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DIY Fork Project

mount DIY Celestron SCT
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#1 Tak North

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Posted 27 March 2016 - 09:46 PM

Like many other threads posted here in this forum, I’ve finally decided to start my “large” telescope project after years (decades) of procrastination, design modifications (indecision), life changes, etc. I’ve collected various pieces and parts over the years (some dating as far back to RTMC swap meet acquisitions in the early 80’s), made numerous iterations to drawings and plans over that time, even talked about this for years here on CN. But I’ve never actually pulled the trigger and just started building it. Until now…

 

I’ve always been a fan of big fork mounts. Likewise, I’ve always wanted a big C14 for the iconic reputation of the scope, and when OPT had a screaming sale on New C14 (non-HD) OTA’s a couple of years ago it solved that problem. However, what to do about the mount? My challenge is to design and build a fork worthy of the mighty C14.

 

Some of the design considerations for my project include:

  1. Visual use only: As described in a different thread, I have no interest in AP. This is purely a visual system, however that does not mean I can compromise on quality. The mount needs to be rock solid, and I am a stickler for mechanical details which also adds to my end user experience IMO.
  2. Built in Stages: Initially Alt-Az to get the system into operation ASAP. But can be upgraded to an equatorial configuration later during the summer observing down-time.
  3. Multiple OTA’s: I’m looking to build a complete observing system. The C14 will be the high power OTA, but I want to tandemly mount a large, low-power RFT (with Binos) for wide-field work later on as well.

Phase I : The Fork Mount

 

Machining the main bearing plate:

Image00003_zpspg7h8io5.jpg

 

Image00005_zpsol3liy5b.jpg


Edited by Tak North, 27 March 2016 - 09:47 PM.

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#2 Tak North

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Posted 27 March 2016 - 09:49 PM

Machining the main gear mount plate (gear courtesy of Jim Egger):

Image00012_zpsqce6d3ws.jpg

 

Image00009_zpsugphymvt.jpg

 

Image00008_zpsq1mqnodp.jpg



#3 Tak North

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Posted 27 March 2016 - 10:05 PM

Edited - Here is the design drawing for the Base:

DwgBase_zpsvebys11o.jpg

 

All the finished parts of the Main Azimuth Base:

Image00028_zpst4hwflt0.jpg

 

The completed assembly:

Image00030_zpsudm9v4xq.jpg

 

Mounted on my Losmandy Pier:

Image00037_zpsayrhjrue.jpg


Edited by Tak North, 28 March 2016 - 09:39 PM.

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#4 dmcnally

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Posted 28 March 2016 - 01:19 AM

That's a very cool and ambitious project.  I used to have an orange tube C14 with the sand cast forks.  What are you planning to do for fork arms?

 

Dave



#5 Tak North

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Posted 28 March 2016 - 02:01 AM

That's a very cool and ambitious project.  I used to have an orange tube C14 with the sand cast forks.  What are you planning to do for fork arms?

 

Dave

 

Thanks Dave. Yes, I just think that there is something about a big cat that belongs on a big fork. I know the trend today is to put them on a GEM, but for me at least it's a shame you can't get the old aluminum C14 forks anymore.

 

Anyway, here is a teaser of the fork - AKA the McMaster mount:

 

The fork as shipped to me from McMaster-Carr:

Image00024_zpsjstcimm8.jpg

 

And all of the parts unboxed:

Image00026_zpsouqbcj88.jpg

 

-Derek


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#6 lambermo

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Posted 28 March 2016 - 03:25 AM

Cool project ! Subscribed to followup posts :)



#7 dmcnally

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Posted 28 March 2016 - 09:03 AM

Anyway, here is a teaser of the fork - AKA the McMaster mount.

Hmmm... Extruded T-slot aluminum.  Okay, that's an interesting choice and will save a bunch of machining.  Are you going to use something besides the slot connectors (like welding) to tie it all together and keep it rigid?

 

Dave



#8 gregj888

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Posted 28 March 2016 - 10:24 AM

I had a fork welded from 2" x 3" Aluminum tubing for my 8" Houghton.  It worked OK, but is pretty "springy", twisting the base member.  I'm in the process of rebuilding it now.  The plan is to add some bent sheet aluminum to try to "beef" things up.  I'm also adding diagonal plates where the arm attaches.

 

I can't tell quite what you are planning and Unistrut is much stiffer than the rectangular  tube I used.  Watch the cross sections, the base piece needs  to resist twist.  I found some design information later, will post if I can find it.  Basically, no sides should be parallel to each other, helps reduce vibration/tuning fork effects.   Cross sections are real important. 

 

I agree with Dave, interesting choice and a better one than mine.  Would not be surprised if once don, you add some welds.

 

Please keep us posted, I have a 20" coming.  Current plan there is to use composites (fiberglass,with carbon reinforcement).



