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Best way to enlarge focuser hole on Starblast 6 OTA?

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#1 Phillip Creed

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Posted 23 May 2018 - 09:01 AM

I just purchased a used 6" f/5 Orion Starblast.  The plan is to replace the 1.25" Starblast focuser with a 2" GSO dual-speed focuser.

What's the best method to enlarge the focuser hole?  Aviation snips?  Hole saws?  Besides removing the optics, anything else I should do to the "patient" prior to "surgery"?  I've got access to a few local machine shops, but I'm trying to find out what's involved if I choose the DIY route.

Clear Skies,

Phil



#2 howardcano

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Posted 23 May 2018 - 09:57 AM

How about a nibbler?:

 

https://www.ebay.com...i4AAOSw3UZa2spT



#3 JGass

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Posted 23 May 2018 - 09:58 AM

Maybe these nibblers?



#4 tomykay12

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Posted 23 May 2018 - 10:22 AM

Probly a Dremel tool.


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#5 Ishtim

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Posted 23 May 2018 - 10:26 AM

I made a paper template with concentric rings for old and new holes, centered it, taped it down and used a Dremel (steel tube scope)

.



#6 epee

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Posted 23 May 2018 - 10:38 AM

Dremel with a cut-off wheel sounds like the way to go. Of course remove the optics. Forming a loose pouch of painters tape underneath the area you're cutting and drilling will help minimize clean-up inside the tube. Simply apply long pieces of wide tape across the work area, but leave the middle portion, directly under the area you'll be cutting, slack so that you do not cut the tape. This way any metal dust will adhere to the tape without getting all over the inside of your tube. 


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#7 Phillip Creed

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Posted 23 May 2018 - 10:43 AM

Many thanks on the suggestions.  Seems like the Dremel tool is the way to go.

 

Another question--should I center the new focuser and mark/drill the four mounting holes before enlarging the focuser hole?  Seems to be the better route to ensure alignment with the existing secondary, but I was just curious what others have done.

Clear Skies,

Phil


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#8 kfiscus

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Posted 23 May 2018 - 11:10 AM

Here's another way to enlarge the hole.  This method isn't cheap but can make a nicer cut and do it in one step.  This method works very well on sonotube and fiberglass, your thin steel tube can also work nearly as well.

 

The key is using 2 hole saws that have been "nested" together on the same mandrel.  The inner one is a little larger than 1.25" and has to fit the existing hole nicely.  It guides the slightly larger than 2" hole saw.  I would run this rig BACKWARDS and at high speed to get the smoothest cut.  Even better results would be gotten if you made a sacrificial chunk of 2x4 wood that was curved to fit the inside of the OTA and had it firmly duct taped inside the tube to help stabilize the mandrel.  You're only cutting/wearing your way through 1/16" of steel.  Since it's curved, you're only cutting a little of the tube at a time and making a lot of sawdust at the same time.  Run a vacuum and then use a strong magnet inside a sandwich bag to collect steel filings.  Good luck.


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#9 drneilmb

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Posted 23 May 2018 - 11:12 AM

Many thanks on the suggestions.  Seems like the Dremel tool is the way to go.

 

Another question--should I center the new focuser and mark/drill the four mounting holes before enlarging the focuser hole?  Seems to be the better route to ensure alignment with the existing secondary, but I was just curious what others have done.

Clear Skies,

Phil

That's the order I would use. First get it located where it needs to be, then cut out the big hole to let the drawtube extend inside the tube.

 

And centered templates and multiple hole saws might make it pretty, but the hole is just a hole big enough for the drawtube to pass through and it's completely covered up by the focuser base. You could probably cut it with a rusty nail and have the same level of performance. ;)



#10 Garyth64

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Posted 23 May 2018 - 08:01 PM

Here's another way to enlarge the hole.  This method isn't cheap but can make a nicer cut and do it in one step.  This method works very well on sonotube and fiberglass, your thin steel tube can also work nearly as well.

