Below a better Boring Head Toolpost I made for the 1" diameter internal threading boring bar I recently acquired.  It is based on one of the traditional types (Figure 1).  The holder consists of a central bolt (with a hole for the bar to fit through), a top piece that serves as a large washer to hold the two clamp pieces together, the top and bottom clamp pieces, a 5/8 NC nut, and a standard washer.  The central bolt has a tapped hole in the bottom that accepts the hold down bolt and "T" nut from the lathes lantern toolpost.  Figure 2 shows the parts.

Figure 1

Figure 2

    The 5/8' NC thread on the top of the central bolt is the first set of "operational" threads I cut on the lathe.  Figure 3 shows the part just after I finished the threads.

Figure 3

    I added a feature to this holder I have not seen in any other one.  This holder was built specifically to hold the threading bar (though I will add holes for two more in the future).  The bar requires the bit to be aligned with the tip of the bit at the center height, and with the bit set in the proper relation to the lathe axis.  Trying to do both at the same time was more than I wanted!  To solve this I drilled and taped for two holes, one on either side of the bar location, in the clamp pieces.  The holes were drilled through both pieces with the correct size tap drill, then the lower piece was taped and the upper piece was drilled for clearance.  The upper clamp piece was also counter drilled to recess for the screw head.  Last the top piece/washer was drilled to expose the screw heads.  These changes can be seen in Figure 2.

    To use the holder I install the bar and tighten the screws and clamp nut snug but still allowing for adjustment.  The bar is then positioned roughly in the proper alignment.  A center is placed in the spindle and the tip of the bit aligned on center.  The screws are then tightened to lock the bar in place rotationally.  The work piece or chuck is then used with the center gauge to align the bit with the lathe axis, and finally the clamping nut is tightened to complete the setup.

    The two holes in the top, for access to the screws, will be opened out to slots in the near future, so as to avoid being cut on the sharp edges.

Dimensioned Drawings

Materials

1.    The rings and cover were turned from 2 3/4" Diameter HRS round stock
2.    The Body was turned from 2" Diameter HRS round stock
3.    5/8 - 20 hex nut
4.    2 each  10 - 24 X 2" machine screws (per boring bar)
5.    A "T" nut and screw
6.    5/8" washer

    Note: These dimensions are for my 12" Atlas lathe.  Check the center height of your lathe, it may be slightly different, and other dimensions may need to be altered for other makes and sizes of lathes, or stock on hand!

Holder Body

* This dimension should be 0.010" to 0.020" longer than the depth of the topslide "T" Slot (with the "T" nut installed), to allow the assembly to slide in freely.  Drill and tap the boss to fit the "T" nut screw.

Construction Notes:

    Only one hole is needed in the body.  This should be bored for the LARGEST tool used.  The body can then be rotated to line up with the other holes in the rings.

Holder Outer Rings Top View

Holder Outer Rings Side View

    The lower ring was initially turned the same height as the upper ring, then faced to the center height before the hole for the bar was machined.

The hole for the boring bar is drilled (with the complete assembly in place on the topslide) undersize with both rings clamped together.  The hole is then finished by boring (all the way through) with a 0.015" shim between the upper and lower rings.  This allows the rings to clamp the bar with the shims removed.   I used a temporary pin to hold the two rings in alignment during boring (see Figure 2).  Place this pin were it will be out of the way for boring all 3 bar holes, and their screws (if used).  180 deg. away from one of the first screw holes works.

    The hole in the body is then enlarged with the rings removed.  I used parallels on either side in conjunction with the cover to clamp the body for this operation.  The hole is made 0.100" in diameter larger then the LARGEST bar the holder is being made for.  Only one hole is needed for all the bars used.  The hole is rotated in relation to the rings to align it with the holes for the bar being used.

    The screw hole placement is determined after the bar hole is finished.  The holes are drilled for tap size with the cover installed .  The upper ring holes are then clearance drilled, the lower ring holes tapped, and the cover is drilled for clearance with the screw heads.

    Even with a 1" bar 2 additional smaller diameter bar holes can be bored (3 total).  Three is about the practical limit, unless REALLY small bars are being planned for.

Holder Cover - Top and Side Views

    If you drill the 0.375" (3/8") clearance holes as shown above they may break through as seen in Figure 2 above.  The body could be turned a little smaller in diameter and the rings turned correspondingly thicker to give more room for the holes.  Alternately the heads could be turned down a little so that a smaller hole would be needed.

    On mine, I also counter sunk the upper ring to allow the heads to sit below the cover.  Whether that is needed is the choice of the machinist.

    Making a dedicated "T" nut and screw for this holder would be helpful, when changing from a regular toolbit holder to the this holder.

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