# Why did the lathe take off 6 thousands when the cross feed was indexed at 2?



## dansawyer (Apr 12, 2019)

The lathe is a South Bend 9 in reasonably good condition. The tool is a freshly sharpened 1/4 HSS. The project is to turn a 1.500 cylinder down to 1.350. The set up is a 3 jaw chuck with a live center. I had turned the cylinder down to 1.355 The turn was clean and symmetrical. All measurements were made with a tenth interpreting micrometer. 
The step I was attempting was to turn off 2 thousands. In order to do this I dialed the crossfeed in to 0. I then moved the apron to align with the surface to be cut and used the compound feed to just contact the work. I then move the apron clear of the cylinder and dialed in the crossfeed one thousand. I then engaged the feed and slowly took the cut. This was clean with no chatter. When the cut was finished I polished the surface with 600 grit to clean it up. 
I then measured the cylinder with the micrometer. It measured 1.349. The process took off 6 thousands. I had planned on 2. 
I am at a loss as to why or how this happened. 
Does the tool bend down with the load and take extra material? For learning I would like to understand this better. How should the the holder and bit be aligned to take a small cut?


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## mmcmdl (Apr 12, 2019)

Spring cut ? How much did you polish off ? Best example would be to rough it down within .010 and take a spring cut , measure , and take final cut allowing for whatever you may polish off . ( If your tool was to bend down , you would be below C/L which would result in less material being cut .) That IS assuming you were on centerline to begin with .


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## mikey (Apr 12, 2019)

Don't know, Dan. Your procedure should have taken off 0.002" off the diameter. I doubt you took that much off with sandpaper. Is it possible that the cross slide migrated during the cut? Did you lock it before cutting?


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## MontanaLon (Apr 12, 2019)

Was your compound angled toward the headstock, toward the tailstock or perpendicular to the bed?

If it is angled toward the headstock the cutting forces will try to push it away from headstock and that will push the whole compound clockwise and the tool deeper into the cut if it has any play in the gib or if it isn't tightened enough.

I know it doesn't seem like it should do it but geometry can be a *****. If the compound is perpendicular or angled toward the tailstock tool movement is out of the cut and you end up taking less than you should. Assuming of course a right to left direction of cut. Reverse everything if you are cutting left to right.

And it doesn't have to move much. You dialed in 2 and it took 6 which means it could have taken as little as 2 thou movement of the compound to put you there. How much wiggle in your compound?


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## lordbeezer (Apr 13, 2019)

Did you do all your turning getting down to 1.355 adjusting compound and cross feed or just cross feed..maybe between doing two adjustments something got alittle off.


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## richl (Apr 13, 2019)

I am curious as whether all the other cuts you made to get to that point were what the compound dial indicated.  
I am not familiar with southbend lathes, if you dial in 2 thousandths are you getting 2 thousandths or 4thousandths?


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## Tozguy (Apr 13, 2019)

dansawyer said:


> used the compound feed to just contact the work.



Was the lathe running?
I have always had a problem getting an accurate zero by visually judging when the tool contacts the work when stopped. 
Even with the lathe running it is very difficult to get back to a true zero setting.
And there are other factors that come into play to take only a .001'' cut. 
For the finest cuts I would lock the cross slide and use the compound set at 30 deg. to dial in the cut. 
If it were me I would not even attempt to cut .001'' and would sneak up on final dimensions with abrasives.


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## MarkM (Apr 13, 2019)

Could be a whole lot of things.  When you touched off you never used as an example rolling papers or something.  How do you know how much you plunged in making contact?  If you left a visible mark no matter how small it is something.  Playing with .001 I think you are asking for trouble.  Leave .005 -.010" for a finish pass with hss. 
You may have been above centre and when you took the cut not cutting enough you were rubbing and cutting somewhat and down pressure put your tool on centre and took off more than you wanted.  If you touch off take a pass under power feed to determine what you plunged touching off before adding more to the cut.  
On tougher to machine material I ll take two finish passes to equal pressure so my last cut is rid of deflection.
I d go and see where your tool is on centre or not.  Take a face cut and see if it leaves a pip.  I bet your not on centre.


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## cathead (Apr 13, 2019)

Some lathes remove the amount indicated by diameter and some by radius.  This means that if one's lathe removes by radius, 
 .002 inch indicated on the cross feed will remove .004 inch from the part.  My Springfield "Ideal" was like that so I made bigger dials with 
twice as many graduations so the marks would read by diameter, a huge improvement.    

I realize that does not account for why your machine took off more than expected but just mentioned as a point of interest.  

