My views on soft jaws

This is where bits of wisdom will be stored, a frequently asked question section with answers.

Moderators: Harold_V, WJH

kvom
Posts: 1163
Joined: Thu May 08, 2008 11:38 am
Location: Cumming, GA

Post by kvom » Wed Aug 06, 2008 6:39 pm

I get the soft jaws :) Now, how does that spider work?

User avatar
BadDog
Posts: 4513
Joined: Wed May 17, 2006 8:21 pm
Location: Phoenix, AZ

Post by BadDog » Wed Aug 06, 2008 7:01 pm

When you bore (or otherwise modify) soft jaws, you need to "set" them in the correct final orientation and "locked" against the scroll. This makes sure the machined feature fits properly when the jaws are loaded, and that the jaws are stable while machining. It also makes it very accurate because any wear or inaccuracy in the scroll or jaw is accounted for.

The spider allows you to adjust the screws so that the jaws can easily be fixed at whatever optimal position you desire. As Harold mentioned, if you already have a feature in the jaws that's close to (or exactly) what you need, you can adjust the spider so that you only take a skim cut to true things up. However, the way I have done it, that is not possible because I have to find "something close" to clamp on, but that also won't get in the way while clamped. I've even got a primitive version of his "spider", but it didn't work out well (screws are too light and aren't square, so it side loads and flexes), so goes unused. I really need to make a good one...
Russ
Master Floor Sweeper

Harold_V
Posts: 17014
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Question about soft jaws

Post by Harold_V » Wed Aug 06, 2008 11:51 pm

SteveR wrote:If you don't have two piece jaws, could you take the one piece jaws and machine off the top/jaw, just leaving the bottom/scroll section and adding the mounting holes for the new top/soft section?
The problem with this idea is that hard jaws are just that. Hard. You would have success in removing the unwanted portion, but it's unlikely the core will be soft enough to drill and tap. If that can be overcome, yes, it's a viable idea. Annealing the jaws is not a good option unless annealing and re-heat treating can be accomplished in an inert environment. Any scaling of the jaws in heating would render them pretty much useless, so that option should not be considered.

Soft jaws generally are keyed by some means, which takes the load off the (mounting) screws. They are intended to hold the top jaw on the master jaw, nothing else. The holding ability of the chuck is accomplished by the keying mechanism. Any altered jaw design would have to include such a feature.

Harold

rohamm
Posts: 51
Joined: Wed Nov 09, 2005 10:03 am
Location: Nashua NH

This is timely

Post by rohamm » Thu Aug 07, 2008 8:32 am

I've read this thread in the past, (I think I've tracked down every fine word Harold's written on this board), and it sure made sense.

Recently I needed to renew one of the shifter tumbler gears on my Rockwell 11. It's bronze and the teeth were all chewed up, making it the last real problem with that lathe. I turned off the teeth and set out to make a ring gear from a new Boston gear.

The time I did this before for the other tumbler gear I had no chuck, so I cobbled up a purpose-made chuck to hold the gear for boring in the lathe. Now, though, I have a really fine Cushman w/two piece jaws and an assortment of aluminum jaws, so it was time to do it right.

Well, not *really* right - I figured I could skip the step of making a spider and used three short 1/4" rods in addition to the piece of iron pipe I keep chucked in the master jaws for installing, removing and carrying it. Naturally, all those round pieces rolled around pretty good but I figured that being round, the master jaws would push them to the true center.

Not so, involute-breath (If anyone remembers Johnny Carson any more). While the skim cut off the soft jaws looked good and resulted in a perfectly-sized bore, they were so far off center that when the ring was fitted to the bronze hub, it couldn't make a complete turn, but jammed against its idler.

I'd found the old purpose-built chuck by then and managed to rescue the miscut new gear and with the help of a little JB Weld got the whole thing back together and working.

Lesson learned: the spider needs flats that meet squarely with the master jaws, otherwise side-loading and binding renders it useless.


Something else I've been mulling over is the dictum to tighten the chuck jaws using the same hole every time. Is that generally the accepted practice, or is it specific to preparing to cut soft jaws? I'd always thought I'd been doing the right thing to tighten a drill chuck by putting the key into all three holes.

BTW, there's apparently no such thing as 'the last problem', at least with my lathe: after running the best it ever has for about a minute, the vfd crapped out, probably from the record sustained humidity we had around here at that time.

Harold_V
Posts: 17014
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: This is timely

Post by Harold_V » Thu Aug 07, 2008 2:40 pm

rohamm wrote:Something else I've been mulling over is the dictum to tighten the chuck jaws using the same hole every time. Is that generally the accepted practice, or is it specific to preparing to cut soft jaws?
You don't have a choice, not if you want the jaws to run concentric with the spindle.

Depending on the chuck, each hole will tighten the jaws differently. I attribute the problem to shifting the scroll -----the result of the load being applied by the socket selected. The scroll loads against it's mounting lug, which has clearance to allow the scroll to rotate without binding. Consider, also, that the scroll is heat treated after it is generated, and may not be perfectly round, or concentric with it's bore. If you select a different socket each time, you can expect the jaws to have runout (more or less, depending on the condition of the chuck). That's also the reason that you should hold all parts to a narrow tolerance when they will be held in soft jaws, and it's equally important that the machined jaws are machined to the proper diameter. Any variation of the location of the scroll due to size often results in jaws that do not run true.
I'd always thought I'd been doing the right thing to tighten a drill chuck by putting the key into all three holes.
You were (and are) doing the right thing. The difference being concentricity is a non-issue with a drill motor. Each socket you select with a drill chuck will also shift the mechanism, taking up more slack. A drill that readily slips when only one hole is tightened generally will drive with no problems if all three are tightened. It's good practice for that purpose, but defeats the purpose of soft jaws if you expect the jaws to repeat. It is mandatory that one socket, and only one, be used for soft jaws. My chuck came marked from the maker, so the socket that runs best with factory jaws will be used. Owners of fixed jaw chucks are all well served to test their chuck to identify which socket runs best. The difference from socket to socket is often substantial. It is for that reason that some universal chucks have only one socket.

