Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Centring Error Detector
    From: Brad Morris
    Date: 2014 Jan 26, 21:48 -0500

    Peter

    The repeatability is due to all the faces meshing at the same time.  So 360 teeth vs 360 teeth. All teeth engage, every time.  The accuracy is due to the precision of cutting the teeth.  You could cascade them but it will be a mighty expensive sextant.  I'll leave it up to you to calculate how many you would need to stack up to get to 0.1 arc minutes (6 seconds) steps, throughout a 120° range.  

    The Hirth interface probably could be 3d printed, but I'm unsure if 3d printing can achieve the surface finishes required.  Additionally, the steps are discrete steps.  There's no way to get a mid-step position. 

    Brad

    On Jan 26, 2014 9:32 PM, "Peter Monta" <pmonta---.com> wrote:

    Hi Brad,

    I've been thinking a little about this---pardon me if I go off on a small tangent.

    Could this technique be used for sextant construction?  With these face-meshed gears one can make highly repeatable angle settings.  With several in cascade, one could have high resolution too.

    For example, imagine a 3D-printed plastic or metal plate with 360 ridges on the top surface, something like the lines of a protractor, but more extended radially.  (With a plate radius of 60 mm, the pitch will be 1 mm, quite doable with FDM 3D printers.) It will mesh with a similar network of ridges in an adjoining plate, and under a suitable preload and with kinematic averaging over such a large surface area, repeatability will be good.  Now put two of them in cascade, the second with 361 ridges/teeth.  Now the settability goes from 1 degree to 10 arcseconds, small enough that one could just set it close and wait for a touch of a celestial body.

    The index mirror would be mounted on the top plate, there would be a thin "idler" plate sandwiched in the middle, and the bottom plate would carry the horizon mirror and scope and shades as usual.

    To calibrate, make several sextants and run them against each other, perhaps under autocollimation with a camera doing the measurements.  With enough (random?) angle settings on each sextant, an error model could be estimated.  (Yes, that's pretty hand-wavey.)

    Could be better than a vernier plastic sextant, and cheaper too.  Could think about a hybrid construction with thin plastic or metal plates bonded onto a high-stability platform like carbon-fiber honeycomb.

    Cheers,
    Peter

    ps: best to "skew" the ridges so they're not radial but instead canted something like a herringbone gear, the better to smooth out any printer anisotropy.

    View and reply to this message: http://fer3.com/arc/m2.aspx?i=126703

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site