Workbench Creations

What an interesting project. Good pics, too, to illustrate what you write.

Just a few questions: What is a 'gn'? You refer to the bolt (arrow) being 440 gn, or 150 gn. Is that 'grains' perhaps? What's that in grams?

Also, why mix English and metric units, as in fps and joules? Just curious why you don't put it all in metric.

Good post. Thanks.

Yes, grains, an old imperial unit for small weights commonly used for weighing arrow heads, gunpowder, small projectiles.
1 grain was the weight of a grain of barley.
I bought a set of digital scales on the internet, it weighs in gramms, pennyweight, grains, and carats.

Only cost £8 It's the sort of thing drug dealers use (so I'm led to believe)

I use the conventional units used in archery circles, but then convert to Kg M/s to get Joules for the engineering context.
Draw weight is still most commonly measured in pounds, draw length in inches, arrow weight in grains (or ounces for heavy warbows).
I personally have no idea what a Joule feels like, I prefer things like horse power or foot pounds. If I keep doing the conversions and arithmetic I may eventually get a feel for it.
If I'm talking to the man (or pretty lady) in the street I quote arrow speed in mph.

Del

or pretty lady) in the street I quote arrow speed in mph.

she had you all aquiver ?

Cool!

When the world money melts down and we are hunting to eat in the suburbs, I wanna be your neighbor Del.

Nice work.

Del,

Excellent design, supurrb craftsmanship, great pictures.

Thanks - KJK

A beautiful design, and really, IMHO fairly sophisticated for a 'primitive'. The crossbow is an appealing weapon, being loaded and ready to point and shoot. Kudos on making one with no metal parts.

Very nice project and very nice documentation. I saw a cross bow made entirely of wood with a very interesting release mechanism years ago. The trigger mechanism was a piece of wood on a pivot pin on the bottom of the bow with two dowels fitted to the forward end that fit into two holes the went all the way through the bow and extended through the other side. When cocked, the string was held by the dowels and the arrow track was between them. When you pressed up on the wooden lever the dowels were pulled down releasing the string and firing the arrow. I thought this was a very interesting and fairly simple mechanism. Sorry, I don't have any documentation to show. I don't remember where the bow was from.

well i like your work and engineering here in this crossbow and i have some questions about the trigger mechanism - i know i could look up and study this subject but i don't want to - i want to ask you what you think is the best trigger mechanism to let the string go. i build mechanisms quite a lot and like to talk about them.

Consistently good work.

If my memory works well, both the Ruger and ahh, Woodsman target pistol's trigger was copied from swiss museum crossbows. You are in good company!

This one goes in my personal survival reference manual. I am especially impressed by no metal, the trigger mechanism and bolt retaining clip. Seems like the whole thing could be build with nothing more than a machete ( a SHARP machete!). What about instructions in how to make bolts?

hi again let me rephrase my last question - i am not trying to make crossbow mechanisms or much like them but i like to think about good ways to do things mechanically and in my first thought i imagined a fairly strong spring retracted metal rod that was set free by the trigger to be pulled quickly away and release the string. quick and balanced and counter absorbed so everything is smooth. can you tell me what you think of that sort of mechanism?

Have a look at this blog entry and part II, it shows the trigger mechanism. It's the 'standard' and most simple proven over the centuries. Modern versions have smaller parts and a much shorter trigger.
Use google images for trigger mechanisms, it's what I did a couple of days ago to look at safety catches for crossbows.
Your proposed mechanism would doubtless work, but as the rod pulls down maybe the string would tend to ride up and over it bouncing up over the back of the bolt.. the only way to find out is to try it.

Sir Ralph Payne-Gallwey's book on the crossbows has extensive drawings of rigger mechanisms.
Del

The higher the ratio of crossbow weight to bolt weight, the more efficient. The bow doesn't kick back as hard, absorbing less energy.

