If you place a plate of thickness, t, between one jaw of a 3-jaw chuck and
now bore a hole,
what is the offset of that hole from the center of the original rod
(diameter, D).

One source says it is 2/3 of t. This might be = 1 / (1 + cos 60 deg) but I
failed to derive this with trig.

Anyone know how to derive this mathematically?
Anyone know if this is true?

thanks
-- - - - - - - - - - - - - - - - -
Bill Graves RKBA!

On Fri, 22 Apr 2005 20:47:42 GMT, "William Graves"
wrote:

If you place a plate of thickness, t, between one jaw of a 3-jaw chuck and
now bore a hole,
what is the offset of that hole from the center of the original rod
(diameter, D).

One source says it is 2/3 of t. This might be = 1 / (1 + cos 60 deg) but I
failed to derive this with trig.

Anyone know how to derive this mathematically?
Anyone know if this is true?

thanks
-- - - - - - - - - - - - - - - - -
Bill Graves RKBA!



The formula is a good bit more complicated than that. Take a look at the
ECCENT.ZIP archive on my website for a program that implements the correct
formula.

Regards, Marv

Home Shop Freeware - Tools for People Who Build Things
http://www.geocities.com/mklotz.geo

On Fri, 22 Apr 2005 20:53:42 GMT, (Marvin
W. Klotz) wrote:

On Fri, 22 Apr 2005 20:47:42 GMT, "William Graves"
wrote:

If you place a plate of thickness, t, between one jaw of a 3-jaw chuck and
now bore a hole,
what is the offset of that hole from the center of the original rod
(diameter, D).

One source says it is 2/3 of t. This might be = 1 / (1 + cos 60 deg) but I
failed to derive this with trig.

Anyone know how to derive this mathematically?
Anyone know if this is true?

thanks
-- - - - - - - - - - - - - - - - -
Bill Graves RKBA!



The formula is a good bit more complicated than that. Take a look at the
ECCENT.ZIP archive on my website for a program that implements the correct
formula.

Regards, Marv

Home Shop Freeware - Tools for People Who Build Things
http://www.geocities.com/mklotz.geo
Greetings Marv,
Thanks for the posting. Years ago my boss and I tried to figure out a
formula to figure out how much the offset would be for a certain sized
shim in a three jaw chuck. Neither of us being math whizes didn't
deter us. Nevertheless we never did figure out and exact way to
determine the offset. I made up a chart from actual measurements but
I've been bugged for a long time about this problem.
Cheers,
Eric

On Fri, 22 Apr 2005 15:57:58 -0700, Eric R Snow wrote:

On Fri, 22 Apr 2005 20:53:42 GMT, (Marvin
W. Klotz) wrote:

On Fri, 22 Apr 2005 20:47:42 GMT, "William Graves"
wrote:

If you place a plate of thickness, t, between one jaw of a 3-jaw chuck and
now bore a hole,
what is the offset of that hole from the center of the original rod
(diameter, D).

One source says it is 2/3 of t. This might be = 1 / (1 + cos 60 deg) but I
failed to derive this with trig.

Anyone know how to derive this mathematically?
Anyone know if this is true?

thanks
-- - - - - - - - - - - - - - - - -
Bill Graves RKBA!



The formula is a good bit more complicated than that. Take a look at the
ECCENT.ZIP archive on my website for a program that implements the correct
formula.

Regards, Marv

Home Shop Freeware - Tools for People Who Build Things
http://www.geocities.com/mklotz.geo
Greetings Marv,
Thanks for the posting. Years ago my boss and I tried to figure out a
formula to figure out how much the offset would be for a certain sized
shim in a three jaw chuck. Neither of us being math whizes didn't
deter us. Nevertheless we never did figure out and exact way to
determine the offset. I made up a chart from actual measurements but
I've been bugged for a long time about this problem.
Cheers,
Eric

It's a lovely exercise in trigonometry...

For the benefit of folks who want the actual formula:

w = width of chuck jaws
d = diameter of workpiece
e = required eccentric offset
r = d/2
root3 = sqrt(3)
p = required packing thickness

if (w root3*e)

{p=1.5*e}

else

{p=1.5*e-r+0.5*sqrt(4*r*r-3*e*e+2*e*w*root3-w*w)}


Regards, Marv

Home Shop Freeware - Tools for People Who Build Things
http://www.geocities.com/mklotz.geo

On Fri, 22 Apr 2005 20:53:42 GMT,
(Marvin W. Klotz) wrote:

On Fri, 22 Apr 2005 20:47:42 GMT, "William Graves"
wrote:

If you place a plate of thickness, t, between one jaw of a 3-jaw chuck and
now bore a hole,
what is the offset of that hole from the center of the original rod
(diameter, D).

