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Author Topic: Sextant - how to rebuild a cheap one to working condition(warning lots of pics)  (Read 10913 times)
Captain Quinlin Hopkins
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« on: September 26, 2009, 10:16:25 pm »

Hopefully this will help out those of you that want a bit more out of that cheap sextant you've bought.  

I'm going to attempt to show what it takes to rebuild one of the cheap $20 brass sextants into something you might actually be able to use.  This may take a bit of time, so have a bit of patience and we'll see where this goes.(I'm honestly not sure myself yet).

This is the one I chose because I could pick it up today in the store, and it's solid brass.
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=66096

Here's what you get when you open the box:
Spoiler (click to show/hide)

It's surprisingly shiny and if you wanted to drop in in a display box or use it as an accessory, it's ready to go.  Personally I'm really into making sure that if it looks like a duck it should quack like a duck.  I think a Sextant should be able to be used as intended, not just for show.  But if you wanted if for a bit of shiny on your outfit, this would definitely work as is.  

The manual states that it functions, but should not be used as a navigation device.  When you take a bit of a closer look you can understand why.  First thing I noticed was that the movement wasn't smooth.  Apparently there is a bracket for the filter lenses actually placed under the index arm.  
Spoiler (click to show/hide)

Then we look closer and notice the frame is bent.  This is from the stamping process when they punch this out of a large plate with an industrial sized, sextant shaped, hole punch.  You can see the left leg is straight, and how far off straight the right leg looks.  This is solely due to the frame being bent.  You'll also see the gap under the steel straight edge I placed on top of the frame.  
Spoiler (click to show/hide)

The bracket holding the magnifier for viewing the minutes on the dial, is also twisted.
Spoiler (click to show/hide)

The glass filtering lenses are also a bit out of alignment.  They should be equally spaced and in line with the sight.  As any deviation of a straight shot through a glass causes the image to jump off to the side a bit.  Do that x 3 and there's not telling how far off your measurements might be.  Both sets are off as shown here.  
Spoiler (click to show/hide)

And another shot of one of the legs compared to a right angle.  All of this is caused by the frame being bent.  The leg is actually straight.  
Spoiler (click to show/hide)


So, Here's the plan.  I'm going to start off by disassembling everything off the frame and using an arbor press to try and get it straight.  Probably have to use a vice on the magnifier as it's too small to hold in the arbor press.  I'll then have to move that one bracket out from under the arm so it can swing properly.  I'll know more about the mechanical side of it when I tear it apart, but these are pretty simple so I doubt there will be many surprises.  

As far as cosmetics, the back and front are in decent shape, but the sides still have the cut marks from the die when it was punched out.  so I'll try to work a bit of magic on the edges and finish off that polished look.  Then it will be onto the electro-etching to give it a bit more style.  

I'll do my best to document with photo's or video as I can so you will see if it's worth all the fun(or hassle to some) to get this done.  After all, for $20 and a bit of time it can't turn out too bad.  I'll try to get updates at least weekly, but will track it the entire time, so hopefully you won't miss a step you're interested in.  

There will be large amounts of electricity and lasers...so stay tuned.  





« Last Edit: September 27, 2009, 05:09:54 pm by Captain Quinlin Hopkins » Logged

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« Reply #1 on: September 27, 2009, 12:05:35 am »

Northerner solution:
'it it wit' 'ammer Smiley
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« Reply #2 on: September 27, 2009, 12:31:31 pm »

Twenty American DOLLA?  Shocked
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Captain Quinlin Hopkins
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« Reply #3 on: September 27, 2009, 05:22:58 pm »

You could indeed hit it with a hammer, and get it fairly close, but you're also going to introduce dings,(low spots) and that makes it harder to get the etching pattern on.  It's not worth the effort of grinding it flat again, might as well start with a new piece of sheet stock if you plan on doing that.  

And yes it's a $20 import from India.  I could start one from scratch, an in all likelihood, will post that project later.  But some of that will be beyond the hobby level for most.  I'll show alternatives as I go along for those that have no tools with each stage.  

