On 2010-02-13, Michael Koblic wrote:
"DoN. Nichols" wrote in message
...
[ ... ]
BTW
can you part the piece out by having the parting tool at 90 degrees to
the
usual direction? Cutting into the piece at the right angles with the tool
lined up along the lathe bed? I have seen it done with a wood lathe.
No! The blade of the parting tool would need to be curved like
a parenthesis on a radius to match the cut to be made. This is called a
trepaning tool. A standard parting tool would bind on the outside
below the cut.
Makes sense! Like making a hole in the head. Do I take it that you need
various sizes trepanning tools to cut different radii?
Yes -- you could make one by grinding a HSS tool bit to the
proper radii -- including clearance.
[ ... ]
Here is a question then: Say you have a 9" swing lathe and a 8" diameter,
0.25" thick plate of mild steel than needs a 6.5" hole bored in the
centre.
You cannot hold it on the outside in a chuck. You cannot clamp it outside
on
a face plate, even a wooden one. Not enough space for the clamps. What do
you do? Would making an 8" sacrificial face plate and super gluing the
work
piece to it work? Would you cut the hole with a mill on a rotary table?
None
of the above?
What I would do is to mount the faceplate (which is probably
larger in diameter than the chucks), coat the surface with double-sided
tape, approximately center the workpiece, and using a live center with a
flat pusher in it -- or a turned piece of aluminum with a large center
hole drilled not all the way through, and use the tailstock and this to
press the workpiece against the faceplate. Better if your pusher is
turned to press near the OD and relieved a bit inside that.
This is used to press the (slightly oversized) workpiece firmly
against the plate to get the tape to grip firmly. Then (using very slow
speeds) drill through the center with a bit which fits your lathe --
perhaps a 1/2" one unless you can find a Morse taper shank drill to fit
the tailstock taper.
I take it at this stage you have removed the pusher and relying solely on
the double sided sticky tape.
Yes. Keep the lathe speed slow, and the cut depth small to keep
the forces down.
Hmm ... for the drilling, you could make a special holder which
held a thrust bearing assembly with a bore larger than the drill which
you would use to start the hole. You could crank the carriage into
position (with the bearing well centered) to maintain extra pressure
while drilling, since you don't need the carriage for the actual
drilling.
Hmm ... another way to hold it fairly concentric (assuming that
your stock is truly round and near the desired size) would be a ring
which slips over the OD of the workpiece and (with less than 1/2" radial
size around the workpiece) and drill and tap four holes at 12:00
o'clock, 3:00 o'clock, 6:00 o'clock, and 9:00 o'clock and clamp it to
the faceplate with two steel bars going from 12:00 o'clock to 3:00
o'clock and from 6:00 o'clock to 9:00 o'clock so they don't interfere
with the spindle -- and the ends have to be cut to clear the bed. This,
perhaps augmented by the double-sided tape, would hold things firmly in
place.
Hmm ... two rings -- one which is a fit on the faceplate, and
the other on the (larger) workpiece, held together with bolts might make
getting things centered a bit easier.
Anyway -- with these approaches, you could use higher spindle
speeds than you would find safe with just the tape.
In any case -- use alcohol or acetone to scrub down the surface
of both the workpiece and the faceplate to make sure that they are both
free of oil and grease for maximum adhesion of the tape.
Then, using a boring bar, bore through the workpiece slowly
increasing the size until it is the right ID.
Then -- put on the 3-jaw chuck (assuming that you don't need the
OD to be truly concentric with the bored hole) expand the jaws inside the
bored hole to grip the workpiece so you can turn the OD to something
close to concentric.
From then on it is a piece of cake - it is that first step that I could not
solve without engaging in what I considered a rather sporting activity. Glad
to know it is an acceptable way to do it.
One of many ways. I've suggested some others above which take
more time making fixtures, but which make sense if you are making
multiples of these (e.g. the scales for your sundials).
If you need the two to be truly concentric, turn the OD while
still holding the workpiece against the faceplate with the live enter
and pusher adaptor. Then shift to boring the OD.
