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 !!

CNC Lathe Headstock - Alignment Check and Adjustments

We've all been there ... Crashville. It's not a place you want to visit frequently ... but inevitably, we all make a visit. When I first started in CNC, one of my mentors told me "If you don't bump it once in a while ... you're not experimenting ... not trying new approaches ... and not using it to it's full potential." Well ... I'm not sure about that but there is a little truth in the concept.

Once you visit Crashville, you may notice some cutting errors and quirks developing in your workpieces. We are about to embark on a new series here in our Making Chips blog ... dealing with some of those unintended consequences of your visit to Crashville.

The first ... tapers developing on your CNC lathe when turning or boring ... the result of your headstock being bumped and not being square to the X axis ways. The result is that as the turret rides on the machine axis ways ... and the headstock and ways are not "square" ... you will be machining a taper. The amount of taper is the result of the amount of the mis-match between the headstock and the ways. It's important to remember that the ways ( base ) of the machine and the headstock are not one-piece ( normally ) ... the headstock is bolted onto the base. The illustrations below will give you a better idea.

So in our first post in our series ... we would like to point you in the right direction and give some tips on re-aligning your headstock.

As the above pic and notes convey, the headstock is normally bolted onto the machine base ... making the X / Z ways and the headstock independent of each other. Normally ... there are alignment screws on the headstock assembly that allow you to move the headstock and thus align it "square" to the machine ways. When you visit Crashville ... oftentimes one of those un-intended consequences is that that alignment is off because the headstock may have moved. So how do you get the correct alignment back?

There are a couple of methods ... let's start with my favorite ... the one I consider the simplest and the one I used most in the field.

First ... you'll need a piece of stock. Qualifications? You need a good material, easy to machine yet with the ability to produce a good finish. I didn't like using aluminum ... I preferred some grade of steel like cold rolled or similar. We want to insure that everything we do is reflected in the material ... not the workholding. So it's best to use hard jaws on the chuck ... and you want to make certain that the chucked material is not flexing ... so the material diameter to overhang factor should be appropriate to insure that the material isn't flexing when you're cutting. You also want to have a good length sticking out of the chuck ... after all the longer the area to measure the better your readings and the better your adjustments. Yes ... there are a lot of factors to consider here ... but you're a machinist !!! You know what to do and what is appropriate.

It's important to note here also what may be obvious ... don't use the tailstock. We don't want any mis-alignment in the tailstock to reflect in our measurements.

Next ... chuck up the material and clean up the stock by cutting the material the entire length. Take whatever cuts you need to clean up the stock ... just make sure the last cut is a nice finish cut and leaves a nice finish. Usually using MDI or the job / feed manual options are the best method. Creating a program is a little overkill and using the handwheel may result in an uneven cut and finish.

Now measure the diameter at the furthest and closest points to the chuck along the turned diameter. Not the same? That's the reflection of your headstock mis-alignment.

To adjust ... you'll need to find those adjusting screws on the headstock ... the above illustration might shed some light on where they might be and how they work. You will need to slightly crack the bolts holding down the headstock body to it's base ... then use the adjusting screws to move it in the direction you feel you need to move it to re-align it. Tighten everything back up ... and take another skim cut on the material. Repeat and re-adjust as necessary until the results are to your satisfaction. What should that be? As close as you can get it. If the material length in the chuck is short ... it really needs to be spot on because obviously the error will get magnified on a longer piece of material. As with everything you do as a professional machinist ... do it to the best of your ability.

One valuable hint : Place an indicator somewhere on the headstock to measure the amount you move the headstock with the adjusting screws. This will help you understand the relationship between the amount of movement with the amount of taper correction.

Another method which some people prefer ... instead of using a piece of stock and turning the diameter ... they will use a test bar. A test bar is a piece of stock that has already been machined and usually ground ... it's perfectly straight and true. They mount it in the chuck ... indicate it in ... and then use an indicator mounted to the turret which they then run back and forth along the test bar in Z as they adjust the screws on the headstock. This method works fine also ... but you need a qualified test bar to start ... and there are more variables that come into play. Is the bar indicated in and running true? Is your indicator on the turret reflecting the actual center of the test bar ... etc.. For my liking ... too many other variables ... and a piece of stock is simpler, more readily available and cheaper.

So as you can see ... this repair is not that hard ... a little time consuming ... but the result will leave you with a more accurate machine tool and a lot more money in your pocket ... the amount you'll save in a repair bill.

Hope this helps you recover from your inevitable visit to Crashville ... and insures you are MAKING CHIPS ... good chips ... for years to come !!

Happy Chip Making !!

As always ... don't forget check out all our Real World machine shop software at our website :

www.KentechInc.com