Jon Kerr

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About Jon Kerr

  • Rank
    Advanced Member

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  • Gender
    Male
  • Location
    - Benfleet, Essex, UK
  • Interests
    Learning more about Blacksmithing!

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  1. Jon Kerr

    Complete Beginner

    Ah I see, thanks. Is the same true of annealing? Is the "weakening" proportional to the carbon content?
  2. Jon Kerr

    Complete Beginner

    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. *sigh* ..................................................................... ITS STRONG ENOUGH FOR A FEW LOGS!
  3. Jon Kerr

    Complete Beginner

    Of course, you're absolutely right on all points! I do actually happen to be a structural engineer....! 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.
  4. Jon Kerr

    Complete Beginner

    Beautiful! Thanks for the idea.
  5. Jon Kerr

    Complete Beginner

    Great suggestion thanks Snuffy! Anyone got any more? The flaming curmudgeons had me worried for a second so I even had to break out the pencil to prove it.....! (No disrespect to TP! All in jest.) For those interested (probably not many...!): Bending Stress is S=My/I (where M is Moment, y is distance from neutral axis, and I is Second Moment of Intertia). For a rectangular bar as in my firepit legs, I=bh^3/12 (where b is the width, 40mm, and h is height, 20mm). Since we're interested in maxiumum allowable mass in my firepit, we can rearrange S=My/I to give M=SI/y. The yeild strength of mild steel is in the region of 370MPa (so S=370MPa). Therefore, the maxumum allowable Moment M= 986642. Moment= Forcex distance, and Force=massxgravity. So F=1973N. Therefore...... The allowable mass on each leg before bending is (believe it or not) 200kg. Since the firepit has 3 legs, thats a total allowable mass of 600kg. There's no way we'll be putting more than 25kg (50lbs) of wood in the firepit at any given time, so we have a healthy Factor of Safety of 24. You never know... someone might one day wish to do a similar calc.
  6. Jon Kerr

    What did you do in the shop today?

    Good idea- Thoman Powers suggested similar. I think its a symptom of the photos, as I said its bigger than it looks and the stock is thick (half inch). I've just stood in the pit without the legs bending, and I weight 250lbs. The pit will never experience more than 50lbs of wood in operation.
  7. Jon Kerr

    Complete Beginner

    Like I said, the picture makes it look smaller than it is. Those legs will be easily strong enough to survive logs being thrown onto the fire. Scrolls to contact the ground is a good idea though, and definitely something I'd consider in future if I made a smaller version with thinner stock legs. I believe my buddy has already added a drainage hole, and has purchased a small circular grill to add into the bottom to allow some ash to settle out.
  8. Jon Kerr

    What did you do in the shop today?

    I finished forging the legs for a big firepit. My first proper forging project complete! Couple more photos in my thread here.
  9. Jon Kerr

    Complete Beginner

    Well, I finished the 3rd of the firepit legs and handed them over to my friend. He's got them welded up straight away and is really pleased. First happy customer. I feel like a real blacksmith (I say customer in the loosest sense, although he did pay me with four bags of high quality charcoal!) The pictures make it look small, but its actually huge. The dish is over 3 1/2' feet diamater, and each of those legs are forged from 1 1/2" by 1/2" stock. (Note. The dish was obviously not forged by me, only the legs. He purchased the dish online.) In other news: My fire management is improving, slowly. I used approx 3kg of charcoal to forge the last leg, which is an improvement. It still seems like a lot. I'm learning where the sweet spot is in terms of air flow for heat. Next step is to wire up my new foot switch so the air automatically shuts off when I step away from the forge to the anvil. Next session I'd really like to try some bottle openers with a dice-twist. My fiance's birthday is coming up this month, and our wedding is in August. Does anyone have any really nice gift ideas I could forge?
  10. Jon Kerr

    Knife Newb seeking input

    based on the context, I'm pretty sure he meant "IS".
  11. Jon Kerr

    Complete Beginner

    I 3D printed a nozzle and adaptors for my new forge blower. Attached a picture of it all hooked up. Also attached a pic of my current set up, for what its worth. Took Frosty’s “trench” suggestion and added the extra brick. Hopefully that will help my consumption issues. Will try breaking the charcoal up next time and hopefully that will solve my metal heating issue. Hopefully with the trench, new blower and smaller charcoal next session will be less frustrating! Perfectly, I’ll do as you describe thanks. What about really tiny chips (are these called “fines”?)? Do I need to seperate them? I’ve seen charcoal prep tables consisting of a series of metal meshes to let small bits drop through.
  12. Jon Kerr

    What did you do in the shop today?

    I gathered that! but I still can't think of a kind of plug that needs a dedicated cutting implement?! Is this something specific to blacksmithing?. Electrical plug? nope Bath plug? Nope Ear Plug? Nah. Rawl Plug? Maybe?? Unlikely. Googled: Plug tobacco? EDIT: Upon further googling, it must be plug tobacco? Now such a beautiful tool makes sense!
  13. Jon Kerr

    What did you do in the shop today?

    Looks great but..... whats a plug cutter?! Beautiful. Indeed! I didn't fancy my chances at getting all four on the ground- especially since the firepit will be on grass often. These legs are easily heavy-duty enough for just three anway.
  14. Jon Kerr

    Complete Beginner

    Ah! Unfortunately I can't make it this weekend which is a real shame as I'd have popped up and said hello. I'll try breaking it up- I hadn't realised it needs to be so small! Any tips for this? I assume the dust etc is bad, so you don't want charcoal coated in dust produced during the breaking up? I've seen examples of a mesh cutting table, where the dust/tiny bits drop through to a lower layer. Is this the best option? Would you use a hand axe or just smash with a hammer? I'll add another firebrick as you describe to try and achieve the "trench". That sounds like it will work great for the long stock I'm using.
  15. Jon Kerr

    Complete Beginner

    Sorry gents, I AM running a JABOD. I've followed Charles' design pretty much to the letter. I've upgraded to the forge blower shown above (NOT PICTURED), attached to a 1" ID Tuyure. I wonder whether the size of my charcoal is now an issue? Some pieces are 2" x 3" pieces? I try to break it up a little as I go by bashing it with my rake. Since the edges of my fire bowl have broken away with use, the size of the bowl is growing a bit. I guess that could be one of the problems?