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Making soft jaws for a lathe chuck

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kvom:
In preparation for building the Halo engine, which is at a smaller scale than I had tried previously, I knw I wanted to make a set of soft jaws for the 6" 3-jaw chuck I bought to mount on the rotary table.  The first step was to remove one of the hard jaws and measure it.  The hard jaws are aligned on the lower jaws via a sqaure horizontal boss and a lateral groove.  Having squared 3 pieces of aluminum, I milled the boss on the bottom.



The soft jaw will be taller than the hard jaw it replaces, but that's a good thing.  It will be able to be bored more often.  Next the lateral groove is milled:



And then the clearance holes for the two mounting bolts.  On almost all chucks like this, the holes will be centered on the boss, allowing the jaw to be reversed.  On my (metric) chuck, the hole centers are 38mm apart.  Since my DRO has both English and metric units, I was able to center on the boss and drill 19mm on either side.



The next step is to counterbore the holes.   Here you need to measure how far the threads will be exposed on the bottom, how tall the bolt heads are, and the diameter of the bolt head.  Since the hard jaws are reversable, the outside bolt is shorter than the inside.  Given all of the measurements, I could calculate the depth of the dountersink.  To drill it, I used a 5/8" endmill.  Since I don't have a DRO on the quill, I used the table of the mill to control the depth.  One way to do this would be to touch the face of the mill to the top of the jaw, lock the quill, and crank the table up the necessary amount.  However, that's too much work.  My process was as follows:

1) With the endmill at the top, raise the table so there's an inch or so of clearance.  

2) Lower the table the amount of the countersink depth.

3) Lower the endmill to touch the top of the jaw, and set the quill stop.

4) Raise the endmill, and then raise the table the same amount it was lowered in step 2.

Now I could use the spindle as if I were drilling, and the quill stop will give the correct counterbore depth.  Using a vise stop made it quick to position and drill each jaw in succession.  Of course, I needed to set a different depth for the outer holes.



With all jaws done, here they are on the chuck:



If I needed to chuck something small I might need to mill angles in the ends of the jaws, but for now these should do fine.

bogstandard:
Very nice Kirk, you are now well on the way to getting super precision from your lathe.

I personally would get the angles put on there first, and very lightly bore the centre out all the way thru them. Because they are ali instead of soft steel, you will need that precision bored area for holding your setting slug, as if it isn't exact, it will deform the jaws out of shape as you are boring the outer face, so when the slug is taken out, the newly bored area will spring back out of shape, and so you will lose your precision.


Bogs

chuck foster:
thanks kvom......................ANOTHER project to add to my looong to do list  :dremel: :scratch:

chuck  :wave:

Mike K:
Very nice, Kirk.  I hope you don't mind a couple beginner questions...

How do the jaws engage the scroll mechanism?

And how does that vise stop work?  Looks like a simple project I could do, but I can't wrap my head around it's use.  Looks like a clamp.

Mike

kvom:
The soft jaws just connect to the lower hard jaws, which engage the scroll.  The soft jaws just replace the upper jaws.  That's why you need 2-piece jaws.

The vise stop is indeed just a clamp.  I made it the first session of school.  You can get the identical thing at Enco for a few bucks, but it's an easy project.  The center pin is just a SHCS with a small spring around it.  The side shafts are a press fit in one jaw and a sliding fit in the other.

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