I may be stupid but

[SillyOldDufferModerator @sillyoldduffer]

Posted by Steviegtr on 26/06/2022 01:21:05:

From a miriad of answers, i have decided to have a go at some measurements to see the difference of tightening 1 keyhole & doing all 3. …

Steve.

Good idea, experiments are always fun. I suggest there are two possibilities to investigate:

1. Tightening the pinions one after the other causes one or more jaws to move and alters the job's concentricity.
2. Tightening the pinions one after the other increases the grip.

The first we can all test by clocking a rod in the chuck, First note how far the needle deviates min to max when the first pinion only is tightened, then same measurement after the other two pinions have been tweaked to take up any slack.

Second is harder, because we have to detect an increased pressure on the workpiece after the second and third pinions are adjusted. I suggest a narrow diameter manometer might be good enough. The apparatus is a steel tube sealed at one end and held in the chuck. The other end is connected by a hose to a clear U-tube, and the whole filled with water dyed red. When the steel tube is squeezed, the water level in the U-tube will register the pressure change by rising. I predict tightening all three pinions will increase the pressure on the tube. The thickness of the steel tube's wall determines the system's sensitivity. Some maths will tell us how much extra pressure, if any, is applied.

Dave

 

Edited By SillyOldDuffer on 26/06/2022 19:19:13

[Neil LickfoldParticipant @neillickfold44316]

We have an old 3jaw chuck at work. If you use 1 key on some bright bar, it will be around 0.15mm to 0.2mm tir. Then tighten first on the jaw by the logo (Pratt) then rotate chuck clockwise(away from you) then just nip onto the next two keys, it will generally be better than 0.02mm. Until I was shown about this, I would not have believed it. It was a great trick to learn of this particular chuck. So it seems that as they wear, different trick can be used and they become individuals as to how they work. The aspect I like about the grip tru series, is that it adjusts the chuck in the plane and the swash of the part remains the same. With old 4jaw chucks, you need 2 indicators to try and get the part running true. Any chuck that is over tightened by holding onto a part that is just using the very front of the jaws, (lost of parts 2nd ops ) don't grip it so tightly. It will last a very long time.

Regular maintenance and cleaning of the scroll and pinions will make them last a lot longer, even on a 4 jaw chuck. Using a vacuum cleaner will also help if cutting things that are dusty and dry.

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello,

I have just checked out the single pinion chuck. It is a conventional scroll type and 4" diameter. I don't need to make a back plate as it has a Myford thread!

So screwed it onto the lathe and checked it out with a DTI and different diameter silver steel rod. Average run out was 3 thou, so I am MORE than happy with that!!! I still think it is of Chinese manufacture, no identifier, only a couple of numbers. The finish on the scroll looks excellent, but then all the machining looks very good.

Maybe the Myford thread is a clue?

Andrew.

[Mike PooleParticipant @mikepoole82104]

I think the runout of a three jaw chuck should not really matter, even if it can be improved by a tightening regime there is nothing you can do if it is out unless you have a griptru chuck. A three jaw chuck is a quick and dirty device to hold a job to be turned at one setting. If further operations are required and concentricity is important then remounting in a collet or 4 jaw independent is required. Working between centres is the best option if the job is suitable. It may be the case that concentricity is not critical so replacing the job in a 3 jaw will not matter but other jobs will require perfect concentricity in which case the three jaw will not be the tool for the job. It may be interesting to explore the performance or a three jaw at various diameters and tightening regimes but unless you are going to clock the job every time it matters then you can never be certain where you are. Even if you establish that a chuck has low runout and repeats accurately you never know when it may have collected an embedded chip in the scroll unless you check every job that is critical.

Mike

[Andrew TinsleyParticipant @andrewtinsley63637]

Mike,

I am well aware that a 3 jaw scroll chuck is a quick and dirty solution to work holding with any accuracy. I quoted the low run out, because it implies that the chuck was manufactured with more than the usual degree of accuracy. The chuck is unused as far as I can tell, so what it would be like after extensive use, I don't know.

Regards,

Andrew.

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