Slot And Tab Fins... Why Don't All Vendors Do This?

Discussion in 'High Power Rocketry (HPR)' started by K'Tesh, Mar 14, 2019 at 1:01 AM.

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  1. Mar 14, 2019 at 1:01 AM #1

    K'Tesh

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    Recently, I was perusing Apogee's videos, and the construction build for the Zephyr came up. I have to say that I was pretty impressed by the way the fins and the CRs were set up. Now I'm wondering... Why don't all MPR/HPR vendors use slot and tabs for fin alignment on TTW designs? So long as they're using a laser cutter, or a CNC machine to cut their fins and CRs, it seems like a small amount of work needed to result in a large benefit to the customer.

    The biggest benefits I can see is easier assembly for the customer, no more fins that are out of alignment (also improving aesthetics), and reduced roll in flight.

    Largest drawback *MIGHT* be some reduced strength of the fin due to the smaller tab, but with the slot holding the leading and trailing edges firm, I'm guessing that this would be negligible.

    Thoughts?
     
    Last edited: Mar 14, 2019 at 4:57 AM
  2. Mar 14, 2019 at 1:17 AM #2

    timbucktoo

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    Probably cause there’s no demand for them & it would increase cost.
    But instead of speculation, why not just ask the few vendors yourself.
     
  3. Mar 14, 2019 at 1:23 AM #3

    Theory

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    Increased cost.
     
  4. Mar 14, 2019 at 1:29 AM #4

    K'Tesh

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    Actually, I sent an email to Binder Design before I posted this...

    How would it increase cost? If the stuff is laser or CNC cut, other than the initial cost (in labor) for designing the tabs and slots, you would use less material, thus have a VERY slight decrease in weight. I don't see a reason for the cost increase.
     
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  5. Mar 14, 2019 at 1:33 AM #5

    dhbarr

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    Laser cutting is fairly new at the common-for-a-hobbyist level?

    I want to see crenellated fin roots with matching receiver holes in the MMT, but we're not quite there yet.
     
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  6. Mar 14, 2019 at 1:36 AM #6

    rharshberger

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    Lasing units have a finite life before losing efficiency and power, more cutting distance equals fewer parts. The lasing unit is probably the single most expensive part of a laser cutter. They wear out just like a router bit, only not as quick.
     
    Last edited: Mar 14, 2019 at 1:44 AM
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  7. Mar 14, 2019 at 1:39 AM #7

    K'Tesh

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    The response from Mike was that the CNC parts need to be deburred. So there is an increase on labor costs.
     
  8. Mar 14, 2019 at 1:46 AM #8

    Locksmith

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    I typically do something similar to all my rockets, I will laser cut the centering rings to hold square nuts for the launch lugs. Makes nice solid connection and I am able to remove or change them any time.
    Never thought of doing it to hold the fins, maybe on my next build.
     
  9. Mar 14, 2019 at 3:32 AM #9

    K'Tesh

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    Another reply from Mike mentioned the pain of having to update the instructions. That said, after the additional labor is completed for them, I can't see any other cost impact with the instructions.
     
  10. Mar 14, 2019 at 12:08 PM #10

    Theory

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    one must remember that the manufactures are looking to maximize their profit margin. this is no slight on them as they are in business to make money, not as a favor to the "rocketry world."

    every additional cut, be it with a laser, CNC or knife costs money. every modification to instructions costs money. every added recovery piece costs money.

    this all needs to be accounted for in the final price of the item, and if all of that adds up to another $10 - $15 / kit, that is a very real cost that might lose a sale.

    need to look at all economies of scale here and not just "a few additional cuts"
     
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  11. Mar 14, 2019 at 12:20 PM #11

    dhbarr

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    Laser cuts have an extremely low marginal cost, far lower than a mill. At a guess it's on the order of other QA/QC measures e.g. it would get lost along with the hours cost of inspecting samples and occasionally tossing out warped bits.
     
  12. Mar 14, 2019 at 12:29 PM #12

    tomsteve

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    the cost of the machinery has to be factored into the cost of the product. companies dont take a loss on machinery costs- the price for machinery,maintenance, upgrades, and replacement are figured into final unit price.
     
  13. Mar 14, 2019 at 2:11 PM #13

    dr wogz

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    Isn't this the same as the LOC goblin fin attachment?
     
  14. Mar 14, 2019 at 2:32 PM #14

    JohnCoker

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    Most likely inertia: it's hard to change the way things have always been done. I doubt the marginal cost is the issue, it's more like "if it ain't broke, don't fix it."
     