One source says it is 2/3 of t. This might be = 1 / (1 + cos 60 deg) but I
failed to derive this with trig.

Anyone know how to derive this mathematically?
Anyone know if this is true?

thanks
-- - - - - - - - - - - - - - - - -
Bill Graves RKBA!



The formula is a good bit more complicated than that. Take a look at the
ECCENT.ZIP archive on my website for a program that implements the correct
formula.

Regards, Marv

Home Shop Freeware - Tools for People Who Build Things
http://www.geocities.com/mklotz.geo


A really excellent set of useful utilities!

One question on the ECCENT program.

This needs accurate knowledge of the width of
w - the little flat part at the end of the jaw that
contacts the workpiece. This is fiendishly difficult to
directly measure accurately. Your program has an elegant way
of avoiding this problem by making the spacer in the form of
a slotted tube.

However it's not always convenient to find or
fabricate a suitable tube. I think that an alternative
possibility is to simply find the largest size drill rod
that will enter the fully closed jaws of the three jaw
chuck. The effective jaw width w would then be accurately
defined by the sides of the equilateral triangle that
circumscribes that drill rod size.

Is this a reasonable approach and could it be
incorporated in your program?

Jim

On Sat, 23 Apr 2005 05:30:08 +0100, wrote:

On Fri, 22 Apr 2005 20:53:42 GMT,
(Marvin W. Klotz) wrote:

On Fri, 22 Apr 2005 20:47:42 GMT, "William Graves"
wrote:

If you place a plate of thickness, t, between one jaw of a 3-jaw chuck and
now bore a hole,
what is the offset of that hole from the center of the original rod
(diameter, D).

One source says it is 2/3 of t. This might be = 1 / (1 + cos 60 deg) but I
failed to derive this with trig.

Anyone know how to derive this mathematically?
Anyone know if this is true?

thanks
-- - - - - - - - - - - - - - - - -
Bill Graves RKBA!



The formula is a good bit more complicated than that. Take a look at the
ECCENT.ZIP archive on my website for a program that implements the correct
formula.

Regards, Marv

Home Shop Freeware - Tools for People Who Build Things
http://www.geocities.com/mklotz.geo


A really excellent set of useful utilities!

One question on the ECCENT program.

This needs accurate knowledge of the width of
w - the little flat part at the end of the jaw that
contacts the workpiece. This is fiendishly difficult to
directly measure accurately. Your program has an elegant way
of avoiding this problem by making the spacer in the form of
a slotted tube.

However it's not always convenient to find or
fabricate a suitable tube. I think that an alternative
possibility is to simply find the largest size drill rod
that will enter the fully closed jaws of the three jaw
chuck. The effective jaw width w would then be accurately
defined by the sides of the equilateral triangle that
circumscribes that drill rod size.

Is this a reasonable approach and could it be
incorporated in your program?

Jim

I hadn't thought of that technique for determining 'w', but it makes sense.

If:

d = diameter of largest rod that will enter closed jaws

then:

w = d*sqrt(3)

Regards, Marv

Home Shop Freeware - Tools for People Who Build Things
http://www.geocities.com/mklotz.geo

The formula is a good bit more complicated than that. Take a look at the
ECCENT.ZIP archive on my website for a program that implements the correct
formula.
Regards, Marv

Thanks, Marv. although I was not able to derive a formula - I see that I
had completely missed the bit about the width of the chuck jaws. This gets
interesting as I assume the compensation is because the jaws are no longer
tangent to the workpiece. In my case, the jaws have a slight curvature!

I will take a look at the C source code you so nicely provided and see if I
can follow the math.
thanks a lot for your work.
-- - - - - - - - - - - - - - - - -
Bill Graves RKBA!

On Sat, 23 Apr 2005 20:35:24 GMT, "William Graves"
wrote:

The formula is a good bit more complicated than that. Take a look at the
ECCENT.ZIP archive on my website for a program that implements the correct
formula.
Regards, Marv

Thanks, Marv. although I was not able to derive a formula - I see that I
had completely missed the bit about the width of the chuck jaws. This gets
interesting as I assume the compensation is because the jaws are no longer
tangent to the workpiece. In my case, the jaws have a slight curvature!

I will take a look at the C source code you so nicely provided and see if I
can follow the math.
thanks a lot for your work.
-- - - - - - - - - - - - - - - - -
Bill Graves RKBA!


The source code isn't going to help you much since all it does is implement the
final, very messy equation.

I still have my derivation of the equation and will send it along to you via
email after I scan it.

