HSS
12th January 2012, 12:11 AM
Last Thursday I had an incident turning a large diameter platter I thought it would be worth detailing what happened.
In November I recieved a commission to turn a blackwood platter around 1050mm diameter x 45mm thick. I have turned 5 similar sized platters before without any dramas.
I client ideally wanted the platter before Christmas so in mid December I purchased the timber (150x50mm kiln dried and in the yard for around 25 years with no visible defects once dressed).
I laminated the boards with megapoxy and then glued on a block of beech to give me something to screw the faceplate onto.
This is were the first problem may have occured...it was a warm day and the epoxy I used on the beech mount was beginning to go off. This mean that when I applied weight to it (about 30kg sat on top, since I couldn't get a clamp on it) the setting epoxy prevented a tight bond between the timbers, staying as a 2mm layer between the blackwood and beech.
I didn't think anything of it since I have used weights to 'clamp' blocks with epoxy many times before and it has always held; if anything the wood breaks before the epoxy bond.
Anyhow, I mounted it on the lathe the next day and turned the back. The beech mount performed without a problem, despite heavy roughing cuts. I left it on the lathe over night and came back the next afternoon to find a 50mm split in one of the boards. I attributed this to the heat in the workshop (it was high 30's, low 40's in Adelaide leading up to Christmas and dry as usual). With the delay that this would cause (cutting out the split and re glueing) I decided I wouldn't get it done in time for Christmas and put it away in the cooler office area.
I returned in the new year to find that the split had grown and the platter had also bowed.
I was able to cut out 4mm of the platter to remove the split. I then made an incredibly effective clamping jig to re-glue the 2 halves (I didn't take any photos but probably should have and then posted them on here). But I think clamping the already bowed platter may have put strain on the laminated block, weakening the epoxy bond; almost 'popping' it off in effect.
Anyhow, visually it seemed fine and I didn't even think what I'd done would be a problem. I remounted the platter on the lathe and began facing it to get rid of the bow. To do this I was using the carriage rather than doing it by hand (because I can! also, you can take very controlled, light passes and it ends up dead straight).
Well within 5 mins of starting, the mount let go and the platter was splintered across the workshop. One second I was winding the carriage across the face, the next second there was a loud bang, the platter was gone and big splinters were falling on the top of my faceshield. I heard my colleague swear and run down the other end of the workshop. the platter ploughed into a large fabricated galv box next to the lathe and sheared into numerous pieces; the largest of which fractured and knocked over a piece of 3"x1" queensland maple I had leaning against my timber rack and continued to cartwheel a further 17m through the air before hitting the back wall of the workshop, bouncing off and landing on the mezzanine.
See the attached photos and the explanation to the chain of events in the following few seconds:
a-the offending faceplate mount that "let go" of the platter
b-the distance from the lathe to the fabricated galv box
c-the dent where the platter first hit the galv box (about 2in deep, right on the corner)
d-the dent in the platter from the galv box
e-the platter re assembled showing how after hitting the box it sheared into numerous large splinters that exploded in all directions
f-a detail of the damage
g-the largest fragment (around 3-5kg maybe) which another colleague saw cartwheeling through the air and hit the back wall above the mezzanine
h-the fractured queensland maple (around 2.5m from the lathe)
i-the view from the lathe up to the mezzanine where the largest fragment landed
j-showing the distance from the lathe (bottom right) to the mezzanine (top left)
Needless to say, it was lucky no one was hurt. I have no doubt that if I had been in the way I would likely be dead or at least have broken something, depending on where it hit.
The lathe was spinning at 716 rpm. Obviously the surface speed on the outside of a 1m platter is fast and it has significant momentum but I would have never expected it to dent the box like it did, shear in pieces, snap a plank of wood (rather than just knock it over) and continue to fly 17m through the air only to be stopped by a brick wall. From what my mate saw, it was not at the end of its trajectory either!
So, attributing factors:
pour exoxy bond
anbient temperature (causing bowing and possible effecting the epoxy???)
re-clamping bowed platter straining epoxy bond
I did have screws penetrating throught the epoxy and into the platter but only about 8mm as I couln't afford to loose too much blackwood trying to get rid of screw holes (in the end I lost aroun $450 of blackwood!)
I have no doubt that epoxy is strong enough alone to hold such a platter as I have used it numerous times in this sort of application.
Anyway, I'm not wanting to turn people off large diameter turning. It can be dangerous, don't get complacent (where all the safety gear just in case, stand clear as much as possible) and be aware of the forces involved. If you agree with the attributing factors I have listed, don't make the same mistakes.