Yes -- the super glue (or a bearing mount Loctite) would work in
place of the double-sided tape. You will need to use heat to release
it.
Right. I allowed for that. I thought the holding would be better with SG
than the tape.
It depends. super glue forms a rather brittle joint, so if you
have an interrupted cut or otherwise bump things you could break the
bond. The double-sided tape -- especially sufficient area of
double-sided foam tape -- will give a very good grip, and be more immune
to shocks.
[ ... ]
Of course if the plate which you are machining can tolerate some
holes in its surface -- perhaps in places which would be machined larger
later -- drill and tap for bolts to hold it to the faceplate..
And yes -- you could use a rotary table and the double-stick
tape or glue again on the mill to do the same thing. The finish of the
bore will probably not be as nice, however.
That is what I thought. And using my boring head to 6" on my mini-mill is
probably pushing it :-) I did manage 3-1/8"...once!
Yes -- the torque needed goes up fast with increasing diameter.
The rotary table and a "slot drill" (two flute endmill) of
perhaps 1/4" diameter will probably work fairly well.
Note that you can probably use a larger diameter 4-jaw on the
lathe than the 3-jaw because the jaws are easier to adapt to not stick
out as far. My 12x24" Clausing has a 6-1/4" 3-jaw, and a 10" 4-jaw.
In your particular described situation, a 9" swing lathe, and an
8" diameter workpiece, *if* your jaws for an 8" 4-jaw chuck have the
last step only 1/2" thick, you could just barely grip the 8" diameter
workpiece with the jaws extending only partially beyond the OD of the
chuck. Be very careful when doing this:
1) Make sure that it does not hit the bed or the wings of the carriage,
and that the tool can reach fully through the workpiece without
bringing a part of the carriage into contact with the jaws.
2) Don't crank down too tight on the jaws. Extended even a single
step beyond the OD of the body puts an extra stress on them.
OK, it sounds like additional reason to use the faceplate method.
Yes.
I picked that example so you *could not* use the 4-jaw.
Actually -- you *could* -- barely -- with modified jaws. It is
just more difficult to make jaws for a 4-jaw than soft jaws for a 3-jaw
which is already fitted with two-piece jaws.
Think of the special soft jaws available for the Taig/Peatol
lathe. Those will handle a workpiece significantly larger than the
chuck body. For a 5" chuck -- just make large pie jaws to replace the
hardened top jaws -- *if* the machine comes with a chuck with two-part
jaws.
Hmm ... also, this sounds like an argument for a gap-bed lathe.
The first few inches of the bed can be unbolted and lifted out to allow
working with a larger diameter workpiece. There is some debate whether
the removed section can ever be put back as precisely as it was
positioned when the bed was first ground, but there are things where the
gap bed is a winner.
I have run into that
problem several times. I really wanted to know what is an accepted way of
dealing with pieces that have a diameter approaching the lathe swing.
Lots of ways to do it -- depending on the lathe, and how many
of the same size workpiece you are making.
What you are making is relatively thin so you don't have to
worry about it clearing the carriage.
Or -- you could perhaps make extended soft jaws for the 3-jaw
chuck if it has two-piece jaws. You can extend them far enough to grip
the OD of the workpiece, and make them of aluminum so you are less
likely to damage the bed if you don't adjust things quite right.
Thanks a lot. I shall try this on a smaller scale (4") and be ready to duck
if things fly at 550 rpm :-)
The larger the diameter, the more grip surface you have between
the double-sided tape and the workpiece and faceplate.
But in any case, I would not consider 550 RPM to be a good idea.
First -- if the tape does let go, it could throw things rather
hard.
Second, 6.5" diameter means 935 SFM, which strikes me as a bit
fast unless you are using something like carbide tooling and a free
cutting mild steel like 12L14.
Based on the cutting speeds chart for HSS tooling in the
Jorgensen steel book:
Steel SFM
---------------
1018 125
12L14 300
4140 120 (annealed)
Stainless steels below
Steel SFM
---------------
304 75
416 160
Enjoy,
DoN.
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