grinding a three jaw chuck

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John Hasler
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Re: grinding a three jaw chuck

Post by John Hasler » Wed Apr 18, 2018 2:22 pm

IMHO only the Haas design and the ring and pins design can produce a purely radial load. I lke that Haas design. You could drill extra holes to allow a single plate to cover a wide range, and the plate is simple enough that it would be easy to make extra plates for more range. The grippers could be made by making a four-hole design, drilling it for the pivots, and then cutting the three pieces apart.

The scroll is going to push the jaws to the side but it will do so in use so you want that.

Harold_V
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Re: grinding a three jaw chuck

Post by Harold_V » Wed Apr 18, 2018 3:30 pm

ctwo wrote:
Wed Apr 18, 2018 1:30 pm
You are drawing an absolute conclusion from an unfavorable procedure (or setup). The result could very well be perfection, as unlikely as that may be.
Correct. Based on my previous experiences in using soft jaws, it's more likely you'd achieve compromise results than acceptable results. With hard jaws, it may be hard to determine, though, as they won't run dead true under almost all circumstances, unlike soft jaws that are prepared for a given diameter and used only there (under normal conditions). In my case, where I use soft jaws almost 100% of the time, I use them the same way one would use hard jaws. They don't run dead true in all cases, as the scroll and scroll clearance create eccentricity, depending on the diameter. I have found that using a different socket will, under random conditions, create a better condition than the designated socket, but thus far it is not predictable.
Now, I see two opposing vectors in this setup, each working on opposite sides of the jaws, and would very nearly balance, and result in a single radial force. I imagine this allows the jaws to settle where they may rather than holding them in some other orientation

Which I imagine the 4-hole disc would more likely do, so perhaps that is not as good. Certainly that device applies a similar force to the sides of the jaws, like so many other commercial solutions. I recall seeing a Haas company video where they used a jig similar to this: https://i.ytimg.com/vi/Zlt1-3-6hso/maxresdefault.jpg Notice this jig also only contacts the side (angled) surfaces, but it's interesting feature is in its floating design. I'd have to understand your thoughts on these before introducing the next idea.
The link image, in my opinion, is exceptionally good for performance. While the sides of the jaws (the angular portion) is where contact is made, that the pin location allows for the jaws to pivot, seeking the favored side when loaded, that most closely duplicates the chuck in use. To be clear, if is far superior to a spider, which must be manually located properly, otherwise it, too, introduces error.
Seems the scroll is going to push the jaw to one side, too.
What ever influence the scroll may have over the operational location of the jaws should not be overcome. That's the beauty of the system in your link. If, in tightening the chuck, the jaws have a tendency to roll to one side or the other, that's exactly what you must allow them to do. In a well worn chuck, it can be extreme. In one that has tight slides, maybe not so much.

One thing I should have commented on. When using a spider, when it is placed deeply inward, unless the adjusting screws have pointed ends, the square contact area of the heads (which is what I use) can prevent the jaws from being firmly loaded in the flexed condition. My chuck is tight enough that I don't suffer from that problem, but being off center is a real problem, and has, on several occasions, given cause to a set of jaws that do not run as true as possible. I should take a lesson from my own ravings and use a small radius end point instead of using the square ends of socket head cap screws. You'd still have to seek center with them, though. Otherwise you still risk tilting the jaws to one side of the other, neither of which condition is acceptable.

H
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ctwo
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Re: grinding a three jaw chuck

Post by ctwo » Thu Apr 19, 2018 12:08 am

I think all we really need is a fast enough spindle. ;)

Anyway, sure for soft jaw applications and what you are typically working too, then I agree it gets much more critical. Hard fixed jaws already seem to be compromise, and a lot of times I have a gummed up shim in there. There is some motivation to revisit that item - it's on my rotab now.
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
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Harold_V
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Re: grinding a three jaw chuck

Post by Harold_V » Thu Apr 19, 2018 1:09 am

ctwo wrote:
Thu Apr 19, 2018 12:08 am
I think all we really need is a fast enough spindle. ;)
Chuckle!

