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Mr Crispin grinds his jaws

Dabbler said:
It seems like it should always be a larger radius than any part you might chuck,
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I think that would be the ideal world. But unless you grind the contact face parallel to the base of the jaw, this is not going to happen for parts which are larger in OD than the diameter at which the grinding took place.

Dabbler said:
Too small and you'd be chucking on 2 points per jaw
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This is what happens when chucking a part larger than the grind diameter: two lines of contact per jaw. I have not found any problem centering parts of larger diameter. One has to “feel” for it seating by slightly turning the part in the chuck as you start snuggling up the jaws.

My Röhm 3J has a super accurate scroll which is very snug inside the body with an absolute minimum of radial play. Thus the chuck is accurate at all diameters.
 
Dabbler said:
MrCrispin does it an alternate way.
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I sure like the way he approached it compared to the others, and his explanation was very clear.

Perhaps one day this will be necessary for me to do, but at the moment it’s all “academic” youtube watching at the end of the day when I’ve no energy for other things.
 
I find it a little funny...On Renzetti's jaw truing video he states that the purpose is for the jaws to hold the material in a solid and even manner to reduce chatter. He states that concentricity is specifically dependent on the wear of the scroll.

With Crispin, he states the purpose is to improve concentricity, which I'm sure it is now better than what he started with, but we didnt see what the TIR was before. It's too bad he didn't check TIR on a piece that was of the same diameter as what he ground the jaws at, but I would hazard to guess that it would have been almost zero.

I like watching both of these guys videos, but I'm in the Renzetti camp on this issue. Though I do like Mr. Crispins jaw dollies.
 
thestelster said:
I find it a little funny...On Renzetti's jaw truing video he states that the purpose is for the jaws to hold the material in a solid and even manner to reduce chatter. He states that concentricity is specifically dependent on the wear of the scroll.
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Hmmmm..... He said that but his description focussed more on concentricity. LOL!

Do as I say, not as I do?

I guess he is allowed.....

thestelster said:
With Crispin, he states the purpose is to improve concentricity, which I'm sure it is now better than what he started with, but we didnt see what the TIR was before.
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Hmmmm...... I wonder if he did what I did! Just assumed it was bad and needed fixing! Maybe he made it worse! Lol!

You are right in any case, we need before and after.

thestelster said:
Though I do like Mr. Crispins jaw dollies.
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Me too.......
 
On TIR before vs after grinding…

Because of the bell mouthed jaws on both the Röhm and the PB chuck, I would get TIRs in the tens of thou range, depending how the part was gripped and how much torque I put on the key; the more torque, the worse the grip. I could (with some effort) actually move the part around by hand and easily bump it with a soft blow hammer to zero TIR.

So it thought that was a good thing (being able to bump to zero TIR) - until I had trouble making accurate parts and, you guessed it, chatter.. I broke many HSS blades and carbide inserts during parting. Never dared to power feed for the parting op either.

After grinding out the bell mouth, all these problems went away. No chatter, and power feed to part is the only way to go. This is for all types of materials: from non-ferrous to 4140HT and SS, etc.
 
StevSmar said:
That’s good information! Maybe I will end up grinding my jaws once I’ve taken the training wheels off my lathe capabilities.
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Keep em handy after you do. No matter how much we learn there is always what we don't know we don't know........
 
When I bought my 6" 3-jaw Bison 2-part jaw chuck, long before I thought home grinding was even a possibility, I noted Bison spare jaw sets were readily available if it ever came to that. They aren't even horribly expensive. The question is are these plug-n-play manufactured to yield the same runout tolerance as buying a brand new chuck (assuming the scroll & slots are within spec). Or are they meant to be replacements that shops grind themselves? Because you actually don't hear about many shops with their own TPG. I have always assumed that jaw faces were given a finish grind as an assembled unit, which makes sense on solid jaws. But for 2-part now I'm not so sure because they are relying on the jaw base slots/keyed features. Who's up for a road trip to Poland? LOL
 
RobinHood said:
After grinding out the bell mouth, all these problems went away. No chatter, and power feed to part is the only way to go. This is for all types of materials: from non-ferrous to 4140HT and SS, etc.
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Yup. The common human action is to direct attention to the cutter, blade, setup, material, speed, feed, fluid... and that's totally understandable. But also important, usually more important, to examine any source of free play (lead screw, dovetail slides) or non-uniform grip (chuck related) for operations like parting & finishing & repeatability which are more stringent.
 
