Pantograph

Discussion in 'Composites' started by Jstorrshall, Dec 28, 2018.

Help Support HomeBuiltAirplanes Forum by donating:

  1. Dec 28, 2018 #1

    Jstorrshall

    Jstorrshall

    Jstorrshall

    Member

    Joined:
    Sep 3, 2013
    Messages:
    20
    Likes Received:
    3
    Location:
    Melfa, VA USA
    So, has anyone ever used a pantograph for hot-wire cutting? It would seem an obvious trick to try, in a down-scaling mode.
     
  2. Dec 28, 2018 #2

    FritzW

    FritzW

    FritzW

    Well-Known Member

    Joined:
    Jan 31, 2011
    Messages:
    3,233
    Likes Received:
    2,854
    Location:
    Las Cruces, NM
    With a pin in a slot on template? ....or?

    1221__35103.1506029041.451.416.jpg Like a router duplicator except with a 2D template on one end and 3D (hotwire) on the other.
     
  3. Dec 28, 2018 #3

    wsimpso1

    wsimpso1

    wsimpso1

    Well-Known Member

    Joined:
    Oct 19, 2003
    Messages:
    5,625
    Likes Received:
    2,866
    Location:
    Saline Michigan
    You would likely need two, one for each end. Then talking points on both ends too. Sounds like complicated for no advantage to me... I too am sure someone has done it too...
     
    Topaz likes this.
  4. Jan 1, 2019 #4

    Jstorrshall

    Jstorrshall

    Jstorrshall

    Member

    Joined:
    Sep 3, 2013
    Messages:
    20
    Likes Received:
    3
    Location:
    Melfa, VA USA
    Shouldn't be all that much trouble to make a coupled pair so that you don't need two people. Alternatively use one (like the router version) to move the foam block around on a platform/vertical wire cutter.
     
  5. Jan 2, 2019 #5

    Markproa

    Markproa

    Markproa

    Well-Known Member

    Joined:
    Feb 9, 2017
    Messages:
    76
    Likes Received:
    30
    Location:
    Bellingen, NSW. Australia.
    Simple if both end profiles are the same, difficult if not, like a tapering wing.
     
  6. Jan 2, 2019 #6

    Tiger Tim

    Tiger Tim

    Tiger Tim

    Well-Known Member

    Joined:
    Apr 26, 2013
    Messages:
    2,772
    Likes Received:
    1,534
    Location:
    Thunder Bay
    I guess for cutting foam this “pantograph” would take your input at one end of the wire and duplicate it on the other? I’m trying to picture exactly how such a rig would work but it might be a neat way to cut cores solo, especially if the rig could be tweaked to make one end work at a reduced scale for tapered planforms. I’m not sure it would be able to handle washout though, and you’d be sunk if you wanted the airfoil to change from root to tip.
     
  7. Jan 2, 2019 #7

    wsimpso1

    wsimpso1

    wsimpso1

    Well-Known Member

    Joined:
    Oct 19, 2003
    Messages:
    5,625
    Likes Received:
    2,866
    Location:
    Saline Michigan
    The thing everybody misses about a pantograph or similar setup is that the precision of the device to do as good a job as two humans with templates is actually pretty high. For a single pantograph to follow cleanly, you must have no slop in the bearings at all of the joints and drag in those same bearings must be low. Then to drive a second tracer, the second tracer will lag behind the first one in proportion to the forces and the stiffness of the linkage between them. Yeah, it won't just lag, the lag will be bigger when forces are high and smaller when forces drop, like when you pause at a corner.

    So it would have to be stiff and have zero slop and little friction. Begins to sound pretty massive to me.

    Then let's consider the rest of the problem. Most wings have a little washout built into them, they taper, and sometimes the foil at one end is different from the other end. Also, any spar cap troughs start out deep at the root end and taper to practically nothing at the tip. You can taper the part with a pantograph (by gearing), and you might be able to twist it (by setup), but you play merry hell using different foils and big differences in trough depth with such an arrangement.

    The folks who have taken the human element out of hot wiring and made fancy parts have driven the frame for the hot wire with 2D CNC on each end. This can make for equivalent or better quality than the typical homebuilder's scheme:
    The frame for the wire can be much stiffer because the weight matters little to machine tool;
    The wire can be kept at constant temperature and constant low speed throughout the cut;
    The profiles are independent of each other - they can be programmed for each end;
    The entire shape can be run as a single long piece of foam instead of doing several short forms and putting them together later.

