Ball or Knob Cutting

[asallwey]

In the September/October 2008 issue of HSM Andrew Wakefield authored an article on cutting dovetails. He presented an excel spreadsheet method for computing X & Y movement. And this was great!

He also referenced a much earlier work on Ball cutting by someone else, and said his spreadsheet calculation method could be used to cut balls or knobs. Well, even though I'm a bit math challenged I tried to do this, unsuccessfully. So, might anyone have a spreadsheet that would calculate the X & Y movement for cutting balls or knobs? I would really appreciate it! Or even a clear description of the formulas involved would work.

Thanks,

Alex

[JCHannum]

Marv Klotz has an excellent site with many programs for the shop.

http://www.myvirtualnetwork.com/mklotz/#shop

Scroll down to the second program, Ballcut.zip.

[TGTool]

Klotz's program also works nicely for putting a crown on knobs, if you want a large radius instead of a flat top just as an esthetic touch. You give the program input for the size of the stock and the size of the radius you want and it quite nicely puts out just the number of X-Y's you need for the job. A really nice utility.

[oldtiffie]

Ballcut.zip

Marv Klotz's "Ballcut" program is in a zip archive at:

You should carefully read the ballcut.txt file within that archive as a square-cut tool (like parting-off tool) is required.

Marv's "collection" (read in full):

http://www.myvirtualnetwork.com/mklotz/#shop

The link to ballcut-zip:

http://www.myvirtualnetwork.com/mklotz/files/ballcut.zip

I have copied and pasted the ballcut.txt file here:

I don't turn very many spherical shapes on the lathe and when I do the
application is seldom critical. Usually the shape is decorative (e.g.
ball-ended tool handles) and so doesn't need to be very precise.

I built a ball turning attachment but, frankly, it's a pain to set up
for a one-off non-critical job.

A much easier way to do it is to make a number of plunge cuts with a
cutoff tool ground with a squared off end. This yields a 'staircase' shape
that approximates the shape of the ball. After making the cuts, I slather the
work with marking out dye. Using a fine file, I then file the shape until all
the dye disappears - voila, a quite acceptable ball.

Input the diameter of the sphere you require and the angular step
size. The smaller the angular step, the less filing you'll have to do (but the
more cuts you'll have to make). When the program runs it produces an output
that looks like:

================================================== ==========================
Incremental Sphere Turning Data
Sphere diameter = 1.0000 in
Stock diameter = 1.0000 in
Angular increment = 5.0000 deg

N = cut number
XF = axial (along lathe bed) position of tool
DX = increment in x from last cut
YF = depth of cut
DY = increment in y from last cut
WD = work diameter resulting from depth of cut YF

N XF DX YF DY WD

0 0.000 +0.000 0.500 +0.000 0.000
1 0.002 +0.002 0.456 -0.044 0.087
2 0.008 +0.006 0.413 -0.043 0.174
3 0.017 +0.009 0.371 -0.043 0.259
4 0.030 +0.013 0.329 -0.042 0.342
5 0.047 +0.017 0.289 -0.040 0.423
6 0.067 +0.020 0.250 -0.039 0.500
7 0.090 +0.023 0.213 -0.037 0.574
8 0.117 +0.027 0.179 -0.035 0.643
9 0.146 +0.029 0.146 -0.032 0.707
10 0.179 +0.032 0.117 -0.029 0.766
11 0.213 +0.035 0.090 -0.027 0.819
12 0.250 +0.037 0.067 -0.023 0.866
13 0.289 +0.039 0.047 -0.020 0.906
14 0.329 +0.040 0.030 -0.017 0.940
15 0.371 +0.042 0.017 -0.013 0.966
16 0.413 +0.043 0.008 -0.009 0.985
17 0.456 +0.043 0.002 -0.006 0.996
18 0.500 +0.044 0.000 -0.002 1.000
================================================== ==========================

Set your compound so it's motion is parallel to the lathe bed. Zero
the crossfeed dial with the tool tip just touching the work (YF = 0). Bring
the left edge of the tool up against the (faced off) surface of the work and
zero the compound dial. This is the (XF = 0) position.

Move the compound to the left until the dial reads 0.002 (step 1). Now
plunge cut until the crossfeed dial reads 0.456. You will have produced a
'step' with diameter 0.087 on the end of the work. Move the compound until it
reads 0.008 (increment its position by DX=0.006) and cut to depth 0.413
(DY=-.043 less than last cut) to produce a second step of diameter 0.174.
Continue as indicated. When you reach the last step you will have cut a
stepped approximation to a hemisphere of diameter 1.00" on the end of your
stock.

