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HSS
12th Jan 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.

oldiephred
12th Jan 2012, 02:07 AM
A rough mental calculation indicates that the speed of the outer rim of the plate was around 7000 FPM using your original dimension of 1050 dia. That seems fast unless that is a hard, dense wood. Any catch at all could be dangerous.:oo:
It is great that no one was hurt.

dr4g0nfly
12th Jan 2012, 06:12 AM
A very worthwhile tale, well told. It's these learning experiences that make the forums more useful than just asking Q's and getting answers.

Someone once said 'Don't trust to luck, rely on it.'

I think you need to allow a little time for your 'Luck' quotient to build up again!

Just glad that you and everyone else in the building came out unscathed.

Drillit
12th Jan 2012, 09:19 AM
Hello HSS,
I tend toturn big bowls/platters and I appreciate your information. I have found (from experience) that turning at a slower speed until the block is in balance is the preferred safety position. I turn at about 350 - so at 250mm it is 3500 rpms, 500 - 7000. I tend to use a large glue chuck on the outboard or if inboard I use that chuck and bring up the tailstock for added support. To reverse I use the glue chuck, several sizes and attach to the turned foot etc. Having said all of that, sometimes wood factors come into play and maybe that was what happened in your case. I might add that for the glue chuck I use a good thick smear of hot glue on a heated chuck and then snap if cold with a frozen lump of steel that I keep in the old frig. The glue chuck is simply a large round of aluminium (130mm) tapped out and threaded to fit the spindle thread. Works very well with a packer to aid easier removal. Hope this helps a little. Please take my comments as constructive. Regards, John M.

Scott
12th Jan 2012, 09:44 AM
What would of been more effective in your narrative would be a description of your feelings. Because as I read this I felt sick and nearly lost control of my bowels. Thanks for sharing though, your disclosure of this situation has made me think a litle harder on how I should be doing this kind of work at the lathe.

steck
12th Jan 2012, 09:53 AM
Wow!
I bet that had your heart thumping!

NeilS
12th Jan 2012, 11:47 AM
Thanks for sharing this with us HSS.

We all need to learn from these incidents and a post event analysis shared with everyone is an invaluable contribution to our collective well being.

I have only ever had one 'projectile' off the lathe. A gum vein parted letting a slice come off the the side of a deep bowl. I eventually found the shard up in the rafters with razor sharp edges that had done some damage on the way. Fortunately it missed me as I think it would have sliced through my face shield had it hit it. Lesson for me: Never use a blank with gum veins, there is enough other wood in the world to see me out.

I share the above because the slice coming off the side of the blank was slung away at very high speed, whereas intact blanks that come off tend to drop slowly while they continue to spin and only get going when they hit the deck.

There was probably still enough spin in the blank when it hit the galvo box and split up for the centrifugal forces to take over and fling the shards away as they did. Had the blank begun to split up as it left the chuck block that would also explain the projection distances.

What a relief that you have come out of the incidence unscathed (except the $s) and the wiser for it, John.

rsser
12th Jan 2012, 05:27 PM
Blackwood is one of those timbers that move in the heat and maybe you got some minor internal checking. A bit of platter flex, high speed and too deep a cut for the circumstances?

Also, kiln-dried timbers appear to be more brittle than air-dried as far as I know.

The splintering of the wood in the pics might hold the answer, at least if that's how they were before the ricochets.

Good to see the Sapporo tonic in the pic.

Live to fight another day. Maybe get a motorbike helmet and chest armour ;-)

kamel
12th Jan 2012, 05:54 PM
it's very dangerous:o

Dez Built
12th Jan 2012, 09:25 PM
Lucky escape buddy!!
Does anyone know if there is a certain grade of perspex we should use in our face shields to protect against this type of thing??

This has only ever happened to me on smaller stuff, although my heart races for few minutes after!!!

Scott
12th Jan 2012, 09:27 PM
Lucky escape buddy!!
Does anyone know if there is a certain grade of perspex we should use in our face shields to protect against this type of thing??

This has only ever happened to me on smaller stuff, although my heart races for few minutes after!!!

You wouldn't want to use perspex, it'll shatter and slice your face. Polycarbonate is what you should be using, it's actually what they use for bullet proof glass.

NeilS
13th Jan 2012, 11:30 AM
As sjt said, Polycarbonate is what you need and make sure it is marked High Impact according to the AS/NZS 1337 Standard.

rustynail
13th Jan 2012, 12:20 PM
When a timber platter becomes a flying saucer!
One of the good things in life is to be blessed with a great mentor. As an apprentice I was so endowed.
Some 42 years later I still often hear his voice in the back of my head as I undertake yet another brainstrainer. A favourite of his was "wood talks to you" or "the wood will show you." We young fellas thought he had lost the plot. With time, I have come to understand what he meant. Once, we were visiting a mill, his comment to me was, "That saw doesnt sound happy." Next thing the blade disintergrated, with a large piece embedding itself into a post close to where we were standing. On another occasion, I had set up a large piece of forrest red gum on the lathe to turn a large wheel hub.
Old Ron came over and said," What did it say?" I said, "I dont know, it was too big and heavy to listen." He said, "The wood has spoken.....Start it slowly."

wheelinround
13th Jan 2012, 01:04 PM
Good to know you and others were not injured.