Most old lathes(mine in particular) turn cones to some degree due to wear near the head stock area.  I tend to do test cuts and
measuring when getting close to the final diameter.  The last bit of removal is done with a file if great accuracy is needed.  
What a joy it would be if I could rely on the dials only for accurate cuts but I like my old lathes enough to keep them around.

The tool needs to be on center and have the right geometry for light cuts.  A narrow radius tool is helpful too as well as having the
set up as rigid as possible, not to mention a sharp cutter.


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## Technical Ted (Apr 13, 2019)

My speculation is that when you picked up the work piece by "just touching" with the compound, you actually were into the work by .001 or two... It's difficult to "just touch" the piece without actually taking a cut, even if you use layout dye or a marker to mark the work piece. You could have told for sure if you had taken a partial cut after "just touching" the piece without dialing the compound in any more...

One nice thing about using HSS cutters in the lathe is that you can turn off you lathe spindle without disengaging the feed and take a measurement after just starting the cut rather than checking it after the pass is complete. Even with carbide, you can stop the power feed, stop the lathe spindle and then check the diameter before taking the whole cut. I rather have the first 1/8" or so undersize than the whole work piece! Note that if you don't disengage the power feed before stopping the spindle using carbide you are very likely to chip the edge of the carbide. HSS, especially with light cuts, will not chip and can be re-started without issues.

And, I would NEVER polish/file a work piece after a finish cut without measuring it first! The first 0.001" or so will come off fairly easily, but beyond that it comes off a lot harder and takes a while. But, that first 0.001" can disappear quickly! 

Ted


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## wildo (Apr 13, 2019)

Welcome to the hell that is the South Bend 9... Hitting your numbers on this lathe can be a serious PITA. It's an extremely capable lathe given its size, but there's a lot of flex in that machine. All I can say is spring cut, spring cut, spring cut!!  Of course, also make sure that you're using a tool with a very small nose radius if you're trying to take small finish passes like that. But in the end, having worked with my SB9a practically every day for the last 1.5 years: it's springy. I consistently need to take 3-4 spring cuts before the tool stops cutting.


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## projectnut (Apr 13, 2019)

It sounds like you're using both the cross feed and the compound to attempt to hit your target.  Personally I would use either one or the other, not both.  In most cases I use the micrometer dial on the cross feed only and lock down the compound.  First off I would leave a minimum of .015" of material after making the initial cut.  After making the initial cut make a second cut without moving the handle.  This "spring cut" shouldn't remove more than a couple thousandths, but it should assure the tool has removed all the material intended to be removed without going past the target. 

Now measure the stock and see how much still needs to be removed.  If you originally intended to remove .015" and actually removed .017" after the spring cut you can expect similar results on each subsequent pass.  If you need to reposition the tool to take a measurement I world back off the cross feed crank one full turn so you have the same reading on the dial only .100 or .125" further away from the material depending on the graduations on your dial.

Once you determine how much material remains to be removed reposition the dial one full turn to advance the tool to the exact same position it was in when you took the last cut.  Now advance it the amount you wish to remove minus the additional amount that was removed on the "spring cut".  This should get you to the target without going over it.  If your machine has a lot of play in it you may need to make an additional cut.

The key when moving either the cross slide or compound lead screw is to back it off enough to eliminate any play between the lead screw and the nut when returning it to the cutting position.  In most cases backing off the handle a full turn  will be sufficient to eliminate the play and insure the dial is reading the true position of the tool when returning to the cutting position.


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## markba633csi (Apr 13, 2019)

No matter what the dials say you need to develop an eye for material removal. When you have done lots of turning you will be able to "smell" the depth of cut with surprising accuracy
Especially true on a springy lathe like Atlas or SB
Mark


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## mickri (Apr 13, 2019)

As a newby I often run into this same thing.  The cut you think that you are making is different from what actually happens.  And I have never been able to figure out where I went wrong.  Now when I am coming close to the final diameter I never try hit it with a final cut.  When I am at this point I take a minimum of 4 spring cuts and sometimes more until I don't see/feel anything being cut.  I measure after each spring cut. Then I will try a .001 cut and measure after the cut.  No more than this as I sneak up to the final diameter.   I have found that you absolutely can't try to re zero your dials.  You can't remove the work.  Everything has to stay the same.  If you change anything everything changes and you are basically starting from scratch.  It is not unusual for it to take 15 to 20 minutes for me to sneak up on the final diameter.  Even with this if I get within .001 of where I want to be I feel that I have done as good as I can.


