Cutting Eccentrics in a 3 Jaw Chuck

[rake60]

I have been properly set straight that this is wrong, but it works in most
cases. In fact it works perfectly for the .050 throw called for in most of
Elmer Verburg's engine plans.
Shimming one jaw of a 3 jaw scroll chuck 1.5 X the required throw will get
you real close if not perfectly indicated to cut the throw.
In this case I used two 1mm thicknesses of brass (.078") and the
indicated run out was exactly .050"

It's 1/2 ounce of steel spinning 800RPM. Of course you wouldn't want to chuck
up a heavy part in such a way.
I hate indicating. If there's an easier way to get a perfect result, I'll use it!

 

[wareagle]

That setup doesn't look to be in the interest of a long lasting machine tool. -At least to my untrained eye!

Any problems with the shims loosening up? How about the piece moving within the chuck? Cutter digging in? Etc?

 

[rake60]

The major diameter of the stock is 5/8"
Not enough weight to make any noticeable out of balance condition.
I've made 6 eccentrics by this method and haven't thrown one out yet.
A single piece shim would be better.
I wouldn't try to rough it with one cut!
The way I see it, set it up and spin it for a few minutes.
If it doesn't feel safe in your mind, then it probably isn't!
I've never felt uncomfortable with this set up myself.
If you aren't comfortable with it please DON'T use it!

 

[wareagle]

The last sentence is a very true and wise statement.

The pictures are misleading in size relationships. It's hard to judge how large the items is that you are seeing. No more mass than there is there, an out of round piece of stock can easily be out of balance more than this set up.

Ingenious method, though!

 

[lugnut]

rake60, that's a neat idea, but wouldn't a 4 jaw chuck work better, assuming you had one? I have a hard time thinking of simple solution like yours not being a real machinist. Thanks for the cool idea
Mel

 

[rake60]

My biggest problem with my 4-jaw is it's size. It's 7-1/4" on the OD and
the jaws don't have the travel to go to center. So... If I chuck up the little
eccentrics I was having to shim jaws anyway. I only use my 4 jaw for
square or larger uneven stock. A 4" 4-jaw chuck is on my wish list.
She just hasn't figured out where to get it yet.
Maybe I'll leave the computer on this page when I go to bed some night.
http://www.littlemachineshop.com/products/product_view.php?ProductID=1588&category=566826475 :wink:

 

[tattoomike68]

I do that same trick. my tools are to be used, Im not trying to make them last forever and a little shim to offset a part is nothing to a chuck. It does not know the part is not round and does not care.

When rounding the offset part out a little care is needed on the pounding interupted cut but thats just common sence to take it easy at that point.

I do some wood on my smithy too and have played with putting more than one center on the end to turn weird parts, I just slow it down and take smaller bites.

 

[te_gui]

Another neat trick if you are cutting a lot of eccentrics is to get a small 3 jaw and put it in your 4 jaw, then you just open and close the 3 jaw once its indicated where desired.

 

[Stan]

This is a simple way to make small eccentrics. The out of balance thing is a no brainer. If you do the same job in a four jaw, not only is the stock the same distance off center, but you now have the weight of two chuck jaws off center.

 

[deere_x475guy]

I know this thread is talking about using a 3 jaw to cut eccentrics but I thought I would post how what I do with my 4 jaw. I cut this eccentric last weekend for the tubing bender I am making. I use my 4 sided 5C collet block in the 4 jaw when I can. It took me longer to swap out the 5C collet chuck for the 4 jaw then it did to indicate the eccentric.

 

[BobWarfield]

I love my collet blocks: cheap and handy.

A set of 5C collets, OTOH, not so cheap!

eBay was my friend.

I too am used to 4-jaws for eccentrics. I had heard of putting a small 3-jaw in the big chuck, but the collet block is a great alternative idea.

But that's not the only way to use a collet system. You can make up a "pot chuck" specifically for turning a particular eccentric:

Pot chucks are useful for a lot of things as you can see from the photo. I don't know why a feller couldn't build a pot chuck suited to something other than a 5C system either.

Best,

BW

 

[mklotz]

A 4jaw chuck is the right solution. Centering is easy if you follow the instructions that I posted in another thread.

If you do want to pack things in the 3jaw, you can use the ECCENT program on my page to calculate the required amount of packing. The text file that accompanies that program also describes a safer way to do this. Basically, you hold the part in a piece of split tubing. The split allows one jaw to contact the workpiece while the other two jaws contact the OD of the tube. By properly sizing the tube, one can achieve the required offset in the part.

Nevertheless, a 4jaw is both safer and, IMO, easier.

 

[Hexbasher]

Of course you wouldn't want to chuck
up a heavy part in such a way.

i was spinning 55 pound burn-outs parts like this today at work (yes i work saturdays and end up staying an extra 3 hours on side stuff, made some more edge finders)

800 rpm fine, 1100 the whole lathe was jumping...

2000 rpm and she was a beauty

ahhhhh, the sh!t you do in a jobber shop

 

[rake60]

The scariest eccentric I ever cut at work was on a 12 foot diameter gear.
Naturally it WAS a 4 jaw set up on a large vertical boring mill.

The hub of the gear was approximately 21 inches in diameter with a 3 inch throw called for.

A 21" part running out 6 inches isn't a big deal.

Stick that in the middle of a 144" gear and spin it fast enough to get a decent finish on it.
When the high side of that gear come around at you it makes you wonder if all your affairs are
in line.

Rick