# Question I always wanted to ask.



## sigp2101 (Dec 9, 2013)

How do you center the center of your radius into the center of the rotary table that is centered to the spindle?

Here is my problem, trying to cut radi of about 6mm in small peace of the metal.
First I want to mount four jaw chuck to the rotary 4" table, has to be perfectly centered. Then all that has to be centered to the spindle, then I have to mount part and indicate it to the center of the table and chuck at the same time, then bring center of the radius to the center of the spindle to be able to make offset of 3mm. What would be proper logic procedure?

Oppps I just realized it is wrong location for this post. Can mods move it into appropriate location?


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## Tony Wells (Dec 9, 2013)

The 4 jaw does not have to be perfectly centered. Eyeball it, or scale from the table edge, just get it close. Chuck the part to be machined and indicate it true by using the 4 jaw. Spin the rotab to do this. After the piece is running true, put your indicator in the machine spindle and sweep the part true. Then offset the radius of the cutter and your desired cut radius. If possible, leave a little stock for finishing and do it in a couple of passes.


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## sigp2101 (Dec 10, 2013)

Tony Wells said:


> The 4 jaw does not have to be perfectly centered. Eyeball it, or scale from the table edge, just get it close. Chuck the part to be machined and indicate it true by using the 4 jaw. Spin the rotab to do this. After the piece is running true, put your indicator in the machine spindle and sweep the part true. Then offset the radius of the cutter and your desired cut radius. If possible, leave a little stock for finishing and do it in a couple of passes.



Thanks for that input it is greatly appreciated. 
I am kind of still confused, you are explaining what needs to be done rather than how. 
I would like to know how to indicate irregular shaped part to the center of the roto table that is obstructed by 4 jaw sitting between part to be machined and rotatable. How do I put that center of my radius over center of the rotatable. Usually if there is a hole in the center of the radius it would be easier but I do not want hole there. Just can't picture it in my head. Not even sure if I am explaining myself right.

SIGP2101


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## Tony Wells (Dec 10, 2013)

Well, you must have some point of reference for indicating the part to the x,y center of the rotary table. It doesn't have to be a hole, or a boss....depending on how accurate you need this to be. Let's say it's just a simple layout crossed line. Then you will need a wiggler with a point, or at least a sharply pointed pin in the spindle. 

It kind of all happens together. Imagine the pointed object (wiggler or whatever) being 1/4" off the x,y center. Rotate the part held in the chuck. The arc, or circle described by the point would be .25R or .50D. No matter what you do with the part, adjusting the jaws, nothing you can do will bring it to a zero radius or diameter, because you are off center of the rotary table. But if you start out just dialing the x,y table to the layout lines, then rotating the table, you will see how far off you are by means of the chuck grip.

Now you could maybe get a head start by simply putting a pin in the 4 jaw and indicating it true with respect to the rotary table, and then sweeping the pin true with the spindle. This will give you an accurate x,y location of the center of whatever arc you wish to cut. After that, put the part to be machined in the chuck, and go to the pointer and finding the center of the desired arc, whether a pin, hole, center punch, or simply a crossed line. 

Maybe I confused you more. I hope not. Let me know and i can try again!


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## Cobra (Dec 10, 2013)

My normal order would be to centre the rotary table to the spindle and then mount the chuck to the table and centre it to the spindle.
install your part and again centre to the spindle or the chuck, which ever is easiest for you.  You can then move the mill table to establish your radius.


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## sigp2101 (Dec 11, 2013)

So today i started to machine indicator bar that will fit snugly into center opening of the rota-table. Bar will have pointy tip on both ends and upper end will be used to indicate center to the spindle of the mill. At the same time will be high enough to protrude through 4 jaw chuck. Once it is centered indicator bar will be lifted and mount into spindle and its pointy end will be used to assist in mounting working part to the center of desired radius. This will eliminate compound error effect of indicating every component separately but will it be accurate enough is to be seen.    

I hope I am on the right track. Thanks to all who contributed so far.


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## ScrapMetal (Dec 11, 2013)

Here's what I do to get the rotary table situated on my mill.

First of all, my rotab has hole in the center measuring 1".  I turned a "slip fit" 1" stainless round x 4" long on my lathe.  When I am ready to line up the rotab on the mill I use a 1" endmill holder on the mill.

I place the 1" stainless shaft in the endmill holder without tightening it in so that it can slide in and out easily.

I crank up the mill table so that the face of the rotab is about an inch from the mouth of the endmill holder with the stainless shaft, in the endmill holder, resting on the face of the rotab.

Now I just adjust the X/Y of the mill until the stainless shaft slips in to the hole in the center of the rotab.

At this point it's very close to where it should be so I let the table down and remove the stainless shaft.

I grab my indicol holder, install it on the mill and use a dial test indicator to get it "just right".

It sounds like a lot of messing around but it actually goes quickly.

-Ron


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## chuckorlando (Dec 12, 2013)

Could you chuck up a piece of round stock thats center drilled. True it in the 4 jaw with a dti. Then use a wiggler to center the chuck to that hole? Center punch your work and use the same wiggler to center the punch mark to the head


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## sigp2101 (Dec 12, 2013)

ScrapMetal said:


> Here's what I do to get the rotary table situated on my mill.
> 
> First of all, my rotab has hole in the center measuring 1".  I turned a "slip fit" 1" stainless round x 4" long on my lathe.  When I am ready to line up the rotab on the mill I use a 1" endmill holder on the mill.
> 
> ...



That is pretty similar to what I was thinking. Like that approach. That is sure way to center rotatable to the mill spindle. Now continuing from there, 4 jaw chuck has to be affixed to the rotatable and object's radius center trued to the center of the rotating table.  This is the part I am difficulties with. Having 4 jaw chuck, that holds the part, between rotating table and part itself  you just lost your reference point. Cant see it, can't access it. But you have one in the spindle of the mill if you didn't move mill table at all.