Harold

User avatar
BadDog
Posts: 4513
Joined: Wed May 17, 2006 8:21 pm
Location: Phoenix, AZ

Post by BadDog » Thu Aug 07, 2008 3:03 pm

The 3 jaw on my new lathe came with a section around one socket painted bright red (almost looks like nail polish). Haven't checked it yet, but I would assume that is the one that results in the most concentric location of the hard jaws, and likely used for consistency with whatever soft jaws they made. In any case, that socket is easily identified and rotation of the chuck is quite distinct. Probably not a bad idea, though not exactly aesthetically pleasing...

I also do what you describe on keyed drill chucks. You will (in my experience) always find that each subsequent hole will tighten a bit more with no additional torque applied to the key. You'll get more rotation on #2, and almost none on #3.
Russ
Master Floor Sweeper

User avatar
SteveM
Posts: 6525
Joined: Mon Jun 27, 2005 6:18 pm
Location: Connecticut

Post by SteveM » Thu Aug 07, 2008 3:03 pm

BadDog wrote:In my opinion, the best bet is to get a new chuck with 2 piece jaws (when you can find a deal on one).
Agreed. I did just that. Got a used Hardinge with no jaw tops for $60 from www.plazamachinery.com and made a backplate.

I machined the soft jaws from aluminum. See how I did it here:
http://www.chaski.org/homemachinist/vie ... hp?t=76590

You can see my chuck and how I did the backplate here:
http://www.chaski.org/homemachinist/vie ... hp?t=76302
http://www.chaski.org/homemachinist/vie ... hp?t=76335
http://www.chaski.org/homemachinist/vie ... hp?t=76527

If you go to this post, you can see a pic of Harold's spider and some soft jaws I made to fit over my existing jaws:
http://www.chaski.org/homemachinist/vie ... hp?t=73256

You might want to read that entire post as it has a lot of good info on chucks.

Steve

User avatar
tornitore45
Posts: 1499
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Post by tornitore45 » Thu Aug 07, 2008 8:41 pm

I still do not understand a detail here.

Let' say your soft jaws have an axial lenght of 1.25" and I want to grip a long bar 1" in diameter.

I set my spider so that it will slide into a 0.990" ID tube.
I place the spider as deep as it goes into the open jaws and clamp.

Now I bore the soft jaws to 1" removing about 0.005 each.
I bore as deep as I can without hitting the spider.

Remove the spider.

Place my 1" bar.

As far as I can tell the soft jaws will grip on the 0.990 left over the spider rather than the 1.000 dia.

What did I miss, clearly this procedure is no good, but I can not visualize a vay to bore the soft jaw all the way throug the 1.25" axial leght while they are clamped on the spider.

How is it done?
Mauro Gaetano
in Austin TX

User avatar
SteveM
Posts: 6525
Joined: Mon Jun 27, 2005 6:18 pm
Location: Connecticut

Post by SteveM » Thu Aug 07, 2008 8:57 pm

On mine, the soft jaws grip smaller than the jaw bases, so if I use the spider on the bases and bore the jaws to size, then I can grip at the diameter thru the spindle.

Of course, at some point, I will have bored up to the bases and will either have to make new jaws or I will only be able to grip on the face of the jaws (which is actually good for shallow large diameter parts like wheels).

Steve

User avatar
BadDog
Posts: 4513
Joined: Wed May 17, 2006 8:21 pm
Location: Phoenix, AZ

Post by BadDog » Thu Aug 07, 2008 8:57 pm

You typically want a bigger step than that anyway.

So lets say you set the spider around 0.900 and clamp the jaws on it. And let us further assume that the spider obscures the rear most 0.250 (that is the amount we can't bore) of a set of jaws 1.5" tall. And let us assume that we are doing a second op on saw cut 2"(+) x 1" pins chamfered and faced on each end.

So now we bore the jaws to 1.000 +0.000/-0.002 (don't want it to be oversize) and an exact depth of 1.250. Then remove the spider and stick in the first pin. The pin will seat to a depth of 1.250 against the shoulder, and this forms a reliable depth stop. Clean up and face the first side, along with chamfer. Then reverse. Carefully face second side to center of tolerance range and then set a stop (or DI) on the bed way for repeatable finishing to accurate length (relying on that shoulder). Soft jaws are VERY concentric when set/bored on-size, so you could even (set stops and) turn a shoulder (or groove for clip/o-ring, or whatever) now.

Rinse and repeat...
Russ
Master Floor Sweeper

User avatar
tornitore45
Posts: 1499
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Post by tornitore45 » Thu Aug 07, 2008 9:15 pm

Thanks Badog and Steve, now I understand.
I missed the fact that is possible to clamp on the master jaws and they are usually at the larger radius.
Of course not all jobs go through, here all sorts of steps are possible.

I looked at 2 pieces chucks but there little selection on 4" plain back.
Anyway my next chuck will be 2 piece jaws.
Mauro Gaetano
in Austin TX

User avatar
SteveM
Posts: 6525
Joined: Mon Jun 27, 2005 6:18 pm
Location: Connecticut

Post by SteveM » Fri Aug 08, 2008 6:36 am

I didn't see any 4" chucks at Plaza, but if you can fit a 5", they have:

184A. Brand New Bison 5” 3 jaw with 2 pc jaws plain back, $175.00

And you could do a lot worse than getting a Bison. My dad loves his.

You could also email Joe at Plaza and ask him if he's got one.

Steve

Post Reply