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On 7/4/2018 at 1:00 PM, Jon Kerr said:

You never know... someone might one day wish to do a similar calc

Jon,

I'm not a structural engineer, but if I remember my old statics calculations I think there should be some kind of allowance made for the effects of point loading a distance from the location of the moment transfer to the leg of the assembly.  My texts indicate that yield strength for mild steel (A36) should be on the order of 250 MPa.  Also, if I read your calculations above correctly, with an allowable moment on each leg of 986,642 N-mm, and an allowable force of 1,973 N, you are saying that your moment arm is 500 mm?  This is likely the length of the leg, but doesn't appear to account for off center loading in the bowl.

I think the calculations you did are great, but when you do your calculations it is best to look at the potential failure points, rather than just the bending ratio for the steel legs.  In this case I would be most worried about the steel bowl buckling at the weld connection rather than the well engineered legs.  The location where the new legs cross the existing cylindrical rim may provide some additional support, but I would have considered putting in  additional welded reinforcing gussets at those three points to try to limit the stress on the body of the bowl.  I'm not sure how hot the steel bowl can be expected to get, which may also influence the strength at those points as well.

As regards gifts, the colonial style kitchen/barbecue implements as demonstrated by our member jlpservices are a good alternative as well.  Of course she makes forging them look easy, so be sure to give yourself extra practice time and stock to get it right.

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39 minutes ago, Latticino said:

I would have considered putting in  additional welded reinforcing gussets at those three points to try to limit the stress on the body of the bowl.

A triangular stretcher connecting the three legs (just above the center twist) would also help.

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On 7/9/2018 at 10:41 AM, Latticino said:

I'm not a structural engineer, but if I remember my old statics calculations I think there should be some kind of allowance made for the effects of point loading a distance from the location of the moment transfer to the leg of the assembly. 

Of course, you're absolutely right on all points! I do actually happen to be a structural engineer....! :D In reality, in my real job, I would do this sort of analysis in FEA (Finite Element Analysis) so i could take into account all the material properties, geomety, and plot stresses on the 3D model. My hand calc was incredibly crude and done more for amusement than anything else.

In actual fact the yield strength I used should probably be even lower than 250MPa due to the weld affected area. Perhaps 180Ma would have been more appropriate.

The bend in the leg is also a stress raiser and this would result in a concentration.

The buckling of the bowl is indeed also a probable failure point!

Its worth remembering though that the 180/250MPa is the YIELD stress, at which point the material starts to deform through the plastic region (and won't flex back). The failure stress, whereby something actually breaks, will be higher.

With a 3 legged structure (much like a 3 point hoist/lift) you're likely to get a fairly even load distribution through the legs, even if the firepit was stacked off centre.

By my hand calc, the "Factor of Safety" was 24. Even taking the lower yield strength of 180MPa (approx half my value), that leaves a Factor of Saftey of 12 (on yield). Even if the dish begins to buckle (and I doubt it), its highly unlikely that any catastrophic failure will occur causing burning embers to tip over the floor!

 

All that said........ for anyone wishing to build a similar firepit, I would probably not copy the design of my legs. They were done that way more to account for my lack of forging skill than any other design choice. Taking Thomas Powers suggestion of an additional large scroll, (on each leg) to sit under the centre of the dish would be a good idea, though it would use at least double the material for the legs. Alternatively, welding stiffeners under the centre of  bowl where it attaches to the legs wouldn't hurt.

 

 

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*sigh*

On 7/9/2018 at 12:17 PM, ThomasPowers said:

Weld affected area and not the forge affected area?  (Much larger and probably more affected over a greater area...)

True!- although if the legs were quenched after being in the forge (though not tempered), would that then increase their strength again?  I have no idea how it works. I need to read a metallurgy book I think.

 

..................................................................... ITS STRONG ENOUGH FOR A FEW LOGS! :D

 

 

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If the steel is low enough in carbon content quenching them will have NO effect on the hardness/toughness. If they have way too much carbon in them quenching will result in them shattering.  In between amounts vary along this spectrum in proportion to carbon content. 

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I'm sorry I don't understand that question.  Do you mean the softness achieved?  Lower carbon content reduces brittleness and so could be seen as the opposite. So weakening in yield strength, hardness, charpy test,...?

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  • 2 months later...

I finally got back around to doing some blacksmithing..! In fairness, I’ve had a busy couple months since we just got married.

In the mean time, I’ve been watching lots of forging YouTube videos and reading old threads on this site to try and soak up as much information as possible.

 

Firstly, I wired up my forge blower properly. It needed a startup capacitor and I wanted to add a foot switch. It works great. I can actually control the air flow by turning the fan on and off rather than using the valve.

 

I’ve been desperate to try my hand at turning a ball pein hammer into an axe/tomahawk, so thats what I tried to do today. I’m really pleased with how it turned out. No idea what the steel is so it may not hold an edge- in future I’d like to repeat the project and forge weld in a high carbon tip. For now though, I just wanted a simple axe for splitting kindling!

 

Lessons learnt:

  • I’ve started breaking my charcoal into much smaller lumps, approx 1” square. Much better.
  • My forge is working much better with the blower and foot switch. Much quicker and easier to heat metal.
  • My main problem now is my tongs are pretty rubbish, and I need to make a couple of new pairs for specific stock sizes. I’m having trouble hanging on to the workpiece, and its causing the stock to bounce. I can’t possibly turn the work piece between blows as I barely have hold of it as it is!

 

Any tips/comments/criticism about the axe would be appreciated!

Ive also attached some pics of my current set up for any other beginners who might be interested...

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I finished my axe!

Rubbish by most standards but I’m really proud of it.

