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    pseudo compound pistol slurbow

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    pseudo compound pistol slurbow

    Post by Gnome on Sat Nov 09, 2013 10:44 am

    I've been on a kick this fall of focusing on finishing up a few stalled projects before starting anything new. Almost two years ago I started a weapon designed to fire small throwing knives. You can probably come up with a long list of reasons that was a bad idea without thinking too hard, and you'd be right! I couldn't make it work, I think my fabrication skills weren't quite up to par with my noobish enthusiasm. I put a lot of time into designing and building parts, though, and didn't want it to be a total waste, plus I needed a test bed for my first lock mechanism that doesn't use a roller nut. So I converted to fire regular bolts.

    I didn't do much in-progress photography, but I wanted to get a photo of it before I broke it down one last time for final tweaking and finishing.

    When I get it broken down I'll post pictures of the components. The body is cut from a retangular alumium tube, the prod is two 80# fiberglass pistol prods, the nock ends and pulleys are carved from Delrin, as are the trigger mech components. The bolt tracks are UHMW plastic, attached to wooden inserts for rigidity. There are two tracks, over and under, a remnant of the original sliding-truck design. I guess that technically makes this a slurbow? I'm not convinced that it's truly a compound, though, despite the pulley action, since the string ends are attacked to the tiller rather than the limb ends. Anybody care to weigh in there with an opinion?
    Thanks to Zidar for the turnbuckle idea. It's pretty cool to be able to experiment with adjusting brace height versus power stroke on the fly. As currently configured the brace height is 2.5" and the powerstroke 8.5". I think I'll go try maximizing the powerstoke by making larger diameter cams and lengthening the string. I haven't bothered measuring muzzle velocity yet, as I've still got it rigged up with paracord until I make a real string for it.

    It fires full size crossbow bolts, albeit the smallest commercially available: MTech 14" long, 5/16" diameter shafts. If you have experience with these or have read reviews, you know they're crap, at least if you go into it thinking they will be useful out of the box for a medium powered prod. These shafts are much too flimsy to be fired from a 150# prod more than once, and the substance they use to attach the components to that shaft cannot be called "glue" without insulting a host of hard-working adhesives across the globe. They were perfect for this project, though, since I wanted something lighter and was planning on taking them apart and reworking them anyway.
    More later,
    Gnome


    Last edited by Gnome on Sun Nov 10, 2013 9:17 am; edited 1 time in total (Reason for editing : pulleys, not cams)
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    Progress

    Post by Gnome on Thu Nov 14, 2013 3:33 pm


    Finally feel like I'm getting somewhere. Once I made a string for it I set up the Chrony and ran some test shots with the original pulleys. That was disappointing, averaging only 170 fps and it became quickly obvious that I still had some work to do on the trigger mech. So I made 1.5" diameter pulleys to replace the 1" originals and re-served the string where it rides on the pulleys, replacing my regular serving thread with something much finer I found in my shop- I think it was some kind of synthetic polyfilament kite string. I fixed the balky spring in the lock mech and now I'm averaging 224 fps! I was worried that the main problem was too much mass on my limb ends slowing things down, and a bigger pulley would just make it worse. The tips and pulleys are Delrin with a couple of small aluminum washers anchored on a 3/16th inch steel pin. I may replace the pins with aluminum if I think it can take the stress.

    The main mech components and internal rails:

     The trigger and intermediary toggle are carved from black Delrin, the "claw" from white Delrin. I didn't make any drawings or plans for this one, I just grabbed scraps and started drilling and carving until I got something that worked. There's a blade spring that presses the back end of the toggle up, a compression coil spring behind the claw to push it forward, and a small, weak, wire spring that serves as a bolt retainer.
    After a couple years of toil and irritation, this build is finally getting to the fun part!
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    Re: pseudo compound pistol slurbow

    Post by Gnome on Sun Nov 17, 2013 3:49 pm

    I've been thinking about making new limb tips, replacing the 3/16" steel axles with 1/4" aluminum, which would require carving new Delrin bits since the current ones couldn't be drilled out without weakening them too much. At first I was going to do this to save weight, then after a few test shots earlier this week it became apparent that the holes in my pulleys were a tad too big- the pulleys wobbled and took an angle when drawn.
    Could have done this today when I had some free time, but I finished 5 bolts a few days ago and decided just to have some fun shooting it instead. And I did, this thing is the closest thing to a "nail driver" that I've ever built. I shot a few nice, tight groups and was thinking about improvements I could make, was drawing it for another go when BAM! One of those limb tips failed and parts went pinging all over the shop!
    No getting around it now, time to carve new tips. Damn! those were a pain to make. Maybe it will go better now that I know a little bit better what I'm doing.
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    Re: pseudo compound pistol slurbow

