Derek wrote:
I have a small HF lathe that my kid & I putz on, no high precission
work etc.

I'm keen to learn any tricks from the old masters regarding turning a
piece down to a specific diameter. For example, say you need to turn
a 10 mm rod down to 8.5 mm, are there any fancy compound slide angles
that equate closely to the dial, or do you guys just "shave and
measure"?
Bear in mind this is rudementry bushings & such, but I would like
to learn a better way.

Any tips much appreciated.
Thanks.

From my personal archives:

'make your final cut and the cut just before the final cut the same
way, i.e. use power feed on both or neither and make sure your depth
of cut is about the same for both. if you want your final cut to be 5
thou then stop rough cutting when you are at least 10 thou away (and
make a pass or two without changing the depth after your last rough
cut), and then make a pre-final cut of half the remaining depth, then
re-measure what is left and make the real final cut.

The problem you are having is that the actual cut depth is not a
linear function of tool bit position. There are all kinds of non-
linear digging in, and machine deflection effects that can happen,
that are not worth trying to predict, hence the above technique can
help."

From my personal experience: when you are starting your final cut, turn
just a little bit, maybe 1/16", then MEASURE. If you're right on, keep
going, otherwise FIX IT.

GWE

I have a small HF lathe that my kid & I putz on, no high precission
work etc.

I'm keen to learn any tricks from the old masters regarding turning a
piece down to a specific diameter. For example, say you need to turn
a 10 mm rod down to 8.5 mm, are there any fancy compound slide angles
that equate closely to the dial, or do you guys just "shave and
measure"?
Bear in mind this is rudementry bushings & such, but I would like
to learn a better way.

Any tips much appreciated.
Thanks.

Derek wrote:
I have a small HF lathe that my kid & I putz on, no high precission
work etc.

I'm keen to learn any tricks from the old masters regarding turning a
piece down to a specific diameter. For example, say you need to turn
a 10 mm rod down to 8.5 mm, are there any fancy compound slide angles
that equate closely to the dial, or do you guys just "shave and
measure"?

That fancy compound angle would be 5 3/4 degrees, where 10 thou movement
of the compound will equal 1 thou straight in.
This doesn't mean you can actually work in tenths of a thou, especially
on that machine! It only gives you a larger effective spacing between
thousandths graduations.
Woops, you're working in metric--same deal, it's a 10:1 ratio regardless.
If you can set the compound to half that, 2 7/8 degrees, you don't have
to think about doubling what you cut away to get the actual diameter.

Ken Grunke

--
take da "ma" offa dot com fer eemayl

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There are several Yahoo groups devoted to these machines that are very
active and helpful. Spend 5 minutes each evening reading the difest and
you will acquire a lot of good information on using your machine.

Derek wrote:
I have a small HF lathe that my kid & I putz on, no high precission
work etc.

I'm keen to learn any tricks from the old masters regarding turning a
piece down to a specific diameter. For example, say you need to turn
a 10 mm rod down to 8.5 mm, are there any fancy compound slide angles
that equate closely to the dial, or do you guys just "shave and
measure"?
Bear in mind this is rudementry bushings & such, but I would like
to learn a better way.

Any tips much appreciated.
Thanks.

Grant Erwin wrote:

Derek wrote:

I have a small HF lathe that my kid & I putz on, no high precission
work etc.

I'm keen to learn any tricks from the old masters regarding turning a
piece down to a specific diameter. For example, say you need to turn
a 10 mm rod down to 8.5 mm, are there any fancy compound slide angles
that equate closely to the dial, or do you guys just "shave and
measure"?
Bear in mind this is rudementry bushings & such, but I would like
to learn a better way.

Any tips much appreciated.
Thanks.


From my personal archives:

'make your final cut and the cut just before the final cut the same
way, i.e. use power feed on both or neither and make sure your depth
of cut is about the same for both. if you want your final cut to be 5
thou then stop rough cutting when you are at least 10 thou away (and
make a pass or two without changing the depth after your last rough
cut), and then make a pre-final cut of half the remaining depth, then
re-measure what is left and make the real final cut.

The problem you are having is that the actual cut depth is not a
linear function of tool bit position. There are all kinds of non-
linear digging in, and machine deflection effects that can happen,
that are not worth trying to predict, hence the above technique can
help."

