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.

[Gas_mantle]

No probs 

Mind you there are 1 or 2 out there who do enjoy the fiddly bits of this hobby that the rest of us hate !

Personally I can't stand making 1 off jigs and mandrels etc but some guys seem to love it.

[zeeprogrammer]

Mind you there are 1 or 2 out there who do enjoy the fiddly bits of this hobby that the rest of us hate !

It just points out that there are many facets to any hobby. Some facets are more interesting to some people than others.
It's the same argument as 'do you enjoy the journey or the destination'?

I'm not into tool making although the occasional jig is pretty interesting.

As we all already know, (and I apologize for saying the obvious) whatever makes it fun and enjoyable.

[gerritv]

Since I guess I started some this later commentary by saying I used a shim to offset in a 3 jaw., no I am not lazy nor am I afraid of truing stock etc in a 4 jaw. For a small offset I looked at the 3 jaw and determined it was not a risk or problem. A large offset, yes I agree the grip would be a risk and issue. But 0.030 is not much.

Gerrit

[Flyboy Jim]

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.

Chris.........I remember seeing you do this on your Double Acting Twin Beam Engine build when I was first getting started. I just assumed that was how it was done with a 4 jaw chuck, so did the same thing. Really easy to set up that way. Then when I added my "Marv inspired pump center" it added even more versatility to the process.

Now that said.........I can see the benefit for understanding how it can also be done with a 3 jaw chuck. It certainly adds to the knowledge base.

Jim

PS: It's interesting to see how we all approach this hobby. At this point, early in my machining journey, I'm still really enthralled with the set up and process of making parts. If suddenly I end up with an engine that runs, that's just the frosting on the cake. I can see where, further down the stream, the end result might end up being more of the focus.

[10KPete]

When I was a young lad, an old machinist explained it to me thusly:

"A machinist makes chips. If he makes the chips correctly he has a good part left over."  Joe Bonney

Then I heard this from another machinist, in the same shop, when a designer brought him a drawing: "Don't tell me to 1/2" ream the hole! I know how to make holes! Tell me the diameter and the tolerance that's needed!!"

Later on, another old machinist said this (he was from Portugal):

"You got no set-up, you got nothing"  Paul Machado

I'll never forget those guys, and many others who guided me early on.

I don't think it matters much how you skin the cat.....

Pete

[Lew Hartswick]

Marv's humorous post is about the truth for me. A good example is the post on Burr puzzles. I was intrigued by the idea of  making the parts.
But after I made the soft jaws and developed the technique of making the cuts etc. the finished device is just more collected junk.  I'm not into engines at all the machining that is interesting.  Just the other day someone wanted me to make a few "flywheels" of different moment of inertia to demonstrate the change in current / acceleration of a DC motor, the resultant "trip back" through  my college physics book, Sears and Zemansky, and the resultant integral math was just GREAT!!!   :-)
   ...lew...

[mklotz]

...Just the other day someone wanted me to make a few "flywheels" of different moment of inertia to demonstrate the change in current / acceleration of a DC motor, the resultant "trip back" through  my college physics book, Sears and Zemansky, and the resultant integral math was just GREAT!!!   :-)

Next time, Lew, just ask me...

http://www.homemadetools.net/forum/flywheel-inertia-analysis-48288#post68898

[Lew Hartswick]

...Just the other day someone wanted me to make a few "flywheels" of different moment of inertia to demonstrate the change in current / acceleration of a DC motor, the resultant "trip back" through  my college physics book, Sears and Zemansky, and the resultant integral math was just GREAT!!!   :-)

Next time, Lew, just ask me...

http://www.homemadetools.net/forum/flywheel-inertia-analysis-48288#post68898

BUT it was such fun doing it with the old text from 1955 and even following the integration .  I was "intuitively" suspicious of the text when I saw the 1/2 M (r2 sq + r1 sq) for the annular cylinder so had to go through the whole business to see why it was.  For all those that just want an answer your link will be a great help.  If they are concerned about the how/why it's a good thing to do the math . :-)  I'll take you up on some of the other things in your fabulous collection of items though.
   ...lew...

[mklotz]

BUT it was such fun doing it with the old text from 1955 and even following the integration .  I was "intuitively" suspicious of the text when I saw the 1/2 M (r2 sq + r1 sq) for the annular cylinder so had to go through the whole business to see why it was.  For all those that just want an answer your link will be a great help.  If they are concerned about the how/why it's a good thing to do the math . :-)  I'll take you up on some of the other things in your fabulous collection of items though.

Just do the integrals in your head; no need for paper. :-)

Reminds me of an old math joke (and math jokes are very rare).

At a cocktail party, a smart-ass senior approaches Norbert Wiener (world famous mathematician for the Philistines) and challenges him with this problem...

Two trains, a mile apart are approaching each other on the same track, each traveling at 60 mph.  A fly on the front of train A flys at 5 miles per hour to the front of train B, turns around and flys back to train A.  He continues this shuttle until both trains collide, crushing him into goo.  How far did the fly fly?

