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Crossbows - Everything about Building, Modding, and Using your Crossbow Gear

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» Fascinating reconstruction
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» Cocking via moving the bow.
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» Claw crossbow prototype
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» how to Build a non Chinese repeating crossbow?
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» High Draw Weight PVC Crossbow Build
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    making rdt crossbow cams

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    Post by Hermit on Thu Oct 23, 2014 10:41 am

    Hi again all.
                   As some of you will be aware,I was a follower of,and enthusiastic contributor to the forum
    ,and then dropped off the radar.This was due to problems with internet providers,and internet reception,not lack of interest.I fell in love with patandjali's rdt crossbow,and decided to make one for myself.Step one is building the cams,so I obtained the material and got to work.I made steel templates for the cams as an aid to present and future construction.I started work on the cams themselves,using pantandjali's methods,and hit a problem.Using a hand held jigsaw to cut through 3/8th ins. aluminium plate accurately just was not going to work for me,maybe I am using a different grade of aluminium,or my saw was not accurate enough,I don't know.I solved this problem by chain drilling to remove the excess  material.This method works,but is much more time consuming,as it means a lot more filing to shape.Obtaining Teflon for the cam bushings was a problem,but I was able to obtain small bronze bushings 1/4ins.I.D which should work.I will undercut the cam pivot pins in the centre to reduce the bearing surface.I have not done ay work on the cams or the bow this summer,as here in the north,summer is a busy time.The next step for me will be cutting the grooves in the cams,and I will post how it was done on here when they are completed.I have submitted this posting,because making the cams is probably the most difficult job in constructing this crossbow.I mentioned earlier that casting these cams would be much easier,and intend to give it a try when I have completed this crossbow,as once I am set up for casting,many other usefull crossbow parts can be easily made.
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    Post by Anatine Duo on Sun Oct 26, 2014 5:26 am

    Hi Hermit, great to see someone else is going to attempt this!

    I was planning to cut my cams from plate the way pantandjali did.  

    Looking at my new store bought compound the cams are incredibly lean, breathtaking bit of engineering, 3/16" thick but over 5" across (hope I don't drop it)

    Backyard sandcasting won't give the strength to weight ratio I would want (at least the way I have done it, lost foam in greensand or petrobond, unknown scrap aluminum) for cams

    As for sawing accurately you could saw thinner plate and laminate together, this could also make the grooves easier to file.  I've been considering making the cams from carbon fiber
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    Post by Hermit on Sun Oct 26, 2014 10:16 am

    Sorry A.D.,but I have to start this reply with a disagreement.I think that casting the cams will work,and will provide sufficient strength.In order to achieve this you would require wooden patterns,as opposed to lost foam.Also,you can use scrap that is known to be good i.e.scrap from small engines(which when you think about it have to be strong enough to contain multiple explosions) there are also special fluxes you can buy that can be added to the melt and will purify it.Another problem I could forsee,was the grooves in the cams,I didn't want to file them,so I checked out Patandjali's videos again. 
    As far as I knew,you couldn't grind aluminium,so I did some research,and found that you could,and grinding discs for an angle grinder are available.Making a fixture to hold the angle grinder(out of plywood)and accurately grind the groove should not present much of a problem,and that is my next step.
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    Post by Gnome on Tue Oct 28, 2014 6:24 am

    Hey Hermit, welcome back. I've been brooding on an RDT design myself, inspired by Patandjali's work.
    Aluminum can be a difficult material for the hobbyist/home builder to work with. I have no experience casting it so I can only wish you luck with that, but I do have some recent, still painful experience with an angle grinder. I wanted to cut some slots in the wall of a rectangular tube, and thought I'd make quick work of it using the edge of a grinding wheel since the slot was to end up being over 3/8 wide. After about a minute the edge of the grinding wheel was completely loaded, and my trusty old angle grinder, a companion of many years, made a sick whining noise, emitted a puff of smoke, and expired.
    So I went and bought a brand new angle grinder. After 2 minutes, it screamed in agony, belched out tremendous billows of black smoke, and expired. Obviously defective, I returned it for a full refund! Wink I bought a different model and got smart, finally- I used a cutoff wheel instead of the grinding wheel and killed no more defenseless power tools that day.
    Anyhow, I think you'll find the aluminum loading on the grinding wheel to be troublesome, if not a deal breaker. In addition to becoming less efficient, a loaded grinding wheel becomes less accurate as the shape actually changes, and tends to mush the metal of the work piece around instead of actually removing it. A smaller surface area works better, i.e. cuttoff wheel vs. grinding wheel, but the loading is still an issue.
    I guess I might start the groove by cutting a guideline in the center with a cutoff wheel, then shape it out using round files. a clean, sharp file will cut away aluminum almost as fast as a grinder, and you'll have a lot more control over the process. Good luck and keep us posted.
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    Post by Hermit on Tue Oct 28, 2014 10:30 am

