2 inch EP / Filter - what thread size?

Greg..
dia, of filter is 49mm and TPI is 0.75mm.

Jim

Greg Campbell wrote:

Just unwrapped my first 2" filter and was surprised to see that my
budget 50mm Plossl isn't threaded! I have access to friendly machinists
(beer makes a great bribe) who can remedy the problem. What is the
"official" thread size/pitch??

Thanks!

-Greg

I believe that it is a 48mm x 0.75mm thread.

How can "threads per inch" be expressed as millimeters? :-)

Del Johnson



"Jim Barclay" wrote in message
...
Greg..
dia, of filter is 49mm and TPI is 0.75mm.

Jim

Greg Campbell wrote:

Just unwrapped my first 2" filter and was surprised to see that my
budget 50mm Plossl isn't threaded! I have access to friendly machinists
(beer makes a great bribe) who can remedy the problem. What is the
"official" thread size/pitch??

Thanks!

-Greg

Correct. Thread pitch (p) is expressed as a distance, and can be measured as
the axial distance between thread roots. Technically, it is measured at the
maximum pitch diameter, but it comes out the same. If the profile is double
threaded, you'd have to count two. Threads per inch (n) is the inverse of pitch
(n=1/p). So in this case, n would be approximately (0.75 mm)^(-1) * 25.4
mm/in=33.9 threads/inch.

The basic diameter, in this case 48mm, is equal to the maximum major diameter
for external threads, and the minimum major diameter for internal threads. This
is not the same as the size of the "bore" for internal threads, which is
actually somewhat {[0.06*(p^2)^(1/3)]=0.05mm} larger to allow for tolerances.

If you have access, you can find out more than you ever wanted to know about
threads from the military specification MIL-S-8879C.

Eric Albrecht
e2 Telescope Design
http://www.e2scopes.com


Del Johnson wrote:

I believe that it is a 48mm x 0.75mm thread.

How can "threads per inch" be expressed as millimeters? :-)

Del Johnson



"Jim Barclay" wrote in message
...
Greg..
dia, of filter is 49mm and TPI is 0.75mm.

Jim

Greg Campbell wrote:

Just unwrapped my first 2" filter and was surprised to see that my
budget 50mm Plossl isn't threaded! I have access to friendly machinists
(beer makes a great bribe) who can remedy the problem. What is the
"official" thread size/pitch??

Thanks!

-Greg