    Post by Gnome on Sun Nov 24, 2013 10:38 am

    Finally got the new limb tips fabricated. Much sturdier now, and with the axle switched to 1/4" aluminum from 3/16" steel, I managed to keep the weight of the whole assembly the same- 16 grams for each limb. One of the old ones on top with 1" pulley, and the brass pulleys that started me down this path.
    .

    Next I had to fix the grip. I didn't have to shoot this thing too many times before I realized the trigger had to be farther forward to be comfortable.
    Before:

    I thought about making a brand new grip from some beautiful exotic hardwood in a true target pistol style, and maybe I will, but it was quicker and cheaper just to modify this one, so I cut it in two and added an insert:

    Obviously I'm not too concerned with looks at this point, but it feels so much more natural. I don't know what kind of wood that is, I think I salvaged it from a crate or a pallet. Plenty hard and tough, but not too pretty. I stained it and brushed on a fast drying water-based satin finish, left a bit rough for better grip.
     I determined that the boost in velocity I acheived when I installed the larger diameter pulleys probably had little to do with any mechanical advantage of the pulleys themselves, but is most likely due to the string being slighly shortened, I wasn't being very scientific at first about how much I tightened the turnbuckles. Ordinarily a shorter string would result in a taller brace height and shorter power stroke, but in this case due to the larger pulleys the power stroke was actually increased by about a quarter inch.
    A few images to better illustrate the geometry.



    I'm calling this one done, save modifying the arrows. Turns out they're a bit too flimsy- shooting them at short range in my shop, with such minimal vanes, I've bent a few of them. I plan on gluing quarter-inch oak dowels inside the aluminum shafts. Anyhow, my real purpose was testing trigger mech ideas, and that's what I'm going to move forward with in my next build. On to bigger and better things!
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    Re: pseudo compound pistol slurbow

    Post by andrey.factor on Sat Mar 22, 2014 10:42 am

    I must say this is pretty cool. I hope you dint mind me taking the concept into a concept with my carbon fiber. Also if you are interested I make 46 mm plies form carbon fiber and Peak. That I made fist for my carbon fiber dual bow reverse draw I charge $5 each plus USPS shipping  coves my costs as I do this for fun not profit. I do Solid modeling for manufacturing  that is my day job 

    [size=16.363636016845703][/size]
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    Re: pseudo compound pistol slurbow

    Post by Gnome on Sat Mar 22, 2014 3:15 pm

    Welcome to the forum, Andrey. Funny that you woke this thread up today, I was just working on this one this morning! Yet another redesign, after the mech failed on me earlier this week. It dry fired while I was arming it, luckily my fingers were still in the way so the prods weren't damaged. Mad 
    Looks like you tried to post an image but it didn't take- I'd like to see whatever it was, so review how to host an image and let me see it!
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    Re: pseudo compound pistol slurbow

    Post by Samuel Wilhelm on Mon Mar 31, 2014 5:31 am

    the Receiver and Handle looks like it's inspired from a M1 Thompson. looks pure Badass, i can imagine the Terminator using this:P
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    Re: pseudo compound pistol slurbow

    Post by Gnome on Mon Mar 31, 2014 6:31 pm

    Thanks, Samuel. Stay tuned, this piece has been back on the bench lately for some major revisions. I ditched the pulleys entirely, it was an interesting experiment and made for an accurate, smooth shooting piece, but it actually robbed a significant amount of power. It also was a pain to work on, with the string ends hooked on the back like that, I had to unstring it to work on the release mech, and the mech needed a lot of tweaking! I may build a true compound version in the future, but for now it's strung traditionally and I'm working on an automatically engaging safety to avoid a repeat of my bruised fingers from a couple weeks ago.
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    Not longer even superficially resembling a compound pistol slurbow

    Post by Gnome on Sun Apr 06, 2014 2:47 pm

    I guess if you mess around with something long enough you either break it or get it right. I think this collection of odds and ends has finally decided what it wants to be when it grows up:


    So I started having problems with dry firing and automatically assumed it was the delrin trigger mech that was failing. After quite a bit of tweaking and headscratching, I figured out it was actually the small blade spring that advanced the toggle into the lock position- I hadn't treated the steel right and it lost it's springiness, causing the mech not to engage fully into the locked position.

