Rookie Lathe Soft Jaw Questions

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rmac
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Re: Rookie Lathe Soft Jaw Questions

Post by rmac »

In a recent video , Stefan Gotteswinter makes a set of soft jaws for grabbing the inside of a part. When machining the jaws, he preloads the jaws in the usual way by clamping down on a piece of round stock. Somebody in the comments points out that it would probably be better to load the jaws in the other direction, to better simulate the condition when the jaws are pushing outward against the inside of the actual part.

That makes sense to me, but I've been wondering what kind of contraption you'd use so constrain the jaws when machining them. Would it make sense to make a giant spider with the screws pointing in instead of out? How about just wrapping a big hose clamp around the jaws? Other ideas?

-- Russell Mac
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Harold_V
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Re: Rookie Lathe Soft Jaw Questions

Post by Harold_V »

Restraining the jaws in the wrong direction is a terrible idea. Concentricity is lost by that procedure, but an even more important feature is lost. Full contact of the part with the jaws. I've yet to see a three jaw that had zero clearance of the jaws in the slides, and I'm highly unlikely to do so, for obvious reasons. When the jaws are restrained in the opposite direction, the jaws tilt one way, then the opposite way when the part is inserted. This condition is further exacerbated by chucks with badly worn or sprung slides. By contrast, such a chuck can still yield acceptable results when the jaws are loaded in the proper direction.

One of the ways jaws can be restrained is by a ring with pins. The pins insert in the holes by which the jaws mount to the master jaws, so the chuck can be tightened in either direction with exceptional results. If the ring is made properly, with the holes at true 120° intervals, side loading of the jaws is eliminated, which can be the source of issues with concentricity. The use of the mounting holes is of further benefit, in that the jaws are fully loaded in the slides, whereas they may or may not be when they are restrained too near the scroll. The problem, of course, is a ring that allows for specific size settings. A little imagination would lead to something functional without expending a serious amount of money. I've never gone there, so I can't offer anything more.

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RSG
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Re: Rookie Lathe Soft Jaw Questions

Post by RSG »

I use the rings as well rmac!

But I don't bother with pins to stabilise it. My method is quick and dirt, first pre-load the jaws with one of the disks close to the size that will work to achieve the correct dia so a slot can be machined on the outer edge of the jaws for the ring to sit evenly with all outer jaw surfaces now concentric. Then remove the loading disk and install the ring to reverse pre-load the jaws to do the work.

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GlennW
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Re: Rookie Lathe Soft Jaw Questions

Post by GlennW »

Bolt length can vary.
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rmac
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Re: Rookie Lathe Soft Jaw Questions

Post by rmac »

All good ideas! Thanks.

Glenn, what you show there is what I meant by a "giant spider".

I'm still wondering if a big hose clamp would work ... kind of an adjustable ring, if you will.

-- Russell Mac
Richard_W
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Re: Rookie Lathe Soft Jaw Questions

Post by Richard_W »

rmac wrote: Sat Mar 19, 2022 9:51 am All good ideas! Thanks.

Glenn, what you show there is what I meant by a "giant spider".

I'm still wondering if a big hose clamp would work ... kind of an adjustable ring, if you will.

-- Russell Mac
I have not used hose clamps, but don't see why it wouldn't work on smaller sized chucks. Although it doesn't take much pressure with a chuck for the hose clamp to fail. In the shops when nothing else was available, I have used bailing wire.

Richard W.
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NP317
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Re: Rookie Lathe Soft Jaw Questions

Post by NP317 »

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Richard_W
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Re: Rookie Lathe Soft Jaw Questions

Post by Richard_W »

rmac wrote: Sun Feb 27, 2022 9:32 pm
rmac wrote: Still need to make that spider.
Well here we are almost six weeks later and I finally made a spider. It's threaded for 1/4-20 screws with an OD of 1-1/4". For times when that's too big, I like Hanz's suggestion from a long time ago to just use a socket, although I haven't actually tried that myself.

I put the three extra holes in my spider so I could try an idea from one of Joe Pieczynski's videos for accurately indexing the threaded holes. The second picture shows that setup. If I remember right, Joe's video shows several variations of the same general idea.
I used hex nuts for my spiders. One large one and one small nut. 1/4" thread on the small nut and 5/16 on the large one. I didn't bother to be real accurate with the drilling and tapping. I used the flats to index and found center with a scale. They have worked fine for me for decades on both CNC and manual lathes. Often I cheat and use small 1/4 and 3/8 drive sockets when the desired bore is smaller than the spiders I have. When you use sockets it give you a stepped range and are right there in your box, so they are handy. I used a 3/8" steel dowel pin just a few days ago.

Richard W.
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