If there is interested in building a sextant from scratch, I'll do another on that with printable patterns, etc. later in another post.
« Last Edit: September 27, 2009, 05:27:03 pm by Captain Quinlin Hopkins » Logged
Captain Quinlin Hopkins
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« Reply #4 on: October 05, 2009, 05:59:52 pm »

Not much of an update as I took the weekend off to relax and enjoy the rain.

We're in pieces now.  I've taken the first piece off and you can see from the back side that no work was put into cleaning it up if it wasn't to be seen.  So after flattening it(you could put two match sticks under it and still not touch) I proceeded to grind off the rough bits from stamping you can see in the first photo.  You'll notice the rough edge folded over from the stamping process.  The entire back and part of the front hidden under a bracket were never polished.  
Spoiler (click to show/hide)

After that I took it to the buffing wheel and proceeded to give it a decent shine.  
Spoiler (click to show/hide)

I've not cleaned up the crevices yet, but you get the general idea.  I'll have to take a dremel to it finish out the polish.  I'm currently using a white rouge(hard to believe they actually get away with labeling it as such on the box eh?) which is designed for steel, thus not as bright as possible.  

A few things I noticed along the way.  One of which is that the hole for the pivot is off center(.050" ,1.27mm), which can only be corrected by removing material from the outer edge of the round part shown.  the second is that the hole is .281"(7.14mm) and the shoulder of the pivot bolt is only .250"(6.35mm).  I'll probably have to space a small shim around it to get it to ride properly.  

I'll try to get a few shots of the polisher in action, and the wheel used for grinding.  It's a fairly interesting wheel, you can actually depress the surface of it using your fingernail.  It's like a polishing dense foam.  If you've never used one before I highly recommend it.  

That's it for a bit anyway.  I'm trying to get some free time to see what light transmission and haze readings I get from the lenses, but haven't had available machine time yet.  Have to remember to hop in there early in the morning and I'm having a dickens of a time remembering that at 5am.  

Hopefully I'll have the entire process documented soon with the wheel numbers and such.  And we can then get on to more interesting projects.  

edit: October 10, 09
I'm going to just add to this post to keep from bumping my own work. 
I've just made the adjustments to pull a bit of slop out of the works..  the .250" pin that held the arm in place has a .280 hole it rides in.  Completely unacceptable.  So with a bit of brass stock i've laying around

I cut a piece to fit exactly around the pin.  .250x 3.14 = .785. 

The stock I had was .020" thick, which would cause it to be too large to fit properly, you can either sand the piece to the proper thickness, or simple use a reaming tool to enlarge the hole to a .290 diameter.  Depending on the tools you have. 

pictures of the sequence
1. dimension of screw
Spoiler (click to show/hide)

2. pi times diameter for the length of the bushing from flat stock
Spoiler (click to show/hide)

3. Cut bushing, I cut the length off first, then the sides, made 2 just in case one  doesn't function properly.  You can of course also make a second if you need more thickness than the shim stock you have available. 
Spoiler (click to show/hide)

4. Reamed out hole to fit the thickness bushing I'd just made
Spoiler (click to show/hide)

5. Bent bushing around pin and inserted it in the hole
Spoiler (click to show/hide)

6. Installed screw with the aid of a block of lead.  lead makes a wonderful hammer for brass, as it doesn't mar the surface.  brass can in turn be used as a non-marring hammer for steel, etc. 
Spoiler (click to show/hide)
Spoiler (click to show/hide)

7. put the washer in place to maintain the gap as it was shipped. 
Spoiler (click to show/hide)

Then of course added the arm and checked the movement.  Much improved!.  All slop removed.

And yes, I do realize that by this point it's going to be easier to create one from scratch that doesn't require all the re-work to have the proper fit and function.  But It's good to have this here so I'll continue.  After I make one of my own design, the two can be compared, and hopefully otherswill decide to make their own.  Or you cna just take bits and pieces of this you feel you might want to use on one of the cheap imports. 



