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    Match target crossbow project

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    Kali
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    Match target crossbow project

    Post by Kali on Fri Apr 02, 2010 3:48 am

    After more than one month of trial and error with my RDT design crossbow, I decided to build a "classical" match target crossbow, and to put "on hold" the RDT design. In order to do so, I ordered a 95lb (and a 150lb too) taiwan bows, and start working.
    The stock will be 1''x2'' section and 30'' long made from solid 6060 dural I'm planning to use 0.2 inch diameter arrows that will slide on a teflon rail inserted into the aluminum stock. I already milled the stock in order to accommodate the teflon rail:

    The trigger mechanism will come inserted (dead fit) in the 0.5'' groove (with clearance for the trigger lever):
    The limbs will be fitted into a "riser" and will be hold by these bushings: (machined from aluminum)
    The infancy stage of the shoulder rest:
    I'm also planning the use non-optical sites siding on dovetail tracks and adjusted by fine threads (i don't have at disposal micrometer threads) (i'm working on the design now)
    There are 3 trigger options that i'm debating about which to use:
    1. "classical" excalibur style:
    2. A russian forum inspiration model (the original model was also posted by IVO on "trigger section"). I used ball bearings for increased smoothness:
    3. a simple romanian design with both holding bolts above the rail (much easier to fit into the small groove left for the mechanism): (I've seen this mechanism working greatly on a 170lb crossbow)
    And if we are talking about precision crossbow, these are the tools that i'm working with for acquiring this:

    In the last picture there is not the rail inclined with 0.1 degrees, but the holding table Smile
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    Re: Match target crossbow project

    Post by Kali on Fri Apr 02, 2010 4:06 am

    I also like to mention that all aluminum pieces are 6060 dural and all steel pieces are carbon steel (other than screws - that are those "high resistance steel" - harder than HSS). The bolts holding the moving parts into the trigger mechanism are made from chrome-vanadium steel (outsourced from heavy duty screw-drivers used in automotive industry)
    The maximum tolerances for all machined pieces is 0.05 mm or 0.002 inches (this is the best that i can obtain with the Chinese built milling machine)
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    Re: Match target crossbow project

    Post by Ivo on Fri Apr 02, 2010 5:43 pm

    Great job Kali Very Happy

    So the reverse limb project is postponed until better days huh? That's fine, don't let it bother you...all best things are still ahead of us! sunny

    Now the trigger selection you have is very interesting, the simplicity is a good factor here...the 1st and 3rd are of the more traditional 90 degree sear....I like the 3rd a bit more than the first - it is really unloading the sear with the extended lever arm that's combined with the latch... and the 2nd(one from Zmeelink) I like for a similar reason - it utilizes a transitional lever and has an angles sear(not sure is it's proper to call it that, so if anyone know please correct me) which distributes the pressure in a slightly different manner than the 90 degree sears and thus is a less demanding area where softer materials can be used.

    The third kinda caught my attention in a few areas, the sear area in particular...any reason why it was reoriented and extended downward sort of like a finger pressing down....I assume it was to make things more compact and move things down into the trigger housing, but I still would like to hear if there was anything else.

    Again great job and I wish you luck on this one...hope all goes well with this project.




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    Re: Match target crossbow project

    Post by Kali on Sat Apr 03, 2010 12:27 am

    The romanian guy who designed the third trigger did tested several alignments and this one was the easiest release model. However he didn't recommend me so much this design for a match target crossbow, considering the long trigger release movement (for a 0.2'' contact zone, the lever moves about 0.5'' to release). He recommend me the second model, considering it's smoothness and shorter lever travel.
    On the other hand, I'm pretty reluctant to use Zmeelink design because is very complicated and I didn't succeed to make it work properly yet (considering the very confined space, i couldn't fit the springs there .. and should be 3 of them).
    And again - I'm concerned about the movement parts bolts that will be nearly impossible to be fit in when the mechanism will be mounted into the rail.
    Also this guy give me another idea - to try to machine the trigger mechanism housing form a solid aluminum block (not to use anymore the locking plates) and to use bronze bushings for bolts for increased strengthens.
    The mechanism housing itself will have to be precisely 12mm thick in order to be fit into the rail, this will left me with the housing walls thickness of 4mm each and the steel pieces also of 4mm (instead of 6mm used up to now). Using this design with both bolts above the rail, I can machine the housing to be thicker there (20 mm by example - to match the rail thickness) ... I have to draw this to help you understand the idea Smile
    Thank you for your support,
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    Re: Match target crossbow project

    Post by Ivo on Thu Apr 08, 2010 7:22 pm

    Yeh, guys with the second design mentioned that it had a nice short pull( 0.5 cm)...I think yours has a rather short transitional lever, but then again you are battling the tight space, so compromises compromises compromises...did you give Robin's design another though, did it go well?

