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    Compound twinbow pistol (posts vs pulleys)

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    Post by Mooki on Fri Mar 01, 2019 7:18 am

    Cheers,

    I'be been thinking, on-and-off for the last 35 years or so, about building a small crossbow, guess this was triggered by a souvenir xbow from San Marino ages ago...
    Anyway, the direction my mind is currently wandering is sort of inspired by the "Twinbow".
    Being the first build I naturally started off with the fun parts without having a clear picture of the final design.
    I used some discarded kitchen drawer handles I had lying around, they act as the stock and arrow rest, with a classic roller nut as release.
    Tried it out with some rubber bands and everything seems to work fine (with that comparatively low load).

    I'll assume the twinbow is familiar enough - since my draw is pretty long for such a short bow I'd like to add some kind of compound system - however since pulleys are a bit pricey for decent ones I was thinking of doing a POC using posts instead of pulleys.
    Any ideas as to how much this will affect performance? (hence the thread title)

    Build so far: Total length 32cm, probably using 30cm prods from spring steel 2.0x35mm (tapered ends to 15mm), riser fixed in the middle approx where the trigger is.
    Compound twinbow pistol (posts vs pulleys) Dsc_0410

    Am I way off in my thinking below, about 24cm draw giving roughly 4cm flex per prod end?
    Prods almost straight at rest, middle post/pulley on the fixed riser. My thinking is that string would be approx 165cm total, draw 24cm should "consume" 50cm forcing the posts closer, so at each "crossing" the string needs to be (50/6=8 ) cm shorter, = 4cm per prod end. Good/Bad thinking on my part?

    If so, it should suffice fixing the prods approx 6cm out from the body, keeping the width at 17cm.
    However, that might be bad due to "compound angle" being too steep - thoughts?
    BTW, that's the brass posts in the pic, threaded through with M4.
    Compound twinbow pistol (posts vs pulleys) Dsc_1010

    Any thoughts or feedback welcome //M


    Last edited by Mooki on Thu Mar 21, 2019 6:00 am; edited 2 times in total (Reason for editing : Edited title to better reflect contents)
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    Post by Geezer on Fri Mar 01, 2019 11:47 am

    I can't claim any particular knowledge about pulley-bows versus posts, but I will suggest the great advantage of pulleys over posts would probably be the reduction of friction exchange for greater weight.  Geezer.
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    Post by Mooki on Mon Mar 04, 2019 3:40 am

    Thanks for the reply.
    Decided to go ahead and see if posts are good enough - short answer = No!

    Definitely a lot of losses with posts, way more than I expected.
    Quite noticable when spanning, only the forward prods flex voluntarily.
    It's necessary to manually compress the rear prods to get some kind of balance, and making it at all possible to cock.

    Started off without tapering the limbs, so the bend was noticably focused around the middle.
    A quick trip to my workshop and some angle grinder time fixed some taper, but I should probable have used a more aggressive taper since the bow shape could be prettier.
    As a side note, access to snow helps for cooling the metal when cutting to avoid overheating tempered steel  santa
    Also added a retention for the bolts, and ran into a bit of trouble with it being in the way for my stubby fingers...
    A couple of shots with some chop sticks as bolts, I really need to get some 8mm staves to make some better bolts...
    Haven't measured draw weight yet, also some kind of spanning aid feels more or less necessary, but most important would be some pulleys for performance.
    The way it is now, it's more of a kid's toy - which admittedly should suit me.
    Compound twinbow pistol (posts vs pulleys) Straig10

    Measured the distance between the front and rear posts, at rest and when cocked.
    Rest: 180+180mm (front+rear)
    Cocked: 140+145mm (front+rear)
    Change: 75mm (compounded 6 times equals 450mm, draw is 21cm)
    Calculations give 443mm which seems close enough (to 450mm) considering measuring errors.
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    Post by Anatine Duo on Mon Mar 04, 2019 7:55 am

    I like it!
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    Post by Mooki on Wed Mar 06, 2019 2:23 am

    Thanks  Smile

    It's now disassembled (again) waiting for some 25mm "Bemalon 1000" round stock so I can carve out some pulleys.
    I noticed that the (rather flimsy) aluminium prod-support shows signs of deforming, hardly surprising since all the pressure is transmitted through an M4 with a small washer.
    I'll probably place the prods on the outside of the support when assembling again.

