Lathe tailstock truing issues

[luge007]

Ok, I have about 2 years of " intense hobbying" experience operating a lathe, just not much tailstock setup.  So my issue is, after moving the tailstock to cut a taper, I now have to reset it to true.

I started by truing the face of my .750 mild steel scrap stock, then used a spotting drill to make a center, done.  I slid the stock out about a foot tightened the chuck and locked down the tailstock with the stock in it snugly.  I proceeded to make a light cut in both ends about .900 long and .015 deep for no reason in particular, but enough to make a clean cut.  Next I mic'd the stock to find it's .0055 out of whack end to end, no big deal, adjusted the tailstock until I get an even reading on both ends, done. ?

 Not so fast... I then double checked things by mounting my Mitutoyo test indicator on a short Noga magnetic mount on the cross slide and checked both ends against each other with the tip on 'cutting center' and it was off by .0025.  Nothing I could do from then on would get the two sets of measurements to jive.

Is this 'normal' and is this a sign that I need to replace my live center?

Thanks
M

[rgb1]

Matt, you may get better results using a
dead center for this particular adjustment.

[truck]

Ok, I have about 2 years of " intense hobbying" experience operating a lathe, just not much tailstock setup.  So my issue is, after moving the tailstock to cut a taper, I now have to reset it to true.

I started by truing the face of my .750 mild steel scrap stock, then used a spotting drill to make a center, done.  I slid the stock out about a foot tightened the chuck and locked down the tailstock with the stock in it snugly.  I proceeded to make a light cut in both ends about .900 long and .015 deep for no reason in particular, but enough to make a clean cut.  Next I mic'd the stock to find it's .0055 out of whack end to end, no big deal, adjusted the tailstock until I get an even reading on both ends, done. ?

 Not so fast... I then double checked things by mounting my Mitutoyo test indicator on a short Noga magnetic mount on the cross slide and checked both ends against each other with the tip on 'cutting center' and it was off by .0025.  Nothing I could do from then on would get the two sets of measurements to jive.

Is this 'normal' and is this a sign that I need to replace my live center?

Thanks
M

That .0025 is what you should be seeing. Just move the tailstock .0025 and you should be good.

[luge007]

(rgb1) OK, so I suspected the live center may have bearings that are going and may be deflecting .0025 as it cuts and a dead center should not flex, which was going to be my next test to verify.  What is the best lube to use on a dead center?  Never turned with one other than between a live center and a dead center and a drive dog on the chuck side.  I'd be concerned with galling.  If it cuts better or to zero, I will replace the live center, if it is the same, I am not sure where to look next. The tailstock is solid, the bed is not cracked or racked, I've checked those...

(truck) So 2.5 thou is an acceptable amount of deflection, or I should move the tailstock over 2.5 thou to make up the difference so the 'flexing' brings the total to zero when mic'd AND indicated or should I not worry about how it indicates so long as I get equal cuts at each end?

For most of my work, .002 is accurate enough for parallel, but it would be nice to get that taper down to zero when I do need to make accurate cuts.

Cheers,
M

[truck]

The indicator should read 000 on both ends. You should adjust the tailstock to get rid of the taper. I doubt the 3/4" stock was flexing.
You could also "bump" the cutting tool in .0005" at even intervals to get rid of the taper provided the taper is bigger at the headstock end.
I ran this lathe and that is the way I handled taper.

[Machinist]

Imagine a line running perfectly thru the spindle center.

It seems you may be assuming that the carriage is running perfectly parallel to that spindle center line. A worn live center could throw things  off a little too.  Tailstocks don't always lock PERFECTLY in line with the spindle every time they are moved and locked. It all adds up.

Personally, if I can drill a hole and the drill bit is nicely centered in the work, I leave everything alone. Machines are not perfect. Except Hardinge lathes, Hardinge lathes are perfect!!! Many lathes start out not being perfect, then they wear.  The ways wear, the carriage wears.

I've done the same as Truck, adjust the crossfeed as I'm turning to eliminate or reduce taper.  I have an old, worn lathe, but we had to do the same thing on new CNC lathes at work, change the program to cut a slight taper, to produce work that is not tapered.

Do a nice job, but don't drive yourself nuts unless you have to.

And last but not least, is the machine level?

Steve

[happymecanic]

And last but not least, is the machine level?

Steve

I will add to this: Is the machine level end-to-end? If the bed is only slightly twisted you will never be able to completely remove that taper. You'll need a high-precision bubble level to check that.

[luge007]

Thanks for all the help guys.
I re-ran the cuts on each end of the stock with a well lubed dead center in the tailstock.......The results concluded that there is tolerance stacking going on.  1. my live center needs to be replaced as the bearings have a few thou of flexing going on.  2. the entire tailstock  appears to be moving about half a thou with maybe 20lb of hand pressure as indicated by the test indicator, regardless there is some flex.  So, I am choosing not to drive myself crazy with the last few thousandths and just understand there will be some compensation required if I need to make a long straight cut on a piece of stock.

I probably should have started with the fact that this is a Grizzly 0516 combo machine with the mill removed, but the 'stabilizer foot' for the mill was left on.  It sits on a HEAVY steel table that is perfectly flat and leveled with a machinists level.  The ways have been checked for 'consistency of parallel in the horizontal plane' with the same Starrett .0005 in 10" machinists level I borrowed to check the table (woah, what a tricky tool). In that I mean there does not seem to be any way to make the ways perfectly 'level'  without making the entire machine canted.  The cross slide turns out to be level to the steel table, but checking the tool cutting center in this condition with one of those 'tool indicating levelers' or 'center finders' only results in frustration.  Due to the odd, dissimilar shapes of the ways, there doesn't appear to be a way to make them in the same plane without making the entire machine tipped.  In short, there is no reference plane to work from.  I have resorted to an old school 'test block' to set cutting tools after making sure the tool hits dead center on a facing cut (double checked under 10x magnification while cutting to make sure the last bit is cut and not snapped off).

I can say without a doubt that I will be purchasing one of those 'taper adapters' to shove in my tailstock for the next time I need to cut a taper, resetting the tailstock is a P.I.T.A!  On the plus side, this little adventure has made me aware of the limitations of this machine and to be careful when trying to cut long perfectly cylindrical parts.

Ciao,
M