I think you've done a typo and got it round the wrong way.
The heavier the projectile the higher the efficiency, but only upto a point of course.
The anvil as a projectile has zero efficiency as it doesn't leave the bow.
Del

Just theoretically it depends on how firmly you hold the bow.

If you laid the bow on a frictionless table and could operate the trigger remotely then: if the bow weighed the same as the bolt then half the energy would end up in each.

Of course momentum is conserved, and, you only have a fixed amount of energy stored in the bow.

I suspect that most of the energy in the bow is absorbed by your shoulder long after the bolt has left the string, so I think I agree with Phaddy.

I think maybe there is some missunderstanding of terms here, probably on everyones part.
'Weight of the bow' is all but irrelevant... As it will be being held firmly into the shoulder.
I assumed he meant the draw weight of the bow (often just refered to as the weight of a bow), which is relevant.
If you plot a graph of projectile weight vs speed for any given bow you will find there is a maximium velocity and also a maximum kinetic energy, they will be at different projectile weights.
Obviously the lighter the projectile the faster the speed upto a point where it tops out due to limb mass (this is the only really important part of the crossbow weight). This low projectile weight is generally only used for flight shooting (going for maximum distance).
Similarly, maximum kinetic energy will be with a heavy projectile and quite a way below maximum velocity, the downside of this is reduced range and a more curving trajectory which is a disadvantage in field shooting or warfare.
The solution is to use projectiles tuned to the application, thus a medieval archer would have some light arrows for goading the enemy at long range, these would be lethal to a horse or infantry man without armour. At closer range a heavier arrow with an armour piercing point would be used to deliver maximum penetration to plate armour.
Del

Anyhow if you look at the figures on the blog you will see that the heavier bolt has more kinetic energy than the light one. The weight of the bow hasn't changed in the meantime, and the same energy was put into the system to cock to bow. Thus the heavier bolt is more efficient.
That dissproves the hypothesis that ratio of bow weight to bolt is proportional to efficiency and somewhat supports my assertion.
Del

Looks as though my simplistic reasoning was well off. It would be interesting to know where I went furthest wrong. I suspect it's (more than one) effect of the limb mass. Plus the fact that if the bow weighs considerably more than the bolt then it takes a very small proportion of the kinetic energy so other effects have much more significance.

I think you were thrown off the scent by the ambiguity of the original assertion about efficiency from Phaddy.

If you assume the bow or crossbow is firmly clamped, an amount of energy is stored at full draw. On release this will be dissipated mostly into the bow limbs, string and bolt. We ignore the string as it's relatively light and a good one doesn't stretch too much.
Thus in our simplified view the energy goes into the limbs and the bolt. If we simplify even more (and grossly) we might assume that the limbs and bolt weigh the same and move at the same speed... ( a down right lie for simplicity) in which case the energy would be shared equally between them and the bow would be 50% efficient, a heavier bolt gets a bigger share of the energy and a lighter bolt would get less.
Vastly better brains have analysed the bow and the funny thing is that despite it being theoretically incredibly non linear, the various factors seem to counteract each-other and end up giving pretty linear results.

E.G, As a bow flexes the tips come back in an arc and the string pulls back much further than the bow tips, so the string is actually acting like a lever.
The geometry of the whole thing changes as the bow is drawn, and much of our classical mechanics which is 'for small angles of deflection' goes out of the window. Yet surprisingly the force draw curve of a longbow is again fairly linear!.

Robert Hardy's book 'Longbow' has some very good technical appendices at the back.

Much of the maths and physics gets beyond me but rest assured I can testify that a heavier bolt travelling slower can have much more energy than a light one travelling faster as I've had to pull such projectiles out of the target.
I recently did some tests...I shall illustrate with some facts and figures using my 75# longbow and my 275# crossbow (both on the website)
The Longbow shoots an arrow weighing 28.93 grams at 166 feet per second.
The Crossbow shoots a bolt weighing 54.55 grams at 158.7 feet per second, (the slower speed of the crossbow bolt may surprise you).
Because the crossbow prod has heavy steel limbs it is slightly slower and shooting a lighter bolt doesn't increase the speed much.
If I convert the figures to kilograms and metres/second to get a figure for kinetic energy in recognised units (Joules) I get 37 J for the arrow and 63 J for the bolt.
(Using the formula 1/2 x mass x velocity squared)
You see the bolt is going slightly slower, and will have less range* but has almost twice the energy when it hits home.
* Actually the shorter bolt is slightly more aerodynamic than the arrow.