    I've also discovered that it works quite well as a Nerf gun, been popping off those little bolts all around the living room, surprisingly not every family member is a fan  ???

    I know I have a Chrony M1 somewhere, once I find it I'll do a comparison between posts and pulleys to get some data. Might not matter to anyone but myself, but I find such things quite interesting...
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    Post by Gnome on Thu Mar 07, 2019 4:28 pm

    Mooki, I'm pretty intrigued by what you're doing here. I didn't notice at first but this is a sort of "reverse" Twinbow, you're drawing back through the length of the bows instead of away from it, making the whole contraption more compact. I'm not too sure your construction methods will hold up to any serious tension, but as a testbed project, this is very interesting!
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    Post by Mooki on Mon Mar 11, 2019 4:46 am

    Thanks Gnome.
    Since the thread has gone a bit astray I changed the title, hope nobody minds.

    I had actually forgotten that I changed the draw compared to the Twinbow, this build is rather aimed at being compact so I felt it was a natural mod.
    True about the construction, I hope that moving the prods outside the support, possible also trying to spread the pressure a bit, will help out.
    During the weekend I've made 4 pulleys, ate least 4 more to go - but I'm thinking I might drop the midway-pulleys all together. Shouldn't really matter as long as the string is clear of interference.
    Also used the angle grinder on the prods, adding a little bit of taper and extending the tips, now at 25mm to allow for better "pulley mounts".


    Current prod dimensions are:
    Length: 300mm
    Width: 32mm
    Thickness: 2mm
    Center: 25mm (un-tapered part)
    Taper goes from 32 to 14mm where the tips start
    Prod tips: 25mm long


    Will update with some more pics as I stumble along...  Smile 
    Not very pretty pulleys, but hopefully functional.
    Compound twinbow pistol (posts vs pulleys) Dsc_0210

    Compound twinbow pistol (posts vs pulleys) Dsc_0214

    Compound twinbow pistol (posts vs pulleys) Dsc_0213
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    Post by Mooki on Wed Mar 13, 2019 2:35 pm

    So, some updates.
    First of all, posts really suck compared to even fugly pulleys like mine.
    Using the pulleys, flex is evenly distributed throughout, also the shooting is much "snappier" so performance seems highly affected.

    My little manufacturing setup for the pulleys below:
    Compound twinbow pistol (posts vs pulleys) Manu10

    After reaming out the center hole to 5.5mm for each pulley they were mounted where the posts previously were.
    I decided that skipping the middle pulleys was probably a bad idea, so pulleys all around.
    Looks like making the shafts 25cm each should work well, I'll probably do something bodkin like for the points, not sure about fledging - if I'm gonna go bare, 2 or 3... I'll probably try a bit different styles and see what works.
    Compound twinbow pistol (posts vs pulleys) Dsc_0310
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    Post by Gnome on Tue Mar 19, 2019 5:05 am

    Looking better all the time! The pulleys look great, a lot better than my first go at making them.
    I say yes to fletching. The way it is set up now, two would be easiest, I think. It looks like your rail would clear for a third vane, but your bolt clip would squash any that were angled up that way unless you narrowed it down to less than the width your bolt. I bet your shooting will get a bit snappier still when you remove all the unnecessary weight from your pulley mounts.
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    Post by Mooki on Tue Mar 19, 2019 5:45 am

    Thanks Gnome, always good to get some feedback  Very Happy
    You're right about the bolt clip, it was intended to narrow down but I forgot before riveting.
    If I narrow it a bit I should be able to use vanes up to 12mm height (if 3 vanes).