 

The key is using 2 hole saws that have been "nested" together on the same mandrel.  The inner one is a little larger than 1.25" and has to fit the existing hole nicely.  It guides the slightly larger than 2" hole saw.  I would run this rig BACKWARDS and at high speed to get the smoothest cut.  Even better results would be gotten if you made a sacrificial chunk of 2x4 wood that was curved to fit the inside of the OTA and had it firmly duct taped inside the tube to help stabilize the mandrel.  You're only cutting/wearing your way through 1/16" of steel.  Since it's curved, you're only cutting a little of the tube at a time and making a lot of sawdust at the same time.  Run a vacuum and then use a strong magnet inside a sandwich bag to collect steel filings.  Good luck.

I have enlarged holes this way many times.  I have also used hole saws this way to enlarge existing holes on my telescopes.  I do it with a electric hand drill, just making sure the tube doesn't move.

I will be doing it again soon, because I am replacing a 1¼" focuser with a 2", just like Phillip.

It actually takes more time prepping everything before the hole is made.  Once everything is set, it's zip-zap, and it's done.  It is easy for someone who has done it many times.  For a first-timer, it may be a bit scary


Edited by Garyth64, 23 May 2018 - 08:01 PM.


#11 Garyth64

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Posted 24 May 2018 - 08:46 AM

Here's a picture of the set-up for clarification:

 

The inner saw must be the diameter of the existing hole in the tube.  It must be a snug fit.  The outer saw is the diameter of the new hole you want to put in it.

Attached Thumbnails

  • Double saws.jpg

Edited by Garyth64, 24 May 2018 - 08:51 AM.


#12 Pinbout

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Posted 24 May 2018 - 09:42 AM

Here's a picture of the set-up for clarification:

 

The inner saw must be the diameter of the existing hole in the tube.  It must be a snug fit.  The outer saw is the diameter of the new hole you want to put in it.

I setscrew the smaller hole saw backwards so no risk of teeth catching

 

gallery_106859_3508_21253.jpg


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#13 macdonjh

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Posted 24 May 2018 - 09:53 AM

That dual hole saw trick is cool.  I don't know if my mandrel will accommodate two hole saws, though.  Now I have to check.

 

My thought was: if a drill press is available, use a paper template to center the OTA under a single 2" hole saw on the drill press and then cut the new hole.  You'd have to secure the OTA on the drill press to keep it from moving to make a clean cut.  Of course, you'll have to do that with the dual hole saw method, too.



#14 Garyth64

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Posted 24 May 2018 - 10:25 AM

These are the parts I use, there are other types of hole saws, and their assembly may vary:

 

Double saws a.jpg

 

The hole saws shown here are 1-3/4" and 2-1/4".

 

The large one screws onto the arbor first, (you do not have to use the locking pins that push up into the saw.  If you do, it will not work.)

Double saws b.jpg

 

And then the smaller one is screwed in:

Double saws c.jpg

 

There are different types of arbors, they could have 1/2" diameter, or a 3/8" diameter.  I don't use a drill press, just a hand drill.  Either way, the tube must be secure.

 

Once you do it, you'll be surprised how easy it was.


Edited by Garyth64, 24 May 2018 - 10:27 AM.

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#15 Phillip Creed

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Posted 24 May 2018 - 11:12 AM

Here's another way to enlarge the hole.  This method isn't cheap but can make a nicer cut and do it in one step.  This method works very well on sonotube and fiberglass, your thin steel tube can also work nearly as well.

 

The key is using 2 hole saws that have been "nested" together on the same mandrel.  The inner one is a little larger than 1.25" and has to fit the existing hole nicely.  It guides the slightly larger than 2" hole saw.  I would run this rig BACKWARDS and at high speed to get the smoothest cut.  Even better results would be gotten if you made a sacrificial chunk of 2x4 wood that was curved to fit the inside of the OTA and had it firmly duct taped inside the tube to help stabilize the mandrel.  You're only cutting/wearing your way through 1/16" of steel.  Since it's curved, you're only cutting a little of the tube at a time and making a lot of sawdust at the same time.  Run a vacuum and then use a strong magnet inside a sandwich bag to collect steel filings.  Good luck.

I doubt it's even anywhere near that thick.  From memory, I think the tube thickness is maybe 1/32" based on how it looked when I removed the primary mirror.  Not a surprise, considering it's only a 6"-scope.

 

Clear Skies,

Phil




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