CNC Turret Alignment ... Checks and Tricks

In this post we are continuing our series dealing with the aftermath of that inevitable visit to Crashville that we all make in our production lives. Our last post dealt with the spindle alignment ... this post we're going to take a look at turret alignment. Oftentimes these repairs cannot be accomplished without professional assistance ( and their tools ) ... but there are some repair options available to the layman.

First ... lets take a look at the types of misalignment that can exist.

Turret Body Misalignment :

How to Diagnose :
(1) Mount an indicator to the chuck face or spindle nose face ... with the indicator point against the face of the turret as shown in the illustration below.
(2) Move the X axis up and down and check the amount of deviation along the face of the turret where the indicator is touching.

Repair :
This type of misalignment is very similar to the headstock misalignment we discussed in the last post. As the headstock is usually bolted onto the base ... so the turret assembly may be bolted onto the X axis slide. As the illustration below shows ... turret misalignment that is detected during the diagnosis ... can be fixed by loosening the hold down bolts as shown and shifting the turret around till the alignment is repaired ... very similar to the headstock alignment procedure.

Some turret bodies may also be pinned to the base using taper dowel pins. These pins would need to be removed ... hold down bolts loosened and the turret aligned ... and then the taper pin holes in both the base and turret body should be reamed in line until "clean" ... and then the taper pins replaced. Not replacing and realigning the taper pins will result with the turret body being simply bolted to the base ... and will shift with even the slightest nudge ... not a stable situation and not recommended. No matter how tight you tighten the hold down bolts ... if the turret is equipped with taper alignment pins ... those should be repaired and restored.

Rotary Turret Misalignment :

How to Diagnose :
(1) Mount an ID tool holder on the turret
(2) Mount an indicator to the chuck face or spindle nose face ... move the X axis so that indicator is sweeping the ID of the ID tool holder.
(3) Adjust the X axis so that the two points parallel with X axis slide movement read 0 ... in other words, bring the ID tool holder to the centerline of the spindle.
(4) The remaining two points and their indicator reading will illustrate the amount of rotary misalignment in the turret.

In the illustration above ... looking through the spindle and onto the ID tool holder. Points #1 and #2 can be aligned to zero by adjusting the X axis ... as they are parallel to the X axis "slant" or movement. Points #3 and #4 will reflect the amount of rotary misalignment ... they cannot be adjusted but require a physical repair.

There are a variety of ways the turret mechanism may work ... and therefore the type of physical repair required to realign Points #3 and #4 can vary significantly. The most common method involves a curvic coupling mechanism. One half  of the coupling is mounted on the turret mounting body ... and a meshing coupling is mounted on the back of the turret face. When the turret indexes ... the turret face unclamps from the matching coupling by moving forward ... the turret indexes to the desired location ... and the turret clamps as the coupling faces mesh together ... and confirm the rotary turret alignment. When you visit Crashville ... usually when the turret is in the clamped position ... the turret rotates pulling the both halves of the curvic coupling mechanism with it. So now that even though the turret is clamped ... the curvic coupling has been rotated and the rotary alignment is now off. So even though the turret mechanism appears to work correctly ... unclamps - indexes - clamps ... both halves of the curvic coupling have been rotated as a pair ... and the alignment is off.

This image illustrates the curvic coupling mounted on the turret mounting body.

Oftentimes the coupling on the turret face can be accessed on the front face of the turret ...
but most often the turret face will need to be removed to access the
matching side of the curvic coupling.

repaid of the curvic coupling is best left to a professional ... as most times the turret must be removed from the body to access both side of the curvic coupling. The repair usually entails :

1. Removing the turret face from the turret body
2. Removing the taper pins on both sides of the curvic coupling ... face and body
3. Slightly loosening the hold down bolts for the coupling halves
4. Remounting the turret face and physically moving the turret ... until the correct ID tool holder alignment can be achieved.
5. Removing the turret face again
6. Reaming all taper pin holes in line to clean
7. Replacing the taper pins
8. Tightening up all coupling hold on down bolts
9. Reassembly and recheck.

As you can see ... best left to a professional with the correct tool-set.

Tricks of the Trade ... 

Rotary Turret MisAlignment Work Arounds

But fear not ... there are some work arounds ... not very professional, but work arounds ... to get around the turret rotary misalignment issue.

What effect does rotary axis misalignment have on my machining?

Rotary turret misalignment will show up in many different machining scenarios :

1. Facing ... you will leave that nub at the spindle centerline as the turning tool tip is either above or below centerline.
2. Grooving - Cut-Off ... since the edge of the grooving tool is either above or below centerline ... grooving and cut-off operations do not machine easily with lots of tool rubbing and poor cutting. Cut-Offs will also leave the infamous nub at centerline.
3. Drilling ... since the drill tip is not on centerline, holes are oversize and in the case of a carbide insert type drill ... machining is just not good at all with lots of rubbing and poor cutting.

Work Arounds For a Temporary Fix?
There a couple of options we have discovered through experience over the years that can get you by in a pinch ... until the repairman shows up. Not guaranteed ... but past experience shows that they can work ... but the problem should be corrected properly as soon as possible.

1. Turning - Grooving - Cut-Off Tools : Take an OD tool holder and machine the slot where the tool mounts to open up the tolerance. This will give you room to shim the tool in the holder to bring the tip of the tool onto center. We oftentimes made a holder that was oversize in both directions and kept it hanging around. When it was needed ... we could pull it out and it gave us the ability to shim the tool in both directions ... depending on which way was needed at that particular time.
2. Drilling - Boring : This fix requires a little bit of work but is a good accurate fix. We would machine new bushings for the ID tool holders ... first roughing them out on another machine either manually or with a CNC machine. To finish the hole in the bushing ... we would mount an adjustable boring bar in the chuck of the misaligned machine ... mount the rough bushing in the ID tool holder in the turret ... and finish machine the bushing ID by feeding the bushing / tool holder over the boring bar mounted in the chuck. This will insure that the hole in the bushing in line with the centerline of the spindle. A little bit of work ... but quite easy for a professional machinist. 

So there you have it ... your "crash" course in CNC turret alignment and repair. Of course ... we always recommend that you employ a professional to repair the effects of your visit to Crashville professionally and correctly. But living in the real world we know that sometimes you just can't wait. We hope that the points and tips mentioned here can assist you in keeping your CNC lathe Making Chips ... and profits !!

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Until next time ... Happy Chip Making !!