Re: Machining / Lathe questions

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Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, March 19, 2003 4:10 PM
Subject: Machining / Lathe questions


 > Original poster: "Mccauley, Daniel H by way of Terry Fritz
<teslalist-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>
 >
 >
 > Just a few questions regarding machining . . .
 >
 > 1.  My first question is that I want to drill and tap a large number of 1"
 > steel spheres which I am going to using as
 > corona balls (spark gaps actually).  Anyone have suggestions on how to
clamp
 > these spheres properly.  I guess the proper way to do this to ensure
proper
 > centering would be to use a lathe (see next question), but precision isn't
a
 > hard requirement as they are only going to be used for spark gap balls.  I
 > was thinking maybe taking some material putting perpindicular V-Slots and
 > having the sphere rest in the intersection point.

Most of the machinery supply places (MSC, McMaster, even Harbor Freight)
have double V blocks which are designed just for this sort of thing.
Drilling would be best done in a drill press. Assuming you're not using ball
bearings (which have a very, very hard surface), one trick is to whack the
ball with a hammer (other side of the ball on a block of wood, so it doesn't
deform).  This makes a little flat spot that is much easier to get the first
pilot drill bit started on.

Unless you've got a very inexpensive source for steel balls, and oodles of
spare time to drill them, you might want to look at brass (or steel, if your
fancy takes you that way) finials or drawer pulls.  Home Depot, etc. carry
them.  Last time I checked, they were in the under $2-3 range.

The other thing that you should look into, for electrical and machinining
properties, is Aluminum balls.  Better conductor (much) than steel, a lot
easier to drill and tap.
 >
 > 2.  Secondly, I'm finding that lathe work is becoming a requirement on
 > almost all my new tesla and high voltage projects (as well as others) and
 > I'm finding the very high prices of out-of-house machining to be very
 > restrictive so I think its in my best interest to invest in a lathe and
 > start practicing.  Are there any lathes out there for under $500.00 that
 > would be okay for practicing and would be tabletop sized (not a huge floor
 > mounted lathe)???

You need to do some quantification of what sort of work you want to do, and
to what precision.  The rattiest lathe out there can probably turn aluminum
and plastic to 0.01" fairly easily. How big a work piece do you need to
turn?  The significant numbers are the max diameter you want to turn (it has
to fit in the jaws, and clear the ways), and the max length (distance from
headstock to live or dead center). (By the way, you can turn something on
the end of a long piece as long as the workpiece can stick out through the
headstock.. turning threads on the end of a long rod, for instance, doesn't
need a big lathe, but does need a room big enough to hold everything).
For what it's worth, turning spheres requires a special tool, unless you are
really, really good at turning two knobs at once in a complex way.

Almost all lathes (even the cheap cheap ones) can be used to do decent work,
but...
1) A big issue is how big a cut can you take in one pass.  A more expensive,
heavier lathe is stiffer and will hold the positions better and make a cut
that is more true. (More motor HP on a bigger lathe helps)
2) The repeatability of  a cheap lathe isn't as good. If you were going to
depend on the handwheel calibrations, etc. the lead screws aren't as good on
a cheap lathe.  This becomes an issue connected with #1, because if you have
to take multiple passes, repeatability from pass to pass affects you.
3) Cheap lathes aren't as durable.
4) Cheap lathes won't have some of the fancy features (8 gazillion screw
thread settings, 9 gazillion turning speeds, auto feed, etc.)
5) Cheap lathes may have funky sized accessories (kind of a problem if you
want to use inexpensive stuff that's only available in "inch" sizes and your
  lathe is all metric, or has non-standard sizes for headstocks, etc.)  Maybe
you want to get that 4 jaw chuck, but you find that it won't fit on your
lathe.

Most "real machinists" will turn up their nose at a cheap lathe that can't
turn stuff to better than a ten thousandth, etc... but, you're probably not
a real machinist, you're probably not doing production work, and you're
probably more interested in just getting something turned down or parted
off, etc. mostly by eye or by dial indicator or by trial fit.  For this, a
cheap lathe might very well be all you need.

All of these same issues apply to vertical mills, as well.

As to your pricepoint.. there are two basic approaches

Harbor Freight Salvage (and it's ilk... http://www.harborfreight-dot-com/)
imports all manner of inexpensive Chinese machinery.  Most all of the stuff
has been reviewed on the web somewhere, so you can find out warts and
blemishes that way.  The significant risk you take with Harbor Freight (and
it's ilk), over and above the fact that it IS, after all, cheap machinery,
is that  the machinery might be damaged (shipping) or misassembled or
missing pieces when you get it  (QC for the cheap stuff isn't all that hot).
You need to be willing to return it and get another one (and be willing to
negotiate the shipping, etc. in connection with this) if you get the
proverbial P.O.S. unit. This is non-trivial when the item you're shipping
weighs 300 pounds!  There IS a fair amount of manufacturing variability.
There are also some things from Harbor Freight that are just not worth it (I
understand that their anvils are not very good, crummy steel and tempering,
so not a good deal for the price), so you need to do a bit of shopping
around.

Used Machinery Dealers - Check those yellow pages.. Machine shops and/or
their machinery are being sold all the time.  A 50 year old lathe might look
a bit archaic (Big exposed belt drive), but work just fine, depending on
what it was used for.  Shopping for used machinery is a bit like shopping
for used cars, though.  It's best to make friends with a machinist (and then
spend some time convincing him that you really didn't intend to make your
life's work becoming one, starting with learning how to scrape ways)