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Plastic Focusers

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#1 mic1970

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Posted 19 February 2019 - 06:33 PM

Anyone ever made their own focusers out of metal to replace plastic one?



#2 steveastrouk

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Posted 19 February 2019 - 06:56 PM

Anyone ever made their own focusers out of metal to replace plastic one?

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#3 mic1970

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Posted 19 February 2019 - 10:40 PM

That is cool... Is it for a newt?

 

Anyone ever made their own focusers out of metal to replace plastic one?

 



#4 steveastrouk

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Posted 20 February 2019 - 08:39 AM

Yes, I made it for a friend. It is dual speed. The front thread is 1mm pitch, the back 8mm pitch. The 8mm pitch is an 8 start thread, so its very compact


Edited by steveastrouk, 20 February 2019 - 10:40 AM.


#5 BillB9430

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Posted 20 February 2019 - 10:40 AM

Well, I guess my focusers don't quite qualify, since they are metal AND plastic. Like steveastrouk, I like helical focusers, but find that ones made with both parts aluminum need a coating of lubrication on the threads to provide smoothness and prevent aluminum-to-aluminum galling. Unless an additional sleeve is installed over the threads to keep fingers out of the grease (like individual-focusing binocular eyepieces), the user can have a sticky-fingered problem. The solution I like is to make the focuser barrel from aluminum, but the focuser base from UHMW. That is an acronym for Ultra High Molecular Weight polyethylene. UHMW is machinable, nonbreakable,  and its natural lubricity provides smooth focusing without applying any grease to the threads. 

 

I'll attach a photo of some 2" and 1.25" focusers I've made for RFT refractors I'm in the process of building. The multiple start threading is 4 start with a lead of 4.5 TPI and pitch of 18 TPI. It can be cut on a 1/8" wall aluminum tube and for 2" eyepiece focusers a 1" thick piece of UHMW 3" in diameter can be bored and threaded to make the base. Nylon screws in the focuser tube and UHMW serve to secure the eyepiece and lock the focuser tube when needed. This is a pretty simple focuser and can be made very low profile if necessary. 

 

Unfortunately, cutting multiple start threading requires a lathe, but can be cut even on my old South Bend 9" using the thread dial indicator. It IS possible to make a similar focuser without a lathe by hand cutting a spiral groove in an aluminum tube that is a slip fit in a UHMW base. A spring-loaded teflon plunger then rides in the groove to provide helical focusing action as the aluminum barrel is turned. The 1.25" focuser in the photo with the threaded metal side piece (housing the spring and plunger) is of that design. 

 

I've not seen any other ATM use of this fortuitous combination of materials, but it can provide a very simple, functional, and compact focuser for those able to cut multiple start threads. BTW, I suspect few lathe-owning ATMs have ever attempted cutting multiple start threads, but it is not so difficult as you might imagine if your lathe has a thread dial indicator. It IS hard to find information on just HOW to index the threads, though. - Bill

Attached Thumbnails

  • Helical focusers.jpg

Edited by BillB9430, 20 February 2019 - 11:13 AM.

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#6 steveastrouk

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Posted 20 February 2019 - 11:25 AM

Think I'll have to try your UHMW idea too Bill. Thanks for the tip.



#7 mic1970

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Posted 20 February 2019 - 11:59 AM

That is awesome work, and I would love to figure out how to complete this.  Question:

 

  • One thing I don't get for a refractor is how does the focuser work without spinning the diagonal and eyepieces. 
  • I'm not sure what it is called, but what are you doing for the sleeve that goes around the focuser tube that connects to the telescope tube. 

Well, I guess my focusers don't quite qualify, since they are metal AND plastic. Like steveastrouk, I like helical focusers, but find that ones made with both parts aluminum need a coating of lubrication on the threads to provide smoothness and prevent aluminum-to-aluminum galling. Unless an additional sleeve is installed over the threads to keep fingers out of the grease (like individual-focusing binocular eyepieces), the user can have a sticky-fingered problem. The solution I like is to make the focuser barrel from aluminum, but the focuser base from UHMW. That is an acronym for Ultra High Molecular Weight polyethylene. UHMW is machinable, nonbreakable,  and its natural lubricity provides smooth focusing without applying any grease to the threads. 

