|Robin Graham||26/11/2019 01:42:08|
|727 forum posts|
I've got two lathes - one a bench top Austrian job, the other generic a Far Eastern 12x36. The Austrian lathe came with a Rohm 3-jaw chuck with a single set of jaws - it's only a matter of reversing them and remembering that jaw 1 becomes jaw 3 when putting it them in backwards to grip larger diameters. It works well enough. The other one came with two sets of jaws, which seems to be normal and also works well. Is there reason behind this? Is one arrangement better than the other? Just wondering!
Edited By Robin Graham on 26/11/2019 01:44:25
|Paul Lousick||26/11/2019 02:31:20|
|1451 forum posts|
Is your Rohn-3 jaw chuck self centreing, or do you adjust each jaw independently ? (A photo is always good so we know what you are talking about)
Self centreing 3-jaw chucks have to mesh with a scroll and should be inserted in the corerct position otherwise they will not be concentric with the spindle. They are normally a matched set to suit the chuck and are not interchangeable with other jaws. The grooves in the jaws are curved to match the scroll and cannot be reversed, which is why you have 2 sets. One for holding on the outside and one for holding on the inside.
4650 forum posts
I think the reversible jaws have the back of the "teeth" curved so they can fit in backwards, so there is some strength lost there, and would be more expensive to machine. Take a look at the profile of the teeth on the back of both sets of jaws and see how they compare. A pic might be of interest to viewers.
18312 forum posts
The smaller Emco machines had that jaw arrangement on the lever type SC 3-jaw, my view is that the thinner teeth make it weaker so OK for small machines with small chucks but not on larger chucks. As hopper says the teeth are curve don both faces which gives a flat ellipse shape, don't remember any problems of excessive runout on mine.
Sherline are the same,
EDIT Pic of mine
Edited By JasonB on 26/11/2019 07:58:29
|Clive Foster||26/11/2019 08:21:17|
|2246 forum posts|
Jaw tooth to scroll engagement is essentially single point contact in the middle for all sroll chucks so the teeth probably aren't significantly weaker. Especially not in smaller sizes.
In practice the major disadvantage from the user perspective is needing to change the jaw positions when going from inside to outside mode and viky versa. Far too much confusion potential, especially on the shop floor.
Making the things has to be considered too. Obviously harder to do a dual purpose set of jaws and maintain reasonable concentricity both ways. Remember both sides of the steps have to be good both ways round.
Nothing impossible but it all adds up. When two sets of jaws is the standard that everyone is used to and the manufacturing is sorted there is little incentive to change unless the bidirectional jaws have significant cost or performance advantages. Generally seems to have been seen as a cheaper expedient for small chucks so quality suffered a bit making the idea even more niche.
If bidirectional jaws had been the original standard odds are two sets would have been the niche.
|115 forum posts|
I once owned a 3j scroll chuck with reversible jaws - ancient (it came with a Brittania), maybe a "Crown", 4(-ish) inch size (memory gets rather treacherous here).
IIRC (and I think what Robin means), it isn't that the jaw positions change relative to the slots, it's just that their sequence of engagement with the scroll is reversed, in that "3" requires engagement first, and "1" engages last..
|Mick B1||26/11/2019 09:00:10|
|1608 forum posts|
I think it's exacly a Jason says. The little tommy-bar 3-jaw on my Sieg C0 - essentially a modded Unimat 3 design - is the same.
I'm not sure whether it's weaker than the standard scroll - it is of course a lot thicker in the middle of the 'tooth'.
|Nick Clarke 3||26/11/2019 13:14:09|
811 forum posts
I have just picked up a small lever 3 jaw SC chuck to go in a rotary table. It came with no instructions, but no problem - why would you need instructions - until I read the above.
Is this jaw reversal usual and necessary?
18312 forum posts
I don't read the numbers on my chuck jaws just look at the teeth.
On the two way jaws you do need to swap over 1 & 3 but 2 stays the same unless you do a lot of eccentric turning.
