Button jaws modificationm, Mike's wish, disaster!

[AndyP]

Due to the shape of a bowl and the size of my 250mm button jaws I could not use the buttons to hold it in compression and there was insufficient adjustment to hold the bowl from the inside.

So I cut down some 5mm bolts and attached 4 of the rubber buttons directly to the chuck.






Of course this only gives me 4 holding points and not the normal 8 but for a small bowl I think this should be OK. Will fire it up tomorrow unless anyone advises otherwise.

 

[Mike G]

What's the shape of the inside of the bowl?

If it follows the line of the outside, it will be sloping the wrong way, and could pull off the buttons. Start it up with a long, long broom handle from just outside the workshop door. Make sure you have your camera set to a very fast shutter speed. :lol:

 

[AndyP]

What's the shape of the inside of the bowl?

If it follows the line of the outside, it will be sloping the wrong way, and could pull off the buttons. Start it up with a long, long broom handle from just outside the workshop door. Make sure you have your camera set to a very fast shutter speed. :lol:

The shape follows that of the outside. Look at the 2nd image. The last 1/2" of the rim slopes inwards which is why I could not hold it in compression. The inside of the bowl follows that shape so the buttons do follow the slope.

 

[Mike G]

Oh that's a pity. I was hoping to see it enter low earth orbit...... :lol:

Yeah, you'll be fine with that. In theory, 3 points should be enough to hold it.

 

[Dalboy]

That will be fine don't over tighten the jaws and take small/light cuts and everything will be fine.

 

[DaveL]

That will be fine don't over tighten the jaws and take small/light cuts and everything will be fine.

Only extra thing I might do is bring up the tail stock with a flat centre while making the first cuts, just in case.
Sharpen your tools just before doing this, and rub the bevel to give a nice smooth cut.

 

[AndyP]

Oh that's a pity. I was hoping to see it enter low earth orbit...... :lol:

Yeah, you'll be fine with that. In theory, 3 points should be enough to hold it.

Oh don't worry that could still happen.

 

[AndyP]

That will be fine don't over tighten the jaws and take small/light cuts and everything will be fine.

Will do. I need to work a bit on the shape. It looks very straight on that last photo.

 

[AndyP]

That will be fine don't over tighten the jaws and take small/light cuts and everything will be fine.

Only extra thing I might do is bring up the tail stock with a flat centre while making the first cuts, just in case.
Sharpen your tools just before doing this, and rub the bevel to give a nice smooth cut.

Thanks Dave, I always try and have the tail stock up for as long as possible.

 

[AndyP]

here you go Mike,



Not only did the bowl enter orbit so did half of the button jaws. I had them adjusted on the limit and 2 of them came out of the chuck. My left hand is still shaking where something walloped it at about 1000rpm. No blood just a bruise. this was the 2nd one of what I hoped would be a pair. No more spalted sycamore to make another. Bloody furious with myself :evil: :evil:

 

[Mike G]

Bloody hell, Andy! Are you OK?

 

[Woodbloke]

Looking forward to hearing all the grisly details on Sat...I may be able to offer a solution which would have prevented that 'explosion' - Rob

 

[AndyP]

Bloody hell, Andy! Are you OK?

Yes Mike, shaken and a little stirred and the back of my hand a bit swollen and ego dented. Definitely suffered from shock for an hour or so but no real damage.

 

[DaveL]

Well I am glad you are OK, but I think you may of over tightened the jaws, stressing the wood which then gave way.

 

[AndyP]

Looking forward to hearing all the grisly details on Sat...I may be able to offer a solution which would have prevented that 'explosion' - Rob

The problem was not the bowl exploding but two of the button jaws coming off the chuck because I had wound them out too far. Even the tail stock couldn't hold it. I did check this and waggled all 4 of them before I switched on but I guess the extra centrifugal forces at speed forced them out and off.

What doesn't kill you can only make you stronger, live and learn eh?

 

[AndyP]

Just rec'd a mail from the Axminster Email Club titled

"Get a grip on your woodturning projects."

That's some marketing trick. How the heck did they find out?

 

[Dalboy]

Looking forward to hearing all the grisly details on Sat...I may be able to offer a solution which would have prevented that 'explosion' - Rob

The problem was not the bowl exploding but two of the button jaws coming off the chuck because I had wound them out too far. Even the tail stock couldn't hold it. I did check this and waggled all 4 of them before I switched on but I guess the extra centrifugal forces at speed forced them out and off.

What doesn't kill you can only make you stronger, live and learn eh?

Did you wind them out to get the rubbers to grip in the right place if so you would have been better off getting longer bolts and a second set of rubbers and doubling them up so the bolts were still tight and fully in the threads

 

[DaveL]

Oh I see, the bolts do need to be tight or they will not stay put, I have used spacers under the rubber buttons to bring the grip out to the required position on the work.
Without the bolts being being tight, you stood no chance of turning the bottom of that bowl, as you say lesson learned, you won't do it again.

 

[Deejay]

Morning Andy

Glad you're OK. It is a bit scary isn't it.

FWIW, 1000 r.p.m. is a bit too fast in my opinion.

I've used the same technique on small bowls. I put a sheet of kitchen towel on the jaws to stop any scratching and expand the chuck until the piece JUST stops moving within the buttons. Support it with the tailstock and take very light cuts with a sharp tool, as above. I run the lathe at very low speed until the cuts is continuous (i.e. any eccentricity has been turned off ). Turn off as much as you can and then remove the tailstock support, check the tighntess of the buttons and turn of the last bit .

With a small amount to turn off, Is it worth turning at high speed?

Better luck with the next one(s).

Cheers

Dave

 

[AndyP]

Oh I see, the bolts do need to be tight or they will not stay put, I have used spacers under the rubber buttons to bring the grip out to the required position on the work.
Without the bolts being being tight, you stood no chance of turning the bottom of that bowl, as you say lesson learned, you won't do it again.

You underestimate my stupidity Dave

See this image:


The rubbers are securely tightened with long bolts into the chuck. The problem this time around was I placed the rubbers into the inner of the two holes and by the time that I wound OUT the jaws to securely hold the bowl there was insufficient grip holding the accessory jaws onto the chuck. I did waggle each one before I switched on but with the extra g force at speed 2 of them came adrift.

 

[AndyP]

Morning Andy

Glad you're OK. It is a bit scary isn't it.

FWIW, 1000 r.p.m. is a bit too fast in my opinion.

I've used the same technique on small bowls. I put a sheet of kitchen towel on the jaws to stop any scratching and expand the chuck until the piece JUST stops moving within the buttons. Support it with the tailstock and take very light cuts with a sharp tool, as above. I run the lathe at very low speed until the cuts is continuous (i.e. any eccentricity has been turned off ). Turn off as much as you can and then remove the tailstock support, check the tighntess of the buttons and turn of the last bit .

With a small amount to turn off, Is it worth turning at high speed?

Better luck with the next one(s).

Cheers

Dave

Thanks Dave, I still can't grip properly with the left hand but is is only bruising. I've just checked the lathe and it is on the second wheel so only 670 rpm. I think even at the lowest speed, 350, I would have been in trouble.