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Author Topic: Oh my...........opps! lathe mishap!  (Read 5826 times)

dazzlingdave

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Oh my...........opps! lathe mishap!
« on: February 22, 2016, 12:09:44 PM »

I have heard from many other turners that at one point or another everyone has a close call.

Mine was today!


The 6" delrin top I have started working on this week flew out of the chuck when a tool grabbed a bit too tight when hollowing.

No one hurt, nothing broken, no big deal. (Just a badly bent tool!)

Can anyone tell me what they think caused this? I had just sharpened the tool, and it was working well, taking a good amount of chips off, and then bam it was on the floor!

Any chance I can bend the tool back and have it work again, or once bent....always rebends?





Thanks

Dave

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Jack

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Re: Oh my...........opps! lathe mishap!
« Reply #1 on: February 22, 2016, 02:48:10 PM »

ur a lucky cookie that chisel bent  :o
the reason for the snag is probably got to do with the angle of the tool and not sharpness, just my two cents worth......which isnt much with the current exchange rate  ::)
if it were me id cut the tool off at the bend and fit a new handle, even if you did try to bend it back it would be weakend and this would happen much easier next time   :(

i am sorry for your loss sir  :'(
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dazzlingdave

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Re: Oh my...........opps! lathe mishap!
« Reply #2 on: February 22, 2016, 02:51:58 PM »


i am sorry for your loss sir  :'(

Thanks Jack.
I think the tool was really sharp, it was pulling out some serious chips, and cutting well before the mishap. I am guessing I must have gotten the angle wrong on the rest or something and it grabbed too hard. It happened so fast!

As far as what to do with the tool.......
I was thinking maybe I could keep the the bend, and perhaps sharpen it with a different angle, and have a hollowing tool now! (you might guess I am an optimist!)

Note: I got it back in the lathe, spinning straight, and am still working on hollowing it out!
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the Earl of Whirl

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Re: Oh my...........opps! lathe mishap!
« Reply #3 on: February 22, 2016, 06:19:07 PM »

Yow!  That bend looks familiar but you sure got that a good one.  I don't think I have ever bent anything that much before!!!  I'd love to hear Eric's comments about this.  I do worry about where I bent mine back and hope this doesn't come back to haunt me (good thoughts, Jack).
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Kirk

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Re: Oh my...........opps! lathe mishap!
« Reply #4 on: February 22, 2016, 07:37:12 PM »

I'm glad to hear that you are intact.
Take the handle off.  insert the sharp end into a potato (thermal insulator)
Heat the bend to dull red with 2 propane torches (or a MAPP gas torch)
Hammer back to shape over a block of steel (or hardwood)
Cool in water or oil. 
It is not likely that you will have enough heat to over harden the shank.
Reassemble, Sharpen and Use,

Delrin can be a bit grabby.  In the machine shop we cut Delrin with the blade very near the centerline. Stay sharp, hold on tight.
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yollector

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Re: Oh my...........opps! lathe mishap!
« Reply #5 on: February 22, 2016, 07:52:29 PM »


 People pay good money for
a tool Shaped like that  ;D
      RFC
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Dick Stohr

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Re: Oh my...........opps! lathe mishap!
« Reply #6 on: February 23, 2016, 12:30:50 AM »

WOW, I have a hard time thinking of the force necessary to bend a tool like that. The only thing I can think of is that you were being too aggressive and came to a bad spot in the delrin. If we all have one of these I hope mine happens 50 years from now. Sure glad that you were not hurt and all is well.
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johnm

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Re: Oh my...........opps! lathe mishap!
« Reply #7 on: February 24, 2016, 09:53:15 PM »

I’ll not try to speculate about what happened in this instance but I’ll identify a few things that all of us should keep in mind as we are hollowing and turning in general.

For all turning there is a pair of factors called feed and speed which have to be controlled by the user to stay within the limits of the machine, tool, and material being cut.  Feed is a combination of the velocity at which the tool is moved and the depth of cut (sort of the thickness of the chip removed depending on the shape of the cutting tool).  Speed is the surface velocity of the stock at the cutting edge and is basically the diameter of the stock multiplied by the spindle speed or RPM.  Feed and speed together determine the volume of material removed per unit time or how “heavy” or “light” the cut is.  Higher speed or feed results in a heavier cut, lower speed or feed results in a lighter cut.  In general we could say a heavier cut is more risky and a lighter cut is safer.  On a wood lathe with hand tools, generally the speed or RPM of the lathe is set and occasionally adjusted while the feed is continuously adjusted by the user based on some experience and sense of how the cut is behaving. 

