From: "John Murray" 
To: "'Mirror Matic'" ,
        "Amateur Telescope Makers" 
Reply-To: "John Murray" 


G'day Everyone,
        I've started breaking in my new 5" tool today. I've noticed that
it does not take much for the tool to start slowing down and grabbing
slightly. IE it spins and then slips over and over until finally I put more
grit on with water. I seem to be using a lot of grit. I'm using about 5kgs
on the overarm removing this helps slightly. The swivel itself seems fine.
I'm using a stationary overarm with around 5mm of overhang and a wag of
around 1/4".
        I'm wondering if this could be caused by one of the following.
        1)      With the centre sag reading 0.10mm and the outer ones
0.13mm I am getting a vacuum lock. (spherometer distance is 64.5mm) ie sag
of 0.10 mm and diameter of 64.5 for the measurement. I figure f of approx
13 in centre and f 10 on the outer regions.
        2)      The new tool simply needs more than the two hours (so
far) to match the sag from the previous tool that self destructed.
        3)      I tried a few wets sweeping the blank going approx 5mm
over the edge of each side. This seemed to increase the outer sag readings
while leaving the centre the same. I didn't expect this.
        4)      Perhaps the tool itself is just too small for this
hogging out. 8" blank, 5" tool. 72 rpm turntable and 21rpm eccentric speed.

        I have cast tonight a 160m tool with no problems, (as reported
in an earlier email tonight) I'll try this tomorrow after curing and see
what effect this has. Any suggestions?


Cheers

John Murray
Whyalla
South Australia
Our ATM page is at
http://astronomy.sa86net.com

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