# Help with making eccentric on a 4 jaw chuck



## Captain777 (Mar 28, 2019)

Hi,

I just made an eccentric from a 3/4" brass rod, 5/8" size of the eccentric and the hole (1/2 the valve travel) offset .155" for an eccentric movement of .310" .

I used the 4 jaw with the brass rod centered to within .001" with no problem.  However, when I did the offset for the axle hole, I used my dial indicator on the cut part of the eccentric and used it to get a total indicator travel of .310".

This took me 45 minutes to do this.  I had just set up the initial cut in only 3-4 minutes.  (I seldom do 4 jaw centering, but I understand the  mechanics behind 4 jaw centering.)

Should I have done the offset this way?  Is there a "trick" to moving the part for the offset?  How much do you loosen the other 2 jaws that are "not" to be moved for the offset?  I wonder if I loosened the "off side" jaws too much or not enough.  It seemed as if I would get close to my measurement, and then the turn of the chuck would show being off 20-30 thou.???  Would I have been smarter to just use the outside of the 3/4" brass rod and move it .155"?  Does it make a difference if you place the dial indicator vertical or horizontal?  For a round chuck, it shouldn't make a difference?

So many questions for what should have been a simple operation.

In the end, my hole was drilled and reamed where I wanted it, but for my next eccentric, I sure would like to do it a little faster.

Thanks,

Dan Fuller
Carrollton, Texas


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## Captain777 (Mar 28, 2019)

I forgot to mention,  I am using a Jet 9x20 with a "no name" cheap Chinese 4 jaw chuck.
Dan


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## Cogsy (Mar 29, 2019)

This method might be a little rough for you but it's what I use when I don't need absolutely precise location with the 4 jaw. I measure and mark the spot I need to align to (the centre of the hole to be made or diameter to be turned) and place a small centre punch in that location. I then fit a nice pointy dead centre in the tailstock and align the part in the 4 jaw so the point of the centre fits nicely into the punch mark. It gets you quite close as you can revolve the chuck by hand when you're getting near to aligned and even small discrepancies between the mark and the point of the centre become visible. It's also very quick to perform and typically only takes a minute or two to get set up after you've made the punch mark. For me, getting the punch mark in the right spot then becomes the only difficult part of the procedure.

Edit to add: If you want a more precise location, this method should get you to within a couple of thou quickly and you can use your indicator from there to get spot-on.


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## Mechanicboy (Mar 29, 2019)

Special tool for eccentric disc. The indexing dial is for only the part with 2 eccentric discs when you need correct degree between the two eccentric disc.  If only an eccentric, then you don't need the indexing disk. The tool is marked with how big is the stroke length as I find the tool with the right stroke length when I want to turn the new eccentric disc.

 See at the pictures..


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## Captain777 (Mar 29, 2019)

Cogsy said:


> This method might be a little rough for you but it's what I use when I don't need absolutely precise location with the 4 jaw. I measure and mark the spot I need to align to (the centre of the hole to be made or diameter to be turned) and place a small centre punch in that location. I then fit a nice pointy dead centre in the tailstock and align the part in the 4 jaw so the point of the centre fits nicely into the punch mark. It gets you quite close as you can revolve the chuck by hand when you're getting near to aligned and even small discrepancies between the mark and the point of the centre become visible. It's also very quick to perform and typically only takes a minute or two to get set up after you've made the punch mark. For me, getting the punch mark in the right spot then becomes the only difficult part of the procedure.
> 
> Edit to add: If you want a more precise location, this method should get you to within a couple of thou quickly and you can use your indicator from there to get spot-on.



Thanks,
I guess that just more practice in needed.
Dan


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## BaronJ (Mar 29, 2019)

Hi Captain,

A little trick I've used to turn something off centre is to use the three jaw and a packing of the right thickness between the work and one of the jaws.  Out of habit I make a point of using jaw No: 1 to hold the packing piece.

The centre punch mark and a wobbler works well when using a four jaw.


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## john_reese (Mar 29, 2019)

I think I would do it in the mill.  Indicate on the OD and dial in the the offset for the hole.

There is a formula to calculate the height of the packing BaronJ mentioned.  Unfortunately I caan't remember where I saved it.


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## Hopper (Mar 29, 2019)

That formula is in one of GH Thomas's books, somewhere. 
But it just takes a bit of practice with setting up in the four jaw to get quicker. 
To help, you can set the tool on the centralized section before moving it, then wind the cross slide back by the amount of offset desired (moving further then bringing back to the mark to take up backlash of course). Then move the job over until it touches the tool again. Gets it pretty close then final adjust with dial indicator in the normal manner.


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## kwoodhands (Mar 29, 2019)

I go about it the same way Cogsy does with one exception. I lay out the center,  Then install a tool I made I call a "point". I locate the center, install a spotting drill. After the spotting drill I center drill. This is done on a DP or the mill. I install the work in the 4 jaw, bring the tail stock center close to the work.  With 2 wrenches I adjust the jaws til the TS center is in the center drilled bore. This gets you very close to center. Then install  a dial indicator in the tool post,  A rod with a 60° turned center on the head stock end and 60° bore on the TS end  goes between the TS center and the work. A dial indicator on the tool post . I usually can find center on the lathe and ready to turn in under 5 minutes because the work is almost perfectly centered before using the indicator.  The "point" is just a scrap piece turned to a long sharp angle, maybe 20° or so. You want the point sharp so locating center is easy. This can be done with the point in an electric drill rotating  against the spinning grinder wheel. I use broken taps for scrap metal points.
The points make good scribers too. Loctite a point in a a 6" length of CRS or any metal you have.
mike


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## XD351 (Mar 30, 2019)

Dan, 
Have a look at this video 

One problem can be the shape of the material you are using , round bar can rotate as you crank the jaws across to get the offset you want and that is why in the video Marc uses square stock .


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## stackerjack (Mar 30, 2019)

Cogsy said:


> This method might be a little rough for you but it's what I use when I don't need absolutely precise location with the 4 jaw. I measure and mark the spot I need to align to (the centre of the hole to be made or diameter to be turned) and place a small centre punch in that location. I then fit a nice pointy dead centre in the tailstock and align the part in the 4 jaw so the point of the centre fits nicely into the punch mark. It gets you quite close as you can revolve the chuck by hand when you're getting near to aligned and even small discrepancies between the mark and the point of the centre become visible. It's also very quick to perform and typically only takes a minute or two to get set up after you've made the punch mark. For me, getting the punch mark in the right spot then becomes the only difficult part of the procedure.
> 
> Edit to add: If you want a more precise location, this method should get you to within a couple of thou quickly and you can use your indicator from there to get spot-on.



I find the easiest method is to use two lathe centres. Place the point of the 1st one in the centre pop, then place the point of the 2nd one in the recess  at the back end of the first one, then place the 2nd one in the tailstock barrel.  Now use a DTI on the left hand end of the 1st one.
Jack


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## Captain777 (Mar 30, 2019)

XD351, Very good video.  I also watched "mrpete222" (Tubalcain) on Youtube, video #160.  I think that they both answered some of my questions.  Probably my first problem was keeping the "non moving" jaws too tight, causing the brass bar to 'slightly jump' as opposed to just sliding into position when moving the jaws.  This probably made the DTI readings jump. 
Thanks for all the help.
Dan


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