The cross section of the teeth are about half what they would be on a similar one way only jaw so even if a point or full contact load it will be taken by a lot less metal
|Martin Connelly||26/11/2019 13:33:34|
1399 forum posts
As Clive says scroll to chuck jaw contact is a single point. Since this is the case I knocked up a spare set of soft jaws for one of my chucks using case hardened dowels. They are for thin parts often in aluminium alloy or brass so don't need great force to hold for turning. The jaws are reversible as well.
|1541 forum posts|
It is the case with Emco SL/DB and 3 models the chuck jaws act as inside and outside jaws by swapping jaws 1&3. Also the case with early 5 series chucks that used the chuck key type type key for adjusting the jaws.
I have used the feature on SL and 5 series chucks and found it useful and as accurate as having separate inside and outside jaws, definite advantage is you won't forget where you put the jaws when you want to change over.
|Neil Wyatt||26/11/2019 14:52:45|
17970 forum posts
That would be an ideal short article for MEW!
|Clive Foster||26/11/2019 17:28:39|
|2246 forum posts|
Interesting job Martin.
Reminds me of the eye-wateringly expensive 8" and 4" scroll type lens mounts originally made for the incredibly expensive Ealing Research Optical benches (**LINK**) which used a similar pin system to move the jaws. Given that the ones I had in the lab had something like 0.1 micron centring repeatability if used with care the price was probably justified. Certainly I never needed to worry about things going back in accurately.
Small picture is the only one I can find on t'net of the Research bench. Cat iron, monster heavy and three sets of slides co-linearly accurate to better than tenth of a thou over the whole 6 ft + length. Only part of the kit shown. I had pretty much everything in the heirloom quality, glass doored, mahogany cabinets with a fair few multiple duplicates. Lord knows what it cost Her Maj back in the day, certainly beyond insanely expensive. In the catalogues but never priced. Probably buy an entire street for a similar amount. Not a short one either.
All skipped by an ass with a PhD after I left.
|Andrew Johnston||26/11/2019 19:11:53|
5556 forum posts
One factor no-one has mentioned is work holding. For a chuck with reversible jaws the steps that hold the inside diameter of work the conventional way round, and the outside diameter of the work in reverse, must be compromise shape that doesn't provide much holding capacity. Which is probably why reversible jaws normally appear on small chucks.
It's a moot point anyway, as 95+% of my turning on the main centre lathe is not with the 3-jaw chuck. Instead, in order, I use a collet chuck, 4-jaw independent, large faceplate and small faceplate.
|Robin Graham||26/11/2019 23:33:59|
|727 forum posts|
Thanks for replies - should have said it is 100mm self centring. The tooth profile is as in JasonB's pic, and you definitely have to swop jaws 1 and 3 to get them to meet in the middle. The Rohm chuck is very accurate in both configurations, better than the the bigger (150mm) 3-jaw on the other machine, but that's probably down to one being Rohm and the other unbranded Far Eastern rather than the design of the jaws.
Anyhow, interesting comments and a great tip from Martin C re soft jaws. I've been having trouble sourcing soft jaws for the 150mm chuck, this may be the way for me to go.
|Martin Connelly||27/11/2019 10:42:19|
1399 forum posts
Robin, if you make some then record the process for Neil, he wants a short article. Mine have 1/8" hardened and ground pins bought online. They are pitched at 1/4" and the starting position on the jaws is offset by 1/12" (third of scroll pitch). After driving the pins in to the tight fit holes they were ground back to the correct length to clear the bottom of the scroll groove (slightly over 1/8" protrusion). A metric chuck would probably be similar but with 3mm and 6mm for pins and pitch.
My 3 jaw chuck has a set of conventional jaws and a set of reverse jaws. These soft ones are for situations where neither of the standard sets allow easy work holding.
|Robin Graham||27/11/2019 21:19:04|
|727 forum posts|
Thanks for the further info about your design Martin - I'm certainly going to make a set of jaws along your lines and have contacted Neil about the possibility of contributing an article ( with due attribution of course!) about the build.
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