    Currently it's all in pieces, again...
    Ran into a bit of trouble with my roller nut, I built the trigger system using 2mm steel I had available - however the distance between the rails is 5mm, recipe for disaster anyone...?
    Worked well enough before the pulleys, I think I now get some more draw, so the trigger components are misaligning due to force - spontaneous ejac... sorry... release.
    Good thing I was still using dull plant supports.
    Currenly making a 4mm thick roller nut, probably also the release arm will need to be redone.
    Mean while I'm trying to come up with a simple bolt design.

    And, some pics of course - no thread is complete without loads of pics  Laughing

    Used some scrap pieces I had laying around, some 25x4mm steel and 10mm copper tube.
    Compound twinbow pistol (posts vs pulleys) Dsc_0312

    Soldered the copper to the steel after a quick visit to the bench grinder.
    Compound twinbow pistol (posts vs pulleys) Dsc_0313

    Voilá! Not totally happy though, depending on which method I use I get F.O.C. 24 or 30, quite high...
    Some vanes might improve it a little bit, not sure where I want it, but quite probably way lower.
    I can only imagine using a bodkin tip would increase it even further, I'm unexpectedly fond of the look of this one.
    Hopefully thinning it a bit is enough to get F.O.C. down, together with adding vanes.
    Compound twinbow pistol (posts vs pulleys) Dsc_0314

    Old and new roller nut.
    Compound twinbow pistol (posts vs pulleys) Dsc_0315

    Had to remove the stop from the nut, still works quite well. I currently use some 0.8mm 316LSi TIG wire for springs, seems to work well.
    Some work remains to get the angles of the mating surfaces right, but it's getting there.
    Compound twinbow pistol (posts vs pulleys) Dsc_0316
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    Post by Mooki on Fri Mar 22, 2019 5:38 am

    Since the weekend is upon us very soon I thought I'd add some more pics.
    Re-did the trigger system a bit, removed one of the springs and changed placement of the remaining spring.
    The result is much more secure thanks to the new roller nut, no tendencies to the pistol being "trigger happy".
    Compound twinbow pistol (posts vs pulleys) Dsc_0317


    Weird thing with the arrows (at least to me having more or less zero experience with these kind of things). To me an arrow point should improve penetration - but when shooting just a plain shaft it blew through the box and kept on going skipping around, while the complete arrow got stuck in the middle only penetrating one side of the box.
    I suppose thinking about it this could be due to two things, the arrow head requires a larger hole to pass through, and the increased weight exceeds the optimum quite a bit - so less energy is transferred to the arrow.
    My long term plans will be to try different arrow weights and measure the energy, the weight with most energy should be best suited for this particular bow. Thoughts about this?
    So far I've tried 4 different weights.
    Short shaft, 6g (sharpened tip)
    25cm shaft, 8g (flat tip)
    Arrow 1, 16g
    Arrow 2, 17g
    Best penetration I achieved with the 25cm shaft, so I guess this means that 6g is too light, and probably 16g is too heavy.
    Measuring speeds and calculating energy should give more detailed information.

    *** Edit to the above ***
    I found that increasing the pre-draw (is that even a word?) made all arrows just blow through my cardboard box like it was rice paper. I should probably find something to put over the nick one of the tipped arrows made in the floor before my SO takes notice...


    Below the 'super duper penetrator shaft' and the regular bolt.
    Compound twinbow pistol (posts vs pulleys) Dsc_0319

    Decided to trim the arrow head a bit, and also try a new one from 5x15 steel and brass tube - although that is probably the wrong way to go, my third try will be from som 2x15 steel.

