
around the circle. This operation took me 10 mins or so. Then use the
"half round" to file the shark's teeth off and down to the line and tidy
up the circle. Also took about 10 minutes.
Cheap tools, half an hour, and a nice finish.

Good luck.

Bob

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