Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?

On 05/25/2010 03:02 PM, Spehro Pefhany wrote:

Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?

Yes, but...

Why not start by eyeballing one axis -- say the Y axis. Then use the
edge finder to find the X dimension each way, and split that. Then set
your X axis to that center point, and do the same with the Y axis. If
you really want to be anal, repeat with the X axis, just to assure
yourself that any error is lost in the noise.

That way you'll be finding an edge that's more or less perpendicular to
the travel of the edge finder, which should lead to more accuracy.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

"Spehro Pefhany" wrote in message
news

Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?


I do this a lot. You're over thinking it.

put the spindle about at the cneter by eye.
Zero X axis by going to one edge with edgefinder
Find to other ege with edge finder. record reading and move exactly 1/2 way
back.
Zero Y axis at one edge with edgefinder
Find other edge with edgefinder, record reading and move exatly 1/2 way
back.

You're at the center of your circle.

Karl

On Tue, 25 May 2010 17:29:23 -0500, the renowned "Karl Townsend"
wrote:


"Spehro Pefhany" wrote in message
news

Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?


I do this a lot. You're over thinking it.

put the spindle about at the cneter by eye.
Zero X axis by going to one edge with edgefinder
Find to other ege with edge finder. record reading and move exactly 1/2 way
back.
Zero Y axis at one edge with edgefinder
Find other edge with edgefinder, record reading and move exatly 1/2 way
back.

You're at the center of your circle.

Karl


Thanks, Karl and Tim!


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On May 25, 5:02*pm, Spehro Pefhany
wrote:
Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?

Another option might be to pinch a straightedge between a blank in the
spindle and the edge of the disk. Adjust until the straight edge is
perpendicular to the x-axis. Then move to the center of the disk on
the x-axis (allowing for the diameter of the blank). (Similar to the
trick of setting a tool bit when turning a round on a lathe.)

On Tue, 25 May 2010 17:04:43 -0700 (PDT), the renowned "Denis G."
wrote:

On May 25, 5:02*pm, Spehro Pefhany
wrote:
Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?

Another option might be to pinch a straightedge between a blank in the
spindle and the edge of the disk. Adjust until the straight edge is
perpendicular to the x-axis. Then move to the center of the disk on
the x-axis (allowing for the diameter of the blank). (Similar to the
trick of setting a tool bit when turning a round on a lathe.)

Nice trick, thanks!


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Tue, 25 May 2010 17:23:15 -0700, the renowned "Paul Hovnanian P.E."
wrote:

Clamp the disk to a rotary table. Eyeball the center. Then use a dial
indicator as you turn the table to measure the error. Correct the X and
Y axis as needed.

Sort of analogous to centering a part in a 4 jaw chuck on a lathe.

I guess if I had a setup that would hold a dial indicator in the
spindle and would reach the outer rim, I could just rotate the spindle
and adjust X/Y. Might be easier since there is no spindle lock on my
X3.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

"Karl Townsend" wrote in message
news.com...

"Spehro Pefhany" wrote in message
news

Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?


I do this a lot. You're over thinking it.

put the spindle about at the cneter by eye.
Zero X axis by going to one edge with edgefinder
Find to other ege with edge finder. record reading and move exactly 1/2
way back.
Zero Y axis at one edge with edgefinder
Find other edge with edgefinder, record reading and move exatly 1/2 way
back.

You're at the center of your circle.


If he has a DRO. If you have to allow for backlash it gets tricky...

--
Michael Koblic,
Campbell River, BC

On 05/25/2010 06:14 PM, Michael Koblic wrote:

"Karl Townsend" wrote in message
news.com...

"Spehro Pefhany" wrote in message
news

Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?


I do this a lot. You're over thinking it.

put the spindle about at the cneter by eye.
Zero X axis by going to one edge with edgefinder
Find to other ege with edge finder. record reading and move exactly
1/2 way back.
Zero Y axis at one edge with edgefinder
Find other edge with edgefinder, record reading and move exatly 1/2
way back.

You're at the center of your circle.


If he has a DRO. If you have to allow for backlash it gets tricky...

True. I am DRO-less, and compensate for that by either pulling the
stage in on the back side as I turn the wheel, or by doing something
like measuring the diameter of the thing before hand.

Both are a PITA and prone to error, for both the error can be overcome
with careful work.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

On May 25, 8:35*pm, Spehro Pefhany
wrote:
...
I guess if I had a setup that would hold a dial indicator in the
spindle and would reach the outer rim, I could just rotate the spindle
and adjust X/Y. Might be easier since there is no spindle lock on my
X3.
Spehro Pefhany

The simple version of that setup is an L shaped piece of stiff wire.
Chuck the short end, cut the long one to just clear the column. A
Starrett Last Word will clamp directly on the wire, otherwise make a
little block to fit the test indicator's dovetail. I would size the
hole in the block for 1/4" rod. While you are set up to cut the
dovetail you could make a lathe holder too.

