Author Topic: Does my lathe turn concave?  (Read 1043 times)

Offline steamer

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Does my lathe turn concave?
« on: August 22, 2022, 09:29:28 PM »
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
« Last Edit: August 23, 2022, 08:17:00 PM by steamer »
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Online Vixen

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Re: Does my lathe turn concave?
« Reply #1 on: August 22, 2022, 10:52:44 PM »
Thanks Dave,

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

Mike
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Offline steamer

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Re: Does my lathe turn concave?
« Reply #2 on: August 22, 2022, 11:36:00 PM »
Thanks Dave,

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

Mike

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....
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Offline Jasonb

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Re: Does my lathe turn concave?
« Reply #3 on: August 23, 2022, 07:27:40 AM »
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?
« Last Edit: August 23, 2022, 09:26:09 AM by Jasonb »

Offline steamer

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Re: Does my lathe turn concave?
« Reply #4 on: August 23, 2022, 12:05:10 PM »
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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Offline Jasonb

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Re: Does my lathe turn concave?
« Reply #5 on: August 23, 2022, 12:16:03 PM »
So it's 5tenths deviation for 12" diameter, I was thinking the angle was 0.0005/12" and therefor half that for 6" of movement

Offline kvom

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Re: Does my lathe turn concave?
« Reply #6 on: August 23, 2022, 02:24:17 PM »
It is quite difficult to face so that the center is precise.  You either get a tiny teat or a small depression.  So the concave deflection can be measured only away from the center.  I'd want a fairly large piece of stock to trust the measurement.  With a 6" piece you'd only measure over 3".

Offline Jasonb

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Re: Does my lathe turn concave?
« Reply #7 on: August 23, 2022, 02:33:38 PM »
You are not actually facing anything with Daves method, just clocking against a straightedge.

That was my query as there is likely to be a difference between does my lathe turn concave and does my lathe clock concave.

Confirmed by the likely deflection of toolpost, top slide, cross slide and carriage with wear, material being cut, feed, DOC, tool type, build quality, etc thrown in.


Offline Mcgyver

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Re: Does my lathe turn concave?
« Reply #8 on: August 23, 2022, 04:37:21 PM »
It's easier with everything out of the way.....here's a shot of my DSG during scraping; getting the right alignment to the headstock.  .0005"/12" was the goal.  Always a pita scraping the dovetail's angled surfaces....only good part about it is its done :)


Offline Jasonb

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Re: Does my lathe turn concave?
« Reply #9 on: August 23, 2022, 04:56:04 PM »
Do you rotate the spindle anti=clockwise and take a second reading towards the back of the dovetail with that setup?

Offline steamer

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Re: Does my lathe turn concave?
« Reply #10 on: August 23, 2022, 05:31:14 PM »
It's easier with everything out of the way.....here's a shot of my DSG during scraping; getting the right alignment to the headstock.  .0005"/12" was the goal.  Always a pita scraping the dovetail's angled surfaces....only good part about it is its done :)



Nice shot McGyver!    I rotated the whole saddle!   But that is easier to do than on a DSG!    I also had the opportunity to space the feed screw down, which you might not be able to on that....You "run the table with the hand the dealer gives you!"   
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Offline steamer

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Re: Does my lathe turn concave?
« Reply #11 on: August 23, 2022, 05:37:46 PM »
That DSG saddle probably weighs more than my whole lathe!....   
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Offline Mcgyver

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Re: Does my lathe turn concave?
« Reply #12 on: August 23, 2022, 05:38:02 PM »
thanks...Jason, exactly, sweep the pin then rotate the spindle and sweep it again on the far side

Offline Admiral_dk

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Re: Does my lathe turn concave?
« Reply #13 on: August 23, 2022, 07:33:49 PM »
Thank you very much for explaining this Dave - but I must admit that I really had a hard time following your text .... that is, until I realized that you have swapped the Z and X axes ....

I do Ass'U'Me that we agree that the Z Axis Always is (Edit) along Parallel to the Spindle - No matter if it's a Mill or a Lathe.

Best wishes

Per
« Last Edit: August 24, 2022, 12:08:40 PM by Admiral_dk »

 

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