Cutting threads on a Unimat-3

Dr. David Robinson   	
Feb 18 1997, 1:00 am

Newsgroups: rec.crafts.metalworking

As promised, I've written up some notes and drawn plans for the required
parts to adapt the Sherline screwcutting attachment to a Unimat-3.
I'll attach the notes to this message, but the plans are just hand-drawn.

Notes on using the Sherline screw-cutting attachment on a Unimat-3.
-------------------------------------------------------------------

Here are some details of how I made adapters to use the Sherline
screw-cutting attachment on my Unimat-3.  The key is to use the attachment
points and leadscrew drive slot on the Unimat that were intended for the
power feed accessory.

Four main parts must be built:

    1.   An arbor fits on the lathe spindle in place of the drive pulley
         and holds the Sherline drive gear and handwheel.
    2&3. A sleeve (with pressed in coupling) is clamped in a bracket.
         This assembly fits into the leadscrew hole on the left end of
         the lathe bed and attaches to a threaded hole in the headstock.
         The Sherline gearbox clamps to the sleeve.
    4.   A shaft fits inside the sleeve and attaches the Sherline gearbox
         to the leadscrew.

I have hand-drawn plans for these parts, but have not (as yet) made
electronic drawings.  Notes on construction and use follow.

By the way, no modifications were made to any of the original Unimat or
Sherline parts - so the attachment can still be used on a Sherline lathe.

FIRST STEP: Carefully study the instruction manual for the Sherline
screw-cutting attachment, available at:

    http://www.sherline.com/3100inst.htm

Parts List (doesn't include metal stock)
----------
10-32 x 1" allan head capscrew (to fit bracket)
10-32 x 3/16" allan head capscrew (to locate shaft in sleeve)
10-32 x 3/16" setscrew (to fit handwheel)
M6 x 22 allan head capscrew (to attach bracket to headstock)
4-40 x 1/8" pan head screw (fits arbor, key for gear and handwheel)
3/32 dia x 3/16 roll pin (to fit shaft)

Construction notes
------------------
Drill through sleeve and coupling separately with letter `N', loctite &
press together, re-drill assembly letter `N', ream 5/16".

Mount arbor directly on lathe spindle to turn 9/16 dia.

Shaft can be made from 5/16 dia CRS.  Turn down end for press fit into
short piece of 3/8 dia CRS that has been drilled through 9/32 - loctite &
press together, then face assembly to length.

Drive slot in shaft is easy to make with a hack-saw and file, if shaft
is first drilled 1/8" in lathe.

Make sleeve a bit long (say 2-1/4"), then assemble arbor with drive gear,
bracket, sleeve & Sherline gearbox with gears.  Mark sleeve length and
finish to length.

Put one of the large washers from Sherline kit on arbor before installing
4-40 screw with loctite.

Usage notes
-----------

Can be used with motor in place - just remove spindle pulley.

To disengage, loosen 10-32 screw in sleeve and pull shaft partially.

I can install (or remove) the complete adapter in less than 30 seconds,
after which the screwcutting attachment is installed exactly as per the
Sherline lathe.

Bracket & sleeve can remain assembled together permanently, and can even
be left in place while using the lathe (but belt guard can't be closed).
In any event, the assembly can be removed in a few seconds by removing
the single bolt that attaches it to the headstock.