#9 Tak North

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Posted 28 March 2016 - 10:31 AM

Dave, Greg,

 

Thanks, yes this will definitely be an experiment for me as well. I've worked with the t-slot material before so I am pretty confident it will work. But I'm not a Mechanical Engineer and I can't tell you what the resonant frequency or expected flexure will be up front. This will be largely a Proof of Concept, and it's quite likely the design will need minor improvements along the way. Also, it may respond one way in Alt-Az mode where the loads are much different than in EQ. We''ll see!

 

Next up is to finish the two Altitude/Dec bearing assemblies before I can start assembly of the fork...

 

-Derek


Edited by Tak North, 28 March 2016 - 10:32 AM.


#10 gregj888

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Posted 28 March 2016 - 01:17 PM

Derek,

 

Mine "bounces" after being touched, but is fine when tracking.

 

Doing an almost complete rebuild.  Replacing the DIY gear track worm with a surplus Byers (supposedly) wheel and worm.  Beefing up the fork and replacing the stepper system (Mels) with a Sidereal Tech model 1.  DEC has never been right, so fixing it as well.

 

T strut/Unistrut is pretty stiff.  If you can get it tight enough, it will "work" but how well is going to be the issue.  If it does and you are happy with it,  might have someone weld the seams here and there to really stiffen it up. 

 

Greg



#11 Tak North

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Posted 28 March 2016 - 03:05 PM

Greg,

 

I would definitely like to see some pictures of your project.  Feel free to post them here if you don't want to start a separate thread. Good idea about the seam welding.  My idea is to take advantage of the modular strut system allowing for easy configuration changes as my system evolves, but that decision might not be up to me.  I've got a MIG welder so it should be easy if it comes to that. 

 

I'm very confident that this design will work in Alt-Az mode with no problems. And I’ve got an Argo-Navis with ServoCat to install later so tracking will be more than adequate for visual use only. I’m only thinking about changing to EQ mode later to fully take advantage of the fork design by eliminating the Alt-Az meridian hole. Also, at my latitude the fork would only be tipped over 29° from vertical anyway, so the torsional load on the lower fork cross member will be much lower than that in an area such as yours. Again, I’m not doing much high powered math or engineering up front, so trial-and error will be my friend!

 

I’ll post some of my design drawings here in a bit - guess I should have started with those first huh? :)  But that’s me, too eager to jump right into the machining without doing enough preliminary work first! However you can see a small snippet of one in my Avatar.

 

-Derek

 

 

 

 



#12 Tak North

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Posted 28 March 2016 - 09:45 PM

Here are some of my preliminary (primitive) design drawings:

 

Side:

DwgSide_zpsggpeiaeo.jpg

 

Front:

DwgFront_zpsoxjtg1vw.jpg

 

End:

DwgEnd_zpsxq8o9sr8.jpg

 

Front with the test RFT attached:

DwgFront2_zps3gpmom4p.jpg


Edited by Tak North, 28 March 2016 - 09:45 PM.


#13 dmcnally

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Posted 28 March 2016 - 09:56 PM

Mine "bounces" after being touched, but is fine when tracking.

Mine was mounted outside.  The mount became a tuning fork when then wind blew.

 

c14-03.jpg

 

Dave



#14 Bob4BVM

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Posted 29 March 2016 - 01:31 PM

Great idea to add the tandem scope mount to the fork.

I also like the fork mounts for their simplicity and (potential) ruggedness.

Not to mention the versatility of using it alt-az or switching to eq by simply sticking a wedge under it.

 

If it were me, for a big C-14 I would be going a lot stiffer than the Bosch strut you are using.

I have a partially-built fork, based on a big truck front spindle & brake drum.

The forks are 3x8x3/8" steel tubing and mount very simply, they are welded to a plate which is then bolted to the existing drum lug bolts.

The drum rides on the same set of dual (inner/outer) Timken tapered roller bearings which originally supported the front end of the truck.

Yes, it is all very heavy, but was designed for the permanent pier in my ROR observatory.

Another one of my many scrapyard-based (:)) ATM projects, I have about $15 into it so far.

One of these days, if I get to retire...



#15 Tak North

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Posted 29 March 2016 - 03:13 PM

Bob,

 

That sounds awesome! Post a pic if you get a chance. I’m a big fan of massively over-building too when it comes to stability. It’s the only way you can be sure without some fancy Engineering degree!  ;)

 

I know it’s hard to tell from my drawings but the fork won’t just be single pieces of extrusions bolted together to make a spindly fork. They will be doubled and tripled up, strategically constrained in multiple axes with hundreds of fasteners. Hard to describe in words. Hopefully I’ll have most of the machining done in the next couple of days so I can start assembling the fork itself later this weekend.

 

 

-Derek

 

 

 

 



#16 Tak North

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Posted 29 March 2016 - 10:41 PM

Got one of the Alt/Dec bearing modules completed.