« Last Edit: October 10, 2009, 02:38:58 pm by Captain Quinlin Hopkins » Logged
PockyNightmare
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« Reply #5 on: December 28, 2009, 03:36:23 am »

O.O 20 freaking american dollars...

this is... almost... 17 euro  Shocked

is there a way to buy sextants in any kind of shop? (i hate online-shops)
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Captain Shipton Bellinger
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« Reply #6 on: April 02, 2010, 07:57:45 am »

What ho! Captain Hopkins,

How did this project progress? I'd love to see an update.

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« Reply #7 on: April 03, 2010, 05:31:15 pm »

I'm going to attempt to show what it takes to rebuild one of the cheap $20 brass sextants into something you might actually be able to use.  ...

Then we look closer and notice the frame is bent....The bracket holding the magnifier for viewing the minutes on the dial, is also twisted. ...The glass filtering lenses are also a bit out of alignment.  

This is a generic problem with pseudo-antiques. But at least the Harbor Freight sextant is cheap. Restoration Hardware has overpriced decorative objects with equally bad, often worse, manufacturing tolerances.  I once saw a bin of metal rulers which differed in length as much as 2mm.  That's embarrassing.

For a few dollars more, they could have found a decent stamping company.  Stamping out objects cheaply without bending them is a routine industrial operation. The thing should cost $25 and be within tolerance.
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Captain Quinlin Hopkins
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« Reply #8 on: April 06, 2010, 06:23:37 pm »

My apologies for replying late.  been a bit too busy to check in as often as i'd like. 
I've progressed a bit, but no photos yet, then the project was relegated to the back shelf for a bit as work decided they wanted me here more then i'm home. 
Looks as if I've a few more weeks of this before any real free time develops.

Feel free to slap me around a bit more in a few weeks if i've not updated.  I often get distracted by new shiny objects and tend to forget a project until I run across it again. 
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arcwelder
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« Reply #9 on: June 19, 2010, 06:35:54 am »

*slap*slap*

Seriously though, how's it going?
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« Reply #10 on: July 28, 2010, 12:06:33 am »

A gentle SLAP# and a big tickle!!

I would like an update too.... Fingers crossed you have found some free time.
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Captain Quinlin Hopkins
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« Reply #11 on: July 28, 2010, 01:09:20 am »

Oh. hello there.  Progress......  I'd love to tell you I was finished, but I've unfortunately been on 70 hour weeks since the last update.  It sits quietly, whispering to me from the recesses of my desk.  or perhaps that's my assistant.  Haven't seen her around lately.   Where was I again?  Oh yes. updates.  I've scheduled in some budget this week (thanks to your prodding) to get a brazing torch and one of the new Dremels with the actual decent motor and a polishing kit.  You can of course polish by hand, it's just a matter of more time.  I seem to be lacking time recently.  Thus the shortcut. 

You don't have to spend the $100 on the kit I'll be spending, feel free to drop in the local hobby shop and you'll find a cheap version for under $10 that will work as long as you don't stress it out too much.  I've one here from a harbor freight(by coincidence the same place I picked up the sextant).  I'll officially open the drawer and set it back on my desktop in the morning so I don't forget about this project before the weekend. 

I'll need to work on the etching design I'd imagine, and find one of the good old battery chargers to get a decent etching tank going.  At least that is simple enough and easy to do bit by bit. 

Thanks for the prod.

Hey, anyone else hear that scratching sound?   
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PockyNightmare
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« Reply #12 on: July 28, 2010, 01:19:59 am »

captain... how long didn't you see your assistent?

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Captain Quinlin Hopkins
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« Reply #13 on: July 28, 2010, 01:50:42 am »

oh my....pardon me as I need to run back to my office and check on something.  

Mumbling.  Though I do keep a bug out bag...should be plenty of water....rations could last quite a while....
« Last Edit: July 28, 2010, 01:55:51 am by Captain Quinlin Hopkins » Logged
PockyNightmare
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« Reply #14 on: July 28, 2010, 02:44:53 am »

D:
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