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DIY 2" helical crayford focuser - materials...?

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#1 furrysocks

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Posted 28 January 2013 - 12:15 PM

I've mocked up a prototype - photo will follow. I've got a single thickness of 18mm ply with four skateboard bearings (I think they're 808s, I forget) screwed into it at an angle.

I've been using a length of PVC drainage pipe as the drawtube, into which a cheapo 2x barlow body friction fits perfectly - which would be my 1.25" EP holder. There are a couple of problems with the PVC pipe... it's not perfectly round so friction in rotating is not even, and it's not perfectly straight so I'd want to spread the bearings over a greater vertical range (eg two thicknesses of 18mm ply) to minimise wobble. Whatever - PVC pipe is probably not the best material.

However, given an 8.5" f/7.6 scope, I really could do with a 2" focuser. I'm currently looking on eBay at 54mm and 57mm aluminium straight pipe for automotive coolant systems. They quote wall thickness of 1.6mm. In imperial measurements, that approximates 2 1/8" and 2 1/4" pipe, with wall thickness of 1/16".

Now, EP nosepieces are 2", or 50.8mm, I assume. I'd drill a hole in the tube and epoxy the thumb lockscrew's nut to the outside, so I don't need great wall thickness or internal clearance.

I've got two options... go with the 54mm which should be "spot on" for size, or the 57mm which I would have to shim internally to fit.

I have a lathe but no tooling, so I could conceivably turn the pipe and sand/abrade the internal diameter if necessary.

I guess the question is less "will it fit", as there is really only one way to tell/test, but more "is it easier to possibly (and marginally) make it fit by buying the 2" internal diameter pipe, or plan from the start to shim a 2 1/8" internal diameter pipe?"

Any thoughts?

#2 StarStuff1

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Posted 28 January 2013 - 02:54 PM

You have learned through experience that PVC pipe is nowhere near perfectly round. Some work with a lathe is almost always needed.

Precision 2-in ID aluminum tubing is available from several vendors.

#3 furrysocks

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Posted 28 January 2013 - 04:02 PM

I might have saved money by realising I needed this when I bought the lengths of aluminium channel for the trusses.

#4 ccs_hello

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Posted 28 January 2013 - 05:44 PM

Although this camera lens helicoid semi-DIY is for refractor (due to its long length), the helicoid section may still be quite useful.

BTW, 2" eyepiece port's dimension is 48mm.

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#5 furrysocks

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Posted 28 January 2013 - 06:09 PM

BTW, 2" eyepiece port's dimension is 48mm.

:shocked: Are you saying a 2" EP fits in a 48mm hole - or what is the EP port?

I haven't heard of helicoid. I'm assuming regular helical focusers rotate the EP - I guess you don't want that with a camera, for example... or star diagonals. Still... tubing is reusable and bearings cost pennies.

#6 furrysocks

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Posted 28 January 2013 - 06:21 PM

Here's my prototype.

Attached Files



#7 ccs_hello

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Posted 28 January 2013 - 07:15 PM

Sorry, thought the wrong thing. 48mm is for thread to mount a filter.

#8 ccs_hello

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Posted 28 January 2013 - 08:00 PM

... I'm assuming regular helical focusers rotate the EP - I guess you don't want that with a camera, for example...


Indeed that is true. This is why a double helicoid is used (one coarse thread for movement; and a much finer-pitch, counter-rotation-direction thread on the other side to defeat the lens itself being rotated.

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#9 furrysocks

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Posted 28 January 2013 - 08:01 PM

Ah, ok - thanks. I've still to get a 2" EP into my hands.

#10 careysub

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Posted 30 January 2013 - 06:16 AM

Aircraft Spruce has 2.125" aluminum tubing, wall thickness 0.065" (ID 2.120") and 0.125" (ID 2.000") for about $8 for one foot.

http://www.aircrafts...tube_6061t6.php

#11 furrysocks

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Posted 30 January 2013 - 06:24 AM

Cheers - I'm in the UK. "Automotive" pipe available at about that price. Aluminium wholesalers is more like £12+P&P for 8'. I'll get something and make it work... ;)

So far I've worked out that I want something without a seam to make my life easier, and ideally anodized but for a DIY job, happy with naked.

Need a 2" EP in hand before I start.

#12 furrysocks

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Posted 30 January 2013 - 12:27 PM

For better or worse, I've ordered the raw material for the drawtube - expecting it next week.

54mm x 1.6mm x 1000mm aluminium pipe

#13 astrobeast1

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Posted 30 January 2013 - 02:01 PM

well that would be for the better as opposed to plastic tube

#14 furrysocks

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Posted 30 January 2013 - 03:23 PM

Indeed!

How accurate, whether there's a seam, etc... I don't know. Some minor work expected, perhaps none required...?

#15 furrysocks

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Posted 01 February 2013 - 11:56 AM

Out of stock - will find another source.






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