fatigue in a hardwood

[condeesteso]

Someone out there will be saying 'told you so'... but I didn't anticipate this:

A small brass mallet I made some time ago (one of a pair, this is the larger one). The handle is ebony. I only ever use it for tap adjustments (sometimes planes, the other day adjustments to the assembly of a big desk).
The following day went into the workshop, had occasion to pick this mallet up and the head fell off (just as I picked it up).

I had something similar happen a while ago - Jim had made me a fine lignum mallet and used some of my spalted holly as the handle. Some time later that handle failed in the same place. Probably to be expected with spalted holly but the ebony surprised me.
The plan is to try and re-centre the brass head, bore it out much bigger, put some relief on the shoulder so handle can have a radius at the internal between smaller and larger diameters, and use a proven material - box, even ash maybe (I suspect box here, or maybe Lemonwood).
The break on this one is a really clear example of shock-fatigue, and I imagine the grain in ebony is very short.
So in future I will consider the choice of handle materials with more care for the technical rather than aesthetic.

 

[Jacob]

Is it a material problem? I think it's the design.
Say the spigot going in to the head is 20mm dia - if this was a one piece straight 20mm rod rather than your shaped one with a shoulder, it would give a bit and absorb shock. As it is all the impact is taken at the neck, where it broke, as, on impact, the head will not only bend the spigot but also tighten against the shoulder of the handle.
The answer is no shoulder. A straight through handle instead - like most hammers/mallets in fact

 

[Phil Pascoe]

Ebony is brittle at the best of times. You could keep the same hole in the head and drill well into the handle, epoxying a piece of threaded rod into both - the ebony is then merely decorative.

 

[Peter Sefton]

A very handsome mallet indeed, I think the failure is a mixture of elements - the spigot may be a little small or possibly undercut/scored when the shoulder was being turned.
The materials in question are both very hard and they have no give or flex; ebony can be quite brittle when being worked. The fashion has become to use these rich and highly prized timbers for tools and I understand why, but traditionally shafts would be made from straight grained Ash or Hickory both chosen for their working properties.
The third factor could be it was used for your assembly job, maybe it just tipped it over the edge.
My students make both carpenters and carvers mallets on our long course along with other tools - the brief for the carvers mallet is Ash shaft with a lignum head and the spigot is 25mm diameter, this may not be the most exciting of combinations but it is tried and tested. We save the Ebony for inlays and details.
Peter

 

[AndyT]

If it's any comfort I had a similar thing happen to me. I'd made a (much more ordinary) mallet and used holly for the handle just because I had a piece of suitable size. In ordinary use it snapped off at the neck. I've replaced it with ash which is what I should have used in the first place.

 

[Phil Pascoe]

An odd piece of yew makes a nice handle too. I'm a poet......

 

[condeesteso]

Thanks all, agreed, wrong material and a basic error in design. I plan to open the hole up in the head quite a bit, but still have a little bit of shoulder (maybe 1.5mm), then put a decent radius on the transition from handle to spigot. If the dia of spigot increases say 50%, cross area goes up well over double, and the shoulder will help distribute stress whereas the tight corner concentrated it. Ash would be excellent I am sure, but I will probably 'risk' lemonwood (a bit like box).
I know straight through / one diameter is best Jacob, but this was only intended to be a tapping device. If the next mod fails, the one after is probably going to be one solid lump of brass :lol:

 

[bugbear]

...but I will probably 'risk' lemonwood (a bit like box).

Lemonwood has enough flex that people use it for self-bows, so that might well be a good choice.

http://georgeandjoni.home.comcast.net/~ ... nwood.html

BugBear

 

[condeesteso]

I'd forgotten that BB - I get mine from Timberline and I recall Robert there saying bow makers used it. Lemonwood it will be then. (He also calls it Castello box and it seems it is good for recorders too, that's handy to know.)

 

[bugbear]

I'd forgotten that BB - I get mine from Timberline and I recall Robert there saying bow makers used it. Lemonwood it will be then. (He also calls it Castello box and it seems it is good for recorders too, that's handy to know.)

Googling gives as species:

Castello Boxwood: Calycophyllum multiflorum
Lemonwood: Calycophyllum candidissimum

Lemonwood is also called dagame and lancewood, but "true" lancewood is "Oxandra lanceolata".

Timberline say that "Calycophyllum multiflorum" is known as "Genero lemonwood"

Now you known why botanist use Latin - "simple" names are anything but.

BugBear

 

[condeesteso]

Indeed, all a bit confusing - but a useful wood nonetheless.
Got it fixed, the spigot is now considerably bigger diameter, with roundover in the shoulder.

A couple of things I have discovered with these - I made this from 1 1/2 round bar which was wasteful as 1 1/4 would suffice (it ended up a fraction below 1 1/4). The handle major dia is about the same as the head and it falls nicely into the hand with thumb and forefinger holding the lower part of the head in a relaxed grip - this gives a lot of control over where you tap and how hard etc. Weight came out 280gm, and most of that is the head. Length is just under 130mm (5").
If you don't have one of these, I personally recommend them, particularly if you set up planes (esp. old woodies, infills etc) and if you make cabinet-proportion joints a lot. I'm not saying mine don't fit :shock: but the odd tap here and there... (note the top of the head is used just as much as the side).

If the head drops off this one day, I'll eat a hat and let you know.