To cut a complete sphere we need to use the same calculations on
the other "side" of the completed hemisphere. When I want a sphere I generally
start by relieving the stock so that a cylinder of length equal to the sphere
diameter is formed on the end of the stock, attached by a "stalk" to the stock.
That way I can touch the *right* edge of the tool to the left side of this
cylinder to establish the XF=0 position. Then I proceed as before, moving the
tool to the *right* as I make the cuts indicated in the print out.
Alternatively, you can set the right edge of the tool on the "equator" of the
hemisphere and continue cutting to the left working backwards through the cuts
in the print out, but I find that technique a bit confusing.

If you're making a ball handle that will be threaded onto a shaft or
whatever, drill and thread the stock, then cut the hemisphere as outlined
above. Cut off so that the overall length of the cutoff piece is equal to the
sphere diameter. Mount the half-completed sphere on a suitably threaded
spigot (or the target shaft if that's possible), reinsert in chuck and cut the
other half of the sphere.

In the simple example above the diameter of the sphere matched the
diameter of the stock (1" in both cases). If we had wanted a 2" radius
cut on the end of the 1" diameter stock, the program would report:

================================================== ==========================
Incremental Sphere Turning Data
Sphere diameter = 2.0000 in
Stock diameter = 1.0000 in
Angular increment = 5.0000 deg

N = cut number
XF = axial (along lathe bed) position of tool
DX = increment in x from last cut
YF = depth of cut
DY = increment in y from last cut
WD = work diameter resulting from depth of cut YF

N XF DX YF DY WD

0 0.000 +0.000 0.500 +0.000 0.000
1 0.004 +0.004 0.413 -0.087 0.174
2 0.015 +0.011 0.326 -0.086 0.347
3 0.034 +0.019 0.241 -0.085 0.518
4 0.060 +0.026 0.158 -0.083 0.684
5 0.094 +0.033 0.077 -0.081 0.845
================================================== ==========================

After step 5, YF becomes negative - we're cutting into material that
'isn't there' if the stock is only 1" in diameter - so the program doesn't
report these 'impossible' cuts. In this case it would probably have been
better to use a smaller angular increment to more closely approximate the
radius on the end of the stock.

This same procedure can be used to approximate shapes other than
spherical. I had to make some handles for a miniature Victorian drill press
I'm building. The handles were smoothly curved. Not being the world's most
adept freehand turner, I described their shape mathematically (spline fit to
diameters measured at a number of points) and built a program similar to
BALLCUT to produce the required cutting schedule. Once the staircase shape was
visible it was quick work with a jeweler's file to make very acceptable
handles.

Should you have need of such a capability, let me know and I'll try to
generalize the handle-shaping program and put it up on my website. You can
reach me via email at: [email protected]

Update 11/00:

The generalized version of ballcut mentioned above is now available on
the website as PROFILE.ZIP.

Several people have requested a version of the program where, instead
of using a fixed angular increment to determine the size of the step in x, a
fixed x step size is used. Rather than write a separate program, I've included
the ability to use a fixed step in x in the existing program. A fixed step in
x makes tracking the cuts a little easier at the expense of having to make more
cuts to get the same definition as when using a fixed angluar step. Fixed
angular step remains the default but it can be overridden when the program is
run.

[asallwey]

Thanks all, I hit his site periodically. I do use his program on those occasional times when I have made a handle (and it works very well). I was just looking for something in excel.

Alex

[Mcgyver]

here's a screen shot of a very simple one i did in excel....i put a little graphic on it as i found with infrequent use i had to re think and decipher what was what each time....if you want one, send me note







ps, dp, its easily changed, to your point though i just changed it.... but half a thou doesn't much matter for this app, you hit it with a file and emery to finish

[dp]

McGyver - The X column is fixed point at 3 places but the increment is 4 places so each entry is rounded up by .00005.

[dp]

ps, dp, its easily changed, to your point though i just changed it.... but half a thou doesn't much matter for this app, you hit it with a file and emery to finish

You know how we like to work at sub-angstrom accuracy here - can't be too careful

[oldtiffie]

Sublime happiness

Yep - you just can't be too careful here Dennis.

I'd have substituted subliminal for sub-angstrom - for that "feel good" euphoria - something like a Revivalist meeting.

http://en.wikipedia.org/wiki/Subliminal_message

http://en.wikipedia.org/wiki/Revivalism

http://en.wikipedia.org/wiki/Revival_meeting