Appropriate G50 for cnc lathe?

Gillby · Feb 14, 2019

Hello, I have a question for you experienced cnc machinists.
I had a disagreement with a co worker, and i would like to know other people's opinion.

So here was the situation.
A Small 304 stainless round part about 1.05 long and 1.25 of outside dia. For a 2nd operation, it was being grabbed by 1" od, .500" deep with correct soft jaws , and pressure of 150psi. So 1.25" od is sticking out .550" long from the jaws. And it's supposed to get turn down to .500" on od.
The lathe is doosan 220 series with 6inch chucks rated for max 6000 rpm.

What would be the proper best G50 S and g96 s values for the optimum tool life and efficiency?
The inserts are carbide and rated for stainless steel. Cut depth of .075" radially.
How do you guys normally set your g50 lines?
Please share me your opinions
Thank you

Delw · Feb 14, 2019

you set the g50 for what ever you feel is the safe max rpm you can run per your set-up. based on a crap load of things(mostly experience) jaw pressure, weight of part, how long tools are hanging out,bar feed work in chuck or collet, using a chuck or collet etc etc.
G50 sets you max rpm.
in older controls you also use G50 for your u and w shifts as well max rpms

Gillby · Feb 14, 2019

Delw said:
you set the g50 for what ever you feel is the safe max rpm you can run per your set-up. based on a crap load of things(mostly experience) jaw pressure, weight of part, how long tools are hanging out,bar feed work in chuck or collet, using a chuck or collet etc etc.
G50 sets you max rpm.
in older controls you also use G50 for your u and w shifts as well max rpms

So with all the information I gave,
What do you think is the safe max rpm?
Thanks

Delw · Feb 15, 2019

Gillby said:
So with all the information I gave,
What do you think is the safe max rpm?
Thanks

I Have no clue, its your machine or your the one running/been running it. My machines are different. you should know what it can expect with that set-up and the way that machine runs.. if its a new to you machine run it at half max then work up from there. this is were experience takes over. experience meaning time with that machine.
if you really must know, a quick check is to load a part in there. start uping the rpms at 500rpms at a time till you feel vibration via manual feed over ride. you dont need to run a part just turn the spindle.
those little chucks you can spin pretty fast with chuck jaws. but you have to ask yourself how good is your chuck,how rigid is your set-up, is your actuator running with in a few tenths. seen a few machines over the years that wouldn't spin half of max cause the actuator was out .010., or the chuck was worn out. cant always rely on a chuck pressure gage.

Gillby · Feb 15, 2019

Delw said:
I Have no clue, its your machine or your the one running/been running it. My machines are different. you should know what it can expect with that set-up and the way that machine runs.. if its a new to you machine run it at half max then work up from there. this is were experience takes over. experience meaning time with that machine.
if you really must know, a quick check is to load a part in there. start uping the rpms at 500rpms at a time till you feel vibration via manual feed over ride. you dont need to run a part just turn the spindle.
those little chucks you can spin pretty fast with chuck jaws. but you have to ask yourself how good is your chuck,how rigid is your set-up, is your actuator running with in a few tenths. seen a few machines over the years that wouldn't spin half of max cause the actuator was out .010., or the chuck was worn out. cant always rely on a chuck pressure gage.

Well the chuck is running within .001" and the part in the soft jaws run out .002".

One machinist with 5 years of exp was saying g50 s2500 and g96 s400 is good enough while the other machinist with 18years of exp says g50 s2500 is way too fast for 304 SS. And drops the g50 to 1000.
So who's wrong and right? Who's is being more efficient in production cnc shop.

guest · Feb 15, 2019

Clamp the spindle (G50) at 3000. That will allow you up to 400sfm at the .500 diameter. I doubt you will get very good tool life at 400 sfm, so set your G96 to 300 to start and see how long your insert lasts. If it squeals like a banshee, use the spindle override to slow it down and adjust your programmed (G96) speed to where it's happy.

If you have some magic carbide that likes more than 400 sfm in 304 you can bump up the G50 to 3500. But you don't, and you may need to go down to 275 or 250 (G96) to get better tool life. Every lathe is different and every piece of 304 is different, and not very fun to machine.

Is the part drilled through? What is the wall thickness? If it's not thin wall, you can up your chucking pressure to 200 or 250 psi, look in the manual and don't go over the recommended pressure and don't distort the part.

The faster you run the 3-jaw, the lower the clamping pressure gets. The chuck should have max rpm on it, don't exceed that in any case.

The 6000 rpm is only for the collet chuck.

edit to add: I just reread the OP.

The lathe is doosan 220 series with 6inch chucks rated for max 6000 rpm.

So if this is correct, maybe you can use your 3-jaw chuck that fast, I'd bring it up gently the first time, lol.

My lathe will do 6K, but I only do it with the collet chuck. I try to keep the 3-jaw under around 3500.

guest · Feb 15, 2019

Gillby said:
Well the chuck is running within .001" and the part in the soft jaws run out .002".

