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Does my lathe turn concave?

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steamer:
I was asked by Vixen and Per  "How do you measure the squareness of your cross slide ( Z axis) to the axis of the spindle?  and Why should it be slightly concave?  (  Something like .0005" in 12" diameter is the specification for most lathes  Always concave!   Never convex.


The reason a lathe should be set up this way is to accommodate for lathe deflection as a function of tool cutting force pulling on the spindle.  You always want to faced surfaces to touch at the OD when mated, and that is how it's done.      measuring it is not hard at all!

You need a chuck  ( 3 jaw or 4 jaw...doesn't really matter but I find 3 jaws work best for this)  and tenths reading indicator, and a parallel that you can trust to be straight   Take the time to check it on a surface plate and make sure it's good!...or it will throw you off.

I put the parallel in the chuck sitting on 2 jaws and clamped gently with the third.  Don't crush it.  just tight enough to stay put while rolling the spindle over by hand.

Now roll your x axis out as far as you can,and mount the indicator on the cross slide     Bring the indicator up against the straight edge, and while gently rotating the spindle,  and lightly tap the straight edge until you get a zero-zero reading showing that the straight edge is now square to the axis of rotation of the lathe.    Easy enough.

Now

lock you z axis tight, no movement of the saddle at all,   and zero your indicator against the straight edge.    With the straight edge in the horizontal position ( 3 and 9 oclock)   slowly advance your x such that indicator is traveling along the straight edge, towards the center of the spindle rotation.    As you proceed to center you should see a positive deflection of the indicator with a ratio of .0005"/12" diameter.   Meaning that for a 12" lathe, and a cross slide travel of 6" you should see approximately .0005" positive deflection.  With Zero cutting force you would make an "inny" or a concave face.   NEVER convex.

For you metric brethren,   that is about 13 microns over 305mm diameter.   

It works, and is really easy to measure.   

Dave


Fixed the typo

Vixen:
Thanks Dave,

Sounds so easy to measure, when you know how.  :ThumbsUp: :ThumbsUp: :ThumbsUp:

Mike

steamer:

--- Quote from: Vixen on August 22, 2022, 10:52:44 PM ---Thanks Dave,

Sounds so easy to measure, when you know how.  :ThumbsUp: :ThumbsUp: :ThumbsUp:

Mike

--- End quote ---

It really is.    Caution!    the spindle itself needs to be properly aligned with the lathe before you do this test.    The spindle should point up .0005"/12", and also point forward .0005/12"...for very similar reasons...deflection.       I measured mine when I rebuilt Samantha Bell with a 1" test arbor 12" long with a 3MT shank.    As I got the lathe it pointed down and to the rear .004"/12!!!!!!  and as it had never been off the bed, it came that way from SB!    So they didn't always get them right!.   

Now the $64 question is how do you align your Z or you headstock ?     You scrape the V's in the direction you want them to move.    I did one cycle of scraping to rotate the saddle and the headstock which means opposite sides of the V's .    Then I would do 2 cycles of scraping for bearing,   Then if required scrape for alignment again.

What I found was that both the headstock and the saddle were REALLLLLLLLLLLLY sensitive to scraping for rotation, IE  a very light pass will make a big change in position and I blew right past the mark on the saddle once, and had to go back!....

PLEASE  PLEASE.....don't try scraping your lathe unless you know what you're doing....

Jasonb:
Dave, your measurements show that the cross slide will move at a very slight angle to the lathe axis with the dti mounted

What effect will cutting under load(s) make to the reading as there will be forces pushing the top and cross slide away from the work and there may well be uneven wear in an old lathe that could give more of a dome  or trumpet than a conical concave surface as you are not measuring point to point which the straight edge does?


"As you proceed to center you should see a positive deflection of the indicator with a ratio of .0005"/12" diameter.   Meaning that for a 12" lathe, and a cross slide travel of 6" you should see approximately .0005""

Should that not be 0.00025" over a 6" distance?

steamer:
Cutting forces tend to turn the spindle towards convex...especially if it's older and worn.  The same can be said of a rough facing cut turning the saddle.

The old saw is rough a facing cut inward, and a finish facing cut outward.....is probably based on the above.

A good read on the matter is "Connolley    "Machine Tool Reconditioning"

And no    .0005/12 diameter is the spec   

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