 

I'll attach a photo of some 2" and 1.25" focusers I've made for RFT refractors I'm in the process of building. The multiple start threading is 4 start with a lead of 4.5 TPI and pitch of 18 TPI. It can be cut on a 1/8" wall aluminum tube and for 2" eyepiece focusers a 1" thick piece of UHMW 3" in diameter can be bored and threaded to make the base. Nylon screws in the focuser tube and UHMW serve to secure the eyepiece and lock the focuser tube when needed. This is a pretty simple focuser and can be made very low profile if necessary. 

 

Unfortunately, cutting multiple start threading requires a lathe, but can be cut even on my old South Bend 9" using the thread dial indicator. It IS possible to make a similar focuser without a lathe by hand cutting a spiral groove in an aluminum tube that is a slip fit in a UHMW base. A spring-loaded teflon plunger then rides in the groove to provide helical focusing action as the aluminum barrel is turned. The 1.25" focuser in the photo with the threaded metal side piece (housing the spring and plunger) is of that design. 

 

I've not seen any other ATM use of this fortuitous combination of materials, but it can provide a very simple, functional, and compact focuser for those able to cut multiple start threads. BTW, I suspect few lathe-owning ATMs have ever attempted cutting multiple start threads, but it is not so difficult as you might imagine if your lathe has a thread dial indicator. It IS hard to find information on just HOW to index the threads, though. - Bill


Edited by mic1970, 20 February 2019 - 12:00 PM.


#8 BillB9430

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Posted 20 February 2019 - 01:04 PM

Hello mic170,

 

The focuser is usable "as is" for Newtonian reflectors by attaching it using machine screws through a board curved to fit the main scope tube and tapped into the bottom of the white multiple-start-threaded UHMW ring. The eyepiece will spin when focusing, but that is no problem, though helical focusers present a little more difficulty with zoom eyepieces. For those it would be best to provide a larger diameter knurled top ring than is seen on my focusers to provide a securing grip when turning the eyepiece "zoom" ring with the other hand. Changing magnification of a zoom would be a two handed operation. I do not normally use zooms, hence the smaller ring. 

 

With refractors, your question is spot-on, since a spinning diagonal is useless. For my refractor builds, I have placed the diagonal prism (or mirror) within the tube, so that the helical focuser holds just the eyepiece at right angles to the main tube. This is similar to the optical arrangement of military "elbow" telescopes. Familiarity with the elbow scopes AND a nice post and photo some time ago by CN member "Mirzam" showing his use of a Newtonian focuser with mirror within the tube for his refractor build provided a start point for my thinking on this modification. My thanks and credit to "Mirzam" for that post.

 

For the attachment of the white inside threaded UHMW base to the flat wood board mounted on the side of the refractor tube, I just tap three 8-32 FH machine screws from underneath into threaded holes in the base of the UHMW "ring".     Hope this helps. -Bill

 

That is awesome work, and I would love to figure out how to complete this.  Question:

 

  • One thing I don't get for a refractor is how does the focuser work without spinning the diagonal and eyepieces. 
  • I'm not sure what it is called, but what are you doing for the sleeve that goes around the focuser tube that connects to the telescope tube. 

 



#9 mic1970

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Posted 20 February 2019 - 01:54 PM

Bill when you have time... can you post some pics?  If yes, please and thanks.  I'm having a problem visualizing the refractor?

 

Hello mic170,

 

The focuser is usable "as is" for Newtonian reflectors by attaching it using machine screws through a board curved to fit the main scope tube and tapped into the bottom of the white multiple-start-threaded UHMW ring. The eyepiece will spin when focusing, but that is no problem, though helical focusers present a little more difficulty with zoom eyepieces. For those it would be best to provide a larger diameter knurled top ring than is seen on my focusers to provide a securing grip when turning the eyepiece "zoom" ring with the other hand. Changing magnification of a zoom would be a two handed operation. I do not normally use zooms, hence the smaller ring. 

