If this was a perfect world, drill bits would produce perfectly round holes and drill chucks would run perfectly true without any run out. This scenario, of course, does not exist in real life. The fact of the matter is that every hole drilled with the common breed of twist drill will never end up the same exact diameter as the bit it was drilled it. It will exibit about 4 to 8 thousand of an inch of run out as measured with a dial indicator. Even the bits themselves, unless of top quality, are rarely true to their claimed size by a few thousands too large or small. If the hole needed is to house a sliding part such as a piston, it must be made to very tight tolerances and with holes smaller than 1/2", it is better and more convenient to drill slightly undersize and ream to final size. When we begin reaching the larger bores, it becomes more practical to bore the hole out as large reamers can be very costly and you need a rather large machine to use them. The big danger in boring relatively deep holes ( more than one inch deep ) is the possibility of ending up with a slightly tapered condition. This could be caused by flexion of the boring bar itself or a lathe spindle that is not square to the longitudinal carriage an bed. That is why you should always chose the most massive and least flexible boring bar that will fit and clear the drilled hole to be bored, assuming that the machine alignment itself if perfect. Don't assume it is.

The common tool post type boring bar will usually have a round straight shaft for the first 1-1/2" or so tapering to the left toward the front tip, with a left cutting carbide insert brazed at the tip. During boring, the left point created by the front and left edges is oriented at a slight angle to the inner left wall of the hole and at slightly above the center line to provide adequate side edge clearance and prevent rubbing during the cut. Do not allow the full side edge of the cutter to attempt to cut.

To begin fabricating the holder, you will need about 2-1/2" of 1" square aluminum stock. Hold it centered in a four jaw chuck and face both ends clean to a length of 2". Machine a 3/4" diameter cavity about 1/16" deep to create a raised bearing point around the perimeter of the stock end. Spot drill and drill a centered #11 hole all the way through the long end of the work piece. Remove the stock, reverse it on the chuck and counterbore the hole with a 1/4" bit 1/4" deep ans square up the hole bottom with a 1/4" end mill to provide a seat for the head of the 2" 10-32 cap screw needed it to attach the tool post to the cross slide. With a 1/8" thick 3/8" square nut and the 2" cap screw, install the tool post to the cross slide " T " slot, making sure that one of the sides is perfectly square the lathe spindle. The best way to do this is to screw a small face plate that is known to be true across its surface to the spindle and run the carriage toward the face plate holding the yet loose tool post so the front face is flat against the faceplate surface. Hold it in that position and tighten it down. Remove the face plate and install the drill chuck with a #1 center drill. With the cross slide, position the post so that the point of the center drill is 1/4" from the front edge. Proceed to with the center drill followed with by a 1/8", gradually enlarging the hole 1/32" at a time until you reach 11/32". Finish by reaming to a 3/8" diameter. Locate and drill two #21 vertical set screw holes through the top on the tool post and through the center line of the 3/8" hole about 1/4" from the front and rear edges of the tool post. Tap these with a 10-32 tap and insert two 1/2" 10-32 set screws. A 3/8 diameter shank boring bar is inserted and rotated to bring the upper surface of the insert to a horizontal plane. Depending on the side edge clearance angle of the carbide insert of the bar you may be able to use the tool as is, or it the tool post may have to be jacked up off the cross slide surface about 1/32" with two small pieces of brass shim stock. Boring is performed by advancing the tool into the hole drilled hole and slowly retracting it before advancing the cross slide toward you a few thousands for the next cut. Remember that when you are turning down a diameter, you advance the cutter forward toward the rear of the lathe and toward you when enlarging a hole. Happy boring!