I look forward to your comments.
In November I recieved a commission to turn a blackwood platter around 1050mm diameter x 45mm thick. I have turned 5 similar sized platters before without any dramas.
I client ideally wanted the platter before Christmas so in mid December I purchased the timber (150x50mm kiln dried and in the yard for around 25 years with no visible defects once dressed).
I laminated the boards with megapoxy and then glued on a block of beech to give me something to screw the faceplate onto.
This is were the first problem may have occured...it was a warm day and the epoxy I used on the beech mount was beginning to go off. This mean that when I applied weight to it (about 30kg sat on top, since I couldn't get a clamp on it) the setting epoxy prevented a tight bond between the timbers, staying as a 2mm layer between the blackwood and beech.
I didn't think anything of it since I have used weights to 'clamp' blocks with epoxy many times before and it has always held; if anything the wood breaks before the epoxy bond.
Anyhow, I mounted it on the lathe the next day and turned the back. The beech mount performed without a problem, despite heavy roughing cuts. I left it on the lathe over night and came back the next afternoon to find a 50mm split in one of the boards. I attributed this to the heat in the workshop (it was high 30's, low 40's in Adelaide leading up to Christmas and dry as usual). With the delay that this would cause (cutting out the split and re glueing) I decided I wouldn't get it done in time for Christmas and put it away in the cooler office area.
I returned in the new year to find that the split had grown and the platter had also bowed.
I was able to cut out 4mm of the platter to remove the split. I then made an incredibly effective clamping jig to re-glue the 2 halves (I didn't take any photos but probably should have and then posted them on here). But I think clamping the already bowed platter may have put strain on the laminated block, weakening the epoxy bond; almost 'popping' it off in effect.
Anyhow, visually it seemed fine and I didn't even think what I'd done would be a problem. I remounted the platter on the lathe and began facing it to get rid of the bow. To do this I was using the carriage rather than doing it by hand (because I can! also, you can take very controlled, light passes and it ends up dead straight).
Well within 5 mins of starting, the mount let go and the platter was splintered across the workshop. One second I was winding the carriage across the face, the next second there was a loud bang, the platter was gone and big splinters were falling on the top of my faceshield. I heard my colleague swear and run down the other end of the workshop. the platter ploughed into a large fabricated galv box next to the lathe and sheared into numerous pieces; the largest of which fractured and knocked over a piece of 3"x1" queensland maple I had leaning against my timber rack and continued to cartwheel a further 17m through the air before hitting the back wall of the workshop, bouncing off and landing on the mezzanine.
See the attached photos and the explanation to the chain of events in the following few seconds:
a-the offending faceplate mount that "let go" of the platter
b-the distance from the lathe to the fabricated galv box
c-the dent where the platter first hit the galv box (about 2in deep, right on the corner)
d-the dent in the platter from the galv box
e-the platter re assembled showing how after hitting the box it sheared into numerous large splinters that exploded in all directions
f-a detail of the damage
g-the largest fragment (around 3-5kg maybe) which another colleague saw cartwheeling through the air and hit the back wall above the mezzanine
h-the fractured queensland maple (around 2.5m from the lathe)
i-the view from the lathe up to the mezzanine where the largest fragment landed
j-showing the distance from the lathe (bottom right) to the mezzanine (top left)
Needless to say, it was lucky no one was hurt. I have no doubt that if I had been in the way I would likely be dead or at least have broken something, depending on where it hit.
The lathe was spinning at 716 rpm. Obviously the surface speed on the outside of a 1m platter is fast and it has significant momentum but I would have never expected it to dent the box like it did, shear in pieces, snap a plank of wood (rather than just knock it over) and continue to fly 17m through the air only to be stopped by a brick wall. From what my mate saw, it was not at the end of its trajectory either!
So, attributing factors:
pour exoxy bond
anbient temperature (causing bowing and possible effecting the epoxy???)
re-clamping bowed platter straining epoxy bond
I did have screws penetrating throught the epoxy and into the platter but only about 8mm as I couln't afford to loose too much blackwood trying to get rid of screw holes (in the end I lost aroun $450 of blackwood!)
I have no doubt that epoxy is strong enough alone to hold such a platter as I have used it numerous times in this sort of application.
Anyway, I'm not wanting to turn people off large diameter turning. It can be dangerous, don't get complacent (where all the safety gear just in case, stand clear as much as possible) and be aware of the forces involved. If you agree with the attributing factors I have listed, don't make the same mistakes.
I look forward to your comments.