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The Folding Knife (& Switchblade) Forum The materials, techniques and the designing of folding knives.

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  #1  
Old 08-06-2009, 03:32 AM
chalkeye chalkeye is offline
 
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newbie auto design

Hi guys,

recent lurker, first time poster here. I'm a 3rd year (of 4) Industrial Design student in New Zealand.
Just getting interested in knife making as my pocketknife just broke on me, so I looked at replacing it. Then I ended up looking at the most expensive ones and figured I might be able to make my own. Long story short, I'm here.

I was looking originally at a liner-locker, because I like the slimness of the design.
My pocketknife was a currey-lockspike, which was nice and thin and sits in your pocket easily. I figure if its nice and thin the more likely I am to have it on me. The best knife is the one you have with you!

(sorry, this is a long backstory!)

Being a bit of a nerd for mechanical moving parts I was also interested in autos, but wasnt happy with the thickness required for a button type mechanism. So, I did some sketches and some fiddling and came up with this;


It appeals to me because it is only three layers (liner, blade, liner) thick, and can afford to have relatively thin liners.

Today I found some acrylic and some free time on the laser cutter. A quick and dirty spring from piano-wire and some nuts and bolts and I made a prototype... I didnt really pay attention to the background noise, you might be better off muting it! also, the acrylic is clear so the details will be easier to see in the HQ version...

youtube video of prototype action


I'm interested to hear your thoughts on the design - some things that I'm aware of are;

- made a rough and ready spring for the locking mechanism out of some speargun rubber, it would be better to limit this movement somehow and use a metal spring

- I didnt account for the tolerance of the laser cutter, so the prototype doesnt close all the way that it should

- The spring pivots are poorly placed and so the blade isnt under tension the whole way through the movement - this shouldnt be too tricky to fiddle though. I have planned to recess the spring into one of the liners.

So thats it, i guess. Sorry for the long post! Looking forward to some critique.

Regards

Rich
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  #2  
Old 08-06-2009, 08:58 AM
Ray Rogers's Avatar
Ray Rogers Ray Rogers is offline
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Because of my very slow 21k dial up connection I can't see the video so forgive me if I must ask questions that would have been answered in the film clip.

From the sketch, it looks like you have designed a modified slip joint knife. Doesn't this knife require two hands to open it and to close it? Nothing wrong with that but the market trend has been towards one handed knives for a few decades now.

What is the purpose of the slot at the rear of the spring?

I'm all for building prototypes but I usually use aluminum. Speargun rubber is a surprise, doubt I would have thought of that. Seems like you might have tried the acrylic as a spring. That would have more closely simulated a metal spring and it should have enough flex to survive the application for a while.......


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  #3  
Old 08-06-2009, 09:12 AM
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Don Robinson Don Robinson is offline
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That's an innovative design. I'd like to see a photo of the model. The video is over so fast and because it's acrylic I can't see the spring.

If you actually make one of these there is some tweaking you may want to do with the locking mechanism.
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  #4  
Old 08-06-2009, 03:27 PM
chalkeye chalkeye is offline
 
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Quote:
Originally Posted by Ray Rogers
From the sketch, it looks like you have designed a modified slip joint knife. Doesn't this knife require two hands to open it and to close it? Nothing wrong with that but the market trend has been towards one handed knives for a few decades now.
Hi Ray, you're right, it is a modified slip-joint, but instead of a "slip" it locks in both open and closed locations. The drawing doesn't show the piano-wire spring, it is an auto.

Quote:
Originally Posted by Ray Rogers
What is the purpose of the slot at the rear of the spring?... Speargun rubber is a surprise, doubt I would have thought of that. Seems like you might have tried the acrylic as a spring. That would have more closely simulated a metal spring and it should have enough flex to survive the application for a while.......
You're absolutely right. I can pinch the back of the handle to stop it from pivoting and theres enough flex for it to clear the locking channel. duh! Speargun rubber I just happened to have lying around, so in it went.


Quote:
Originally Posted by Don Robinson
That's an innovative design. I'd like to see a photo of the model. The video is over so fast and because it's acrylic I can't see the spring.

If you actually make one of these there is some tweaking you may want to do with the locking mechanism.
Thanks Don. Here's a photo of the model laid out (I really should have scrounged around for some coloured acrylic! sorry guys).


The spring is extremely primitive at the moment.


It would really make more sense if the spring coiled around the main pivot.. I'll do some more thinking here and have a play.

Don, what do you suggest as far as improvements to the locking mechanism go?

Thanks for your input, I really appreciate it!
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  #5  
Old 08-06-2009, 06:19 PM
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Don Robinson Don Robinson is offline
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Obviously you've resolved any problems with the lock notches.

Looking at your sketches the locks are square with no angles for a lead-in, but your model works fine.

Thanks for the photos.
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  #6  
Old 08-06-2009, 07:25 PM
doublearrow doublearrow is offline
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You may have to do some tweaking with the actual design, but I think you are on to something. I'd like to see it if you do a knife with this. (I've just gotten into making folding knives so I"m no expert, but the model works)


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  #7  
Old 08-06-2009, 07:53 PM
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DaveRuhlig DaveRuhlig is offline
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Very interesting design and seems to work fairly well based on your video. I'm certainly no folder expert (no expert in anythig really) but I suspect you'll have to beef up your spring a bit if you make a metal prototype. Nicely done!
-Dave


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  #8  
Old 08-06-2009, 11:36 PM
chalkeye chalkeye is offline
 
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Quote:
Originally Posted by Don Robinson
Obviously you've resolved any problems with the lock notches.

Looking at your sketches the locks are square with no angles for a lead-in, but your model works fine.

Thanks for the photos.
Thanks Don. I think the model works because of the tolerance of the lasercutter, if i were to make it more precisely I think i would need some kind of lead-in.

I have just been speaking to a man here in Wellington who reckons he has some HSS that is 18% tungsten that he thinks would be suitable for the blade - I couldnt get any more detail from him from that, other than he said it wouldnt need heat treating (?), I could just grind it into shape. Is it worth looking at?
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  #9  
Old 08-07-2009, 07:46 AM
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Don Robinson Don Robinson is offline
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That sounds like M2 HSS. He may have a planer blade or some other tool that has already been heat treated.

M2 would make a very good blade.
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  #10  
Old 08-07-2009, 08:58 AM
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ARCustomKnives ARCustomKnives is offline
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That's a very intriguing design. I like it! I'd definitely like to see this made into a knife.

Or even just a metal prototype.
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  #11  
Old 08-08-2009, 08:30 AM
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SharpByCoop SharpByCoop is offline
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Great work. I'm impressed, too.

Parts for the knife, including auto coil springs, are easily available at www.knifekits.com.

Keep us posted!

Coop


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  #12  
Old 08-19-2009, 08:32 AM
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Geno Geno is offline
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I have made that mechanism for years and call it a toggle lockback.
I have heard it called a French folder too (probably why I quit making it)

If you use a kick spring, they work great.
Coil springs can work too.
They are a good ambidextrious carry design.(either hand auto)
Have fun and hollar if you need help.
God bless, Geno
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  #13  
Old 09-20-2009, 03:38 AM
caseynz caseynz is offline
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hi,im in wellington too.ive made that same style knife too,id be keen to have a chat with you about it if you want.heres the one i made.it uses a leaf spring though.
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