A darn good write up and timely warning for all turners regardless of size. I'll be reviewing this often as a reminder.

Rusty Nail Old blokes know they have learnt or being taught well themselves. One such fellow with me always said "Follow your gut instincts, if in doubt DON'T"

Shedhand
13th Jan 2012, 01:22 PM
Blackwood is one of those timbers that move in the heat ...
Which is why I use it for firewood....it's only glorified wattle after all..

NeilS
13th Jan 2012, 03:42 PM
....it's only glorified wattle after all..


....leaves one wondering why all those top woodworkers included our acacias in their commissions when they were working on the fit-out for our new national parliament...:rolleyes:

wheelinround
14th Jan 2012, 07:53 AM
....leaves one wondering why all those top woodworkers included our acacias in their commissions when they were working on the fit-out for our new national parliament...:rolleyes:


Acacia's are dense timbers just like politicians............also stops the drone as its a good sound insulator keeps it all in the House.:;

NeilS
14th Jan 2012, 10:15 AM
Acacia's are dense timbers just like politicians............also stops the drone as its a good sound insulator keeps it all in the House.:;

..... :rotfl:

tea lady
14th Jan 2012, 11:48 AM
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).

Loverly big lathe you've got there! What do you mean "Using the carrage"? Is it a metal lathe? :hmm:

tells a similar story involving an escaped platter. Only his made it out the door and across the highway! :oo::D Has us in stitches. :rofl:

sjm
14th Jan 2012, 12:02 PM
What do you mean "Using the carrage"? Is it a metal lathe?

Checkout the 2nd photo :)

tea lady
14th Jan 2012, 12:15 PM
Checkout the 2nd photo :):pi: Oh I see! Can wind it across the face like a metal lathe. Does that mean you use a scraper? Puts the work under alot more pressure than a bowl gouge in cutting mode. :shrug: Although I guess if its gonna let go its gonna let go. :shrug:

Paul39
16th Jan 2012, 07:02 AM
Good escape! Luck, higher power, clean living. Doesn't matter, no one hurt.

What is the name of the lovely lathe? I can't make it out.

Acco
16th Jan 2012, 07:22 AM
What is the name of the lovely lathe? I can't make it out.

It's a Wadkin Paul :2tsup:

HSS
20th Jan 2012, 12:18 AM
Thanks, for all the replies, sorry about not getting back to it for a while. As far as speed goes, I find it's a pay off between reduced vibration and a good finish. Having said that, I didn't consider the speed too great for the task. I was aiming for a good finish off the tool to reduce the amount of sanding I'd have to do since blackwood is reasonably soft and the soft grain is chewed out by the sand paper.

Yes the lathe has a carriage like a metal lathe. It moves in both directions and rotates to any angle in between as well. It's a very handy feature and I use it all the time in conjunction with the tailstock that can be wound off-centre for doing tapers etc. It's far quicker than trying to do straights by hand (cheating I know). Anyhow, although scrapers do impose a greater force on the wood, I find this is negated by the control that you gain from using the carriage (with scrapers of course). You can take very light cuts and there is no chance of the tool inadvertently digging in and ruining your day. In the case of this platter it was a failure in the mount, rather than a failure in the turning process. I believe it still would have sheared off the mount at a slower speed, It just might not have gone off with such a bang. a platter moving at 400 rpm is going to hurt more or less the same amount as a platter moving at 700 rpm.

There's another point I noticed whilst turning the edge of this platter prior to the incident. I was using a gouge on the edge of the platter to reduce friction and get a clean cut (no chatter). However, long splinters were coming off the side grain as it came around(reducing my diameter considerably with every splinter). I changed to using a 4mm wide round nose scraper bit in the carriage and the problem was solved; no more splinters.

oldiephred
20th Jan 2012, 07:38 AM
Don't want to be a nuisance but I disagree with what appears to be your feeling about the speed not being significant. The RPM should only be used to determine the surface speed of a rotating disc. A disc of 25mm at 700 rpm will have a surface speed of approx. 200FPM. A disc of 300mm at a speed of 700rpm will have a surface speed of nearly 3000 FPM. Quick mental calc. so don't take them as exact. This means the stresses in the disc at the edge are quite considerable and in the case of soft wood (maybe with flaws) the stresses will overcome the adhesion factor and we get "explosions". A slight amount of excess pressure or slight catch can add to the problem. ON the 300 mm disc. when a piece leaves the surface for other locations it is travelling at the 3000 FPM mentioned above. This is a respectable muzzle velocity for many weapons.

It is very good to know that no one was hurt. There was a report a year or two ago that a turner on Ontario Canada was killed in a incident where a workpiece aperantly exploded and a piece struck him in the neck. Never could get the full details but it was a newspaper article.

rustynail
20th Jan 2012, 11:43 AM
"Long splinters coming off the side grain."
"The wood will show you."

tea lady
20th Jan 2012, 12:11 PM
Yes the lathe has a carriage like a metal lathe. It moves in both directions and rotates to any angle in between as well. It's a very handy feature and I use it all the time in conjunction with the tailstock that can be wound off-centre for doing tapers etc.:CI think is crying now! I'm crying now! :C:U