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## Mitch Alsup (Apr 13, 2019)

When I need something precise (like what you describe) I rough turn the material down until I need 3 final cuts.
I carefully measure the starting point, divide by three, and dial in the DIV-3 number and take a cut.
I then measure the resulting cut, determine the difference between the dial and the cut, and the amount left.
Then I dial in 1/2 the rest and take the next to last cut.
Measure again, and dial again shooting at 0.001 too big. This allows a spring pass (with my tools) to hit the mark. Or some polishing with paper.


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## Shootymacshootface (Apr 13, 2019)

mickri said:


> As a newby I often run into this same thing.  The cut you think that you are making is different from what actually happens.  And I have never been able to figure out where I went wrong.  Now when I am coming close to the final diameter I never try hit it with a final cut.  When I am at this point I take a minimum of 4 spring cuts and sometimes more until I don't see/feel anything being cut.  I measure after each spring cut. Then I will try a .001 cut and measure after the cut.  No more than this as I sneak up to the final diameter.   I have found that you absolutely can't try to re zero your dials.  You can't remove the work.  Everything has to stay the same.  If you change anything everything changes and you are basically starting from scratch.  It is not unusual for it to take 15 to 20 minutes for me to sneak up on the final diameter.  Even with this if I get within .001 of where I want to be I feel that I have done as good as I can.


^^^ This is a very good post. ^^^
You are one step closer to becoming familiar with your machine.


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## silence dogood (Apr 13, 2019)

This exact same situation happened to me when I took machine shop in high school.    I had just finishing polishing the project with 400 grit, it measured right on.  Proudly took it to the instructor and he said it's too hot. Imagine my surprise after it cooled,  It was a couple of thousands under.  When you doing a lot of cutting, a lot of heat is produce and your metal will expand.  Before you make your final cuts. let it cool, then take your measurement.


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## Chuck K (Apr 13, 2019)

If I have to hit a dimension, I leave at least 0.010 for my final pass and leave 1 or 2 for polishing to size.  By the time I get to the final pass I have measured several times so I  know how much I'm removing.  I don't back the crosslide off between passes. I have never had much success trying to take a 0.001 cut and only resort to it when there's only a few thousand material to begin with.


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## rust rescue (Apr 13, 2019)

I was taught by the "old guys" to go for it. Find a tool-speed -and feed that gives you an acceptable cut finish and then divide the remaining needed removal up into equal cuts. Like Mitch suggested. Measure after each cut. If you make several cuts and they all repeat, go for it. If your nerves won't allow this then back off  last cut one or two thousands and file/sand. I try to make that a last resort. When I was machining for a living I used to bore couplings with HSS taking .100" to get down to where five or six cuts of .050 would finish it up and hit it dead on. Of course this was on a big Lodge and Shipley but the same theory/practice holds true on a small lathe. You want to do all your experimenting and wearing in of a newly sharpened bit or new insert when you have material to spare. You have the same tool pressure/spring on every cut so if you dial a value you get the value every time. If you get a combination of speed and feed and bit that works on your machine write it down for the next time. If you only have room for a couple cuts practice on another piece of similar stock to dial it in before you chuck up your piece. Machining stainless requires that you take big cuts to get below work hardening so this method works well.  I ran a DoAll lathe that would get weird when the taper attachment was left on and not in use. Just the drag of pulling it up and down the ways would ever so slightly twist the carriage skewing the cuts. So would somebody  stopping to watch  and leaning against the back panel of the lathe pressing it against the carriage. (only about 1/8' clearance there). Another tip is to leave the lathe running when you dial in a cut. The vibration makes it easier to smoothly advance to a dial reading. Lastly a small pocket magnifying  glass is just the ticket for touching off on a cut or dialing that half thousand. (even when your eyesight is good.) Practice makes perfect but unfortunately not every time.


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## stupoty (Apr 13, 2019)

Sometimes if your taking very small cuts or trying to hit a critical size it's worth putting a dial indicator on the cross slide so you can measure the movement when your close to size.

Stu


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## Shootymacshootface (Apr 14, 2019)

I never really thought about it, but I can almost always get what I want with my mill just by dialing it in. But, I did have to get to know my lathe quite a bit before I  had much accuracy. I did a lot of overshooting.