 

With refractors, your question is spot-on, since a spinning diagonal is useless. For my refractor builds, I have placed the diagonal prism (or mirror) within the tube, so that the helical focuser holds just the eyepiece at right angles to the main tube. This is similar to the optical arrangement of military "elbow" telescopes. Familiarity with the elbow scopes AND a nice post and photo some time ago by CN member "Mirzam" showing his use of a Newtonian focuser with mirror within the tube for his refractor build provided a start point for my thinking on this modification. My thanks and credit to "Mirzam" for that post.

 

For the attachment of the white inside threaded UHMW base to the flat wood board mounted on the side of the refractor tube, I just tap three 8-32 FH machine screws from underneath into threaded holes in the base of the UHMW "ring".     Hope this helps. -Bill


Edited by mic1970, 20 February 2019 - 01:56 PM.


#10 Mirzam

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Posted 20 February 2019 - 02:58 PM

Hello Bill and Mic170,

 

Here is the thread with the picture that Bill mentions above:

 

https://www.cloudyni...a-jaegers-3-f5/

 

The only challenge that I had with the internal diagonal was providing collimation.  It took some iteration to get it perfect, but it has been locked in place for years now without any problem.

 

JimC



#11 BillB9430

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Posted 20 February 2019 - 03:47 PM

Hello mic1970,

I see Mirzam (JimC) was kind enough to post his thread that I referenced, so you likely can figure out the arrangement now. Thanks, Jim.

 

As I mentioned, though I have the focusers built, I am still in the process of constructing two RFT scopes using these focusers and Jaegers 127/629 mm objectives (5" f/5). I have mounted one of the helical focusers on my "test bed" scope-on-a-stick, so I will attach photos of that for additional clarity. The cardboard tube in the first photo is just set in place temporarily to decrease stray light at the eyepiece when testing in daytime. Imagine the prism, housed in the wooden box on the scope on a stick, instead secured in the back central portion of an aluminum scope tube. The focuser and eyepiece will be affixed above it to allow right angle viewing.   Hope this helps.  - Bill

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  • Scope on stick side.jpg
  • Scope on stick no shroud.jpg

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#12 mic1970

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Posted 20 February 2019 - 04:13 PM

Mind blow (yes I'm new and easily impressed).  Now the question.  How do you mount the diagonal  in the tube.   I assume just like a dob.  I guess the size of the mirror doesn't matter as long as it fits.  Do you have to collimate it regularly ?



#13 BillB9430

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Posted 20 February 2019 - 05:23 PM

Hello Mic1970,

 

As I said, the scopes are still in construction, so I don't yet know if I will have to collimate them regularly. I doubt it, though. I plan to close the back end of the tube with a wooden disk securely screwed to the tube around its periphery. A FS mirror could be attached to a 45˚ cut block with RTV silicone cement and fastened to the center of the back disk. As you stated, the size of the mirror is unlimited, so long as it fits in the tube. This is an advantage, since there will be no blocking of light by an oversize diagonal mirror as occurs in a Newtonian scope. Actually, because I want to use my scope terrestrially and for a correct view of the North America nebula, I plan to use a surplus military Amici prism to give a correct image. I'm making the second scope for friends in Montana who have a panoramic view from their house and lots of local wildlife, so that scope gets another Amici and will serve as a "super spotter". The Mil surplus prisms are already mounted in protective housings with screw attachments at the back, so I will just use those, with either small springs or "O" rings to allow collimation of the diagonal.

 

I can vouchsafe that the helical focusers work nicely, but until the scopes are completed and tested, cannot speak with any authority on collimation difficulties and other problems that may arise.  - Bill 

 

Mind blow (yes I'm new and easily impressed).  Now the question.  How do you mount the diagonal  in the tube.   I assume just like a dob.  I guess the size of the mirror doesn't matter as long as it fits.  Do you have to collimate it regularly ?