Well, be careful! :lol:

I've had soft jaws release parts on more than one occasion, due exactly to the thing you suggested. If the jaws are heavy, and some of mine are, enough speed causes them to release. It's a strange thing to see a part slip while cutting--when you know it should be rigidly gripped.

I should make mention---if fragile parts are being machined, I tend to use very light chucking pressure, to avoid distorting the parts. That can include using broad jaws, which exacerbates the problem (greater weight, but less chance of distortion). Because I specialized in small work, it wasn't unusual to find my lathe running @ 2,000 rpm, even with my 3 jaw.
Anyway, sure for soft jaw applications and what you are typically working too, then I agree it gets much more critical.
Thanks for understanding. I realize that I often sound like I've lost my mind, but the world I served demanded exacting performance. The slightest thing would often result in unacceptable results. While the majority of it seems like overkill, if you acquire good work habits, tough work becomes routine. The guy who thinks "close enough" generally struggles when called upon to do tough work, so there's a huge reward in spending enough time in learning to do it right. The best possible lessons are learned that way.
Hard fixed jaws already seem to be compromise, and a lot of times I have a gummed up shim in there. There is some motivation to revisit that item - it's on my rotab now.
I agree that hard jaws are a compromise, a lesson I learned only when I was forced to use soft jaws for non-critical work. If I haven't made mention previously, my three jaw was sitting on the compound of my Graziano. I took my hand off it and leaned to pick up my air hose. Chuck rolled off and landed on the concrete floor, on a jaw, shearing the teeth. The scroll appeared to be unharmed, and has been in use ever since. This occurred way back in the late 60's, not too long after I purchased the lathe.

I inquired of the dealership if they could get me a set of jaws. They said they could, but in the mean time they had a badly used (or abused) set they allowed me to use. They fit poorly, although they did work. I was not impressed with their general condition, nor of their lack of concentric gripping, so I installed the master jaws and bored a set of straight through jaws. I was shocked at the results, so I decided to stick with them instead of the hard jaws. That was a respectable decision, as the set of jaws I requested from them never were delivered.

I am now so comfortable with the soft jaws that I rarely use the hard jaws. There are occasions where they fit the bill for a project, so they do get used occasionally. Mean time, I'm a died-in-the-wool user of soft jaws.

The rotab issue. Because I often needed my lathe when doing mill work, the idea of tying up my three jaw on my rotab wasn't acceptable, so I purchased a nice Rohm (with master jaws), which I used only on my rotab. A permanent base plate was made (about 1" thick aluminum) which mounts directly to the chuck and rotab. Do keep in mind, this was when I worked for gain. I am not the least bit critical of your choice, and would likely do the same thing now if I didn't already have the second chuck (which rarely gets used).

A spider isn't the best possible way to machine jaws, be they hard or soft. However, it is the fastest and least demanding of all systems. With one spider and three sets of screws, a huge range can be covered, while most other methods require rings and other devices in large numbers. It also eliminates the problem of not having on hand a component for the system you may use when you hope to make a quick setup. I use a spider and spend the necessary time (generally just a few moments) to ensure it functions as it should. For me, it was the best solution. I got tired of hunting for the just-right-size piece of stock that would load the jaws, and learned quickly that a spider allows for removing just a few thou from the jaws, so they enjoy a very long service life.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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ctwo
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Re: grinding a three jaw chuck

Post by ctwo » Thu Apr 19, 2018 12:57 pm

Ah, compromise, just not too much... In another context, someone had a comment about absolute safety, and while ideal, may not be practically achieved.

I'd be impressed if you remembered that I bought an import 6" chuck for removable jaws, which is primary on my Logan now. I think it came with a 5" Cushman type, which works OK on the RT. My Jet has something similar and would benefit from some attention - when I eventually get it off the floor...

Such a lost mind is fast becoming a rarity, something to admire, and even cherish...
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...

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