Facing question and wp length problems

I have been in swiss many years, using hanwha cnc lathes, i am pretty happy with the every part and set-up that i made, except one.
I turn a 8 mm bar, 80 mm is part length. Material is 4140. DOC is 2.75 to 0.75 mm..
The problem is i get different lengths from part to part, like +/- 0.04mm.. And I am really stuck with it.
The lathe is ok, i get the same problem while machining this part in other new / old hanwhas, too..
Chucking force is good, barfeeder centerline is good, cutting conditions, oil, tools and tool centerlines is ok.
Running 5k rpms like 0.05-0.08 mm/rev feedrate when turning, and 3k rpms 0.04 mm/rev in cut-off progress.. - I tried different speeds and feeds in cut-off-
I also support the part with a special ejector pin, using spring (soft one) force to push the part out after back turning progress.
It is ok with the machine, when making other parts, harder or softer material i allways get the exact part length in every part.
And cleaning the backside of the part did not solve the problem, i got rid of it.

Am i making a mistake in facing & front radius turn?

O0032
M90032
G0G40T0
M7
M8
M11
G4U0.3
G300X-1.Z82.T0101
G150Z9.5
M10
G4U0.3
G0X20.Z-2.M3S5000
T0
T0202(FACETURN)
S5000M3
G0X8.5Z0.
G99G41G1X-1.F0.07
G1X0.
G2X2.5W1.25R1.25F0.03
G1X8.W4.F0.05
G40G0X20.
T0
....

T0101M3S3000(CUT-OFF)
G50W-9.5
G99G1Z-12.F2.(DEBURR)
G4U0.3
G1Z82.
G0X8.5
M500
M56
M501
G99G1X-1.F0.04

and so on..

Anything wrong with it guys? Any suggestions? Thanks. :S

My guess is that it is the machine, setup, or offset as opposed to the program, as I see no glaring errors in your code.

-Are you positive that your front turn is facing material?
-Are the bars ground? Variation in the bar diameter could be a culprit
-Excess Guide bushing tension could possibly bind up the bar and cause it to slip in the collet. Can you move the bar though the GB by hand?
-Have you tried over feeding in Z+ then going Z- to your cutoff length? for example Z90; Z82;?
-Since you go to Z-12. to deburr/de-chip the part right before cutoff, could a chip sometimes bind up the bar when it goes back to the cutoff length causing the bar to slip just a tad?
--Along the lines of the last one, Do you put a chamfer on the bar every time you retract into the GB? I have seen bar burrs cause variation in the OAL.

Check off those and I'll keep going

Sorry for late answer,

I finished the job with holding tight tolerances as well as possible.

But i am sure there is no problem with the lathe, i checked the part length without pulling it back, but nothing changed.
Bars are ground and i chamfer it before pulling back, too.
I also tried moving the material a little bit forward before cut-off, but it did not work.
I do facing & front radius with a vcgt.

So that..I am suspicious of cut off tool bite, i use an applitec 751R.
And as far as i had checked, the cut off surface is not that clean, and i think this might cause a failure in part length tolerances..
I tried helping the cut off process by making a groove to 4 mm diameter, and so on i finish the job.

I guess a better cut off tool like iscars' would be nicer, i use applitec because of regrinding ability.

What you think, any ideas?

Quote:

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Originally Posted by MCImes

My guess is that it is the machine, setup, or offset as opposed to the program, as I see no glaring errors in your code.

-Are you positive that your front turn is facing material?
-Are the bars ground? Variation in the bar diameter could be a culprit
-Excess Guide bushing tension could possibly bind up the bar and cause it to slip in the collet. Can you move the bar though the GB by hand?
-Have you tried over feeding in Z+ then going Z- to your cutoff length? for example Z90; Z82;?
-Since you go to Z-12. to deburr/de-chip the part right before cutoff, could a chip sometimes bind up the bar when it goes back to the cutoff length causing the bar to slip just a tad?
--Along the lines of the last one, Do you put a chamfer on the bar every time you retract into the GB? I have seen bar burrs cause variation in the OAL.