Wiener instantly responds, "220 feet".

Student: "Very good. sir.  Most people would try to sum the infinite series"

Wiener: "I did."

[Admiral_dk]

OK / Good joke from a mathematical point - but from physics it is rubbish, as there is no way a fly, flying 5mph can fly back and forth between trains traveling at 60mph .... or are you telling me that I missed the point completely Marv ...?... (not for the first time ....).

[Roger B]

I can raise the stakes with off center turning/drilling with a 4 jaw SC chuck 

For my fuel injection components two 2mm shims move the piece of 10 mm square bar the required 1 mm offset in each direction 

[10KPete]

I can raise the stakes with off center turning/drilling with a 4 jaw SC chuck 

For my fuel injection components two 2mm shims move the piece of 10 mm square bar the required 1 mm offset in each direction 

 

Pete

[mklotz]

OK / Good joke from a mathematical point - but from physics it is rubbish, as there is no way a fly, flying 5mph can fly back and forth between trains traveling at 60mph .... or are you telling me that I missed the point completely Marv ...?... (not for the first time ....).

American flies strong like bull.  Two of them can carry off a horse.  That's why we call them horse flies.

Yeah, you missed the point, the joke and the train.  Ever hear of the humor concept called "voluntary suspension of disbelief"?

Maybe it would have worked better if I had used Neils Bohr instead of Norbert Wiener?

[Tennessee Whiskey]

To be honest Marv, if you could have explained it to me and then I could interpolated into shop floor speak, we could have pulled it off. No emojis were harmed in this post .

Cletus

[mklotz]

To be honest Marv, if you could have explained it to me and then I could interpolated into shop floor speak, we could have pulled it off. No emojis were harmed in this post .

Cletus

I don't think I could have explained it to you.

[Tennessee Whiskey]

You might be surprised, but, I'll let you have the last laugh.

Cletus

[b.lindsey]

Made perfect sense to me and funny too. I am easily amused though.

Bill

[mklotz]

I was at Tech when Norbert was still in residence as an Institute professor.  He could be seen strolling the aisles staring at the ceiling in deep thought, bag of salted peanuts firmly clutched in left hand while he used his right thumb to track against the wall so he didn't walk into people.  It's rumored, though the story is probably apocryphal, that on occasion, if a classroom door was left open he would enter the room, circle it, and exit it to continue his stroll.   Probably unconnected but this happens to be a perfect algorithm for finding your way out of a maze - not necessarily the quickest but sure fire.

During the war Norbert was a key player in the development of military radar systems.  So key, in fact, that he was assigned an FBI agent to watch over him.  One of his primary duties was to prevent him from wandering into traffic on Massachusetts Avenue where Tech's primary entrance is located.

Another probably apocryphal story...He arrives at work one day with a note pinned to his lapel.  He says nothing about it and his secretary is consumed with curiosity.  At last she takes him some papers to sign and gets a chance to read it.  It says "Don't give me a ride home today.  I have the car."  Since Wiener, like Einstein, could almost certainly not "get it together" well enough to drive, I assume this anecdote is the stuff of faculty happy hours.  [What is true is Einstein's wife, when asked by an academic organizer what car should be rented for the professor, responded, "Oh, that won't be necessary.  Driving is far too complicated for Albert.]

[Dave Otto]

To be honest Marv, if you could have explained it to me and then I could interpolated into shop floor speak, we could have pulled it off. No emojis were harmed in this post .

Cletus

I don't think I could have explained it to you.

Or me 

[Flyboy Jim]

To be honest Marv, if you could have explained it to me and then I could interpolated into shop floor speak, we could have pulled it off. No emojis were harmed in this post .

Cletus

I don't think I could have explained it to you.

Or me 

Me neither Dave. I was thinking the whole point of Marv's story was that "Some days your the bug and some days your the windshield" (or train in this case)! 

Jim

[Pete49]

Lost me at 220 ft (insert emjoi of choice)

[Admiral_dk]

American flies strong like bull.  Two of them can carry off a horse.  That's why we call them horse flies.

Good one Marv - but I'm not naïve ....

Yeah, you missed the point, the joke and the train.  Ever hear of the humor concept called "voluntary suspension of disbelief"?

No, never, had to Google it - but then again, I'm over average in IQ but only slightly better than Rainman in people skills in a number of situations, so I tend to understand something's literally and do not always get them as jokes ....
It is not like I don't have humour ( I laugh quite a bit reading some of the posts here, books and movies), but I don't practice my own as it tends to show other people as inferiors IQ wise (= not nice) and I prefer to be polite, since I'm absolutely no better than the vast majority of people on this planet.

Maybe it would have worked better if I had used Neils Bohr instead of Norbert Wiener?

Only as far as to know who his was and some of what he did (Niels Bohr) - I will be lying if I claimed to understand all he did with for Quantum Physics.

Best wishes

Per

ps  I haven't calculated if 30 seconds at 5 mph equals 220 feet, but it can't be far off.