I decided to have a go at charring the handle. I also carved out some recesses for some decorative wrappped copper in a couple of places, just for aesthetics.

(For anyone interested in the copper wire process. I drilled a small hole to poke the wire into to start, wound really tight, and drilled another small hole and cut the end just right to poke in to the end hole. Dab of superglue to secure. Held the wire really tight and worked great. No idea if this is a common method but it worked for me, and meant zero wire overlap.)

This was my first try at heat treating. Its still not very hard in the end (tested with a file) so perhaps the steel is just shoddy.

Unfortunately, during the final grind I discovered a hairline crack. I’m not sure what the cause is- I did introduce a cold shut during forging but I was sure I had ground it all out before finishing forging. Perhaps not. Or perhaps caused by  quenching in water rather than oil....

Any comments and criticisms welcome.

 

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I'm sorry for all the stupid questions. The learning curve is incredibly steep without anyone experienced to forge with early on..... I need to find the cash for a class ASAP.

 

I've been wondering about oxidation and fire regions.

Obviously I have little to zero experience of these things and nobody to look over my shoulder and say "you're burning your steel".

Does the axe look over-oxidized to anyone? I don't know how I would tell.

 

I'm still concerned about my fire management, really. It's possible my JABOD set-up isn't exactly perfect in terms of size and depth of the firebowl.

I was previously struggling to get things hot, especially if I stick to the "keep the stock horizontal" rule. Im finding it difficult to maintain the fire to stay hot enough at "ground level" (by which I mean, my JABOD dirt surface level, ie/ the top of the fire bowl) to get steel for forging temperature. With the axe, I allowed the axe to be buried a little lower than the ground level and had much fewer problems getting it hot. That also allowed me to use far less charcoal and forge for longer without adding more charcoal.

I'm now concerned I might have been letting the fire die down too far (not enough charcoal to raise a mound), and shoving the axe right into the oxidising region of the fire.

 

Is there any way to tell what by eye where each of the fire regions are?

 

Another thing I noticed is a green hue to the flames at some point. This could be from staring at the fire too long, or it could be because it was getting dark and I've never forged in the dark before. Whats this? Steel burning? Firebrick burning? Normal?

 

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There are some threads on the forum that show the regions of the fireball. Basically, oxidizing is close to the tuyere, reducing is at the top of the fireball, and neutral is in between.

Now, go grind that crack out before someone gets hurt!

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I think the little axe looks fine other than some hammer control and yeah, grind that crack out. I would guess it's still some of the cold shut. Its best to correct or grind out cold shuts as you go, or practice not getting them. ( there are some methods scattered throughout the forums. )

You can adjust a jabod forge, so if it seems you need to raise the bottom up you can. Is you needed it deeper you can build up around it leaving notches for the stock to go through. 

The flame color could be from stuff in the charcoal. Only thing to really watch out for are the flames and yellow powder look from zinc coated steel. Also don't forge anything chrome plated.  Other than that I have seen different color flames from charcoal and wood fires. 

Finding the right spot in the fire to heat your stock in Your forge, and fire control takes practice. 

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Common location to get  cold shut from forging the hammer face down to a flat, often you get a "birds mouth" cold shut.

DESCRIBE YOUR HEAT TREAT!  I would have gone slightly above non magnetic for the edge and cooler for the body and then quench in warm oil.  If that did not harden it to my suiting. I would reheat and quench in Brine.  Note new smiths often get the surface decarburized and so have a bogus hardness when you test.  File down the edge a bit and see if it gets harder the deeper you file.

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Thanks Thomas,

 

I knew the cold shut was happening but struggled to do anything about it and made the decision to carry on regardless and grind it out later. What I underestimated is how deep it would go. I started off badly and should have hammered the edges down first to prevent the cold shut occuring. My big problem was struggling to hold the piece initally with my crappy tongs (I became better at this later on) which caused my hammer control to be terrible early on.

My next plan, eventually, will be to grind off the edge completely, ensure the cold shut is gone, split the face into a "V" and forge weld in a high-carbon tip..... but this requires some forge welding experience and practice and I'm nowhere near there yet.

 

As far as the heat treat... I'm almost scared to post as I'm sure I did this wrong:

- I thermo cycled it 3 times- up to glowing orange and left to cool naturally in air to non-visible heat.

- Then I heated again to orange and quenched (in water). I didnt check the magnetism at any stage. I didn't have oil available.

- I then tempered in the kitchen oven at 250C for 1.5hrs. I could see straw colour in the body of the axe, and perhaps blue in the edge. I believe this may be wrong also?

- I then ground my edge. I wonder I did too much grinding at this stage and the heat from grinding would mess up any heat treat. How far do you grind before heat treat?

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7 minutes ago, Jon Kerr said:

My big problem was struggling to hold the piece initally with my crappy tongs (I became better at this later on) which caused my hammer control to be terrible early on.

THIS IS WHY WE TELL BEGINNERS TO INVEST IN GOOD TONGS!!!!

(Not yelling at you, Jon; just trying to make the point.)

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Don't bother splitting this one to forge weld in a bit. Save it for the next one. Keep this one as is (other than grinding past the cold shut.) as your first to look back on. 

Mystery steel is tricky to heat treat. First you have to get an "idea" of what it is. Spark test to get an idea of carbon content.  When unsure, start with what Thomas mentioned. 

I like to get things ground as close as I can, just leaving a little to grind after and also not get things Too thin. Around the thickness of a dime(1.35mm) at the thinnest on something like that. The key to grinding After heat treat to not ruin temper is to keep things cool. If its getting hot to hold, cool it in water. 

I personally liked Technicus Joe's youtube videos to learn about making tongs. There are some other good ones as well. 

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