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Workbench Creations is the place for conversation and discussion about do-it-yourself (DIY) projects. This DIY blog will feature projects completed by its owner as well as projects completed by other do-it-yourselfers. Workbench Creations is the place where DIYers can discuss ideas, learn about what others have done, and share their expertise.

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19 comments

Force Draw Curve Plotter

Posted July 22, 2011 10:41 AM by Del the cat

I made this machine inspired by Clarence N Hickman's machine made in 1938.
It's used to plot the force/draw curve of bows, his machine used rack and pinion gearing. I don't have much in the way of metal working facilities so I made use of pulleys string and elastic.


Here's a link to a video explaining how it works. http://s411.photobucket.com/albums/pp195/Del_the_Cat/websitevideos/?action=view¤t=FDdemo.mp4
And here's one showing it working,
http://s411.photobucket.com/albums/pp195/Del_the_Cat/websitevideos/?action=view&current=fddemo-1.mp4
I'm afraid it's cut a bit short at the end to improve downloading speed. I'll try to get a better one later.
I don't know if I'll use it much, but it's fun to see it in action.

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#1

Another pic.

07/22/2011 11:02 AM

Whoops, too late to edit it!
Here's a pic of the pen holder on it's slider, it's tensioned with a bit of hacksaw blade and held in position with a rubber band.
The pen drops back a bit slow giving a spurious plot. The thin vertical black line just right of the hacksaw blade is a piece of elasic, which needs to be thicker.

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#2

Re: Force Draw Curve Plotter

07/22/2011 11:27 AM

Can't get into photobucket, but it looks like quite a project!

I must say, you are quite crafty...

Good show!

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#3
In reply to #2

Fisrt Video Link re-done as it was bad.

07/22/2011 11:43 AM

First link doesn't seem to work, dunno why.
Second seems ok.
I'll try a new link to the first video.
Del

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#4
In reply to #3

Re: Fisrt Video Link re-done as it was bad.

07/22/2011 7:50 PM

The second one does, and gets you to the rest.

Very clever, indeed. Sort of a period plotter that could have existed at the time the bows were state-of-the-art.

Hats off to you!

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#8
In reply to #3

Re: Fisrt Video Link re-done as it was bad.

07/23/2011 6:28 AM

None of them work properly for me, everything except the actual picture/Video.

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#9
In reply to #8

Force Draw Curve Plotter

07/23/2011 7:58 AM

Try this link to my Bowyers Diary where I've embedded the video...
Del

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#11
In reply to #9

Re: Force Draw Curve Plotter

07/23/2011 5:22 PM

I am convinced that there is something wrong with the video itself as it isn't viewable there either, but the great pictures WERE!!!

I will send you an email address via CR4 mail, can you send me a copy of the video please?

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#5

Re: Force Draw Curve Plotter

07/23/2011 3:20 AM

Aw, I wanted to see the pattern after video 3.

Nice force grapher!

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#6

I'm an Idiot!

07/23/2011 4:21 AM

I couldn't figure out why I got a nice big vertical deflection when 'calibrating it' by pulling against a big dial spring balance, yet my bows seemed to give about half the vertical deflection.
Some further experimentation revealed I was reading the Kg scale on the balance, when bows are measure in pounds.
On the up-side I figure I'm twice as strong as I thought
Del

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#7

Re: Force Draw Curve Plotter

07/23/2011 6:25 AM

The second link provided the whole screen for me, except the actual pictures/video....strange effect.....

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#10

Re: Force Draw Curve Plotter

07/23/2011 8:21 AM

Both links work fine, interesting plotter, I hope the bow string is held well or you could end up shooting yourself in a very vulnerable area.

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#12

Re: Force Draw Curve Plotter

07/23/2011 6:59 PM

Very cool! It's nice to see what you can do with wood instead of metal. (and a bit of hacksaw blade.. and a rubber band...)

I'm impressed how smoothly it all moves. I assume the fit has to be quite tight for the moving/sliding parts, since a generous gap would make the plotter wobble... how did you achieve it?

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#13
In reply to #12

Re: Force Draw Curve Plotter

07/24/2011 4:34 AM

I did it by drilling holes on blocks of wood, lapping them out with a bit of spare slider rod which I'd sawn a slot up, this leaves a burr which cuts the hole just a whisker oversize so it slides nicely. I then maunted it all up and screwed it together with just one screw in each block to allow it to line up, then when it ran smooth I added a second screw each side.
To be honest, it's a bit twitchy and seems to have bound up a bit now so some draw weight is wasted working against friction rather than the spring.
There is a lot of excess weight too. Dunno if I can be bothered to make a mkII but I'm sure I could make one work pretty well if I really planed it and used some good quality plywood throughout and had the slide blocks machined from nylon.
It was just fun to make it, and the pen holder works surprisingly well.
Del

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#14
In reply to #13

Re: Force Draw Curve Plotter

07/24/2011 5:16 AM

I make CNC machines using plastic coated chipboard, met angle iron and the start - skateboard bearings, sealed ones. They are cheaply available on ebay, I have sometimes bought 16 bearings for a few Euros at the right time of day, sometimes they are completely stainless......

If you added bearings(I clean mine with meths or denatured alcohol and glue them into the chipboard with Epoxy glue - fantastic) I am sure you would improve matters directly. Double or triple bearings up for heavy loads and stability.

I use both Aluminium and stainless angle glued to the correctly cut end of the chipboard. Check here:-

"Stiction" is also part of the problem and although you would reduce this with the bearings, a small proportion would still be there, you can as good as erase this by "vibrating" the whole unit to keep things moving smoothly.

Look here for a quick explanation of Stiction:-

http://en.wikipedia.org/wiki/Stiction

Its strange that the spell checker on CR4 does not know the word "Stiction"???

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#15
In reply to #14

Re: Force Draw Curve Plotter

07/24/2011 8:16 AM

Cheers, some good tips there. I like the whole wood/metal/epoxy construction method. Knicker elastic is always a good addition too it works on so many levels.
I prob' won't make a mkII as it is of limited use, more a novelty really and I have some Osage wood shipped from Texas which is in urgent need of my attention
Del

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#16
In reply to #13

Re: Force Draw Curve Plotter

07/24/2011 1:58 PM

Ah... I'm familiar with the swelling and shrinking effects of wood with changes in humidity. But keep it stored in a very dry place, I'm sure it will work well enough for the purpose.

(A half hour outdoors where I live is enough to make any whisker-tolerance design in wood a sticking, groaning wreck. Why the doors alone are covered in boot marks, and we are thankful they aren't delicate as well as stubborn ....)

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#17

Re: Force Draw Curve Plotter

07/25/2011 10:47 AM

Very good job Del,

but I'm curious as to why the pull has a different force curve than the return? is that the human factor? If it is measuring force, then shouldn't they be equal at equal draw/return speeds?

cheers

Chris

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#18
In reply to #17

Re: Force Draw Curve Plotter

07/25/2011 11:09 AM

The return plot is just machine problems with friction ignore it completely.
Look into my eyes...you are feeling sleepy, when I click my fingers you will forget about the return plot ...3...2...1 <click>
Del

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#19
In reply to #18

Re: Force Draw Curve Plotter

07/25/2011 11:30 AM

roflmao...

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