    Space was an issue in my case too. I couldn't find a spring small enough and could not fit the ones I had...perhaps a flat spring can be used or the one made from spring wire(bottom left section of the picture).



    Also the trigger housing can be made as a separate module(kinda like in Robin's crossbows) and bolt onto the track directly(rail>trigger) or to a transitional mounting block(rail/block/trigger)

    PS: In your other build you mentioned "screw covers"...I was recently assembling a piece of furniture and it had something similar...would you mind showing the ones you got?




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    Re: Match target crossbow project

    Post by Kali on Thu Apr 08, 2010 10:07 pm

    Hi Ivo,
    Sorry that I didn't posted last days - I've got a new "toy" to play with and I made extensive tests on it:

    The archery shop misunderstood my order and they bring me the full assembly crossbow instead of just the bow. Anyway, I bought it - is a 95lb model.
    The crossbow is pretty ok for it's price (about 100usd), but you got what you paid for ... the trigger is a mess:

    is very hard to pull and makes accurate shooting form standing position very difficult. As well the original aiming system is also very difficult to use, so i mounted a scope (also Chinese).
    the results are like this:
    The shooting distance was 12 meters and I aimed the red circle. The arrows aimed this way went into the green circle regardless of my scope adjustments (it seems that the L/R adjustments are not working at all)
    I also made a F/D curve of the bow, but i will displayed later today
    The screw covers are exactly like these, but smaller:
    I was forced to stop working on my target crossbow for several day as I broken again the pinions in my milling machine and I worked a new drive system (belt driven). Later today I will take pictures with this.
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    Re: Match target crossbow project

    Post by Kali on Fri Apr 09, 2010 11:06 am

    So this is my milling machine belt conversion kit ... sorry for the very rough execution - it was made in just two hours, but is more than functional, reliable and very quiet Smile

    The 95lb bow F/D looks like:
    5cm/ 2'' - 7,5kg/ 16.5lbs
    10cm/ 4'' - 14kg/ 31lbs
    15cm/ 6'' - 23kg/ 50lbs
    20cm/ 8'' - 29kg/ 64lbs
    25cm/ 10'' - 38kg/ 84lbs
    28cm/ 11'' (max draw length) - 45kg/ 99lbs (greater than claimed!)
    All dimensions are the same as Ivo's 150lb bow but the draw length ... I assume that they use exactly the same bows on 95 and 150 lbs crossbows, the difference in power being achieved from longer draw length.
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    Re: Match target crossbow project

    Post by Ivo on Sat Apr 10, 2010 1:46 am

    Hmm...I haven't done an FD curve on that bow...will certainly be an interesting test.

    I have a sort of a model uploaded in Google 3D Warehouse...it isn't of much help in this view, but does look cool...so download it and it should open up in Sketchup to see the actual measurements. Wink


    -this model shows a the measurements for a half of a bow...not a single limb

    In the "Crossbow Shop" forum there is a topic called "Mod by Sopel"...there is some info on tuning this crossbow. Wink

    The bow costs around 17-20 dollars(with tips and string), so you can imagine that the trigger, stock, accessories >>>cost about the same and all together add up to around 100 bucks...that's my math...poor Chinese fellas Laughing

    The trigger on that one is pretty cool actually...being made entirely from sheet metal actually makes it wicked from my point of view. It is also used in the similar PoeLang crossbow(folding limbs) where the trigger is set forward...



    PS: Belt driven system...well...this is real progress Kali...I would bet my arm and a leg that it's much more pleasant to the ear now Smile ...I've checked it out online...the company is German and I would expect good quality from Germans...guess nowadays they also get their machines made in China just as Americans and Russians do...but all in all your decision was very smart in switching to belt drive...was the speed adjustment function effected by this switch?




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    Re: Match target crossbow project

    Post by Kali on Wed Apr 14, 2010 10:49 pm

    Hi there,
    Indeed, the belt system is a real improvement for the milling machine - not just due to the noise level, but also for the smoothness of operation and low speed capabilities. In terms of available speeds, i can use the continuous variation potentiometer to achieve anything between 50 to 1700 rpms - that being perfect for almost any metalworking operations. I didn't worked yet on wood or plastic (where high speed are usually required), but i'm confident that it can handle well.
    New items in the mailbox:
    these limbs were ordered from France and are 100% carbon (also 95lb). They are coming installed in a factory build match target crossbow. I didn't made yet a riser to draw the F/D curve, but, as a first impression : the building quality is excellent and they are very light (120 grams each).
    After another several test with the Chinese crossbow, finally I managed to make it work ok:

    In the red circle (that is just 3 inches in diameter) there are 20 arrows fired from 12m distance. The problem wasn't so much about the crossbow, but about the scope (I opened, greased and the windage adjustments come to work). I also used some slightly longer and heavier arrows.

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    Re: Match target crossbow project

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