Good practice question

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neanderman
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Good practice question

Post by neanderman » Tue Dec 04, 2018 12:56 am

So, at what point do you use a steady rest to support the 'unsupported' end? IOW, how much extension from the chuck is too much?

https://www.youtube.com/watch?v=nG2vd8XuFzs
Ed

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BigDumbDinosaur
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Re: Good practice question

Post by BigDumbDinosaur » Tue Dec 04, 2018 2:45 am

neanderman wrote:
Tue Dec 04, 2018 12:56 am
So, at what point do you use a steady rest to support the 'unsupported' end? IOW, how much extension from the chuck is too much?

https://www.youtube.com/watch?v=nG2vd8XuFzs
Way, way, way, way back when, when I was first learning my way around a lathe's ways, the rule of thumb was to use support (steady rest or tailstock center) if more than six diameters of stock were protruding from the chuck or collet and the operation would involve material removal at or near the unsupported end. Of course, the type of material is important, but I recall six diameters for steel or extruded aluminum was good. Beyond that, deflection and chattering would become issues.
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ctwo
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Re: Good practice question

Post by ctwo » Tue Dec 04, 2018 2:39 pm

I always thought ROT was > 3 diameters.
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pete
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Re: Good practice question

Post by pete » Tue Dec 04, 2018 3:10 pm

Textbook recommendations in the books I have all say a maximum of 3 times the diameter for part length outside the jaws. In practice I've found that to be generally correct. But there's a dividing point where common sense kicks in. For a real large shaft on a big lathe I wouldn't expect any chuck to be capable of properly holding a 2 ft. diameter shaft unsupported that's 6 ft. long. Permanently damaging a chuck can be done both by over tightening them and with too much unsupported weight. How much diameter you have available for the spindle through hole and how much shaft length you have behind the chuck jaws as a counter balance also helps a bit. Then there's what your attempting to do. Center drilling that shaft for a tailstock center is obviously a whole lot different than taking a cut down it's length. Actual tool sharpness, cutting depth and feed rate are also a factors. A honed razor sharp HSS tool is going to exert much less tool pressure than say a generic three sided carbide replaceable tip would. If your lathe does turn parallel and that unsupported shaft shows a slight taper after the first cut then it's going to need more support. That's the method I judge it by. Boring bars will have a maximum they can be expected to bore a hole depth to without some deflection. 3 - 5 times the bar diameter at the absolute maximum for steel and upwards of 8 times the diameter for a solid carbide bar are the numbers I've seen mentioned. So an unsupported shaft is just the same and 5 times it's diameter might be barely possible if you can get away with it. Keith's video you linked to showed a good surface finish while doing that facing cut, so for what he was doing it looked to be rigid enough. My eyeball estimate is he had about 3.5- maybe 4 times the diameter outside the jaws. He certainly knows a whole lot more than I ever will.

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Re: Good practice questio

Post by Glenn Brooks » Tue Dec 04, 2018 3:36 pm

Iam wondering if 6x diameter might be a proper rule of thumb for adding a steady rest to tailstock supported work. With 3x or less, being the cutoff point for unsupported work. I took a night intro lathe class a few years ago at our local tech college, where the instructor advised us to use the trailstock support on anything over 1.5 x work diameter. Although we were turning 3” diameter material down to 1” - so maybe that related to diameter size and material strength for the particular project.

Like Pete said, on my small 7” Dalton, (OXA QCTP and 1/4” bits) diameter size makes a big difference how far out I can machine along the axis without support. Can’t machine 3/4” round stock at 2” from chuck without the tailstock support, for example. With 1/2” to 3/4” diameter bronze round stock it, for small parts, I generally add the tailstock soon as I have enuf room to work the bit between the chuck and live center, often an inch or so.

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Re: Good practice question

Post by Conrad_R_Hoffman » Tue Dec 04, 2018 9:42 pm

I do a lot of unsupported work because I have to. About 6:1 is the limit, and the tool grind matters a lot. You want a sharp small radius tool that doesn't cause much deflection, with just the right amount of rake so the work doesn't chatter. And either take off more or less than you want to take, because what you want to take off is always in the no-man's land of chatter!
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John Hasler
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Re: Good practice questio

Post by John Hasler » Tue Dec 04, 2018 11:04 pm

Glenn Brooks wrote:
Tue Dec 04, 2018 3:36 pm
Like Pete said, on my small 7” Dalton, (OXA QCTP and 1/4” bits) diameter size makes a big difference how far out I can machine along the axis without support. Can’t machine 3/4” round stock at 2” from chuck without the tailstock support, for example. With 1/2” to 3/4” diameter bronze round stock it, for small parts, I generally add the tailstock soon as I have enuf room to work the bit between the chuck and live center, often an inch or so.

Glenn
Use a half dead center for this.It has part of the center cut away to give you access to the very end of the part.

https://www.mscdirect.com/product/details/00100644

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neanderman
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Re: Good practice questio

Post by neanderman » Tue Dec 04, 2018 11:37 pm

John Hasler wrote:
Tue Dec 04, 2018 11:04 pm
Use a half dead center for this.It has part of the center cut away to give you access to the very end of the part.

https://www.mscdirect.com/product/details/00100644
Dang... 'Em's expensive! But nice tip. (No pun intended. Seriously. :D )

Thanks!
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tornitore45
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Re: Good practice question

Post by tornitore45 » Thu Dec 06, 2018 7:54 am

The half centers are a lot more expensive that a regular dead center. I just ground off a regular HSS one. For the few time I use it it work great.
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Richard_W
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Re: Good practice question

Post by Richard_W » Fri Dec 07, 2018 12:40 am

I like these tracer point/extended point live centers.

https://www.travers.com/revolving-tails ... 3-301-787/

Of course the old standard live center is really nice to, but often times to large to get the tool close on smaller diameters.

https://www.travers.com/revolving-tails ... 3-301-783/

Then the CNC point comes in real handy as well for smaller diameters. But not worth much for larger parts. So its nice to have the 3 styles around.

https://www.travers.com/nc-quad-bearing ... 9-203-013/

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tornitore45
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Re: Good practice question

Post by tornitore45 » Fri Dec 07, 2018 7:23 am

Sure, $1200 to adorn the tail stock of a $1200 lathe that has been "turning" out good parts with its original center.
Mauro Gaetano
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earlgo
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Re: Good practice question

Post by earlgo » Fri Dec 07, 2018 9:51 am

These partial centers are what I have been using for years. I think the ground half center is from 1980 or so.
centers.jpg
partial centers
A bit of high pressure center lube and you are good to go. The drill center was a broken drill so it was free.
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