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## Chipper5783 (Apr 14, 2019)

Like Rust Rescue says.  Trying to machine just a skim is a pain.  You had 0.150" of diameter to work with.  Use your early cuts to sort out the speeds and feeds.  Get it down to about 1.380".  Measure, divide the remainder by 3 (it will be about 0.010" each) - so take that 0.010" cut (don't even retract the tool, just crank the carriage back pretty quick - lathe still running - don't worry about the scratch - you'll take that off in the next cut).  Shut the lathe off, measure and go in half the remainder (which ought to be about 0.010" - but maybe a bit different - that's fine).  Make the cut with your established speed/feed - again, crank the carriage back quick, lathe still running.  Shut off and measure.  Advance the tool to your target diameter (allow what ever you want for polishing - say 0.001").  Make the final cut, retract the tool before cranking the carriage back.

This will manage the spring and give the tool enough to get a nice bite, even with a little lathe.  Of course if the lathe is solid and the work piece can handle it - you can take the same approach with heavier cuts.

If you loose track of your spring and are trying to start from just picking up the diameter - like you were, start with very small in-feed (like zero) - until you can figure out where you are at.


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## astjp2 (Apr 17, 2019)

I forgot but what angle do you set the compound to get .0001 per .001 of indicated travel?


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## mmcmdl (Apr 17, 2019)

Looks like 5.77 degrees , but check my math as I'm not positive what you mean . I'm not sure my calculator has adjusted for todays inflation either . 

You asking 10 on the compound to get one on the tool tip ?


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## Asm109 (Apr 17, 2019)

never mind/


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## RJSakowski (Apr 17, 2019)

My practice is to never make a final pass using a touch off as a reference.  Even with a DRO to indicate cutter position, there os uncertainty in the touch off.  If  I have to touch off, as in changing tools, I will  back the tool off slightly and advance until just skimming and measure with a micrometer.  I try to arrange my cutting so that I have material for at least one pass prior to the final pass.  

For critical dimensions, I will make one or more spring passes before measuring with a micrometer to determine final stock removal.  Depth of cut definitely affects the reading.

Regarding backing the tool off when returning the carriage, I have found that if I pull back on the tool post as I return, there is enough flex to provide clearance.  Doing this removes the requirement of resetting my cross feed dial and possible errors that come with it when making sub-thousandths adjustments.


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## mmcmdl (Apr 17, 2019)

Asm109 said:


> never mind/




???


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## Shootymacshootface (Apr 17, 2019)

RJSakowski said:


> Regarding backing the tool off when returning the carriage, I have found that if I pull back on the tool post as I return, there is enough flex to provide clearance.  Doing this removes the requirement of resetting my cross feed dial and possible errors that come with it when making sub-thousandths adjustments.


This is genius!
Thanks RJ


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## epanzella (Apr 17, 2019)

I'm terrible at using dials so I have a DI on my xslide. It helps me be precise and also immediately flags any backlash issues. If you went both ways with the handwheel instead of pushing it to the part all in one direction you could have had some backlash introduced. If the tool digs in a bit sucking out the backlash could easily add  5 thou to the cut.  The DI makes this obvious even to my 70 year old eyes


.


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## Jubil (Apr 17, 2019)

Lots of excellent suggestions here. I have had the same problem before, (over cutting). I have learned to take spring passes occasionally while hogging off metal, sometimes 4-5. The heavier the cut, the more spring passes it takes before it stops cutting. On final passes I do a lot looking, feeling and hearing. Sometimes I can get it to .001 but not often. But I'm learning and it's fun.
Chuck


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## astjp2 (Apr 17, 2019)

I was broaching a 1911 frame, even it had .003 of spring.  I will know better for next time. This is not just known to lathe work.


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## dpb (Apr 18, 2019)

As I believe someone else also said, simply make sure your final 2 passes are the same DOC. If I was turning 1.5 down to 1.0, I would take roughing passes down to 1.05, check my diameter, dial in half the remaining, make the cut, measure, and make my final pass, so, for instance, my diameter progression might be 1.5, 1.4, 1.3, 1.2, 1.1, 1.05, 1.025, 1.000.  Doing this, I can generally hit well inside of .001”, with no spring passes.  The key is that your final two cuts need to be the same.  Don’t sneak up on it.


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## NortonDommi (Apr 18, 2019)

I think you introduced error when you zeroed the cross slide dial then used the compound to 'touch' then went back to the cross slide to dial in the cut.
  The venerable Tony Foale posted the following about making fine cuts and it is one of those "Why didn't I think of that " tips that is a job saver.


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## mikey (Apr 18, 2019)

Where is the original poster? Second page and nothing from him. Not cool.


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## stupoty (Apr 18, 2019)

mikey said:


> Where is the original poster? Second page and nothing from him. Not cool.



Lolz , we need to know if it's working now ! 



Stu


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