Newsgroups: rec.crafts.metalworking
Subject: Re: REALLY Dumb Question.....Question #2 (diam)
From: Robert Bastow <Tubal_cain@hotmail.com>
Date: Fri, 01 Jan 1999 17:34:05 GMT

Fitch,

I couldn't have put it better myself!!

Another way to get that "Last Half Gnats Whisker" is to give the boring bar a
TEENSIE rub (on the top face..NOT the cutting dia.) with a very fine stone or
diamond lap. Take another pass through and the (sharper) bit WILL take a fresh,
but tiny bite out of the hole.

A really high quality, (and Expensive) head..like a Darex..will allow you to
dial in AND GET a 0.0001" increment on diameter.  But they are beyond the
pockets of most of us.

Robert Bastow



Fitch R. Williams wrote:
 
> "Martin H. Eastburn" <oldtree@pacbell.net> wrote:
> 
> >How does one set the diameter that the boring bar will cut out.
> 
> I'm strictly a self taught user of this device and will be interested
> in how the others do it.
> 
> Meanwhile, back at the ranch, what I've been doing on the mill:
> 
> 0.  Locate the work piece under the spindle so that the centerline of
> the hole will be where required with the required accuracy.  How this
> is done depends on whether the rough hole is to be cut on the mill or
> was cut someplace else.  If someplace else, I usually set up
> positioning "stuff" in the form of stops, or pins, or whatever works
> to locate the "mark" that shows where the hole is to be.  I've been
> known to use my edge finder on two blocks or stops and then dial to
> the center of where the hole is to be so that when the piece is
> placed, its quite accurately located - but this isn't always feasible.
> 
> 1.  Arrive at a rough hole size - usually by drilling but I've been
> known to use a hole saw and once used a torch and grinder (to get rid
> of the heat affected zone before boring to size - as close as I can
> but being absolutely "certain" to be undersize by .040" or so at the
> start of the boring operation.
> 
> 2.  I position the boring head so that the end of the boring bar is in
> the hole, and then adjust it so it just makes contact with the hole.
> 
> 3.  Make passes adjusting the cut until the hole cleans up, then use
> the telescoping gage and the mike that measured the part that has to
> fit inside to get a measurement on the starting diameter.  After that,
> since the boring head is calibrated (sort of) its a matter of setting,
> cutting, tracking your progress, getting a feel for exactly how much
> is taken off by a setting of .0xx", etc. so you arrive at the final
> size.
> 
> I am sort of "retentive" about keeping notes on my progress boring
> holes or machining diameters - I always have the calculator right
> there and fill out my litle table during the course of which I always
> make a prediction of what the size should be after the next cut, and
> then measure the error after the cut.  If you havn't done this, try it
> - very instructive about the effect of doc, feed and rpm on the
> performance of your lathe - and an objective indication of its
> rigidity compared to another lathe - like between my SB and my 12 x 36
> import.  The import is so much more rigid it isn't even a contest.
> But I'm digressing here.
> 
> A trick(s) I learned from a machinest friend.  I was boring the
> vertical hole for the feed nut in  the "T" slotted cross slide casting
> from MLA, and was about .0005" undersize. Ok, I know that seems
> rediculous, but it "was" undersize cause the brass (?) part to be
> inserted didn't would not go in there, but it "almost" did if you know
> what I mean - since this was my very first hole boring experience with
> my genuine $129.95 including shipping Criterion look alike boring head
> with carbide tipped bars - and there was this casting I had invested
> hours machining into perfection with dovetails and all sitting there
> on the verge of ruin - so I was nervous.   To top it off this was for
> my Dad to use on his (now my) 9" SB, and I wanted it to be "perfect"
> (OK, OK,OK, already - I confess - I want "almost" everything to be
> perfect, but thats my excuse this time).
> 
> Time to take a break!
> 
> I knew from my experience and tracking tables that if I loosened the
> clamp screw and adjusted I was going to be oversize and the casting
> would be history and itwasgonnabejustawful - the tolerances on the
> drawing are quite tight for this dimension and the other cross slide i
> had for dimensional checks - a SB original normal cross slide because
> Dad had the taper turning attachment on the lathe back in Michigan at
> the time - was "right exactly on" the dimension.
> 
> So I called him up and asked what to do - He asked what rpm I was
> using and the hole size - 80 rpm, I forget the size.  Then he made two
> suggestions, one was to make another pass at about twice the rpm and
> then measure it again.  The second was to chuck up a brake cylinder
> hone (the hole was in the right size range for this) and run it slow
> for a very few seconds.
> 
> The pass at higher rpm did make the hole about .0004 bigger, and a few
> (maybe 5) seconds of honing with the brake cylinder hone finished the
> hole exactly the right size close as I can measure.  And then the part
> "fit" in there "just right" - it stll does - I have that cross slide
> ready to use on the lathe in the garage now.
> 
> All in all, it isn't so different from what one does boring on the
> lathe with the boring bar mounted on the cross slide - which was the
> model for my approach on the mill with the boring head.
> 
> Fitch"interested in better ways for achieving perfection"Williams
> In So. Cal.