FILENAMES:

DoN-mandrel.gif

DESCRIPTION:

This is a drawing of a special purpose lathe mandrel for holding
workpieces with female thread pre-tapped.  Thanks to Don Nichols
<dnichols@d-and-d.com>. Don provided the following description:

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This is the text description of the accompanying image file, mandrel.gif.
I've  been requested to upload it to the dropbox, so here it is.

	The function of this was to hold a piece of brass stock which had
already been cut to (over) length on a bandsaw (the usual $200.00 style),
faced drilled and tapped on a Taig, and which now needed to have a more
complex shape turned in the remaining end and the OD.  The center needed the
1/4-20 tapped hole for the end function of the part, so it plus the already
faced end made a good way to hold it.

	This is intended to be held in the chuck or a collet and to hold the
workpiece in such a way that it can be removed without needing to worry
about damage by pliers applied to the OD.  The technique used was to thread
it onto the matching threaded stud which is the center piece of this tool,
and to lock it in place by turning the cam to both hold it firmly against
screwing onto the stud, and to allow it to be freed for hand unscrewing when
the operations are complete.

	The operations, for those who care, are Turning the OD down enough
to make a clean finish, turn the rest of the length down to 5/8" OD, turn a
runout groove between the larger area and the end of the 5/8" diameter,
chamfer the free end, face the free end, and then thread the OD of the 5/8"
section to a 5/8-27 thread.  In a later operation (because of setup changes)
it is knurled on the larger OD.

	The body of the tool has three diameters.  One is small enough to
fit into the center hole of the chuck, to allow the central part of the body
to be backed firmly against the face of the chuck and firmly gripped.  (The
smaller part may also be held in a collet, if that is preferred, but I
believe the chuck held part to be more rigid.)  The final OD of the body is
small enough to clear the "pretty-turning" and later the knurling of the
largest OD of the part.)  The length of that part is long enough so the
knurling tool has full access to the area being knurled.  (Since I am using
a fairly light lathe, and the workpiece is not being supported by a
tailstock center (for various reasons), I opted to use a scissors style
knurling tool.

	The center piece of the tool is turned down and threaded on the end
to fit the thread in the workpiece.  (I have no dimensions on the drawing,
because everything must be scaled to your needs, not made as a duplicate of
my part.)  This center piece and the cam are of drill rod (water hardening
in my case), while the body is (I believe) HRS.  It was free, and it works,
so I didn't enquire too deeply into its pedigree. :-) The center piece has a
slight projection on the rear to push against the spring held in the back of
the body.  A large setscrew was used to push against the other end of the
spring, to maintain some pressure under normal circumstances.  With a dummy
workpiece threaded onto the center piece, and a washer setting a distance
below the lip of the opening in the body, things were clamped together,
cross drilled (not all the way through, though you could if you wanted to),
and reamed to the proper diameter for a smaller drill rod.

	The center piece was then removed, and the drill rod placed into the
reamed hole, and scribed relative to the ID of the hole which accepted the
center piece.  It was then removed, chucked in a 4-jaw chuck, and offset
from center enough to make a reasonable cam.  (Mine was a 5/16" OD, offset
enough to turn a 1/4" OD which was tangent to one side.)  Once this is done,
a hole was drilled at right angles in the part projecting out the side,
drilled and tapped for a handle with which to turn the cam.

	The whole thing was then placed in the lathe chuck, and utilized in
the production of a lot of those pieces of brass.  The end of the cam was
always chucked in the space between the #1 jaw and the highest-numbered jaw
(#4 in my case, since I was using a 4-jaw universal chuck, which has a
better grip and more mass than my 3-jaw.)

	The actual production work using this was done under the control of
a CNC lathe with a toolpost turret, so it could be loaded, patted on the GO
button, and ignored until time to take the finished part off and place the
next intermediate one in its place.  while it was doing that work, I was
facing, drilling and tapping the next workpiece on the Taig, resulting in a
greater efficiency (and less chance to reach for a coke. :-) It took me a
little less time to do the work on the Taig than the CNC required for the
more complex work, so I was able to keep it well fed.

	I hope that this is of some help to others.

	Good Luck,
		DoN.
		(dnichols@d-and-d.com)