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

[dansawyer]

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?

 

[mmcmdl]

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 .

 

[mikey]

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?

 

[MontanaLon]

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?

 

[lordbeezer]

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.

 

[richl]

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?

 

[Tozguy]

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.

 

[MarkM]

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.

 

[cathead]

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.

 

[Technical Ted]

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