 

Here's the drawing:

DwgAlt2_zpsrsia393k.jpg

From L to R:

Right OTA Saddle

Saddle plate with pressed in 1.750" SS Shaft

encoder (future)

Thrust brg (future/EQ configuration)

Inner Bearing Plate

3.5" ID X 4" OD X 3" L roller bearing

Outer Bearing plate

Thrust brg (future/EQ configuration)

Outer Saddle plate

Outer/Auxilliary OTA Saddle

 

Machining the shaft:

Image00014_zpsgqwhf01v.jpg

 

Machining the roller bearing shaft bushing:

Image00016_zpsucqov74r.jpg

 

Machining, drilling, and Tapping one of the bearing plates:

Image00018_zpsnupp1vvq.jpg

 

Image00019_zpsnubpadn3.jpg

 

Image00020_zpsad1kqkw4.jpg

 

Truing up the Saddle Plate after pressing in the shaft:

Image00050_zpszs4zwemv.jpg


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#17 Tak North

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Posted 29 March 2016 - 10:44 PM

The completed parts for the assembly:

Image00045_zpskvizb8pk.jpg

 

The finished assembly (less the auxiliary OTA Saddle):

Image00046_zpso7q7fqfa.jpg


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#18 Tak North

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Posted 31 March 2016 - 11:52 PM

Got the other alt/dec bearing assembly finished. This one has essentially the same core components as the first, with the addition of:

Worm gear drive
Thrust bearings to

  • Maintain the OTA centered within the fork
  • Locate the Worm Gear concentric to the axis of rotation
  • Provide thrust for the friction clutch

Hand-wheel to allow easy adjustment of the friction clutch preload


Here’s the drawing:

DwgAlt1_zpsihec6jx6.jpg

From R to L:

  1. Left OTA Saddle
  2. Saddle Hub & 1.750” dia SS Shaft (not pressed in)
  3. 1/16” X 6” Teflon Friction Clutch washer
  4. 8” 360 tooth Jim Egger Worm Gear
  5. Worm Gear Hub
  6. 5” ID X 6” OD Kaydon thin profile radial/thrust bearing
  7. Inner Bearing Cap
  8. Roller Bearing
  9. Outer Bearing Cap
  10. 3” ID X 4” OD Kaydon thin profile radial/thrust bearing
  11. Thrust bearing preload hub
  12. Thrust bearing preload adjustment hand-wheel

 

Completed parts:

Image00042_zps99r741ev.jpg

 

Final assembly:

Image00044_zpsqqcuvtj5.jpg

 

One more simple part to make before I can star assembling the fork.

 


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#19 Diego

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Posted 01 April 2016 - 06:24 AM

Wow! I love looking at these pictures!  Very nice work! Very tidy and professional. Did you do all the machining? It is top notch! :bow:  :bow: :bow:  :bow:  :bow:  :bow:  :bow:  :bow:  


Edited by Diego, 01 April 2016 - 06:24 AM.


#20 dmcnally

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Posted 01 April 2016 - 08:58 AM

Hi Derek,

You need to start working on your observatory, preferably with a stout permanent pier, ASAP. :)

 

Dave



#21 Tak North

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Posted 01 April 2016 - 11:24 AM

Wow! I love looking at these pictures!  Very nice work! Very tidy and professional. Did you do all the machining? It is top notch! :bow:  :bow: :bow:  :bow:  :bow:  :bow:  :bow:  :bow:  

 

Thanks Diego,

 

Glad you are enjoying the photos. It's much easier for me too than trying to write a bunch of words! Yes, I'm doing all of the machining myself on the lathe and mill in my small shop. Tidy? You should see the mess I make in the process! ;)

 

-Derek


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#22 Tak North

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Posted 01 April 2016 - 11:28 AM

Hi Derek,

You need to start working on your observatory, preferably with a stout permanent pier, ASAP. :)

 

Dave

 

Thanks Dave. Yes, the "observatory" is definitely on my list! Can't catch a break in the weather though. Stupid wind is still blowing outside! :(

 

Normally this time of year (March-April) is the best observing season for us up here. But this year it's just been snowing & raining a lot. Soon we'll lose most of the darkness and I'll be in Solar mode for the summer.

 

-Derek



#23 Tak North

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Posted 01 April 2016 - 08:13 PM

Finished up the fork adapter saddle. Nothing more complicated than what's seen here. Just three flat plates with a bunch of through/counter-bored & tapped holes. Tapping sucks though. My least favorite thing to do in the shop IMO (next to cleaning!).

 

Image00022_zpsq8w68rkk.jpg

 

Image00023_zpsciwvs5cd.jpg

 

Image00031_zpszoyjnp2x.jpg

 

Image00039_zps6u88w6nr.jpg

 

If all goes to plan, tomorrow will be Fork Assembly Day! :bounce:


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#24 Tak North

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Posted 02 April 2016 - 11:59 AM

Laying the Keel for the Fork. Sorry, this old Navy Vet couldn't resist using that phrase at least once in his life. And this is the closest I'll ever get to building my own boat!

 

Image00048_zpsf3lbxxtp.jpg

 

Now back to work!


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#25 Diego

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Posted 02 April 2016 - 03:51 PM

Tapping sucks though. My least favorite thing to do in the shop IMO (next to cleaning!). 

 

 

:lol:  :lol: :lol:  :lol: I hate tapping also! Guess it's a common dislike among machinist. Cleaning?? What that? 

 

Great work! Keep going!! 




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