# Confusion With Machining A Blank On A Chuck Mounted To Rotary Table



## Machinehead (Mar 19, 2016)

It's a simple operation of milling a .250 diameter out of .375 round stock and putting a hole in the center after machining the diameter. I have my rotary table centered in the mill table, with my X,Y zero set on the dro. Then, I indicated in the 3 jaw chuck to the same zero. After I machine the diameter true to the rotation of the table, shouldn't my hole go directly on center to that diameter when I go back to zero? It appears off center, and I thought it would be good to the outer diameter after machining. Am  I missing something?


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## Richard White (richardsrelics) (Mar 20, 2016)

Depends.  First off, I have to ask so I apologize, did you indicate the rotary table? You state it was centered.  Also there are some things you may not realize.  This is easiest done in a lathe, but it can be done in a mill.  You have to indicate the rotary table, taking into account WHICH direction you last moved the table to get t either the X or Y axis to zero. This is because if you don't, the slop in your table will NEVER allow you to get the hole back in the center. I say never, but I digress, everyone gets lucky once in a while. Sorry for babbling, but in my mind I can do this in my sleep but to describe it in words that are easy for others to understand can be challenging. Once you see my description you will understand why this is best done in a lathe..

Ok, so you must indicate the rotary table, you have to know which direction you moved the table last as that is the direction with NO slop. This needs noted for the X and the Y axis.
Then mount the 3 jaw must be centered to that same point AND with the table being last moved in the same direction as the rotary table. That is gonna be tough.


Let me change the entire course.... Try this...

Buy a piece of .25 stock, using a small V-block, clamp it in a good strong vise mounted to your table, Indicate the .25 stock to zero, lock the table from movement,  you will be centered, then drill your hole, done... You will have so much hair left over....hope this helps...

(EDIT)And if you don't have a good V-block or a vise, use your 3 jaw chuck clamped to the table and indicate the .25 material in there, will work just as well.


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## TommyD (Mar 20, 2016)

Got the backlash going the same way when you come back to your zeros?


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## Ed ke6bnl (Mar 20, 2016)

Not in your guys class, but it seems to me if he has a DRO won't that compensate  for slop in wheels.


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## RJSakowski (Mar 20, 2016)

A chuck mounted on a rotary table will more than likely have some runout.  

When I machine a cylinder using the mill with a rotary table, I get an approximate center and offset the table to mill an oversized cylinder.  The cylinder will now be true the the RT axis.  Indicate the cylinder in the x and y directions to determine the RT axis coordinates and set your DRO to coincide.


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## Bill Gruby (Mar 20, 2016)

I hesitate to ask this, but why are you putting yourself thru all this when all you have there is a job for the lathe. Info so far is all good but you are, IMHO going about it with the wrong machine. Chuck it up in the lathe, turn the OD and bore the hole., done deal.


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## Wreck™Wreck (Mar 20, 2016)

RJSakowski said:


> A chuck mounted on a rotary table will more than likely have some runout.
> 
> When I machine a cylinder using the mill with a rotary table, I get an approximate center and offset the table to mill an oversized cylinder.  The cylinder will now be true the the RT axis.  Indicate the cylinder in the x and y directions to determine the RT axis coordinates and set your DRO to coincide.


One may find the center of rotation of a rotary table mounted on a mill by indicating the convenient center hole provided for this purpose amongst other uses.
Many have a hollow spindle with a reference diameter produced during manufacturing, can't get much closer to center then that.

I have a Walter 18" Horizontal/Vertical with a vernier dial (1 minute resolution)  that is in excellent shape that I would like to unload at a very nice price, with a wheeled storage cart included, it weighs 300+ pounds.
Looks exactly like this.
http://www.maschinensucher.de/bilderuploads/1074679.jpg


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## Andre (Mar 20, 2016)

When using a chuck on a RT, I indicate the work true to the axis then center the spindle over the work I just indicated in. This way I'm not relying on the inside hole of the RT for a reference. Plus, Import 4" RT's don't have a bored hole in the center, rather it's threaded M6 (I think, I chased it with a 1/4-20 UNC tap for convenience)


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## Machinehead (Mar 21, 2016)

Bill Gruby said:


> I hesitate to ask this, but why are you putting yourself thru all this when all you have there is a job for the lathe. Info so far is all good but you are, IMHO going about it with the wrong machine. Chuck it up in the lathe, turn the OD and bore the hole., done deal.



Right now I don't own any tool bits to create a sharp inside corner, so I needed to use an end mill on the RT to get it.


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## British Steel (Mar 21, 2016)

You own a grinder, right?

Dave H. (the other one)


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## Machinehead (Mar 21, 2016)

British Steel said:


> You own a grinder, right?
> 
> Dave H. (the other one)



No HSS blanks. 

Sure I can do the part on a lathe, but my question was referring to rotary table centering, not grinding lathe tools.


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## Bill Gruby (Mar 21, 2016)

PM sent

 "Billy G"


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## Machinehead (Mar 21, 2016)

Ed ke6bnl said:


> Not in your guys class, but it seems to me if he has a DRO won't that compensate  for slop in wheels.



Actually, this makes me wonder if the table moves after, or while I machine the outside cylinder. If I indicate the part and set zero, machine my cylinder then go back to zero, the table (x,y zero on the DRO)  could be in a different spot after the tables backlash is taken up from the cutting forces?


