Physics help

[Torin]

Some of you may know of my current 'fun' project.

Mythbusters did a show a little while ago where they said you could hit hammer faces together and you wouldn't chip them out.

Well, I'm doing a little project to disprove this. However I'm having a little bit of a problem. My current setup is lifting and then dropping a hammer, so that gravity is the only thing powering the hammer hit. After 33000 hits, it hasn't chiped.

I can change it to where it delivers a powered hit, but I don't want to exceed the force that a human would be able to deliver. If I knew the speed that a human driven hammer hits, I could work the math out and drive the hammer at that rate.

Does anybody know how fast this is, or where to find this information?

Edit: The hammer has broken it's shaft and as a long crack in the neck, but nothing on the face yet. And I'm using plenty of protection from possible flying shards.

[HWooldridge]

I'll take a SWAG...for short periods of time, a trained person can hammer 120-180 bpm (this is somewhat dependent on the rebound speed from the anvil and weight of the hammer - a 1-1/2 lb hammer will obviously cycle much faster than a 3 lb).

I have actually timed this on a couple of experienced smiths so it's a good estimate. The other variable is length of the stroke but 18-24" is probably safe.

BTW, I have chipped hammer faces on more than one occasion and another smith spalled a piece from my anvil during a demo so I guarantee that it happens occasionally.

[rthibeau]

If you use irnsgn's swing as a young man, you'll get one result, and if you use his swing now, you'll get another. Probably a difference measured in factors of 10... There is the Velocity of the hammer, Mass of hammer, Force being applied in swinging the hammer, Jr's Anger Factor to give you a formula that looks like:
Derivation of Impact Force Formula
Work = 1/2 Mv12
=> v1 = [(2/M)(Work)]1/2
Converting v1, the mass center velocity, to initial impact point velocity p1:
p1 = (d/r)[(2/M)(Work)]1/2
From the coefficient of restitution formula:
p2 - s2 = c(s1 - p1)
we can find p2
p2 = cs1 + s2 - cp1
Now knowing p1 and p2, we can find v1 - v2 by multiplying by the ratio of the mass center radius ® to the impact point distance from the axis (d) and plugging in the derived values of p1 and p2:
v1 - v2 = (r/d) [p1 - p2]
= (r/d) [(d/r)[(2/M)(Work)]1/2 - cs1 - s2 + c((d/r)[(2/M)(Work)]1/2)]
Impact Impulse = change in hammer momentum = M (v1 - v2)
Impact Impulse
= M(r/d)[(d/r)[(2/M)(Work)]1/2 - cs1 - s2 + c((d/r)[(2/M)(Work)]1/2)]
Simplifying:
Impact Impulse = M{(1 + c)[(2/M)(Work)]1/2 - (r/d)(cs1 + s2)}
Impact impulse is measured in momentum units (kg.m/s) and if we divide impact impulse by the time it operates we will have kg.m/s2, which by Newton's Second Law must be a force, measured in Newtons of force (1 Newton = 0.225 lb). The time to divide by is the dwell time of the hammer on the anvil, which we assume to be 0.004 second for all hammers being compared.
Impact Force = Impact Impulse/dwell time
= (M / t)*{(1 + c)*[(2/M)*(Work)]1/2 - (r/d)*(cs1 + s2)}



So the result is, as you found, a broken handle.

[mike-hr]

We were discussing this a few weeks back, while re-hydrating after an evening forge session. The idea was postulated that hardware store bought hammers are most likely tempered way down to be so soft they won't chip very easily. Maybe you should try re-hardening and tempering the test hammers.

[Torin]

I'll take a SWAG...for short periods of time, a trained person can hammer 120-180 bpm (this is somewhat dependent on the rebound speed from the anvil and weight of the hammer - a 1-1/2 lb hammer will obviously cycle much faster than a 3 lb).

I have actually timed this on a couple of experienced smiths so it's a good estimate. The other variable is length of the stroke but 18-24" is probably safe.

BTW, I have chipped hammer faces on more than one occasion and another smith spalled a piece from my anvil during a demo so I guarantee that it happens occasionally.

Thanks, I'll go with that range. I'm currently working with a 14" stroke, but I can increase it. And I also know several people who have personally chipped hammers. I just need to catch it on video along with some other good information.