    As for me, I kind of like having simple inexpensive tools, getting a friend or two over for a session, followed by food and beers.

    Billski
     
    Last edited: Jan 2, 2019
  8. Jan 2, 2019 #8

    Norman

    Norman

    Norman

    Well-Known Member

    Joined:
    Nov 29, 2003
    Messages:
    2,877
    Likes Received:
    902
    Location:
    Grand Junction, Colorado
    Are you sure about that? I've never cut blanks more than 3' so I'm not sure but I've been told that wire sag will cause noticeable distortion at greater than 4'. Even on small parts of only 1" I can see the wire bend a few degrees as it enters the blank and have to wonder how far the center of the wire is lagging behind the ends on long parts. Also when the wire exits the part the ends come out first showing that even on short parts the wire is "sagging" (sagging is in quotations because the so called "sag" is horizontal indicating friction lag rather than gravity sag). The first "long" cut I ever made was using the "two guys with station marks on the templates" method and I found it awkward. You just can't maintain constant speed and the result is often a washboard (by my standards ~ 0.02 wave height). After a couple of what I considered inadequate results we built a drop bar that clamps onto the edge of the building table and got pretty consistent results. A drop bar is a much simpler device than a pair of pantographs and easily cuts tapered, twisted parts with different profiles at the ends.
     
  9. Jan 2, 2019 #9

    wsimpso1

    wsimpso1

    wsimpso1

    Well-Known Member

    Joined:
    Oct 19, 2003
    Messages:
    5,625
    Likes Received:
    2,866
    Location:
    Saline Michigan
    I would never try to do a hand guided wire cut more than four feet for exactly the reasons cited by Norman, but if you have CNC drives at each end and closed loop control of wire temperature, you can tension the wire and drive it slowly enough to have excellent accuracy and very little lag.

    Here is one thread where a guy does 10' cuts. http://www.homebuiltairplanes.com/forums/showthread.php?t=1429&highlight=wire+cut+core

    There was another thread where full wing cores were cut this way, but I can not find it right now - IIRC they drove from Germany to Poland to pick up the new cores...

    Billski
     
  10. Jan 4, 2019 #10

    Jstorrshall

    Jstorrshall

    Jstorrshall

    Member

    Joined:
    Sep 3, 2013
    Messages:
    20
    Likes Received:
    3
    Location:
    Melfa, VA USA
    Can you post a picture please?
     
  11. Jan 4, 2019 #11

    Norman

    Norman

    Norman

    Well-Known Member

    Joined:
    Nov 29, 2003
    Messages:
    2,877
    Likes Received:
    902
    Location:
    Grand Junction, Colorado
    How about a video:
    [video]https://www.youtu.be/zlWqHXpSttk[/video]

    Hope that worked, had to type it in, cut & paste isn't working on this stpid phone.
     
  12. Jan 4, 2019 #12

    Norman

    Norman

    Norman

    Well-Known Member

    Joined:
    Nov 29, 2003
    Messages:
    2,877
    Likes Received:
    902
    Location:
    Grand Junction, Colorado
    How about a video:
    [video=youtube_share;zlWqHXpSttk]https://youtu.be/zlWqHXpSttk[/video]

    Hope that worked, had to type it in, cut & paste isn't working on this stpid phone.
     
    wsimpso1 likes this.
  13. Jan 4, 2019 #13

    lr27

    lr27

    lr27

    Well-Known Member

    Joined:
    Nov 3, 2007
    Messages:
    3,216
    Likes Received:
    463
    If you can control wire speed well enough, you can prevent the wire from touching foam at all. Then gravity will be the only cause of sag. Cores cut this way are very nice. Anker at rcgroups.com discusses this.
     
  14. Jan 6, 2019 #14

    Jstorrshall

    Jstorrshall

    Jstorrshall

    Member

    Joined:
    Sep 3, 2013
    Messages:
    20
    Likes Received:
    3
    Location:
    Melfa, VA USA
    Norm, Great, thanks!
     

Share This Page

arrow_white