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## BGHansen (Mar 21, 2016)

I use a test dial indicator on the spindle of the mill to find the center of the rotary table sweeping the center hole of the table.  Zero out the DRO at that point.  The chuck for my RT is loose, screwed to a ground plate of steel.  It's mounted to the RT with a couple of T-nuts/bolts to the slots in the RT.  I set the chuck/mounting plate on the RT and roughly center them to the mill spindle with the T-nuts/bolts loose.  I then put a 3/4" dowel pin in a collet in the mill spindle and advance it down to and an inch or so into the chuck.  Then lock my mill spindle and tighten the chuck onto the dowel pin.  Then tighten the bolts that clamp the chuck/mounting plate to the RT.  For grins and giggles I'll sweep the dowel pin in the chuck and am never worse than 0.002" out.

If you have an RT with a chuck mounted to the spindle through an arbor, your chuck should be reasonably centered on the RT.  You should be real close taking a dowel pin in the spindle collet and run it into the chuck like I do.  Then tighten your RT to your mill table.  You may have to do some table movement to get the mounting slots on your RT lined up with the slots on your mill table at the start.

Boy, I'd sure think if you zero'd your DRO once you found the center of the RT, backlash should be out of the picture as long as you're locking your mill's table down at 0,0 before machining.  Your DRO tells you where the sliding detail of the glass slide is within the glass slide itself, not the position of your handwheels.  Unless you have ball screws on the mill, you can probably chuck your table back/forth because of the slop between the table screws and table nuts.  That'll show up on your DRO since the glass slide .  I'd think you'd be good zeroing the DRO once you find center and make sure to lock the table before cutting.

Not to belabor it, but Tom Griffin of Tom's Techniques (google it) has a nice tutorial on centering the RT to the mill.  He uses a tapered plug in his mill spindle.  Advances it to the hole in his RT until it wedges, locks the spindle and bolts the RT to the mill table.  I recall him showing he was within 0.0005" using that technique.

Good luck!  Bruce


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## Bill Gruby (Mar 21, 2016)

The center bore on my RT is a MT2, Last I checked it, it was within .0003 of  the spindle. My mill spindle is also MT2. I just put a dead center in the mill taper and bring the table up till the dead center catches tight. Tighten the hold down bolts and you as close to "0-0" with the Spindle Center as you will ever have to be with the RT. Takes all of 30 seconds.

"Billy G"


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## stupoty (Mar 21, 2016)

Machinehead said:


> No HSS blanks.
> 
> Sure I can do the part on a lathe, but my question was referring to rotary table centering, not grinding lathe tools.



You could grind a broken centre drill their hss, use them a lot for booring bars.

Did you indicate the rotary table then mount the chuck?

I tend to not indicate my rotary table but use a pin holder that goes in the centre hole. And put the pin in a collet.  Most of my rotry work gets bolted to the table not in a chuck though ao thats less handy for you I guess.

If your cutting ally any old steel that is vaguely hard and cutter shaped will cut it in the lathe, I forgot to harden a tool steel bit once, used it for a couple of turning jobs an ally then tried to turn some steel and was confused as to where the point went 

With regard to getting the hole on centre I would do the hole first then crank out to do the outside, no point trying to refind the centre or if you have to do it that way re indicate it.

Stuart


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## Richard White (richardsrelics) (Mar 21, 2016)

Well then drill your hole right after you indicate the cylinder, THEN go out and run around the outside....That should resolve this issue


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## Jester966 (Apr 2, 2016)

Ed ke6bnl said:


> Not in your guys class, but it seems to me if he has a DRO won't that compensate  for slop in wheels.



I just finished installing scales on the Y- and Z-axis of my own mill tonight (X-axis soon), and was initially very disappointed with poor repeatability of these I-Gauging scales.  After some head scratching I figured out the problem, and then immediately thought of this thread.

What I believe he is seeing is like "backlash" between the table and the scale - there is too much flex in the brackets, allowing the scale to lag behind when the table changes direction.  Verified with indicators, for me I'm getting about .005" on the Y-axis and about .012" on the Z-axis.

Solution is, as with any other backlash, to always approach from the same direction.  So in this case when returning to your zero, pass it and come back from the other direction.

Better yet, improve the brackets.


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## British Steel (Apr 3, 2016)

What are the brackets like? When I used to fit scales and heads on machines (several times a week, but still to do it on my own!), I used to make up new brackets every time as the supplied ones were a) pretty bulky and b) multiple parts bolted together and c) looked naff.
If the scales aren't absolutely parallel with the ways, the reader head will drag and the friction will try to move them or the reader out of alignment, leading to backlash and in the worst cases, broken scales and readers...

 To mount the scales I'd use a couple pieces of sturdy (e.g. 10mm) plate, tapped for the scale's fixing bolts, with counterbored holes for its fixings to the lathe/mill and threaded holes for dog-point grubscrews at the corners to a) adjust the plates parallel to the ways and b) pierce the quarter-inch of loose filler and paint found on far-eastern machinery.
When setting up, before attaching the reader head put a DTI on the carriage / table and sweep the length of the scale both vertically and horizontally aiming for (ideally) no deviation end-to-end, maximum of a few thou" - the corner grubscrews make this pretty easy to do, along with the slotted ends of the scales 

For the readers, again there's no substitute for sturdy mountings, and parallelism's just as important! Leave the transit spacers in place while adjusting, and adjust the bracket to the reader, not t'other way around!

Dave H. (the other one)


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