 



#14 ww321q

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Posted 21 February 2019 - 07:37 AM

I hate single pointing threads ! Especially internal. 



#15 steveastrouk

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Posted 21 February 2019 - 10:24 AM

I hate single pointing threads ! Especially internal. 

So do I. I also hate gauging internal threads, and I am always looking for neat ways to do it.

EXTERNAL threads, especially if you cut with a full form insert are relatively easy to gauge - three wires is great - but even a rule of thumb that, when you finish, if your external thread is around 4thou/0.1mm smaller than the original stock, you pretty well nail it first time.

 

I've not found GOOD inserts for plastics and Aluminium though, although Warner make a very limited range of HSS inserts.


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#16 don clement

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Posted 21 February 2019 - 12:34 PM

I hate single pointing threads ! Especially internal. 

I get excellent results doing  internal (and external) single point threading in aluminum  using carbide full form  topping inserts.  One does need an  separate insert for each pitch with full form topping but are rewarded with  near perfect thread form and the PD is concentric with the OD or ID.    To get good surface finish with carbide full topping inserts in aluminum  surface speed must be high. I use the reverse helix method where the spindle is going CW and feed is towards the tail stock to get high surface speed  (i.e. 500 rpm spindle speed) on a manual lathe even when threading up from a blind shoulder. Also I use anvils under the insets to accommodate clearance for the helix angle of the thread being cut. Relton A9 cutting fluid with aluminum. Plastics like Turcite or Delrin don't need lubricants when threading. Attached are pictures of an adapter I made with 4.0-16 UN internal threads up from a blind shoulder. I also made a thread gauge for testing while threading.

 

Don

 

Finished adapter

IMG_1406web.jpg

 

Cutting internal thread relief groove

IMG_1403WebInternalSlot.jpg

 

Internal full profile threading tool

IMG_1404WebInsertTool.jpg

 

Thread gauge made for 4.0-16 UN threads

IMG_1405WebThreadGauge.jpg


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#17 steveastrouk

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Posted 21 February 2019 - 01:22 PM

I get excellent results doing  internal (and external) single point threading in aluminum  using carbide full form  topping inserts.

I took single pointing to mean using tools like this one.

0525203-24.jpg


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#18 mic1970

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Posted 21 February 2019 - 01:44 PM

Awesome idea.... now where can I find one.  Oh.... Dr. Google? 

 

I would love to hear updates on how it is going. 

I plan to use a surplus military Amici prism to give a correct image.


#19 don clement

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Posted 21 February 2019 - 05:22 PM

I took single pointing to mean using tools like this one.

0525203-24.jpg

 

The full profile topping inserts I use are for single point threading only they have a cutting surface at the back end in which when you get to the proper depth of cut also cut the OD. In addition for each thread pitch the single tooth form cutter is the right profile for that pitch. In both cases  the full profile carbide inserts or the tool you show if carbide need fast surface speeds to get a good surface finish. When I use the recommended cutting speeds for full profile inserts there are no burrs left or cleanup needed. This is my internal threading tool. Notice that it is backwards from the threading tool you show. That is because it is used on the back inside wall with the spindle rotating CW and feed is towards the tail stock for RH threads.

 

Don

Attached Thumbnails

  • IMG_1404WebInsertTool.jpg

Edited by don clement, 22 February 2019 - 03:25 AM.

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#20 steveastrouk

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Posted 23 February 2019 - 07:45 PM

Someone asked me by PM what CNC threading looks like.

For all of you who have ever done it in a manual lathe.

https://youtu.be/VQnH_bydMOY


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#21 mic1970

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Posted 11 March 2019 - 10:58 AM

Bill and Mirzam.... thanks for the idea.  I started the first steps of my new build and are going with the internal diagonal focuser.  I started a new thread just for that.  https://www.cloudyni...onal-refractor/

 

See ya there. 




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