[Beefheart]

Hello,

I bought a -98 Okuma LB15 for some months ago, I have made some parts in it without any problems. This week I have made shafts with two grooves for a customer, I had problems cutting the grooves, the width varies about 0.04 mm. I checked the movement on the z axis with a dial indicator and it stops under 0.01 mm when I repeats the movement 5 times fast but if I wait about a minute it goes 0.06 longer, why? what happens when the machine rests for a minute?

[deadlykitten]

motion dynamics even if encoder position is the same, turret position may vary, depending on :
... backlash
... approach direction & speed
... approach type ( rapid or feed )
... slide resistance, depending on wear, heat, lube
... controller motion consistency / repetability, etc
* maybe other factors, like plastic/elastic behaviour

of course, it is considered that tool and fixture, are both rigid

what happens when the machine rests for a minute?

when machine stops, inertia changes, movement chain is broken ... just saying, i don't know your case

also, pls be aware : even if you measure repetability, that value does not represent the machine when is cutting; under load, things are different

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if you wish to increase precision, please share the drawing, the tool ( insert & holder, so to check overhang ), and i will draw you a higher precision toolpath and i will share few tricks, so to be able to increase precision class on older models / kindly

[Beefheart]

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if you wish to increase precision, please share the drawing, the tool ( insert & holder, so to check overhang ), and i will draw you a higher precision toolpath and i will share few tricks, so to be able to increase precision class on older models / kindly

Tanks for answer, all tips are very welcome! I made another shaft after that and it varied in length about 0.07mm. I made 40 peaces of these, 20 mm diameter and length 72 mm. I programmed G0 Z-74. and G01 Z-75. before I started parting the peace of. The shaft was getting shorter after about 15 peaces and varied up and down by some hundreds and the last two was at the same length as the first. I clocked the lathe this evening when it had been turned off for about three hours. I use Iscar tag for parting and Iscar Penta for grooving, and I only have as much overhang as it needs when parting.

[deadlykitten]

hy pontiac, so there seems to be a problem with length variation

few tips :
... be sure that there is coolant flow, especially when the edge reaches small diameters; these can not be achieved by normal coolant nozzles, but special curved ones, or special toolholders
... be sure that there is paralelism :
...... toolholdes shank paralel to X axis
...... insert paralel to X axis ( sometimes, even if toolholder shank is paralel to X axis, the insert is not, because of insert bed wear; if insert is tilted, don't lose time with re-aligning the holder, or re-clamping the insert, but simply just lower the cuting specs )
... be sure that the insert is subject to simetrical wear : for example, if you do a chamfer before cutting, then the insert won't be subject to simetrical wear; you may improve this, by cutting the chamfer with a different toolpath, so to delay the apparition of wear ( depends, just saying )
... is there enough speed ? for example, for a o20mm steel, i would cut it at G97 S2000 F0.06-0.08 G95, and maybe reduce the rpm to few hundreds, just before parting off, so to avoid smashing the window a good insert should keep that spec for at least 8-10 hours, or 100 and more parts, in normal steel

I programmed G0 Z-74. and G01 Z-75

normally, you should only use G0 Z-75; i would write G0 Z-72-3, because i like sugestive codes actually, my parting code is different, i use a soubroutine, inside which i change a few variables

The shaft was getting shorter after about 15 peaces and varied up and down by some hundreds and the last two was at the same length as the first.

is there a sign of wear, after only 15 parts ? did you inspected the insert cutting edge ?

did you looked at the cutting load evolution ? if tools is getting wear, then load should increase, loose it's linearity, stuff like that

in such cases, i align all parts, in a row, puting them in the order they were machined : a quick look, inspecting the back of the parts, tells if there is a problem

what is the insert life spam ? how many parts / edge ? kindly

[underthetire]

Ok, are you indexing the turret when doing these checks? Ive had problems in the past with customers trying to adjust the turret speed with the hydraulic adjustments on the right side of the turret. They usually end up slowing down the clamping speed too far for some reason, and the turret takes a few minutes to settle in for a full clamp.

[Superman]

Is the front of the part still at the origin ?
Has it been pushed back in the chuck by the cutting actin, or tailstock ?

