A Simple Boring Bar Holder



Note Well!
I am not a professional machinist, nor have I ever been one.  Prior to purchasing this lathe, I had used a metal cutting lathe precisely once in my life, for about two hours; that was in 1980, and the lathe I used then was a monster compared to this small, inexpensive machine tool.  Don't take anything written here as a recommendation -- in a machine shop, I'm not qualified to recommend anything other than following the manufacturer's instructions and seeking competent advice.


May 6, 2003: This isn't the first thing I wanted to make, or the first thing I started making on my new lathe; in fact it's a tooling accessory for the lathe.  It's not completed yet, but I started with a chunk of 3/4" x 3" aluminum bar, eyeballed a length that turned out to be 2 1/8" and cut it off with a hacksaw and miter box (no, I didn't think to try my wood cutting bandsaw until the cut was half done -- but I will, since most woodworking tools will handle aluminum with a little care).  Since the tool post that came with the lathe only has 5/8" clearance, I had to thin the edge of the block to fit.  I'd love to say I set it up in my milling machine and machined off 1/16" from top and bottom, a half inch wide to fit the tool turret, but I don't have a milling machine, nor even a milling attachment for the lathe (yet).  I do, however, have a Dremel (nearly 30 years old), a router base to fit it that includes an edge guide, and a 1/4" diameter rotary cutter that looks a great deal like a fine toothed end mill (with about a dozen flutes).  After a little experimentation I found that if I didn't try to cut deeper than 1/32" in a single pass and kept the work dampened with kerosene as a lubricant, I was able to take cuts half the cutter diameter in width without loading up the cutter or overheating the tool.  In the end, I had to cut 3/32" off each face; with the blank milled down to exactly 5/8" thickness, it still hung up on some irregularity inside the tool post before bottoming against the straight edge of the step (which is necessary to keep the boring bar parallel to the spindle when the tool post is indexed on the detent).

Having finished the adventure with using a Dremel and router base as a milling machine, I installed the blank in the tool post to drill precisely at center height.  Lacking a center drill, I started the hole in a prick made by chucking the point of my center punch into the lathe chuck, spinning it slowly, and advancing the carriage until the punch point marked the work.  I then chucked a series of twist drills in the 3-jaw chuck, starting with 3/16" and working up in steps to 3/8" -- which is the size of the boring bar shank I want this block to fit.  I had a minor adventure when I bumped the cross slide feed handle while turning the carriage advance, but was able to realign the existing bore to the drill (fortunately, the drill wasn't in the hole at the time, or I might have broken it).  I was aware that twist drills often cut oversize, and first tried a 23/64" size (the next smaller than 3/8" in my set), but found the shank of the 3/8" drill, which measure .372", wouldn't enter the hole yet.  After carefully drilling the hole with the 3/8" drill, I found it was still quite snug on the shank of the same drill, despite the flutes of the drill measuring .002" larger than the shank -- I'm not certain, but this may be due to raised metal around the size stamp on the drill shank.  In any case, snug is just the way I want it.

Remaining steps are to cut a slot in the block to allow clamping on the boring bar, and trim the block to final size (which is where I'll find out if my bandsaw will cut aluminum reasonably well).  There won't be any drilling and tapping of holes for clamping screws; the slot will be on the tool post side, and the same bolts that hold the block in place in the tool post will serve to clamp the boring bar in its hole as well.  Then, just get some 3/8" drill rod and make up some boring bars...

And yes, there'll be pictures along in a bit -- some are in the digital camera, others haven't yet been shot.

Update, May 7, 2003: I've gotten all the trimming and cleanup done, and there is only cutting the slot to complete this part.  In one of the pictures, you'll see a surface that looks as if it were cut with a badly trammed milling machine -- that's the one I milled with my Dremel, burr cutter, and router base.  After trimming off unneeded material, and facing the cut edges all around, the part is just under two inches along the lathe spindle axis by a small amount less across, 3/4" thick with centered, milled step about 9/16" thick where it fits in the tool post.  When the slot is cut, it will hold a 3/8" round shank.  I'll probably make another one to hold either a 1/4" or 1/2" shank to accomodate using router bits as boring bars (an idea I recently saw on a web page someplace), but I'll have to modify the process a little; on this one, by the time I got it milled down enough to go into the tool post, the hole came out distinctly above center, and with a half inch bore, it might wind up too thin.  I'll also pay more attention on the next one, and put the bore for the bar shank closer to the tool post, so I don't have to saw the slot so deep.

I also got a couple shots of a "not recommended" method of mounting stock (that isn't round or hexagonal) in a 3-jaw chuck.  I had no trouble with this method, but I kept the spindle speed low (the table the lathe sits on started to shake when I got above about 200 rpm with one setup), and it's certainly no substitute for a 4-jaw chuck.  For one thing, the part looks a little odd with the facing marks on all the machined surfaces grossly off center.  OTOH, it does let me face flats on rectangular parts without waiting to have a usable four jaw chuck.

Update, May 11, 2003: Pictures!

Dremel w/ router base, pretending to be a milling machine. Click on the image to see the full size picture.  This is my ancient Dremel, mounted in the router base with a straight sided, flat ended cutter mounted, along with the bar holder blank showing the first couple cuts in milling the step.  If you have sharp eyes, you might be able to see some gouges in the milled area; that was from trying to use a dull 1/8" cutter instead of the much newer 1/4" cutter shown mounted in the photo; the gouges wll milled out when I took the additional 1/32" cut to accomodate whatever it is inside the tool post that prevents a 5/8" high lug from fitting.  After this was taken, I also (eventually) figured out that the rule that applies to milling cutters, about taking the deepest, heaviest cut the machine will handle in order to spread the wear on the teeth, doesn't apply to a 90W Dremel with a twelve-flute cutter; when I was taking 1/16" depth at t time, the cutter was loading up even with only 1/16" infeed per pass.  When I went to 1/32" cut depth, I could take a 1/8" infeed without any sign of stress, and got a better surface finish into the bargain.

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