Compound project

by MRobin Fri Sep 21, 2012 3:23 pm

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Post n°1

Compound project

by MRobin Fri Sep 21, 2012 3:23 pm

First topic message reminder :

Hi everyone
My first thread was about a medieval-ish crossbow characterised by a big draw length which compensated a little for the weak wooden prod. Since I can't get better material for now, I decided to start a new project that allows me to use various material with maximum draw length and minimal bending of the material in question. Therefore I went for a compound system, with four pulleys which increases the mechanical reduction, here's the first idea :

Compound project - Page 3 3darbalete

And I've been working on this over the last couple of days :

Compound project - Page 3 Img0224rp

Those are just cheap weak wooden leaves, if it works I will have something strong.
First I need to see if the string travels nicely through the pulleys.

Compound project - Page 3 Img0222rs

by Hotspur Thu Aug 29, 2013 2:37 pm

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Post n°51

Re: Compound project

by Hotspur Thu Aug 29, 2013 2:37 pm

Fascinating project.

Consider getting your hands on a couple billets of Gordon Composites Power-Tuff glass :
Bingham Archery catalogue Page 3 bottom right:
[url=http://www.binghamprojects.com/Bingham Projects, Inc. Catalog 37.pdf]http://www.binghamprojects.com/Bingham%20Projects,%20Inc.%20Catalog%2037.pdf[/url]

I ordered four 36” ‘sanded-one –side’ .5 “ x 2” they were about US$26 each plus shipping. And maybe some B50 Dacron for the cables?

by MRobin Fri Aug 30, 2013 11:11 am

MRobin

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Post n°52

Re: Compound project

by MRobin Fri Aug 30, 2013 11:11 am

Pretty unexpensive! I'm planning on buying 45# or 50# cheap limbs for the wooden version to work again while I keep building the terminator one. B50 will do the trick for sure.

I'm back to uni on next tuesday so ready for some time without any progress.
Still I'm working on fixing the metal rail to the tiller. 2x8mm bolts at the front and at the rear, with hard wood spacers (cherry). Should be done before studies begin.

by MRobin Sat Aug 31, 2013 1:23 pm

MRobin

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Post n°53

Re: Compound project

by MRobin Sat Aug 31, 2013 1:23 pm

Standby mode activated now, see you probably in a few months.
Yeah, two bolts are missing (no more threaded rod), and long screw tips haven't been cut yet.

by Gnome Sat Aug 31, 2013 1:49 pm

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Post n°54

Re: Compound project

by Gnome Sat Aug 31, 2013 1:49 pm

Wow! Really looks like a siege engine to me. So you're going to take a break for a few months and leave us hanging?
Gnome

by MRobin Sat Aug 31, 2013 2:28 pm

MRobin

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Post n°55

Re: Compound project

by MRobin Sat Aug 31, 2013 2:28 pm

It's likely, but who knows, I may have a few days off before it gets too cold.
I tried to draw the string as far as I could with both hands, locking the tiller between my abs and the ground : had a hard time to get it past the crosspiece. So I guess it was close to 100# at some point, I'll have to downgrade the power a bit if I want to get big powerstroke.

As for looking like a siege engine, I concur, I can still carry it but with 7 kgs it gets on your muscles a bit.

by octavioaugusto.oliveira Sat May 17, 2014 8:06 am

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Post n°56

Re: Compound project

by octavioaugusto.oliveira Sat May 17, 2014 8:06 am

Hello Mrobin

First of all, my congratulations concerning your realizations. Very nice! Please, tell me if you painted the limbs...why do not cut the extremities, not necessary if you dedicate them to this weapon?
I am very enthusiasm to make the first one with a pair of limbs. Indeed the power of my limbs are 35 pounds, too much for me...I am 77 years old you see...
Your stock is a work of an excellent scratch man...I will try to do all my best. In your experience how long fly an arrow shouted by your first weapon with two limbs?
Best Regardas
Octávio Augusto

by Hermit Thu Oct 23, 2014 1:20 pm

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Post n°57

Re: Compound project

by Hermit Thu Oct 23, 2014 1:20 pm

I have just read this thread,and it looks very promising.this build used pulleys,as opposed to cams.I would think that cams would give more of an advantage,but Idon't have sufficient knowledge of exactly how cams work in a crossbow,is there any member who could give an explanation or recommend any literature that does?thanks in anticipation,
Hermit.

by MRobin Thu Oct 23, 2014 7:03 pm

MRobin

MRobin: Tinkerer
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Post n°58

Re: Compound project

by MRobin Thu Oct 23, 2014 7:03 pm

octavioaugusto.oliveira wrote:Hello Mrobin

First of all, my congratulations concerning your realizations. Very nice! Please, tell me if you painted the limbs...why do not cut the extremities, not necessary if you dedicate them to this weapon?
I am very enthusiasm to make the first one with a pair of limbs. Indeed the power of my limbs are 35 pounds, too much for me...I am 77 years old you see...
Your stock is a work of an excellent scratch man...I will try to do all my best. In your experience how long fly an arrow shouted by your first weapon with two limbs?
Best Regardas
Octávio Augusto

Hello, sorry for the delayed reply.
Did not paint the limbs. The extremities have already been cut, I could have cut shorter, indeed. The 4 limb version could shoot roughly as far as a 60# hunting recurve bow I guess. How far exactly, I don't know, probably like 130 meters ? Possibly farther with light carbon shafts...
All the best with your project.

Hermit wrote:I have just read this thread,and it looks very promising.this build used pulleys,as opposed to cams.I would think that cams would give more of an advantage,but Idon't have sufficient knowledge of exactly how cams work in a crossbow,is there any member who could give an explanation or recommend any literature that does?thanks in anticipation,
Hermit.

Cams are probably the best but, just like you, I don't have sufficient knowledge of these things and it probably needs advanced machining tools. This project uses the most basic concept of mechanical reduction with pulleys. By the way, I had some thoughts about why medieval crossbow makers didn't experiment with such design, since they obviously knew about pulley reduction principles on windlass crossbows. And it's not hard to build.

by Hermit Fri Oct 24, 2014 10:25 am

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Post n°59

Making cams

by Hermit Fri Oct 24, 2014 10:25 am

Cams are not difficult to make and can be made with hand tools.If you go to the section of this forum that deals with archived posts on compound crossbows,and pull up the videos by Pantandjali,there is a quick how to on how he makes them,all with basic tools.He also provides drawings of the profile he used.The cam profile is the thing,once you have that,the rest is just cutting and filing not overly difficult if you are handy,patient,and prepared to put in the necessary time.
Cams have a number of uses,they can be used for converting rotary motion to linear,timing events,acceleration,and torque multiplication or reduction.I understand what they do,but don't have the math or physics to design them............frustrating,but that's life I guess.
Hermit.

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