Check off those and I'll keep going

---

In that case I'd say it has to be cutoff varibility, especially if the finish isnt great and you are regrinding inserts. Do you back face the part? That's about your only other option that I can see. You'd probably get the back face for free (time wise) while the main is working.

The interesting side is, i back face the part make a radius to the back end. And it really does not effect too much a new one or grinded one.. :s This is an easy looking job but, it is very messy. A true swiss story..

Huh. Weird. If you're back facing the part and still having OAL issues, to me, that could only be the part moving in the sub, or less likely a worn out machine issue (like backlash, but you dont think that's an issue).

So way to go! you've stumped me.

Quote:

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Originally Posted by MCImes

Huh. Weird. If you're back facing the part and still having OAL issues, to me, that could only be the part moving in the sub, or less likely a worn out machine issue (like backlash, but you dont think that's an issue).

So way to go! you've stumped me.

---

I am stumped, too, overdone myself but no way.. And things do not change even if i machine the part in an other machine, brand new or old one. This is really a weird situation, makes me sick! and is why i asked it here. Can it be chucking 6.8 diameter with 7 mm collet? Chucking force is ok, material does not slip when i hit it to backwards..Plus, i made a special ko pin to eject the part. While sub spindle is picking up KO is not rotating.. The part presses the soft spring, and centering with ko is very good, too.. Maybe a material stress is causing this.. The only thing i have not tried is "not using" the ko pin, but this time sub will struggle to eject the part and make alarms.. I am so drowned by thinking so much detail..

Couple ideas:

1. Cut off surface finish; this can and will cause process variation. Check center height. Check sub-spindle alignment to Guide Bush. Try new insert. Try rough cut then finish cut. (cut to X-1. then go back up X8.5, move Z or W, and take a second pass)

2. There's always some level of backlash, its just whether you can gauge the amount. I've ran Hanwha's before and they are not a high quality machine. I've had to program around backlash plenty of times on those machines. With that, ALWAYS move in the same direction when positioning Z axis. Omit G0 moves and use G1 when positioning Z axis for Cutoff.

3. Every machine has some level of thermal expansion. If the change is linear, then this may be your problem.

So, overall, cutoff surface finish really concerns me...this maybe your culprit.

DC

The exact problem is cut-off surface finish. I found that this problem makes a little difference in part length.
I guess it is just the cut-off tool, applitec 751R..

Quote:

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Originally Posted by DCogswell

Couple ideas:

1. Cut off surface finish; this can and will cause process variation. Check center height. Check sub-spindle alignment to Guide Bush. Try new insert. Try rough cut then finish cut. (cut to X-1. then go back up X8.5, move Z or W, and take a second pass)

2. There's always some level of backlash, its just whether you can gauge the amount. I've ran Hanwha's before and they are not a high quality machine. I've had to program around backlash plenty of times on those machines. With that, ALWAYS move in the same direction when positioning Z axis. Omit G0 moves and use G1 when positioning Z axis for Cutoff.

3. Every machine has some level of thermal expansion. If the change is linear, then this may be your problem.

So, overall, cutoff surface finish really concerns me...this maybe your culprit.

DC

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I'm surprised the Applitec tool is causing problems. I've used Applitec for nearly 15 years and have always had good success.

In this occasion, is that cut-off tool a re-ground tool?

i am pretty happy with applitec but the cut off surface is not that clean. i am using applitec products since 8 years and they are simply ok for me, especially cut-off bites. it is a cut-off tool 751R-2.0

Well that's good. How's the alignment between the sub-spindle and the guide bushing?

How far back are you holding it in the Sub spindle? The closer the better. What Diameter are you holding and What Size Collet are you using? Lastly 4140 sucks. My guess is you are seeing a combination of Backlash, Material Size Flux, And Material being crappy. You're code looks ok. I program in inch so I'm not sure about your feed/speeds but I know 4140 is gummy so you need to push on it sometimes. Whats your parting tool radius? Is it a handed or neutral insert?