Off centre turning in a 3 jaw chuck.

[Gas_mantle]

Hi all

I'm trying to turn a raised spigot off centre to give a 5.25mm throw as part of an eccentric I'm making. I realise I could dial it in the 4 jaw but I've seen some people use packing pieces in a 3 jaw and wanted to give it a try for a change.

Does anyone know what the formula / ratio is to calculate the packing size required for a given throw ?

Many thanks
Peter.

[pgp001]

Here you go Peter

http://ibls.org/mediawiki/index.php?title=Turning_an_Eccentric_Hole_in_a_3-Jaw_Chuck

Phil

[Gas_mantle]

Thanks Phil, on the face of it it seems like it might be easier than using the 4 jaw so I'd thought it would be worth trying and could be useful knowing how to work it out mathematically.

If my grasshopper engine doesn't work I can blame it on you ;-)

[gerritv]

I used a similar method, without the math, to machine the 0.030 offset for Eureka relieving device mandrel. I iteratively added shims to one jaw until it reached the correct position.
Thank you for posting a link to the math, makes it more predictable :-)

Gerrit

[mklotz]

I wrote a program, ECCENT, to do the required math.  It's free for the downloading from my page (see my sig for URL).

Putting a shim under one jaw is, IMO, a rather iffy, dangerous setup.  I wouldn't do it.  If you're really too lazy to dial in the 4jaw, there's a more secure method to use with the 3jaw, assuming the parent stock is cylindrical.

Imagine a tube bored to be a sliding fit on the parent stock.  A slot
of sufficient width to pass one of the three-jaw chuck jaws is milled in this
tube.  The parent stock is inserted in the bore and the whole thing clamped in
the three-jaw chuck such that one jaw passes through the slot to seat on the
parent stock while the other two jaws seat on the circumference of the tube.
Obviously the centerline of the parent stock is offset from the lathe spindle
axis and turning will produce an eccentric.  If we know the OD of the parent
stock and the amount of the offset required, it's possible to calculate the OD
of the tube needed to produce just that offset when clamped as described
above.

The program to calculate the OD is also included in the ECCENT archive on my page.

[Gas_mantle]

Thanks Marv, looks like you have some interesting software there - I'll have a good look through it all later.

I've managed to make the part I wanted and pleased with how it worked but think it would have been a bit iffy had I needed a bigger throw relative to the holding diameter.

[kvom]

For me an intuitive way to do it without math is to use CAD (in my case Draftsight).

1) Draw a circle centered at 0,0 with dimension of stock.

2) From a point on the Y at the eccentric throw distance draw a line that intersects the stock at an angle of 120 from vertical.

3) Draw a second circle whose center is on the Y axis eccentric point and intersects the point where the line intersects the stock circle.  This circle represented the jaws.

4) Dimension the distance along the Y axis bewteen the quadrant points of the two circle to get the packing size.

[mklotz]

You've not taken into account the finite width of the (typically prismatic) chuck jaws.  With the shim in there, the work will be riding on the edges of the other two jaws and thus their width affects the computation.

Look at the attached diagram for a better understanding of what I'm on about.

The effect is admittedly small.  If I use a chuck tip width of 0.125", and your numbers of 2.0 and 0.1, my calculation yields a shim thickness of 0.1497 versus your 0.1462.

If one is trying to do precision work safely, he learns to use his 4jaw and doesn't resort to shadetree tricks like this.

[b.lindsey]

I am not opposed to using the Math, and it is nice to know the theory involved, but given the variability in jaw configurations isn't it just easier to use the 4 jaw, not to mention a more reliable holding of the part? Just my 2 cents.

Bill

[mklotz]

I am not opposed to using the Math, and it is nice to know the theory involved, but given the variability in jaw configurations isn't it just easier to use the 4 jaw, not to mention a more reliable holding of the part? Just my 2 cents.

I'm with you completely, Bill.  Shimming like this is a dangerous redneck kludge and should be avoided.

Why do folks have so much trouble learning to use a 4jaw?  There are plenty of instructions (including mine)...

http://www.homemadetools.net/forum/centering-work-four-jaw-chuck-27241

 floating around the interweb.  These kludges to avoid learning a new skill remind me of the way some folks will do anything whatsoever to avoid learning any math.

[AOG]

I think it's a matter of perception. Most of you guys have been doing this for years and have had lots of practice. I can tell you that when I started in this hobby I read up on centering a four jaw and thought "it shouldn't be that hard". When I went to do it for the first few times, I found out that there is a large gap between comprehension and competent execution. Those initial bad experiences turned me off on the four jaw for a long time. I've done enough with my four jaw now that I don't dread using it like I used to but I can understand why people would look into alternatives.

My 2 cents

Tony

[crueby]

Another 2 cents....

With a dial indicator (mounted mine through a aluminum block that bolts to the cross slide with a t-nut, allows for really quick zeroing by moving the cross slide), centering up the 4 jaw takes me less than a minute. Once one axis is dialed in, the second one goes really quick. For offset eccentrics, I just center it, then offset one axis the amount needed. After a few times, the process becomes second nature. Its confusing which way to move which jaw the first couple times, gets easy quick.

If making lots of parts with same offset, I could see making a jig with the offset built in like you guys have shown so no adjustment part to part is needed.

[mklotz]

I think it's a matter of perception. Most of you guys have been doing this for years and have had lots of practice. I can tell you that when I started in this hobby I read up on centering a four jaw and thought "it shouldn't be that hard". When I went to do it for the first few times, I found out that there is a large gap between comprehension and competent execution. Those initial bad experiences turned me off on the four jaw for a long time. I've done enough with my four jaw now that I don't dread using it like I used to but I can understand why people would look into alternatives.

You guys have the misconception that this hobby is about building engines.  It isn't.  The hobby is all about setting up to make parts.  Sometimes those parts can be assembled into an engine or a useful tchotchke but the real hobby is setting up.

[Gas_mantle]

You guys have the misconception that this hobby is about building engines.  It isn't.  The hobby is all about setting up to make parts.  Sometimes those parts can be assembled into an engine or a useful tchotchke but the real hobby is setting up.

The hobby is about whatever gives pleasure - for me that is building engines - setting up the machinery to make the parts is just one of a number of procedures to do that.

[mklotz]

The hobby is about whatever gives pleasure - for me that is building engines - setting up the machinery to make the parts is just one of a number of procedures to do that.

It was an attempt at humor, Gas, just humor.  I'm sorry you missed it.  I know we love out engines but sometimes it seems we spend more time on setups than we spend on actually making parts.