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Thread: Almost finished powered shooting machine..

  1. #13
    It's an X
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    The more you tell us about your machine, the more I admire the work that has been put into it and the different ways that it can be used.
    It's a bit of a shame that the thread seems to be focussing more on disagreements between posters.

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    Rik (02-12-18)

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    In the Blue Fuzzy's Avatar
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    This looks great. Can it do recurve?
    *Click* *Thunk*

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    Quote Originally Posted by geoffretired View Post
    The more you tell us about your machine, the more I admire the work that has been put into it and the different ways that it can be used.
    It's a bit of a shame that the thread seems to be focussing more on disagreements between posters.
    I've had more than a few years to think about it, while looking at photos at what other people have built.

    One of the benefits of studying the aquisition of expertise has been the demonstration that one should study everything that has been done as the vast majority of advances are built on previous ones.
    I really liked the idea of the linear bearings, and disliked the idea of the effort involved in pulling the bow back.
    I've also seen some horribly made ideas as well as brilliant.
    There are even extremely well made bad ideas.


    Quote Originally Posted by Fuzzy View Post
    This looks great. Can it do recurve?
    Yep. Due to the height of the legs, my own bow's limb tips come close to the ground, but the bow mount can move up quite a distance purely by shifting it up that grid of 15mm spaced holes.
    The release finger simulator hasn't been finalised yet, but I've been told the basics of what I'll need to get the horizontal displacement off finger releases already.
    All that work and research has already been done, so I know what to expect.
    It's also extremely possible to make a fixture to velcro my arm (or someone else's) into the machine and use actual fingers in a shooting machine to measure variation. It will require confirmation measurements to establish that the different arm geometry isn't disturbing things too much, but it should be pretty damn close.

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    Fuzzy (06-12-18)

  7. #16
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    I really liked the idea of the linear bearings, and disliked the idea of the effort involved in pulling the bow back.
    I understand the reason for having the powered draw. What are the benefits of the linear bearings, with regard to drawing the bow? Is it all about the end point of the draw being more precise, shot for shot?

  8. #17
    In the Gold ThomVis's Avatar
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    Quote Originally Posted by Andy! View Post
    It's also extremely possible to make a fixture to velcro my arm (or someone else's) into the machine and use actual fingers in a shooting machine to measure variation.
    I don't know if I want to be strapped to that machine. For a couple of shots, maybe. But for the dozens and dozens of arrows that need to be shot to get dependable data, no thank you.
    I've enjoyed the build log on ArcheryForum, pure engineering p0#n, too bad old pictures stopped working.

  9. #18
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    Linear bearings are just nice, but that's one part of it.
    You've nailed it. They're more precise. Everything has become so much cheaper with Chinese manufacturing and the demand from CNC and the 3D printing industry.
    What used to be hundreds of dollars worth of recirculating ball bearings can now be bought for less than 20 Australian dollars, including postage.
    Getting the rails and bearings cost so little that using anything else seems like a complete waste of time. There are designs floating around that are bits of water pipe over more water pipe.
    Technically, they do the job, but there are a few things which are overlooked with shooting machines on the internet.

    If you can't put the same arrow into the same hole at the same distance, this indicates that your aiming technique or your machine (or both) are inconsistent.

    I've seen video of people showing their homebuilt machines which are absolutely terrible in grouping one arrow. Anyone with a clue should probably realise that something is wrong.

    If you couldn't achieve a single hole group with the same arrow, would you then try and test a group of arrows and claim a meaningful result?

    What if you were using the same arrow and aiming at the same spot but the arrow still wouldn't go into the same hole? I've also seen people do this online.

    Provided that we can be confident that someone with a bow in an adjustable jig can likely point it pretty close, there's a variation going on there somewhere.

    People underestimate the significance of drawlength. Nothing is so enlightening than taking a chronograph to a club and inviting everyone to shoot through the sensors. An extra couple of inches of draw can make an astounding difference in speed from the same model bow with similar poundage

    So how about if one pulled the bow back and had no ability to sense where the stops were? It's quite possible to give your bow a little bit of extra thrust by pulling hard into the stops and storing slightly more energy.

    Currently the hottest accessory in compounds is a strain gauge which can measure the flex of the riser. This shows exactly how much the riser is bending which of course is proportional to how hard someone is pulling into the stops. The idea is to get the indicators exactly the same every time. Jesse Broadwater shows incredible consistency at pulling into the wall using this training device.
    Does this seem like random coincidence to you?

    The linear bearings provide a very consistent surface to work with. The round rails provide accurate location reference and are excellent to clamp onto with the hard stop.
    The two linear bearings align with the edge of the box that holds the drive nut and the hard stop contacts two of those surfaces extremely consistently. I have a dial gauge which magnetically attaches to the frame and I can use the .01mm scale to test if the slide stops in the same place every time.

    As the slide comes back, it hits the gauge and the needle stops at exactly the same spot when the slide then contacts the hard stop. It is easily less than the lines that indicate the 1/100th of a mm spacings.

    I feel pretty confident that I've designed this to effectively minimise variations in draw force caused by draw length variation. Linear bearings are a large part of this because of their precision fit.

    Spot Hog tell you to line up a pencil line on the manually cranked hooter shooter.
    I've seen one guy use a paper clip to sight onto a ruler.
    I've seen bits of tape.

    This doesn't exactly fill me with confidence that these designs are precise enough, so why would I trust the results?

    A wise man once told me that it's not what you test, it's how you test.

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