From my personal experience: when you are starting your final cut, turn
just a little bit, maybe 1/16", then MEASURE. If you're right on, keep
going, otherwise FIX IT.

GWE

As exemplified by the last thing I turned, where my cross slide was
being very consistent, and I was cutting less and less material each
time, working on a 0.75000000 OD. On the _very last cut_ that should
have been _perfect_ I got a chip caught in my cross-slide threads and
took off about an extra 0.010. Worse, I had been lulled into thinking
everything was going swimmingly so I got about 200 thou into the part
before I realized my mistake.

I backed off a couple of turns, put the cross-slide knob _exactly_ where
it had been, and the rest of my part came out at 0.750 to within the
tolerance of my micrometer. Now if I just edit the drawings to show the
step on the last 1/4 inch of that part it'll be perfect.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Wed, 02 Feb 2005 08:59:55 -0800, Tim Wescott wrote:

I backed off a couple of turns, put the cross-slide knob _exactly_ where
it had been, and the rest of my part came out at 0.750 to within the
tolerance of my micrometer. Now if I just edit the drawings to show the
step on the last 1/4 inch of that part it'll be perfect.

Isn't that...you know...cheating or something?

Dave Hinz wrote:

On Wed, 02 Feb 2005 08:59:55 -0800, Tim Wescott wrote:


I backed off a couple of turns, put the cross-slide knob _exactly_ where
it had been, and the rest of my part came out at 0.750 to within the
tolerance of my micrometer. Now if I just edit the drawings to show the
step on the last 1/4 inch of that part it'll be perfect.


Isn't that...you know...cheating or something?

If the drawing and the machining match how could it be cheating?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Wed, 02 Feb 2005 10:41:56 -0800, Tim Wescott wrote:
Dave Hinz wrote:

On Wed, 02 Feb 2005 08:59:55 -0800, Tim Wescott wrote:


I backed off a couple of turns, put the cross-slide knob _exactly_ where
it had been, and the rest of my part came out at 0.750 to within the
tolerance of my micrometer. Now if I just edit the drawings to show the
step on the last 1/4 inch of that part it'll be perfect.


Isn't that...you know...cheating or something?

If the drawing and the machining match how could it be cheating?

Well...changing the drawing to "as built" rather than "as designed"
after the fact, just seems wrong or something. (shrug)?

On Tue, 01 Feb 2005 20:53:20 -0800, Grant Erwin
wrote:

Derek wrote:
I have a small HF lathe that my kid & I putz on, no high precission
work etc.

I'm keen to learn any tricks from the old masters regarding turning a
piece down to a specific diameter. For example, say you need to turn
a 10 mm rod down to 8.5 mm, are there any fancy compound slide angles
that equate closely to the dial, or do you guys just "shave and
measure"?
Bear in mind this is rudementry bushings & such, but I would like
to learn a better way.

Any tips much appreciated.
Thanks.

From my personal archives:

'make your final cut and the cut just before the final cut the same
way, i.e. use power feed on both or neither and make sure your depth
of cut is about the same for both. if you want your final cut to be 5
thou then stop rough cutting when you are at least 10 thou away (and
make a pass or two without changing the depth after your last rough
cut), and then make a pre-final cut of half the remaining depth, then
re-measure what is left and make the real final cut.

The problem you are having is that the actual cut depth is not a
linear function of tool bit position. There are all kinds of non-
linear digging in, and machine deflection effects that can happen,
that are not worth trying to predict, hence the above technique can
help."

From my personal experience: when you are starting your final cut, turn
just a little bit, maybe 1/16", then MEASURE. If you're right on, keep
going, otherwise FIX IT.

GWE

I haven't used my lathe much lately (a 9" SB), so I went out this
morning and tried to see how close I could get. I turned a piece of
brass using the 3 jaw to near 1/4" for an inch or so, then mounted a
1/4" collet in place of the 3 jaw for turning the other end to as
close a 1/4" as I could. I pretty much followed Grant's method.
Getting to the final cuts I pretty much ignored the dial graduations
and just nudged the cross feed a little. The result was a diameter of
..2498 inches, with a runout of about .0001 inches, and a taper over
and inch or so of 0.0002 inches, using a Starret 436X micrometer.
Surprisingly, the runout on the other end using the 3 jaw was only
about .0002 to .0004.

Tom