The shape of a western peg top provides an interesting set of changing parameters that the turner must accommodate compared to a simple cylinder.  While turning the outside diameter as the top profile takes shape, for a single pass the diameter changes significantly and thus the surface velocity correspondingly changes and the user has to adjust the feed of the tool to adapt to this change.  Then from pass to pass the diameter has been reduced and thus the surface velocity for the same RPM setting has correspondingly been reduced so an adjustment (perhaps small) of the feed might also be in order.  For outside turning with other parameters staying the same, the decreasing diameter results in a lighter cut which is inherently safer.  However for hollowing cuts, the diameter is getting larger so the effective surface velocity is increasing which results in a heavier cut from pass to pass if other parameters remain the same.

Be extra careful while hollowing to keep the cut above the centerline of the work.  While turning the outside diameter if the tool tip dips below the center line, the work forces the tool down while the material rotates away—reasonably safe.  But while turning the inside diameter if the tool tip dips below the center line, the work again forces the tool down but now the work piece is in the way and the engagement of a catch is amplified.

Keep the extension of the tool beyond the tool post as short as possible.  This reduces the lever arm against which you must work.

While hollowing diameters that are larger than the entrance aperture, the tool will be angled into the work so pay attention to the tool exit point to avoid the shaft of the tool from contacting the work piece.   Rubbing the tool on the crown of the top there could cause the tool handle to move unexpectedly upward and move the tool tip below the center of the work while the tool lifts above the tool rest causing a catch.

Pay attention to the contour of a hollow and how it changes as the pass of your cut proceeds.  In particular consider the profile of your tool and if at some point in the pass, the amount of surface contact between the tool and the inner profile could change.  A sudden increase in contact length will suddenly make a heavier cut which could result in a catch.  A sudden decrease in contact length will change the effect your applied force will have perhaps driving the tool deeper into the work piece resulting in a catch.  A sudden change in contact position could result in a torque on the tool causing an uncontrolled rotation and catch.

Be careful when hollowing from a large diameter to a smaller diameter.   The depth of cut can quickly increase as the pass is taken and any issues may result in jamming the tool farther into the work because your cutting force is applied in that direction.  It is a safer cut to move from a small diameter to a larger diameter inside because the force you apply will tend to pull the tool out of the cut if some problem occurs.  Be aware though of the increasing surface velocity as the cut moves to the larger diameter and compensate for the corresponding heavier cut.

Be careful of swarf build up inside the top while hollowing.  With rotation of the work, loose chips are forced to the wall of the work where they can interfere with the cut.  Long stringy swarf can wind around the tool and pull it in unexpected ways.  Bunched up swarf can require you to apply more force to the tool to compress the swarf or move it out of the way and when it does move, the extra force you are applying could inconveniently increase the depth of cut. 

Keep the tool rest smooth and free of dings and pits.  The lathe tools are typically harder than the tool rest so it is pretty easy to put defects into the surface of the tool rest.  As you are making a pass, the tool may snag on the rest thus redirecting the force you are applying in some uncontrolled way.

Regularly check the tightness of your work holding method--in particular lower quality scroll chucks clamping on the outside diameter of the work piece.  The general design of the scroll and jaws coupled with the forces exerted by rotating it at high speed can cause the jaws to back out thus loosing holding force.  As the lathe spins, the chuck jaws have an outward radial force on them.  This force is countered by the scroll to jaw tooth friction in their inclined plane geometry, but the force from rotation might be enough to rotate the scroll allowing the jaws to open.   You can perform a little test of your own chuck by opening the empty chuck about midway between its extremes and measuring the diameter of the jaws.  Turn the lathe on and off a couple times and again measure the diameter of the jaws to compare to the starting diameter.   You might notice the jaws have opened a little.  With a work piece clamped tightly in the jaws, friction between the scroll and the jaw teeth will be higher and the unwanted scroll motion will be minimal.  However, you may have noticed that after removing a work piece from a chuck, the surface has an imprint of the jaw profile.  This means the local work piece material gets moved by the jaws as they are tightened.  Over time the compression of the work introduced by the jaws can relax by deformation of the material (especially plastics which can flow under force) thus reducing the clamping force and consequently the friction between the jaw teeth and the scroll.  On a large job that takes a lot of time (or perhaps progress is delayed/interrupted by life) this relaxing can be a problem especially when coupled with multiple starts and stops of the lathe.  Things may function well until a small catch occurs and the now lighter clamping force could be insufficient to properly retain the position of the work piece thus amplifying the hazards.  A little shift of the work during a cut can greatly change the depth of cut with basically instantaneous consequences.

