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getting rid of holes in leafsprings

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Hermit
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getting rid of holes in leafsprings

Post by Hermit on Tue Jan 14, 2014 12:19 pm

Since joining the guild,I have seen several postings from members who have wanted to use a leaf spring to make a crossbow prod.The project killer that they come across,is the locating hole in the middle of the spring.Although I have'nt done it myself,I believe there is a way to do it,and retain the strength of the leaf spring.Get an experienced welder to fill in the hole with an arc welder.Leaf springs are hardened and tempered,and generally speaking,it is not a good idea to expose them to enough heat to ruin the temper,rendering them useless as a spring.Arc welders create metal melting temperatures instantaneously,and a leaf spring has some mass.It takes time for heat to travel through metal and reach tempering temperature,if the metal does not exceed tempering temperature,obviously, temper will not be affected.If heat can be rapidly removed from the weld area,then I think that the small local difference,where the weld meets the edge of the hole will not compromise enough hardness to make a difference.Brownells,the gun supply people sell a compound for this purpose,but I believe the same result can be achieved by packing and wrapping the leaf spring with wet rags as closely to the weld as possible.Weld strength is not an issue.All welding rods are numbered,the most commonly used rod is 6011.The first 2 numbers indicate the point at which the weld,when properly done,will break or fracture,with 6011,the breaking point is 60,000 lbs. per sq. inch.Welding rod also comes in different materials,such as mild steel,stainless steel,cast iron,and various steel alloys,so if you're bothered by having 2 dissimilar steels mixed together,a good welder should know what rod is compatible.I own an arc welder myself,and use it,but I know I am not skilled enough to be sure of a perfect weld,that is why I recomend a skilled welder,it would take him(or her,lets not be sexist)no more the a minute or two to do this job.I see no reason why this method will not work.having said that I'm sure there are members out there who have more experience in this area than I do,and I would appreciate their thoughts or criticisms.
                              Hermit.

Rizzar
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Re: getting rid of holes in leafsprings

Post by Rizzar on Tue Jan 14, 2014 1:02 pm

Sorry to intervene, but welding will heat the area and completely destoy the temper there.
It does not matter how short or long you do it, the temperatures reached at that point will be far over limits, that is how welding works. Metal is melted and so combined/filled.
The only thing you do is creating a larger area of structual difference than the hole was before.


If you weld any area of a prod you will have a potentially dangerous bow that in best case will bend over at that position (if you are lucky enough to have softened the steel by heating and not so stupid to quench the welded area).
In worst case it will break.

I strongly suggest: DO NOT DO THAT!


edit: sorry if I sound a bit harsh, just want to prevent harm.
In fact I was very curious to see something new when you announced this post.
I thought about leaf springs a lot before I came to the point when I realised that building a prod from plain spring steel was actually "easier" for me than dealing with a leaf spring.
But I was hoping you would not bring up the idea of closing the hole by welding, since I had that discussion some time ago with someone that was not willing to hear my advice.

This morning I thought about a thread what disadvantages/dangers i see in leaf springs, perhaps I find some time at the weekend.

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Re: getting rid of holes in leafsprings

Post by Oppenheimer on Thu Jan 23, 2014 2:44 am

Is it safe to drill a small hole in a leafspring to fasten it to the front of a bow? Or is the drilling making the surounding metal too hot and weakening it?  Should it be drilled at slow speed and has some drilling oil for cooling to be used? Whath´s the best way to form the limbtips? By saw and handheld file or some mashine tool?

Has never done this and is intrested to hear about the safety aspect of it from you people who knows this better.
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Re: getting rid of holes in leafsprings

Post by Geezer on Thu Jan 23, 2014 7:28 am

Is it safe to drill a hole in your leafspring for a mounting bolt?  No-no, Bad Idea.
Why?  Any kind of hole through the spring makes it substantially weaker at that point. Given long enough, your spring will fail right through the hole. Just don't go there!  Geezer.
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Re: getting rid of holes in leafsprings

Post by Oppenheimer on Thu Jan 23, 2014 2:10 pm

Thanks for the explanation. Also red Rissars thread "Working with leafsprings" and now I understand Why it´s no god idea. Howevwe the Strizh crossbow has some adjustment screws in the limbs close to the middle. Thats where I got the idea.