[Torin]

We were discussing this a few weeks back, while re-hydrating after an evening forge session. The idea was postulated that hardware store bought hammers are most likely tempered way down to be so soft they won't chip very easily. Maybe you should try re-hardening and tempering the test hammers.

I'm currently working with a hardened hammer. The other thing is that over time the hammers work harden. Their tests were done with only new hammers, and only one time per hammer. Plus it looks like they didn't get it quite hot enough, or quench well enough to harden.

[Rantalin]

Torin, though I applaude your desire to disprove those who have gone ahead and researched this, I feel that the Mythbusters episode thoroughly covered this situation. They measured the force a human can hammer with, and ran multiple tests. They struck hammer on nail, and hammer on hammer; and they got the same results as you. Nothing but broken handles. If you do get some different results, e-mail them, I'm sure they'll run a "myths revisited" episode on it.

[Torin]

Torin, though I applaude your desire to disprove those who have gone ahead and researched this, I feel that the Mythbusters episode thoroughly covered this situation. They measured the force a human can hammer with, and ran multiple tests. They struck hammer on nail, and hammer on hammer; and they got the same results as you. Nothing but broken handles. If you do get some different results, e-mail them, I'm sure they'll run a "myths revisited" episode on it.

I don't need to e-mail them, I'm going to get to present my findings in person.

Also, if they thoroughly cover this situation, can you explain why in this very thread, there is somebody who has had hammers chip out on him more than once? And what case hardening compound did they use to case harden the hammers? And quenching in used motor-oil to increase the carbon content of iron? Are they aware of how slow carbon migration is?

Normally they are pretty good, but they missed a few things on this one. The main thing is I know several people who have small chunks of steel still in their bodies from flying shards from hammers. (Ok, they have probably been coroded away and absorbed by now, but they were never removed.)

The other reason I'm doing this video, is because it is one of the few ways I can stand out from the crowd on their forums that are still yelling at them that you can indeed cut through a machine gun barrel with a katana.

[mcraigl]

Torin,
Count me among those that've been hit by the shrapnel. When I was younger I was driving a geologist's hammer into a tree with a 3lb engineer's hammer when the head of the geologists hammer exploded sending about a 1/4" chunk through my coveralls, jeans, and about an inch into my thigh. Wasn't any fun having pops pull that thing out of there with hemostats and needle nose pliers. Anyway, another thing that I saw wrong with their test is that their "apparatus" struck the faces together too perfectly centered. The average joe out there isn't going to hit them that perfect. Also as to the comment that they had "measured the force a human can hit with"... I remember watching those jokers swinging wildly trying to drive a 16 penny nail in one blow. Are you kidding me? I framed for a living for some time and my hammer only touched each nail twice. Once to set it, once to drive it. They should've measured the "force" while they were visiting Scott Thomas and seen the force from someone who is used to manual labor... Anyway, that's my .02 cents.
McL

[Strine]

rthibeau,

Your derivation forgets to substract the first number you thought. Otherwise it looks very impressive. Maths rules OK

[Brian C.]

mcraigl- I am with you. My grandfather was a carpenter for 35+ years. Had the thumb off of his right hand, held the hammer between the first 2 fingers. Still only used 2 blows when framing. That is some power. Daddy always said that if Pappaw could get those first 2 fingers around your neck, he could probably pinch your head off. :)

[rthibeau]

rthibeau,

Your derivation forgets to substract the first number you thought. Otherwise it looks very impressive. Maths rules OK

The first number????

[HWooldridge]

Torin,

This is the letter I wrote to them after the episode. To date, I have not received a response. I hope you have better luck in person...


< January 1, 2007

Attn: Producers - Discovery Mythbusters

Dear Adam and Jamie,

Let me preface this letter by stating that I thoroughly enjoy your show and find it a refreshing change to some of the pabulum found on TV. Unfortunately, tonight

[irnsrgn]

18 to 24 inch stroke would be a tapping stroke for me, I usually swing from just above my head, so a 36 inch stroke would be more realistic and a few years ago I could get mad and break the handle out of a hammer by hitting to hard, but them days are over.