    Good morning Gnome,thanks for the welcome back.I have known for many years,that grinding aluminium is a no no,but time and technology marches on.The research I did showed that there were grinding wheels out there for aluminium,so I thought I should at least give it a try.I determined that,like patandjali,I would make a fixture for grinding the groove,using my angle grinder.I do however have a back up plan.I intend to make the fixture so that it can also be easily adapted to hold my router,and should grinding be unsatisfactory,I can try routing out the groove with a ball end cutter
                                                          I have much learning to do about my new computer,including digital photography,and how to post pictures on this forum!,but I intend to master it,because as we all know,a picture is worth a thousand words.When I do,I look forward to publishing on here especially if my methods are successful and can be of help to other forum members.I have had much experience with filing over the years,including precision filingH,but I really don't fancy filing those grooves!,it's a personal thing.
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    Post by ali.j on Thu Oct 30, 2014 12:58 pm

    i made a rdt compound crossbow. but i soon made a better cam with hard al ( 7075 ) Alloys 

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    Post by Gnome on Mon Nov 03, 2014 6:52 am

    Hermit, looking forward to whatever hints and tips for working aluminum you can provide! This weekend I was enlarging some 4" deep holes in aluminum, broke two bits and lodged another permanently in the workpiece before it occured to me to lubricate the bit. D'oh! Things went much smoother after that. Good luck with the new computer, it can be a game changer. Sometimes I spend weeks puzzling through a design idea on the computer to figure something out, then again sometimes I just grab the grinder and start making sparks with no idea where I'm headed!
    Ali J's sandwich cams are an interesting idea. Wonder how the weight and durability compare to "billet" pieces?
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    Post by Hermit on Mon Nov 03, 2014 10:49 am

    Ali's idea is an excellent one Gnome,and if you haven't started cam construction yet,is the way to go I think,there are so many advantages.3/8th ins. ali plate,is not easy to come by,and is expensive.1/8ins. would be what I would go with.Strength wise,patandjali's cams are over engineered.If you look at large pulley wheels,with spokes,you will find that the spokes are just about allways 'thinner'than the outside pulley thickness,a 1/8th centre and spokes,with I/8th laminations,would be more than man enough for the job.1/8th ali. plate is much easier to come by,and can be sucsessfully laminated with epoxy and if you have any concerns about delaminating,you can allways put half dozen rivets round the outer rim of the cam.  Thinner ali. pate is much easier to work accurately with hand tools.
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    Post by Hermit on Mon Nov 03, 2014 11:40 am

    When drilling aluminium,the drill bit should be withdrawn frequently,and all the swarf cleaned out of the drill flutes,the deeper you go,the more you should do this.When putting pressure on the bit to cut,let off the pressure momentarily this breaks up the swarf,and makes it easier for the bit to eject,this should also be done often. When drilling larger holes,start with a smaller bit than finish size.Use cutting fluid especially on larger holes.The best cutting fluid for aluminium,is kerosene(paraffin in England)over here,Varsol is a good substitute(one sniff will tell you it's mostly kerosene)and it's also excellent for cleaning off greasy or oily parts.I keep a small dish soap bottle of it handy for working aluminium.If you sharpen your own drill bits,do some research on you tube,there are 'how to' videos on there,and if I remember correctly,drill bits need to be sharpened slightly differently from standard bits for best cutting of aluminium.Hope this is helpful Gnome.............drill on,baby!
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    Post by ali.j on Tue Nov 04, 2014 1:26 am

    I'm barely Find to produce aluminum alloy namber 7075. is very hard al. 
    Thickness is 8 mm.

    i tray to made a beter cam but this aliaj is very very hard and i have not Appropriate equipment to made. 

    making rdt crossbow cams 04_14141509065731051

    making rdt crossbow cams 04_14141509065760822
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    Post by Hermit on Tue Nov 04, 2014 4:31 pm

    You can do it with the tools you have in the pictures you posted Ali,but it  will  take carefull work,and much time and patience,I did it using the same methods that you will have to use.
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