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Re: getting rid of holes in leafsprings

Post by Spyro.Magno on Sun Dec 14, 2014 7:44 am

Just an advice, NEVER try to put a welded piece in to water to cool it even mild steel, you'll drastically lower the joint resistance.

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Re: getting rid of holes in leafsprings

Post by Hermit on Sun Dec 14, 2014 11:57 am

I agree with all the information regarding what not to do on this thread....................but.............with reservations.Of  every mechanical device that has ever been made,not a single one has ever been perfect.Everything  made and used,is allways a compromise.Nothing(including a crossbow)is going to last forever.Applying this principle to leaf springs I would say that yes,you can drill holes in them,the determining factors that are going to apply,are going to be the size and position of the hole,the application,and the amount of force applied in the general area of the hole.For example,there are threads on here regarding a homemade R.D.T. crossbow with limbs of spring steel.At the end of each limb there is a hole for securing the limb to it's mounting.Obviously that section of the spring being securely fastened is going to do very little work,and be subjected to very little stress.Theoretically,that hole will weaken the spring,but practically the application works,and works well.
                                         A spring does not obtain it's properties naturally,those properties are obtained by heat treatment.When a hole is drilled,if done properly,it will not affect heat treatment.Welding however,is a very different proposition.The heat generated by welding will remove the temper from the steel,and the area affected will be just about impossible to estimate,and will generate serious weakness.
                                             In conclusion.............yes,you can drill holes in steel crossbow limbs,but you need to consider carefully,all the factors mentioned above,and apply the well known scientific formula............Suck it and see!!!.
                                              Hermit.


Last edited by Hermit on Sun Dec 14, 2014 12:00 pm; edited 1 time in total (Reason for editing : words not relevant to the text)

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Re: getting rid of holes in leafsprings

Post by Spyro.Magno on Sun Dec 14, 2014 1:36 pm

Well yes and no

What are you saying is correct but you didn't take in consideration what a hole in bridle or iron mounted prod does. 
First of all any hole, notch, sudden change of shape, or even surface roughness affect the resistence of the piece.
The difference in fatigue resistance between a lapped piece and a raw sand casted one is huge, the more rough the surface the faster cracks will develop (even through just rust pitting)
Any of these factors contributes to concentrate stresses in a single area, indipendently from the fact if the material tensile strenght or section is correctly calculated. You may go from a little of multiplying the tensile strenght of the material to 0.7 to as much as to 0.2 (yes the material is rated for 400 N/mm2 but you would allow only a maximun of 80 N/mm2 of force)
One of the first commercial airliners (douglas if i'm not mistaken) after a few months of being put in service they started to break in mid air..... guess why?
The windows had the shape of a square. 
The frame was perfectly capable infact, they flew quite some time but then material fatigue ensued.

Aside from all of this, what happens is this:

In a bridle or iron mounted prod, fastening metod does not result in a joint (it does in theory but not in reality). 
It actually a double hinge (the edges of the stock) mounted beam (the prod)


So in the first drawing the bulk of the flexion is of the edge of the support (joint type of support) and the hole doesn't even matter. Really you just couldnt care less.

In the second drawing, the part of the flexion act in the middle of the prod just where you hole is.
To clarify: you calculate the section of the prod based on the fact that the maximum flexion created by one of the purple arrow act (because the prod is specular) on one edge (the triangle), but the middle has a reduction of the section (hole) and a stress concentration point (hole) where part of the flexion created from BOTH purple arrows just happen to ''bleed'' there.

So hope this clarify a little.

EDIT: Now just imagine welding that hole:

Adding different material: different carbon content and maybe alloy (depending on which rod you use)
Welding: bubbles, slag inclusion, 
Heating: so you heat a tempered material, distant parts won't be affected, near parts will get annealed, the welding area will have different steel composition that affect how it quickly it will cool down and how it will cool down. 

mmm just leave that hole there

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