Lathe turning length to diameter ratio question

[dml66]

I turned a piece of 5/16" diameter 12L14 steel down to 0.2755" in 0.005" increments using a carbide insert cutter, then down to .02750" using 400, 600, 1299 and 2500 grit oiled sandpaper. The material projection from the ER32 chuck was 0.6875", using the L/D rule of thumb I figured I'd be good for up to 0.825", 3 times the finish diameter.

From the chuck end of the cut, my DTI indicates a near-perfect axial zero reading up to about 0.4602", then it falls off rapidly, and linearly to the TS end of the piece. The taper at the end is radially 0.0015" less than the chuck end.

My first thought is this must be due to workpiece deflection but since my L/D ratio is "in-spec", I wonder if there's something else going on?

I've already corrected the lathe for bed twist and don't believe it would cause such a rapid taper at one end of the work while allowing roundness further back.

Also wonder if maybe this might be caused by a wear spot in the ways; either low or high?

I may try using a dead or live center on a piece to see if it's deflection-related, thought I'd ask here first.

Thank you.

 

[WobblyHand]

Probably deflection. What insert are you using? To minimize deflection use a very sharp insert, one that ends in GT, with a small nose radius, or grind your own HSS tool with similar geometry and get it razor sharp. And lower the DOC.

 

[pontiac428]

Put the tip of a test indicator on the back side of the outer end of the work, then push against the work with your thumb. The result will help you visualize the limitations (quirks) of your lathe and your setup. Try this with various lengths, chucks, center support, etc. The exercise is worth a thousand words.

 

[dml66]

Probably deflection. What insert are you using? To minimize deflection use a very sharp insert, one that ends in GT, with a small nose radius, or grind your own HSS tool with similar geometry and get it razor sharp. And lower the DOC.

I'm using a Sandvik-Coromant insert:

ANSI: CCGT 2(1.5)0-UM 1125
ISO: CCGT 06 02 02-UM 1125
Coated: PVD TiAlN+TiAlN

0.2 mm (0.0079") nose radius. I haven't rotated the insert for a while, not out of the question it's not as sharp as it could be.

 

[mmcmdl]

Another way to do beat the L over D issue but it takes an extra step . Turn the shaft on a taper first . The tailstock end .005 approx large and the chuck end way over size . Come back and finish in one pass . The material in the taper will hold stable and straight and the part won't flex . I had to do this on multiple parts on the CNC lathes in the past . Most were over a 10 to 1 ratio .

 

[benmychree]

Coated inserts are not particularly sharp, hence the deflection, a sharp HSS tool would likely make a big difference.

 

[MrWhoopee]

 

[dml66]

Coated inserts are not particularly sharp, hence the deflection, a sharp HSS tool would likely make a big difference.

One of these days I'll try making my own cutters, for now I'm stuck with inserts

 

[jwmelvin]

The taper at the end is radially 0.0015" less than the chuck end.

That’s the opposite of what deflection would cause, right?

 

[dml66]

That’s the opposite of what deflection would cause, right?

That was my thought as well.