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Advice on cutting a focuser opening

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#1 epenna

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Posted 23 October 2019 - 12:14 AM

I've got a 6" dob and I am trying to upgrade the focuser. I bought a crayford style 1.25", but it requires me to widen the focuser hole. 

 

I've been trying to figure out the best way to mark the circle I need to cut, but since i have never done this before, I have a feeling I am not thinking this through correctly. I'm leaning towards cutting out a paper circle that is just smaller than the hole I want to cut, place it carefully, mark around it, and then cut. 

 

Does that make sense? The original hole is kind of jagged, so I am not sure how to find the exact center of the current hole. 

 

Thanks, 

E


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#2 BGRE

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Posted 23 October 2019 - 01:05 AM

If its cut into a flat surface that will work. If the surface curvature is significant the result will be a hole that only a tube with a matching elliptical cross section will fit.
The "center" of the hole should probably lie on the optical axis of the old focuser.

#3 jtsenghas

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Posted 23 October 2019 - 06:33 AM

Do you have, or can you borrow a hole saw set and hand drill?  You may be able to set up with two hole saws with the smaller one shimmed or offset with a nut to act as a guide or pilot bit in the original hole if the tube is made of sufficiently soft material.

 

The good news is that it is not necessary at all to maintain concentricity with the original hole.  Trivial rotations of the secondary mirror during collimation can be made to point the primary and focuser axes at each other.  If you would prefer your focuser to be at a slightly different angle than the original, you can even take this as an opportunity to offset the hole in angle by whatever the difference of your hole diameters are.  The important thing is to place your hole the correct distance along the axis of the tube  for your optics.  You just need to be able to round and center your secondary mirror in your focuser and have adequate focusing range once collimated.

 

What material is your tube made of?  There are a number of ways to do this.  You can even cobble up a marking gauge that will mark an opening a certain amount larger than the original hole by using that original hole as the reference, even if it is jagged.  All of the work could even be done with files and rasps, though it would be best to remove both mirrors if your tube is in fact metal.


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#4 Migwan

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Posted 23 October 2019 - 06:47 AM

Might ask around to see who has a drill press.   That and a hole saw is how I would go.   Good luck.  jd



#5 spacemunkee

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Posted 23 October 2019 - 07:42 AM

Can you back out the drawtube enough to get the focuser base to set agaisnt tube?
If so could eyeball in place and mark the inside with a sharpie. Tape to a pencil or something if needed to reach deep enough. Then just cut outside circle the thickness of drawtube.

#6 Garyth64

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Posted 23 October 2019 - 09:53 AM

You can use the two hole saw with a hand drill, as mentioned.

 

One hole saw fits into the other on the arbor.  The one is the diameter of your existing hole, the second one is the diameter of your new hole.  Piece of cake. 

 

I'll look for the pictures I have posted here on CN before.

 

Double saws a.jpg

Double saws c.jpg

Double saws.jpg


Edited by Garyth64, 23 October 2019 - 10:05 AM.

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#7 MitchAlsup

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Posted 23 October 2019 - 10:02 AM

Hole saw and drill

or

just use a dremel tool and a rotary burr.

 

As to marking the hole, just use a pencil and do it by hand.

 

AFTER the hole is made and the focuser fits the hole, then drill the attachment points using the focuser as a template.



#8 macdonjh

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Posted 23 October 2019 - 12:46 PM

You can use the two hole saw with a hand drill, as mentioned.

 

One hole saw fits into the other on the arbor.  The one is the diameter of your existing hole, the second one is the diameter of your new hole.  Piece of cake. 

 

I'll look for the pictures I have posted here on CN before.

 

attachicon.gif Double saws a.jpg

attachicon.gif Double saws c.jpg

attachicon.gif Double saws.jpg

That's what I was going to suggest, but I've never tried it so I didn't know if the threaded portion of the arbor is long enough for two hole saws.  Now I know.  Thanks.

 

epenna, a hole saw with an arbor is $20 or less at Home Depot, and a hole saw without an arbor is about $10.  The arrangement Garyth64 suggests will cut a nice round hole for your new focuser, centered where your old focuser is.  It will make for a very tidy installation.  The hole saws will cut anything your scope tube is likely made of (steel, aluminum, Sonotube, etc.).

 

It's counter-intuitive, but I suggest using your drill at its highest speed if you use a hole saw.  High RPMs and light pressure (so you're almost filing with the hole saw rather than cutting) will give you a clean hole and minimize the chances of the hole saw "grabbing" and tearing into your OTA.  

 

Oh, and your paper template idea is excellent for locating the screw holes for your new focuser.  Of course, you could also simply hold your new focuser in the hole and mark the screw locations with a felt-tip pen or pencil (if you've put tape on the OTA first).


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#9 epenna

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Posted 23 October 2019 - 04:19 PM

Excellent advice! Thank you all!

 

I think the two hole saw looks easiest.

E


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#10 darkcloud

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Posted 25 October 2019 - 01:09 PM

Here's how I enlarged my focuser opening: https://www.cloudyni...y/?p=9138093.  


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#11 Garyth64

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Posted 25 October 2019 - 01:21 PM

Here's how I enlarged my focuser opening: https://www.cloudyni...y/?p=9138093.  

That will work very well, if someone doesn't want to mess with my suggestion, maybe better.  waytogo.gif


Edited by Garyth64, 25 October 2019 - 01:23 PM.


#12 Volvonium

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Posted 25 October 2019 - 02:48 PM

I recently had to do the same when upgrading a cheap plastic focuser in one of my small scopes to an all metal one-- I had to increase the hole diameter by 2mm and also grind the channel for the rack out a little bit.  With the tube stripped and bare (you don't want flying metal shavings going into your mirror), I simply used a marker on the tube to draw how much i needed to open it up and then used dremel with a grinder bit to remove the material.  It's not too difficult to maintain a curve and overall circular shape if you take your time while grinding.

 

It is far more important to precisely drill the holes for thefocuser baseplate, to ensure that when you mount it, the drawtube is centered and on the same optical axis as the old one.  While you can make adjustments to the secondary to compensate any slight misalignments, it's better to get the baseplate mounting holes done right and as centered to the original as possible.  

 

The hole for the drawtube only needs to be large enough for the drawtube to stick through; its concentricity is mostly irrelevant since the drawtube is a perfect circle and you're looking through that, rather than the hole you enlarged


Edited by Volvonium, 25 October 2019 - 02:54 PM.


#13 epenna

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Posted 01 November 2019 - 05:10 PM

Update:

 

After some consideration, I decided to use the dremel, as I only had to widen out the drawtube opening a few millimeters. 

 

Drilling the new holes for the focuser baseplate was just a bit more challenging in terms of getting the fit right. 

 

Love the new crayford 1.25" focuser: this will be my main planetary scope, so I am glad for the improved focuser action. 

 

Every upgrade creates a new problem: Now I have to move the primary up about an inch so that some of the eyepieces I prefer can come to focus... luckily this is an easy fix.

 

Again, thanks for all the advice.

E




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