How close is close enough?

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Re: How close is close enough?

Post by earlgo » Thu Jul 27, 2017 8:47 am

At the last place I worked (commercial products, not aerospace) the engineers were required to go to a 3 or 4 day class on GD&T. My money says that half of the class had glassy eyes half way through day one. After the class, very few drawings appeared with GD&T on them. Those that did, were immediately questioned by the vendors as to "WIH does this mean?" and "We can't do that." Mostly because their shop guys didn't know what it all meant. Rob't Burns was right about things "gang aft aglee".
(Oh and we were told not to use the concentricity symbol because in reality it meant that every molecule on the part needed to be checked, not just the surfaces. The TIR symbol was for surfaces.)
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Re: How close is close enough?

Post by Magicniner » Thu Jul 27, 2017 12:21 pm

SteveHGraham wrote:What I have been told is that the 4-jaw is supposed to be the standard chuck, and the 3-jaw is really for convenience.
The Collet Chuck is the standard chuck, the 4-Jaw is for stuff that's out of round, off-centre, previously machined or otherwise difficult and the 3-Jaw is for making stuff that doesn't need taking out of the chuck to finish.

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Re: How close is close enough?

Post by SteveHGraham » Thu Jul 27, 2017 2:07 pm

That unusual theory doesn't hold water for some obvious reasons. For one, collets can't be used for large workpieces, which are very common. If a typical machinist who does a variety of jobs used collets as his standard workholding devices, he would have to take the collet chuck off so many times during a year, it wouldn't be worth the aggravation.

The 4-jaw will do just about everything a 3-jaw will do, better, and it will do most of the things a collet will do.
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Re: How close is close enough?

Post by BadDog » Thu Jul 27, 2017 2:51 pm

Not sure about "supposed to be", but if you could have only 1 work holding device for the lathe, you would probably be best served by the flexibility, holding strength, and potential accuracy of the 4 jaw, though that might change depending on what you need the lathe to do. A 12" 4 jaw stays on my lathe most of the time, but the 8" 3 jaw is also there a lot, with Rubber Flex and others taking up the slack once in a while. Much of the time, what I use is whatever happens to be the last thing on the spindle, assuming it will work for the task.

But not sure I could ever describe a collet chuck as "the standard" chuck for a general purpose lathe. Too much need to hold hot roll, previously worked to arbitrary dimensions that must be concentric and coaxial, and of course castings and other things that may not be very round if at all. It certainly couldn't be *my* standard chuck, but maybe for a production shop where everything is made from clean "on size" bar stock. Heavy reliance on "soft collets" might get you further, if you could justify the cost.
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Re: How close is close enough?

Post by Downwindtracker2 » Thu Aug 10, 2017 1:30 pm

I'm a retired millwright. I was the guy that had to make the part fit and work. Our machines varied from 50 year old Japanese to the latest Chinese,with German and some English designed Canadian.. So job shops did a lot of work when they broke. One firm's name rhymed with "they couldn't do anything right" . But the maintenance supervisor got a new motorcycle. I'm just learning but my work fits better.
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Re: How close is close enough?

Post by tornitore45 » Fri Aug 11, 2017 9:33 am

If you are doing production and a collet works then is probably the best option. But if you do a variety of one piece the 4J is a good fit.
After getting the 4J centering routine to the point I can do it hanging upside down the 4J is the default chuck mounted on the lathe. The hassle to change to a 3J is only worth if I have many pieces that are not critical for concentricity or can be done start to parting off in one chucking. Sometime I have to change from 6" 4J to 4" 4J for small stuff.
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Re: How close is close enough?

Post by Magicniner » Sat Oct 07, 2017 8:21 pm

SteveHGraham wrote:If you're too cheap .
Pot, Kettle!

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