    I knew when I decided to rebuild it one more time I was going to do four major changes: add a safety, provision for a cocking rope, a folding foot stirrup, and convert it to a completely standard string geometry. The first three are all about safety. My bruised fingertips were bad enough, but think about this for a second- this think is a muzzle loader, and some of the bolts are just long enough that just the very points stick out the end, in fact I have to push them completely home with a fingertip on the point. Just thinking about trying to explain a 5/16" hole in the end of my finger to my wife is scary business! Oh yeah, and I'm also going to make all my bolts for it an inch or so longer.

    Here are the components of the safety, inspired by bits of a broken staple gun I took apart looking for treasure. When the string pushes the claw out of the way and the sear end of the toggle snaps up, the white UMHW plastic bit just slides under it, so it can't slip into the release position no matter what. Hard to see but there's a tiny hole in the end of the spring guide to hold everything in place, I was using that tiny screw but ditched it for a tiny pin that works much better.


    Ready to arm:


    armed:


    pull and twist and it's ready to fire:


    The new folding stirrup:

    Before I just had a rotating hook that I snagged on a loop of cord I'd cleverly tied around my foot beforehand. That worked about as well as you can imagine, hence back to basics. I wanted to keep the weight down up front, so it's all aluminum.

    The cocking rope:

    This is an adaption of a previous device I made for a previous overpowered pistol crossbow. A single shared handle is much less fiddly to manage, and a further enhancement is to permanently attach it through a hole in the butt block, so I don't suffer any flare-ups of my chronic case of "where'd I leave that #%&@! cocking rope" syndrome.

    And lastly I made a string and a couple of small plastic wheels to go on the nock axles and strung it up, no pulleys, no turnbuckles, no nonsense. I did that mainly because having the string attached to the block at the back meant totally unstringing it if I wanted to work on the mech. Brace height is 3", power stroke 7.5. That's a lot of bend for these little 80# fiberglass prods, I know. Of course I was also curious, and a little trepidatious, to see how my bench jalopy would handle full power.
    I got my answer: Like a champ!

    These are bolts I made for the last iteration:

    Top is the lightweight commercial aluminum bolt I shortened and modified the fletches on to fit in the slurbow track. Flew OK, but bent just about every time. 19 grams.
    Center is a quarter inch oak dowel with a 5/16" 125 grain field point epoxied on front and just enough aluminum of the same diameter on back to give the string a solid face to push, keep it aligned down the track, and hopefully add a bit of turbulence to help keep the point-heavy projectile pointed in the right direction. Only 14 to 15 grams.
    Bottom is the heavy hitter, solid fiberglass with the same 125 grain field point. I carved grooves in back to move the center of gravity forward and, again, hopefully add some air resistance to the tail end. 30 grams.

    I wanted to explain the "ammo" I was using to illustrate the Chrony results I got just now. That 30 gram bruiser averaged 232 fps muzzle speed, and the lightweight above it rang in at 305! That's the first time I've broken 300 fps with one of my homebuilts, I certainly never expected to do it with a pistol. So, in a roundabout way, I can finally say MISSION ACCOMPLISHED. I managed to cram full size crossbow power into a one-hand-wielded package.

    I was snooping around on Geezer's New World Arbalest site the other day, and right on the homepage are these sage words of wisdom:
    "A note of caution: Bows in excess of @150 lb. of draw will be hard to pull, have more recoil and destroy more bolts than the lighter bows. They will not be as much fun to play with, and are more likely to seriously injure or kill your neighbors if you should have a shooting mishap in your back yard. All bows are dangerous if used unsafely, but the STRONG bows are REALLY DANGEROUS! We do not recommend you buy a bow in excess of 150 lb. unless you have some particular purpose for a very strong bow."
    I agree with this sentiment entirely. Unfortunately, I just can't help myself, and this build is the most useless and dangerous thing I've put together yet. I should probably change my signature line to "Kids! Don't try this at home!"

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    Re: pseudo compound pistol slurbow

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