PeterT said:
Who's up for a road trip to Poland? LOL
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Not me. But I'd love to know the answer.

What I can tell you is that I don't like reversing my jaws. The fit is extremely tight. They do not just slip on. On all three jaws, they have to be pulled on with the screw, and pried off with a wedge.

Mine is a no-name chuck that came with my lathe. But I love it!
 
PeterT said:
The question is are these plug-n-play manufactured to yield the same runout tolerance as buying a brand new chuck (assuming the scroll & slots are within spec).
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The answer is yes - ish. Your concentricity is in your matched master jaw and scroll. so you manufacture to, say, +/- .001 TIR. That means for each scroll and master jaw is fitted to be about half that, or a half thou. All you need to do is match grind thousands of jaws from the slot (or buttress in European jaws) to be about half that again, which is a quarter thou from the seating to the gripping, as it were, This is trivial on modern grinding setups. One tenth is typical.

So if you have a quality manufacturer, you won't see enough variance to make a difference.

For those guys with a SG, things can be touched up easily enough to match your particular chuck and jaw position, off the chuck.
 
Yup, but tight fit is the only way to preserve tolerances & facilitate interchangeability. Its very important to clean both surface spotless before assembly. I also give a finger tip wipe of oil on mating surfaces to make me feel good because I'm starting to notice some displeasing discoloration in dormant areas on my lathe/parts from cutting fluid even though its 'oil based'.

I'm about to machine some reverse engineered aluminum soft jaws, wish me luck.
 
PeterT said:
That's encouraging. So replacements would presumably be numbered 1,2,3...?
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I bet not. I bet only the base jaw will be numbered..... But good question. Let's see what those who know what they know, know about that!
 
Dabbler said:
For those guys with a SG, things can be touched up easily enough to match your particular chuck and jaw position, off the chuck.
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That's exactly what I was wondering about when I asked about preferential pre-set diameter of TPG. RobinHood answered with more practical answer - large enough to accomodate the wheel but small enough to not exceed bore. But a surface ground jaw is a flat jaw & that will always be center tangent holding round part.
 
Here is what Röhm has to say about clamping force / grinding / testing / accuracy…

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They do not pre grind anything. All jaws are ground insitu at a given torque. That is where the accuracy needs to be measured / is achieved.

I guess with a TPG and careful set-up this can be done in the shop (no need to send in the chuck to the factory for grinding in the new / replacement jaws).

This is the link to their catalogue from which I got the above screen shots.


I will need to check how Bison, or PB, etc do it… my guess is in a similar manner to Röhm.
 
Thanks! I have never before caught that advice from a manufacturer in their hardened secondary jaws page. This may be due to Rohm's greater accuracy specs. A lot of chucks are much lower accuracy - .006 concentricity for a 1" test bar, for instance.

Pratt Burnerd America doesn't mention this in their replacement "top jaws" brochure:

Pratt Burnerd, like the offshore Asian chuck manufacturers only give a 'repeatability for the same part' specification these days. A less meaningful number that looks good.
 
RobinHood said:
Here is what Röhm has to say about clamping force / grinding / testing / accuracy…
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Wow. I've never seen a specification like that. But it makes a LOT of sense. Specifying a 10NM (7.4ftlb) Torque on the drives also adds a new dimension I've always wondered about. Presumably the 40 (30ftlb) is max and 10 is for specification.

I'd love to see something similar on the lock cams.

Adds a new dimension to the way I'll think about tightening my chucks in the future.

Thanks Rudy!
 
Thanks for looking that up John. Appreciate it.

As with much of anything: the devil is always in the details - and a lot of time they are hidden. Maybe that’s their manufacturing secrets?
 
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