Perhaps most important is to always pay attention!  Turning the outside of a top is interesting and exciting as the top begins to emerge so it seems easier to stay alert.  Hollowing is boring, tedious, and perhaps awkward and progress is not really appreciated until the lathe is stopped and the wall thickness is gauged.  It can be easy for ones thoughts to stray while hollowing, especially on a larger top since the effort is very repetitive.  Take a break if you find yourself daydreaming.
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Dick Stohr

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Re: Oh my...........opps! lathe mishap!
« Reply #8 on: February 24, 2016, 10:32:01 PM »

WOW, what a terrific explanation, well thought out and well presented, thank you John!!!!!!!!!!!
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robtsou

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Re: Oh my...........opps! lathe mishap!
« Reply #9 on: February 24, 2016, 11:31:00 PM »

I'm betting you have more of a burr on the edge than you thought.  The same thing happens to me with newly sharpened tools and never happens with duller tools.  Glad you came out unscathed!

If you want to bend it back, you could try applying some heat at the bend and bending it back. If you clamp the sharp end in a vise and don't go too crazy with the torch, the heat shouldn't spread enough to affect the temper at the end and you'll anneal the steel at the bend.
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cecil

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Re: Oh my...........opps! lathe mishap!
« Reply #10 on: February 25, 2016, 03:12:51 AM »

I have been a Machinist for 45 years. Now that I'm retired and cutting tops with hand tools, it's a whole new world. When I cut the center out I cut the top almost in half then I hallow the both ends and then glue them back together. Cutting the center out with hand tools scares the hell out of me. I'm glad it happen to you and not me. YET. Please keep us up to date on how this big monster is coming out. If you live through it.
« Last Edit: February 25, 2016, 05:17:03 PM by cecil »
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dazzlingdave

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Re: Oh my...........opps! lathe mishap!
« Reply #11 on: February 25, 2016, 10:32:23 AM »

Thanks Johnm, and all who have contributed to this thread.

After speaking with Eric, and reading the detailed thoughts of Johnm, I am pretty sure the main problems were:

Tool rest not being able to be inside the top....so in other words, my tool was at maximum depth into the top, and the rest was very near the handle. (allowing way to much movement)
This was compounded by working too near the center line, and while working from the larger diameter to the smaller diameter part of the spin top. Which when it started to grab, pulled the tool more forcefully into the top causing the tool to bend and the top to be ripped from the jaws of the chuck. (perhaps with the lathe at too high of an rpm as well)

It was amazing how anti-climactic it was. It is a huge piece of delrin, that I am taking a ton of material off. I have a couple of lights right above my lathe. My hands are pretty important to my yo-yo career.....and all that happened was the top fell out of the chuck, and my tool bent. (Thanks GOD)

Back to work, with a few more safety ideas in mind!

Thanks
Dave
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ta0

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Re: Oh my...........opps! lathe mishap!
« Reply #12 on: February 25, 2016, 11:06:37 AM »

I can still not believe how much that tool bent!
Great explanation John!

Take a break if you find yourself daydreaming.
That is why I am always taking a break!  ;D

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Neff

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Re: Oh my...........opps! lathe mishap!
« Reply #13 on: February 26, 2016, 09:35:02 PM »

I would never use a straight tool inside a top, only on the outside.  Only curved blades or gouges inside.  Glad your face is still the same shape!
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cecil

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Re: Oh my...........opps! lathe mishap!
« Reply #14 on: February 27, 2016, 03:59:48 PM »

What to do about tool digging in? What I want to try is turn tool upside down and reverse chuck. So If something grabs tool will bounce away from work area. Try it and if you live Thur it let me know. Also when I'm cutting inside I pray like hell and tell God, I will be better to my wife.  And to give a dollar to the street people and get blessed.  GOOD LUCK.
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