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how I make a pitch lap

Started by Phil Barker, 11/02/2012 06:57PM
Posted 11/02/2012 06:57PM Opening Post
A few years ago i got hold of a large quantity of rosin or pine resin.

I make up pitch by combining with stockholme tar which is used for dehorning cattle believe it or not.

A standard brew is a 7/1 ratio which I heated to melting point stirred and then put into little casts lined with foil.

I then end up with what looks like blocks of toffee which I wrap in tinfoil and store in a container. They can last for years in a cool place.

I take care to ensure they are the same thickness around 5-6mm.

I simply heat a knife over a candle and and cut them squares which are placed on a tool to form a lap.

I usually make a slightly softer brew for figuring tools which are subdiameter.

Pitch needs to be sealed and tends to harden is exposed to the air from my experience. By wrapping in foil and keeping in a sealed tin in a cool place it seems to stay fresh.

Posted 11/02/2012 11:12PM #1
Thanks for the info, Phil.

I just made a pitch lap for polishing my secondary mirror.

I'm about to make a pitch lap for the 12.5" primary I'm working on.

Clears,
Joe

In lumine tuo videbimus lumen.

8O Home-made 10” Dob / Home-made 4” refractor

EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm
Posted 11/03/2012 11:32AM #2
interestingly the recipe for the tar came from Gary Nankivell one of the great telescope makers.

He built the optics for the 1 metre f-6 corrected dall kirkham telescope at Mount John This was in the 1970's possible the first of its type in the world. He also did work on a 9.5 inch replacement doublet lens for the Cooke triplet at carter observatory before his death in 2001.

The origional lens had an unstable element that needed regular refiguring and was no longer useable.
There are still some of his 10 inch f-7 newtonians around which are suberbly corrected. I was lucky enough to learn a few things from him before his passing.

RE making the molds for the pouring of the hot pitch I recommend using tinfoil pressed into a master mold to replicate it. generally I make blocks around 6mm thich and 3 inches by 1 inch.

I can line up the foil molds ready to go. I use a spoon to ensure I put the same ammount of pitch in each 3 desert spoonfuls last time I did this.

good luck