[Beefheart]

Thanks for engaging in my lathe problem. I just checked the backlash and it is 0.2 mm .Think I have to check the ball screw and the related parts.

[deadlykitten]

hi beefheart, i was going to tell you to check the backlash, at least in the zone where you cut the part

however, your errors were pretty normal :
... post 1 : width varies about 0.04 mm
... post 3 : varied up and down by some hundreds

with errors so small, like hundreds, i wouldn't have believed that there is a baklash problem; so far, i was blaming your tool actually, i was also thinking about an issue with your dial indicator

here, during long term setup, dimensions shift with <0.03 after a few hours, because machine gets warmer, so i would expect a few more hundreds from your machine

however, from few hundreds, to 0.2mm backlash, is a long way good luck


ps : now, it does not matter anymore, but :
... if you wish to check if the turret is clamping slowly, like underthetire said, you may need to use 2 dial indicators : one on the turret ( revolver ), and another on the saddle if turret is clamping slowly, then the dial that is on it, will shows some deviation
... a lot of pecking will damage the insert faster, so try to keep it @ a minimum; for your part, i would recomand :

    G97 S2000 M03 M08 M42
    G00 X22.5 Z-75
    G01 X4.5 F0.06789 10 11 whatever
    G01 X-2*0.4 S250 M03 M63 ( rpm is changed during feeding, X axis is not reversed )
    G00 X22.5

[tea hole]

I've seen before where this nut backs off.
The set screw gets loose and the nut backs off allowing some severe backlash.
I'd suggest checking item 21 in the attached.

[Beefheart]

Thanks for your interest and tips, We tighten the nut as tea hole described yesterday and now the backlash is 0.02 mm.

/B

[Beefheart]

... a lot of pecking will damage the insert faster, so try to keep it @ a minimum; for your part, i would recomand :

    G97 S2000 M03 M08 M42
    G00 X22.5 Z-75
    G01 X4.5 F0.06789 10 11 whatever
    G01 X-2*0.4 S250 M03 M63 ( rpm is changed during feeding, X axis is not reversed )
    G00 X22.5

Thanks for the code, new way to cut a part for me. I have a part time shop, working 3 days a week (and weekends) I use Fusion 360 to my Haas wmc and tried to use it for my lathe but I don't like the lathe module so I'm going to program the Okuma manually. The other two days I'm working for a bigger company programming a Mori Seiki NTX using Esprit cad/cam so my code typing is a little bit rusty.

/B

[tea hole]

Thanks for your interest and tips, We tighten the nut as tea hole described yesterday and now the backlash is 0.02 mm.

/B

Glad you found your problem.

[deadlykitten]

hy pontiac

teahole is a rare presence, but when it shows up, it hits i am glad that there was an easy fix for your 98 okuma

you have to run some maintenance, inspections on the machine; like this, you would know the numbers for your machine : look behind the covers, check the screws, guides, clean the chuck, check the lube flow, at least 1 / year; check backlash at each 20-30mm, check turret tir, etc

i have a geometry record for each machine, and like this, i know when something is out of alignment ... if the nut was loose, then you should observe 0.2backlash all over the Z axis, from the spindle, to the tailstock

Thanks for the code, new way to cut a part for me.

it may not work, because of that M63

I have a part time shop, working 3 days a week (and weekends)

so you work for you 3+2=5 days ... thus, your ' main - esprit - job ', is during the week-end your have a shifted calendar

i will share few tricks, so to be able to increase precision class on older models

ok, go and unscrew that nut, put the backlash back to 0.2, and let's deliver nice parts

during parting off, when you said " I made another shaft after that and it varied in length about 0.07mm. ", it was because the cutting load may have moved your turret, towards the backlash; there is no fix for this

during grooving, when you said " width varies about 0.04 mm ", you could have used a gentle approach, like G01 Z_at_least_1/2_screwball_revolution F_gentle, so to keep the backlash behind the nut, in a repetitive manner also, a small doc + gentle feeding, should generate lower cutting forces, keeping the turret steady ... i use such tricks, when i have to deliver tight tolerances

if you wish to deliver tight tolerances, you have to keep the cutting load under a zone that does not generate dif, and to keep the backlash only on one side of the nut / kindly