If your email address is false or you don't hear from me, drop me an email at
mklotz at alum dot mit dot edu.

Regards, Marv

Home Shop Freeware - Tools for People Who Build Things
http://www.geocities.com/mklotz.geo

William Graves wrote:

The formula is a good bit more complicated than that. Take a look at the
ECCENT.ZIP archive on my website for a program that implements the correct
formula.
Regards, Marv


Thanks, Marv. although I was not able to derive a formula - I see that I
had completely missed the bit about the width of the chuck jaws. This gets
interesting as I assume the compensation is because the jaws are no longer
tangent to the workpiece.

It's because the *centers* of the "other two" jaws are no longer the points of
tangency to the workpiece.

In my case, the jaws have a slight curvature!

Depending on how precise your offset needs to be, it may be safe to ignore the
curvature.

-jc-

In article ,
Marvin W. Klotz wrote:

It's a lovely exercise in trigonometry...

For the benefit of folks who want the actual formula:

w = width of chuck jaws
d = diameter of workpiece
e = required eccentric offset
r = d/2
root3 = sqrt(3)
p = required packing thickness

if (w root3*e)

{p=1.5*e}

else

{p=1.5*e-r+0.5*sqrt(4*r*r-3*e*e+2*e*w*root3-w*w)}

The second case seems to involve the edges of two of the chuck jaws
digging into the workpiece. That would render the formula inaccurate:
if the edges are blunted they won't be where calculated, and if they're
sharp they'll dig in, so the workpiece won't be where calculated. It
might also damage the workpiece, although in some contexts that would be
completely acceptable.


--
Norman Yarvin http://yarchive.net

On 26 Apr 2005 17:18:27 GMT, Norman Yarvin wrote:

In article ,
Marvin W. Klotz wrote:

It's a lovely exercise in trigonometry...

For the benefit of folks who want the actual formula:

w = width of chuck jaws
d = diameter of workpiece
e = required eccentric offset
r = d/2
root3 = sqrt(3)
p = required packing thickness

if (w root3*e)

{p=1.5*e}

else

{p=1.5*e-r+0.5*sqrt(4*r*r-3*e*e+2*e*w*root3-w*w)}

The second case seems to involve the edges of two of the chuck jaws
digging into the workpiece. That would render the formula inaccurate:
if the edges are blunted they won't be where calculated, and if they're
sharp they'll dig in, so the workpiece won't be where calculated. It
might also damage the workpiece, although in some contexts that would be
completely acceptable.


--
Norman Yarvin http://yarchive.net

Making eccentrics by shimming on a three jaw is at best a bodge. If one wants
to do it correctly, a four jaw is the tool of choice. The program is aimed at
newbies who still haven't equipped their lathes properly.

If they must use a three jaw, the technique of using a slotted tube is far more
accurate and safer. A program for this technique is included in the archive
that contains the program described above.

Regards, Marv

--

Home Shop Freeware - Tools for People Who Build Things
http://www.geocities.com/mklotz.geo

On Tue, 26 Apr 2005 19:43:10 GMT, (Marvin
W. Klotz) wrote:

On 26 Apr 2005 17:18:27 GMT, Norman Yarvin wrote:

In article ,
Marvin W. Klotz wrote:

It's a lovely exercise in trigonometry...

For the benefit of folks who want the actual formula:

w = width of chuck jaws
d = diameter of workpiece
e = required eccentric offset
r = d/2
root3 = sqrt(3)
p = required packing thickness

if (w root3*e)

{p=1.5*e}

else

{p=1.5*e-r+0.5*sqrt(4*r*r-3*e*e+2*e*w*root3-w*w)}

The second case seems to involve the edges of two of the chuck jaws
digging into the workpiece. That would render the formula inaccurate:
if the edges are blunted they won't be where calculated, and if they're
sharp they'll dig in, so the workpiece won't be where calculated. It
might also damage the workpiece, although in some contexts that would be
completely acceptable.


--
Norman Yarvin http://yarchive.net

Making eccentrics by shimming on a three jaw is at best a bodge. If one wants
to do it correctly, a four jaw is the tool of choice. The program is aimed at
newbies who still haven't equipped their lathes properly.

If they must use a three jaw, the technique of using a slotted tube is far more
accurate and safer. A program for this technique is included in the archive
that contains the program described above.

Regards, Marv
Marv,
I must disagree about the bodge. If a small offset, say .030", is
needed then shimming a three jaw is much faster. In fact I'd say up
about .125" offste is best accomplished with a three jaw. Of course,
I'm talking about adjustable three jaw chucks, where it is possible to
indicate the part. Sort of a 4 jaw, but much faster.
ERS