    Below the 4x25 with copper pipe arrow head (trimmed down) and the 5x15 with brass tube arrow head.
    The weight is almost identical, 9g for copper pipe arrow head and 8g for brass tube arrow head, the shaft by itself weighs 8g.
    Totals 17g for copper pipe (=262 grains) and 16g for brass tube (=247 grains).
    Compound twinbow pistol (posts vs pulleys) Dsc_0318

    So, back to the infamous F.O.C. I mentioned in my latest post - I've seen 3 different ways of calculating this.
    Method 1: Measure the complete arrow from nock to tip (=27cm)
    Method 2: Measure the length of the arrow shaft without head (=25cm)
    Method 3: Measure the arrow to the point where it enters the arrow head or where the insert starts (=24cm)
    They all use the same way of measuring from Rear to Point of Balance (19cm for brass, 19.5cm for copper)
    The different methods give widely differing results.
    The most popular (or maybe most widely accepted) seems to be method 2, but to my mind method 1 seems most logical.

    Brass, method 1: F.O.C. = 20.4%
    Brass, method 2: F.O.C. = 26%
    Brass, method 3: F.O.C. = 29.2%
    Copper, method 1: F.O.C. = 22.2%
    Copper, method 2: F.O.C. = 28%
    Copper, method 3: F.O.C. = 31.3%

    So, I have for Brass a F.O.C. of 20.4-29.2%, for Copper a F.O.C. of 22.2-31.3%
    Leaving me very confused...  scratch
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    Post by Mooki on Mon Mar 25, 2019 2:43 am

    Still trying to figure out where my fishing scales are hidden, until then I don't really know how to measure draw weight.
    This weekend was partly spent manufacturing some bolts with different weights and head designs.
    But, first I took the two finished bolts and tried the out on a piece of plank I found in the shed.
    Compound twinbow pistol (posts vs pulleys) Dsc_0320
    Compound twinbow pistol (posts vs pulleys) Dsc_0321

    At least they got stuck, but only if I hit with the head parallell to the grain, otherwise they bounce.
    I think the tips need to be lighter, thinner and possibly more pointed.
    Found the cut-offs from the prods and thought they looked useful.
    Unfortunately I didn't get the time to try them out, somehow family keeps getting in the way of pursuing hobbies...
    Compound twinbow pistol (posts vs pulleys) Dsc_0322

    I fixed the tips using hot glue, and reinforced with some fishing line coated with epoxy.
    After weighing the new arrows they are still a bit on the heavy side, so I also made some with just copper pipe points.
    So far I have arrows ranging from 6.5g to 19g, different heads and gaffer-fletchings.
    I'm not managing to get the gaffer-tape quite centered, not sure how much the offset will affect flight, at least they're parallell to the shafts.
    Compound twinbow pistol (posts vs pulleys) Dsc_0323
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    Post by c sitas on Fri Mar 29, 2019 8:50 am

    Mooki, you are on the right track. Just keep looking for ways to raise your power stroke. Looking at your design , there might be ways.If they would be enough remains to be seen. Keep up the good work, you seem very handy with the tools you have.
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    Post by Mooki on Mon May 06, 2019 8:21 am

    Thanks for the cheery comment Smile
    Been a while since I had anything to add, so I have refrained from updating my post.
    This weekend I finally located where my Chrony had hidden, I'd apparently left it at my parents cabin a few years back when I was testing different pellets in my air rifle.
    Took it home with me and did a (very) quick check in the backyard.
    Seems it's more of a toy than I thought.
    First of all it's extremely uneven in performance, possible due to the pulleys.
    With the same arrow I got the series [8.5, 4.7, 4.7] Joules.
    I much prefer the first shot, unfortunately I missed the target so couldn't check penetration.
    Another arrow gave me [1.2, -, 4.1] Joules (one shot didn't trigger the Chrony.

    My aim was to try to map which arrow weight gave my the best energy transfer, but with the uneven performance I'm not sure it will be of any use.
    I didn't have the time to test all arrows thoroughly, so I'm still lacking a lot of data, but so far it seems the heavier arrows retain more energy - unfortunately their flight characteristics are similar to those of a rhinocerous  Very Happy

    I'll see if I can figure out why the performance is so uneven, and also make a more complete measurement series.

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