Wholesale Tool sells a carbide insert internal threading bar for about $30.  The bar uses TPG-32X or TPU-32X inserts.  Figure 1 shows the bar setup and ready to thread.  The bar comes in either right hand (order No. 2230-0005) or left hand (2230-0006).  The inserts come in three different nose radiuses (321, 322, & 323), the sharpest nose radius 321 is used for threading, and the other two (which have larger radius noses, giving a better finish) for regular boring.  This bar can be used for standard boring, but not boring to a shoulder, as can be seen in Figure 1.

    The shank of the bar is 1" dia. and the minimum bore is speced to be 0.930" (see below).  After a few correcting mods the bar has performed very well, and is certainly worth the money.  While the minimum bore is quite large it, is perfect for my intended use, cutting the internal threads in backplates and jigs to match my Atlas's 1 1/2 - 8 tpi spindle nose threads.

As mentioned I had to make a couple modifications to the bar as it arrived, to make it functional.  Figure 2 shows the bar disassembled with an insert included.  You will notice in Figure 2 you that one corner of the insert is marked in black.  When one of the cutting edges becomes dulled or chipped I mark it with a black permanent marker, so I do not try to use that point again at a later date.

Figure 2

    As the bar came the clamp piece would not even touch the insert with the screw tightened!  A little investigation showed that the screw head fit the clamp so well, that the clamp could not tilt down to contact the insert.  I ground a taper onto the outside of the Allen head as shown on Figure 3.  The clamp now snuggled down nicely onto the insert.

                Figure 3                                            Figure 4

    The other modification was to the the clamp.  The bar would not fit into the specified minimum bore as the back of the clamp struck the workpiece.  Another trip to the grinder removed the offending edge as shown in Figure 4 (upper left).

    While unexpected the modifications were not hard.  I'm quite satisfied with my purchase, and have used it to make two backplates and a wheel turning  jig for the wheels on my locomotive tender, as well as using it for regular boring.
 

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