- - - Updated - - -



chuckorlando said:


> Could you chuck up a piece of round stock thats center drilled. True it in the 4 jaw with a dti. Then use a wiggler to center the chuck to that hole? Center punch your work and use the same wiggler to center the punch mark to the head



Kind of see where are you going with this but not 100% sure I understand all. Can you elaborate please.

THANKS!


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## chuckorlando (Dec 12, 2013)

Take some round solid stock, turn it so it's true. Center drill it. Then place it in the 4 jaw. Set the indicator on the bed touching the part. Rotate it 360* adjusting the run out. Once this is done you should the center of the 4 jaw or what ever your using should be square to the center of the table. Might even dti the jaw it's self. If the jaw has no or little run out as the table is rotated 360 then it pretty well has to be centered on the table. The spindle would also be pretty square to center as you use the wiggler in a centered drilled scrap piece that was already dti to center in the chuck that was dti to center of the table. So when you put your part in you lower the spindle to just above the work, and dial in the 4 jaw so the center punch mark is square to the wiggler or scribe point or center that your using.

dti (drop test indicate) jaw to table.
dti scrap part to jaw both of these two are side readings as it rotates
use a wiggler to center the head over the centered hole.
center punch your work
line mark on work up with the centered center in head


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## ScrapMetal (Dec 12, 2013)

sigp2101 said:


> That is pretty similar to what I was thinking. Like that approach. That is sure way to center rotatable to the mill spindle. Now continuing from there, 4 jaw chuck has to be affixed to the rotatable and object's radius center trued to the center of the rotating table.  This is the part I am difficulties with. Having 4 jaw chuck, that holds the part, between rotating table and part itself  you just lost your reference point. Cant see it, can't access it. But you have one in the spindle of the mill if you didn't move mill table at all.



I suppose you could use the same method I talked about with one little caveat, make the 1" (or whatever is appropriate) steel shaft long enough so that when it sits down in the rotab hole it is long enough to stick out of the chuck when the chuck is placed on the rotab.

So what you do is to set up the rotab just like my first post.  Get it dialed in perfectly.

Now, lower the mill table and place the longer shaft back in the rotab.  You can get the chuck close to center on the rotab and then once again use the indicol/dti to fine tune/center the chuck on the rotab.

Once that is done you can center the "part" you are working on using the indicol/dti setup.  If the whole mess, rotab/chuck/part, gets off center because you accidently turned your X or Y (been there, done that :banghead: Hey, I was grabbing for the Z and the Y was real close...) you can always re-center everything by going off the outside of the chuck body.

Hopefully that makes some sense 

-Ron


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## sigp2101 (Dec 13, 2013)

More I think about it more I am convinced that solid sturdy indicator bar as explained is way to go.
With this solution you can go quick and dirty or fine tune it just after as continuation of the same process.
Producing such bar accordingly to size of the equipment in each shop is right solution. This bar could be used in conjunction with dead center for roughly centering center mark to the center of the spindle that is centered to the center of the rotary table, right?:goodjob:


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## chuckorlando (Dec 13, 2013)

I must be missing the huge flaw to what I said that would call for, aw well never mind.

So being I'm new, tell me the huge flaw here. Seems to me checking run out from the table to the 4 jaw would center the 4 jaw. Then getting rid of the run out in the part would make the part true with everything under it. Right up to the tool. Our instructor makes a huge deal of everything being nuts on. From the table, up the vise, and so.


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## Tony Wells (Dec 13, 2013)

Chuck, the only thing I will say is that I can offset the chuck 1/4" and indicate a part true to the table, then the spindle.....and then cut whatever I want.


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## ScrapMetal (Dec 13, 2013)

sigp2101 said:


> More I think about it more I am convinced that solid sturdy indicator bar as explained is way to go.
> With this solution you can go quick and dirty or fine tune it just after as continuation of the same process.
> Producing such bar accordingly to size of the equipment in each shop is right solution. This bar could be used in conjunction with dead center for roughly centering center mark to the center of the spindle that is centered to the center of the rotary table, right?:goodjob:



That sounds right.  :thumbzup:




chuckorlando said:


> I must be missing the huge flaw to what I said that would call for, aw well never mind.
> 
> So being I'm new, tell me the huge flaw here. Seems to me checking run out from the table to the 4 jaw would center the 4 jaw. Then getting rid of the run out in the part would make the part true with everything under it. Right up to the tool. Our instructor makes a huge deal of everything being nuts on. From the table, up the vise, and so.



One error (in my opinion anyway) is indicating off the "part" to center the rotab/chuck.  "Getting the run out in the part would make the part true with everything under it." would only be true for that particular part.  As the part itself could be out of true or just some odd shape I would think trying to center from it wouldn't be real reliable.  Another problem is that if the 4-jaw isn't centered on the rotab it might not even be possible to remove the run out with the part as the part could never be center to the rotab.

There may (and probably are) other/better ways to handle this but it makes more sense to me if a person center/squares up the work holders - rotab and chuck, so that you have a "known" point to start from.

-Ron


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## chuckorlando (Dec 14, 2013)

I under stand what you guys are saying. And I see how your getting to the end result. We would never indicate the roto to the spindle though. We would use the spindle to hold an tdi to run the edges for square and work up. The last thing we would even consider is the tool to part. We would square the table to head, roto to table, jaw to roto, part to jaw, and then the tool to part. At least I think we would. 

Admittedly your way seems more straight forward. I'm gonna ask the teacher on monday how he might do this.

Thanks for the info fellas


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