To roughly center something into the range of an indicator I center a
6" ruler on it and move a point in the chuck to the 3" line. Usually
this gets it within 0.010".

jsw

Jim Wilkins wrote:
On May 25, 8:35 pm, Spehro Pefhany
wrote:
...
I guess if I had a setup that would hold a dial indicator in the
spindle and would reach the outer rim, I could just rotate the spindle
and adjust X/Y. Might be easier since there is no spindle lock on my
X3.
Spehro Pefhany

The simple version of that setup is an L shaped piece of stiff wire.
Chuck the short end, cut the long one to just clear the column. A
Starrett Last Word will clamp directly on the wire, otherwise make a
little block to fit the test indicator's dovetail. I would size the
hole in the block for 1/4" rod. While you are set up to cut the
dovetail you could make a lathe holder too.

To roughly center something into the range of an indicator I center a
6" ruler on it and move a point in the chuck to the 3" line. Usually
this gets it within 0.010".

Quick way to get a center (close)
Use a postcard or other rectangular cardstock with crisp edges.
Place the card on the round workpiece such that one corner is at an edge.
Mark where the two adjacent sides intersect the edge of the disc.
Connect those two marks with a line, which intersects the center
Move the card approx 90 degrees and repeat for an intersecting line.
The intersection is the center.

My goodness! What's wrong with using a machinist center square. This thing should be in everyone's tool box or chuck it up in a 4
jaw, eliminate runout and mark with your tailstock. I can't believe you are asking the question.
Steve

"Spehro Pefhany" wrote in message news

Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?

Tim Wescott wrote:

If he has a DRO. If you have to allow for backlash it gets tricky...

True. I am DRO-less, and compensate for that by either pulling the
stage in on the back side as I turn the wheel, or by doing something
like measuring the diameter of the thing before hand.


You are a embedded control guy. How darn retro do you want to be? Break open that wallet
in a dim light so the Benjamins don't get blinded and get yourself a DRO.

If it was a lathe, that would be different for some strange reason IMHO.

Wes
--
"Additionally as a security officer, I carry a gun to protect
government officials but my life isn't worth protecting at home
in their eyes." Dick Anthony Heller

On Tue, 25 May 2010 18:02:54 -0400, Spehro Pefhany
wrote:


Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?


Take a trip to

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

Download

OSBORNE.ZIP

That plus the milltable and an edge finder will do the job.

HTH
rgentry at oz dot net

On May 26, 3:22*pm, Wes wrote:
Tim Wescott wrote:
...
You are a embedded control guy. *How darn retro do you want to be? *Break open that wallet
in a dim light so the Benjamins don't get blinded and get yourself a DRO.
If it was a lathe, that would be different for some strange reason IMHO.
Wes

I don't have one on my mill at home, and don't really miss it.

At work making new parts to drawings the DRO is an enormous help. At
home I more likely fit a repair part to an older one by cut and try,
and it's easier to incrementally determine how much more to remove.
The resettable dials work fine for this.

jsw

On Wed, 26 May 2010 18:40:05 +0200, "Steve Lusardi"
wrote:

My goodness! What's wrong with using a machinist center square. This thing should be in everyone's tool box or chuck it up in a 4
jaw, eliminate runout and mark with your tailstock. I can't believe you are asking the question.
Steve
Junior was so impressed with the centre head the the next time I was
in HF (Port Huron, Mi) I invested $6.99 in their three head square,
took the protractor head for me and gave him the rest!
Gerry :-)}
London, Canada

On May 25, 6:02*pm, Spehro Pefhany
wrote:
Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?

Spehro,

Probes are kinda crappy for finding circles (CMMs aren't even good
with curved surfaces). Not sure about your tolerances, but indicating
the disk would likely be the best (indicator mounted to the spindle).

Why?
-Well, the edge finder prob may roll oddly along the curved edge of
the disk as you likely won't be on center the first try. I mean, you
have to have Y centered to make an accurate X measurement, and the
reverse. When you start, you'll have neither. If I had to do this, I'd
center one axis, center the other, then center the 1st again. You will
likely end up with a slightly different result.
-The disk may not be round. Depending on how it was made (turned,
punched, laser cut, plasma cut, waterjet, cylindrical ground, jig
ground, milled, etc, etc) your edge finder prob won't tell you
anything about its circularity and may touch a high or low point. An
indicator allows you to average highs and lows and find out the
location with the lowest deviation.

Just my $0.02 worth.

Regards,

Robin

"Spehro Pefhany" wrote in message
news

snip-
Does this sound like a reasonable plan?

Not to me. I use a DTI.

Harold

I use a compass myself. Measure the diameter of the circle set the compass
to the radius put one leg of the compass on several points around the edge
and the other leg will cross on the center.


"Spehro Pefhany" wrote in message
news

Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?