  15. Mar 14, 2019 at 3:56 PM #15

    Donnie

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    As a consumer, i'd willingly pay an additional cost for this feature, especially if it saves buying a guillotine jig, having to store it somewhere at home, etc.

    It's the same reason I would purchase a kit that is ready for dual deploy instead of buying a cheaper kit and some extra tubes and couplers to make it happen, though that would at times be cheaper.

    I'm in it for the flying... the building is just a means to an end.
     
  16. Mar 14, 2019 at 4:57 PM #16

    jqavins

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    I'm in it for the endless cycles of sanding and priming. ;)

    OK, seriously, I enjoy the building at least as much as the flying, but I still want kits that go together straight and true; harder building doesn't increase the fun.

    For those manufacturers that laser cut (as opposed to milling) the difference in recurring cost for the affected parts is proportional to the increase in total cut length of those parts. That includes machine life and machine time not spent on something else; it all comes down to cut length. And those parts are only a portion of the kit cost.

    The non-recurring costs of updating the cut patterns and instructions are not trivial, but not very great.

    I'm with John that, at least for manufacturers using lasers for cutting, the cost should easily be outweighed by the price the market will stand for the improved design (a new salable feature).
     
  17. Mar 14, 2019 at 5:09 PM #17

    Bat-mite

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    You don't need a fin jig, just a printer, a piece of foam board, a matte knife, and PayloadBay.com.

    [​IMG]
     
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  18. Mar 14, 2019 at 6:14 PM #18

    boatgeek

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    Having watched a laser cutter running on plywood, the extra machine time for slots/tabs would be measured in seconds for each kit. Optimizing the process of getting plywood in and out would probably offer larger gains in productivity, and optimizing tool path definitely would.

    If the manufacturer was contemplating a price increase due to increased material costs, this would be a way to justify it in terms of benefit to the consumer. That might make the price increase go down easier.

    On the other hand, if manufacturers do this and cut fin slots in body tubes, they have to be really confident in their ability to make those two match. If the CRs aren't slotted for the fins, a couple of degree misalignment on the fin slots probably won't be noticed.
     
  19. Mar 15, 2019 at 3:36 AM #19

    Wingarcher

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    I've made a few designs for customers along this line. Mostly what I did was slotted fins and rings so they straddle each other. As I run a router, there's design requirements to add in to make sure the bottoms of the slots act square. Lasers don't have this issue.
    My goal was to make parts that fit extremely well and so my slotting tolerance was pretty tight. Because the slot slides over the thickness of the material, the material thickness is important. Plywood and other materials I've worked with have pretty lax tolerances on thickness. Even within a sheet of material. A thick sheet ruins the slot fit. I could make them work as one off parts because I drew to match the materials. I wouldn't want to try it in bulk.

    N
    UpscaleCNC
     
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  20. Mar 15, 2019 at 4:13 AM #20

    Zeus-cat

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    It takes time and money to do this stuff. Someone has to modify the existing laser cutting files for both the centering rings and the fins to create the slots. Then someone has to get material and do a test run to make sure the slots in the body tube, centering rings and fins are properly aligned and don't cause an interference problem with other components. If everything looks good they can make the changes for production. If not, then they need to address alignment issues and try again.

    Then someone needs to update the instructions to change the list of materials, the placement of the centering rings and where the parts should now be placed. Then they need to change the bill of material for production by deleting the old parts and adding the new ones. They also need to create new stocking locations for the new parts as they are now different than the old parts (which may still be used on other kits).

    Simple in concept, but it takes time and money to make the changes.
     
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  21. Mar 15, 2019 at 1:38 PM #21

    Wallace

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    Crenellated: Always thought I was at least fairly good vocabulary wise. Ya totally got me with that oneo_O
     
  22. Mar 15, 2019 at 2:32 PM #22

    dhbarr

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    I wish I could think of a better way to say it, but it's the only word I can come up with :)
     
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  23. Mar 15, 2019 at 6:14 PM #23

    rfjustin

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  24. Mar 18, 2019 at 7:30 AM #24

    K'Tesh

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    I understand your concern about tolerances, I suspect that larger vendors have access to materials with tighter tolerances. That said, even a thick sheet is easy to deal with.

    "Test the fit of the fins with the centering rings. They should be snug, but not so tight that they bind. It they are too tight, carefully sand the fin (or the centering ring) until they fit snugly. If they are too loose, carefully apply strips of tape to the same side (one side only) of the fins until they fit snugly. Make sure that the fins remain at a consistent angle from each other."
     

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