[How-To] Indicating a 3 jaw independant chuck

mikey said:
Mark, Bear, I think these chucks are very cool. However, when I first heard of them from Mark, I took a long look at them and decided for myself, they were not worth the cost. I do not own one so my comments are coming from ignorance but it seems to me that a 3 jaw, when used for a first op, would not need adjusting. For a second op, a 4 jaw is faster and should hold more solidly given that the part is held by tension of the screws for each jaw vs a scroll. I can see how putting the part in the chuck would be faster with a scroll but for the rest of it I just don't see the advantage, especially in light of how much they cost.

I can see one if you are trying to avoid having to switch out chucks or if you are dialing the chuck in to make a production run on some nominal stock of the same size but in a hobby shop, these are non-issues.

Again, I am speaking from ignorance. Is
my logic off?
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No offense, but actually, yes... first, you are correct that for a first op, a standard 3 jaw normally would not need indicating.

For a second op, from my experience, it is still faster than a 4 jaw independant unless you also used the 4 jaw for the first op... in which case the 4 jaw would already be very close to being centered anyway...

As far as holding the part securely, you can apply as much holding force with the adjustable jaws as you can with the scroll... or with the independant jaws on a 4 jaw...

Normally, by the time I have a part indicated, the jaws are tighter than what I put on them with the scroll. I have not had any issues whatsoever with the work coming loose, moving, etc.

I did wonder the same thing when I first bought the lathe... but I've since found it to be a non-issue.

Both of my combination chucks were with the lathe when I bought it... along with a 12" 4 jaw. I won't form an opinion on whether the cost is worth having one... that is up to the individual user. I will say that if something happens to this Rohm chuck, I will be looking to replace it with another combination chuck.

My other combo chuck, the 9 inch Union, is worn so badly that the scroll pinions tend to bind up... it is useable, but not easily, so I don't use it at all. It is currently a dust collector.

-Bear
 
markba633csi said:
I want one of those for Christmas, Santa!
-M
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Interesting. I have never seen a chuck like this other than your stande
682bear said:
In response to a question that I was asked on the 'what did you buy today' thread, I decided to do a pictorial thread on indicating a 3 jaw independant scroll chuck. The reason for this thread is both to keep form hijacking the other thread too much, and also to make this easier to find if someone else needs it in the future.

To begin, a 3 jaw independant scroll chuck is like a standard 3 jaw, it has a scroll which moves all 3 jaws together and keeps them somewhat centered, but the jaws are 2 piece jaws, making them independantly adjustable. My Rohm chuck is older and has some wear, so it will only repeat within about .005-.007 using the scroll to tighten.

View attachment 337221

First, I put the part in and tighten using the scroll...

View attachment 337222

Next, I put the indicator on the part. I use a travel indicator mounted on a magnetic base that I just stick to the side of the QCTP.

View attachment 337223

I rotate the chuck and stop with the indicator even with all 3 jaws and note the readings. Here, jaw #1 is at 0.000...

View attachment 337225

Jaw 2 is at 0.010...

View attachment 337226

And jaw 3 is at 0.004...

View attachment 337227

Now, I go to the lowest jaw (#1) and loosen the adjuster on that jaw about ⅓ of the runout (0.003)...

View attachment 337228

Now I check all 3 jaws again. #1 is at 0.003, #2 is at 0.009, # 3 is at 0.001.

Now I will go to the high point, (jaw #2) and tighten it about ⅓ of the runout (0.003).

Now jaw #1 is at 0.007, #2 is 0.006, #3 is 0.001. I'll go to the low point (#3) and loosen it about 0.003.

I will continue to alternate between the high point and low point, tightening and loosening as necessary until I get it running as close as I need it.

If you turn the scroll at all during this process, you will probably have to start over... do all the adjusting with the jaw adjustments.

I can usually get it within a half thousandth of an inch in a minute... 2 minutes tops... but I've been doing this for a couple of years

Finished...

Jaw #1...

View attachment 337230

Jaw #2...

View attachment 337231

Jaw #3...

View attachment 337232

It's really simple and quick once you get used to it. I hope this helps someone...

-Bear
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Interesting. I have never seen a chuck like this, other than your standard 4 jaw independent and scroll chuck before.
 