I fear I've exceeded my brevquot and am in danger of boring you rigid.
Suffice it to say the physics of the bow and arrow is intrigueingly complex, which contrasts delightfully with it's sheer simplicity.
Del

Speaking of range, given the data you present want to swag at *effective range?

*I'll let you define effective

I'd say the primitive bow would be good for (humane) hunting at upto say 30 yards, maybe out to 50 with sights. My longbow and 275# crossbow would be good for hunting at a similar range, but out to about 200yards against a hoard of zombies.

A medieval longbow of 150 pound draw weight pulling back 31" would be effective at about 250 for harrying the enemy. Very accurate and lethal from 100yards in even against the armour of the day.
Doubtless some will argue about the effectiveness of the longbow against armour as it was an ongoing arms race, but there are plenty of dead to vouch for it.
Approx figures for Agincourt 5,000 archers vs 15,000 French who were daft enough to charge into a muddy bottleneck between two wooded areas. So the whole conflict was about the numbers and size of a modern football (soccer) game. Next time you are at a sporting venue, look across the arena and imagine the sky darkened with arrows.
Del

heh heh

he said rigid ...

I found it interesting, I am sure I am not alone!!!

Nice work and craftsmanship on the project. Not to mention interesting.

Great pictures! I'd want someone like you nearby when/if the zombie apocalypse ever happens :)

...I went to a lot of trouble to hammer a stake into my chest last night, but we diddn't get any trick or treaters.
It's not easy preparing a meal with a stake protruding from your chest, it's hard to see when you are slicing up the onions.
I'd even made pointy teeth from an old yoghurt carton .
Still the teeth were tasty.
Yeah, I'm ready for them zombies.
Del

It sounds very creative and quite a sacrifice to stake yourself in the chest!! I only got one small group of trick-or-treaters but they had coats on so I couldn't really see their costumes.

Sounds like you probably have a fair amount of candy left. I did, but it's completely gone now...whoops!

Great project, good pictures and what a simple easily understood mechanism.....

Incredibly brilliant design and workmanship Del!

When the radiated Zombies attack and I've run out of bullets for my AR-15 I want you standing by my side with that crossbow shooting the Nasty SOB's! LOL

Great survivalist tool!!!!

Here's a graph showing how velocity and energy change with bolt mass, the data was gathered using my small bow pistol (50 pounds draw weight) for ease of testing, shooting through my chrono'. I made an adjustable weight bolt by glueing a thinwalled steel tube onto the end of a bolt, I could load this with fishing weights and then put a rubber blunt head over the end.
It illustrated nicely how velocity decreases as mass increases, but energy soon levels out. So for maximum combined range and energy we'd be looking at about a 12gram bolt. Increasing mass beyond 20 grams doesn't gain much energy and starts losing significant range.

The results surprised me (which is the whole point of doing experiments) as I expected the energy to rise and then fall again as there is a v² term

The text is a bit illegible.
Blue line is velocity, vertical scale is velocity (m/s) in steps of 10.
The red line is Energy in Joules, but I've multiplied the vales by 4 to get it to sit nicely on the same graph for comparison. Horizontal axis is grams in steps of 10.
You will see that by the end of the test I was shooting a pretty heavy bolt, literally an M8 coach bolt with loads of M10 nuts on it shoved into my adjustable bolt.
Del

Del, you are a clever old stick. Very interesting and thanks for sharing.

It isn't a stick it's a bolt.

Old UK saying that I was using about Del, not his bolt!!!

(Anyway, it may have started life as a stick, but it just bolted instead!!!)