That's a pretty strong endorsement, Bear. Yeah, I can see that it might be faster than a 4 jaw independent and you're right, I didn't think about the fact that you can tighten each jaw hard enough to hold very securely. I should think that a 4 jaw combination would be easier to adjust vs the 3 jaw because you can use opposing screws. Interesting chucks, I have to admit, and I can see why you like them.

I have a Pratt Burnerd 6 jaw and a Yuasa 3 Jaw set-tru chucks and they're okay. They can be made to run very accurately but I have to admit that I think they are over-rated. You don't need it for a first op and a 4 jaw is more accurate for a second op. Some guys seem to think they are uber-chucks though.
 
We have a 6 jaw set-tru on the lathe in my department at work... I really like it. I originally wanted one for my SB 14 ½, but I'm very satisfied with the Rohm and see no reason to spend the money for one.

-Bear
 
Not having owned one I'm only speculating here, but I'd imagine a 6-jaw would be better at taking the wobble (for lack of a better term) out of a part. Like when you indicate in a 4-jaw down to a tenth, move the indicator over an inch, and it's running out again.
 
6 jaw chucks are good for grabbing thin-walled work pieces because they distribute clamping forces all around the part. However, it is still just a scroll chuck and therefore does not apply all that much force, at least not compared to a 4 jaw independent or perhaps one of these combination chucks. I think a simple 3 jaw grabs tighter.

They are also good for grabbing plastic parts because there is less potential for the part to move in the chuck; it is surrounded by the jaws so you can apply higher cutting forces to it without the part flying out of the chuck.
 
Thanks Bear. And totally agree....those chucks are expensive at 1900.00 from MSC.

but.... Bear was nice enough to post this thread because of my original post on new tools. I scored one of these on EBay and it was new. Like....not even surface rust yet. I had asked him how he centers stuff. Anyway....no I can’t justify 2000.00 for a chuck. But for 250.00 , I couldn’t resist. Still high but hell, my dad died at age 69. I’m 60. I like tools and love this hobby. What the hell. Did I need it. No, but I wanted it and “Like the saying goes....you can’t take it with ya“.
————
Here’s the original post from other thread......

I found this chuck on EBay. When I saw it, I instantly knew what it was. It’s another oddball. And I think some machinists just would prefer to use a 4 jaw. But, I wanted to give it a shot.
It’s a Rohm 3-jaw scroll lathe chuck with adjustable jaws. It’s like an Adjust-Tru.....but it’s. Three jaw. And a little different design since there’s a 1/4” Allen adjustmen screw integral to the jaw. The jaws are reversible.
Rohm 3-jaw self centering 8” lathe chuck- 1800.00

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I took it apart because it was really tight. There was absolutely no swarf in it but the grease was gummed up. Someone put white lithium in it without removing the other clear grease. And yes... Rohm is very specific... this chuck should be greased. Not oil. Grease, so I used wheel bearing grease.
The cost on EBay was 225.00 + 36.00 shipping. This chuck is like brand new. The jaws and scroll and adjusting screws for the jaws are like brand new. They are so damned precisionly machined that the fit is within a half thousandths on the jaws to the steel body.

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And here is why Bear posted this thread. I asked below on tools thread. And I was kind of hyjacking the thread.
Anyway, I’m psyched. Thanks again Bear.
———-
Bear, I’m curious as to your method centering the jaws.
I’ve given it some thought and in my mid.... I’m thinking that the way to go about centering with 3 jaws.... is with dial indicator mounted....to focus attention only on the low point. And then adjust that jaw since the high points will always be midway between the other two jaws. Does that make sense. Am I on the right track ?
Anyway I’m not there yet since I can’t mount the chuck to my other find.... Bison Backing Plate... for 8” chuck and with a L-00 taper. I need to go to hardware for 7/16”x14tpi socket head bolts.
Anyway...between the Rohm chuck and Bison backing plate.... I feel like I have a horse shoe up my butt, finding this and the chuck on EBay at the same time. The backing plate was “ accepting Offers. “
got it for 115.00 plus 21.00 shipping.



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Anyway, I’m psyched. Thanks again Bear.
 
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