One machinist with 5 years of exp was saying g50 s2500 and g96 s400 is good enough while the other machinist with 18years of exp says g50 s2500 is way too fast for 304 SS. And drops the g50 to 1000.
So who's wrong and right? Who's is being more efficient in production cnc shop.

I'm pretty sure neither one said those things.

G50 2500 is perfectly safe, but you can't get to 400 sfm if the spindle is clamped at 2500 rpm. The max sfm you can get would be 325 at the .500 dia. So G96 S400 makes no sense if the spindle is clamped at 2500 rpm.

G50 S1000 is way to slow.

G50 S3000. G96 S300.

You should be able to do a lot better than .002" runout- is the part round? If you bored your jaws, the chuck shouldn't have any runout...

Gillby · Feb 15, 2019

guest said:
I'm pretty sure neither one said those things

G50 2500 is perfectly safe, but you can't get to 400 sfm if the spindle is clamped at 2500 rpm. The max sfm you can get would be 325 at the .500 dia. So G96 S400 makes no sense if the spindle is clamped at 2500 rpm.

G50 S1000 is way to slow.

G50 S3000. G96 S300.

You should be able to do a lot better than .002" runout- is the part round? If you bored your jaws, the chuck shouldn't have any runout...

Well I was originally thinking g50 3000 g96s400 but I was being conservative. And dropped to 2500
And the soft jaws were already bored long time ago for another job, didnt want to re skim the jaws for .002 run out.

the machinist with 18years of experience plus other experienced machinists at the shop were saying I'm wrong. That g50 s2500 is too fast for 304ss. So i wanted to get other people's opinion.

SeymourDumore · Feb 15, 2019

Gillby said:
while the other machinist with 18years of exp says g50 s2500 is way too fast for 304 SS. And drops the g50 to 1000.

I guess the question should be to the 18 year machinist guy asking what he thinks the G50 S2500 means.

My suggestion is G96 S360 for finish cuts to start and go from there, but it has precious little to do with G50 on SS....

guest · Feb 15, 2019

Gillby said:
Well I was originally thinking g50 3000 g96s400 but I was being conservative. And dropped to 2500

the machinist with 18years of experience plus other experienced machinists at the shop were saying I'm wrong. That g50 s2500 is too fast for 304ss.

G50 is just the rpm you clamp the spindle. That's not how you control the speed of the cut. It's the maximum speed you are allowing the chuck to rotate, for safety reasons. So if you're facing a part to X zero you don't max out the spindle.

G96 controls the speed you are machining the material. As the diameter decreases, the rpm increases to keep the sfm constant.

G50 S2500 with G96 S400 means you will cut at 400 sfm until you get to about 5/8" diameter, then the sfm would go down each pass after that, because the spindle is at the clamped rpm of 2500.

G50 S3000 will allow you to get 400 sfm all the way down to the 1/2" diameter because you have another 500 rpm available.

The type of material in the chuck has no bearing on the maximum rpm that is safe to run.

Job Shopper TN · Feb 15, 2019

Ultimately, safety and balance aside, G50 is irrelevant. When facing you will inevitably end up at zero surface footage regardless. So long as your surface footage (G96 S) is set right, the spindle speed limit will have little effect on tool life.

However, safety is another thing. As others have said, keep upping it a little at a time until it starts to sound funny or vibrate too much. In MDI you might run G50 S6000, then start at 2000 or so and override it up until you get nervous!

Another point to consider from a cycle time perspective. Granted, my experience is with Haas machinery, but our new lathe has a maximum RPM of 4500 on a 10" chuck machine. I've always programmed it with a limit of 3000, because on most things in our applications it takes longer to ramp up to or down from, say, 4000 RPM instead, than whatever the reduction in cutting time from the higher RPM is. However, a 6"-chuck not-Haas machine would probably handle this better. Figuring this out is something you can really only do by playing with it.

litlerob1 · Feb 15, 2019

Sounds like your 18 year man, doesn't understand the purpose of G50. That's okay, he probably has a different background.

Generally I program the G50 at whatever the Chucks are rated at. Other than off-center Turning, IMO that's what the rating is for. I have never had anything bad happen as a result. But the wind up-down is a time killer.(sometimes) Point being; determining the correct Max RPM has NOTHING to do with material being Turned, or material of the cutting Tool. Those elements determine the Surface Speed or G96.

R

Gillby · Feb 15, 2019

SeymourDumore said:
I guess the question should be to the 18 year machinist guy asking what he thinks the G50 S2500 means.

My suggestion is G96 S360 for finish cuts to start and go from there, but it has precious little to do with G50 on SS....

The 18years experience machinist knew exactly what g50, g96 mean.
So i asked her what she think is suitable cutting speed is for 304 ss. Because machinist pro app that i use reccomend 450 CS for carbide insert on 304. Which is the reason why i used 400 CS.
And she responded saying values from books and forums do not mean anything, because her 18years of experience tells her 2500 rpm is too fast , doesn't sound right and drops every g50 to 1000 which makes the g96 CS irrelevant since It will max out 1000rpm everytime .
And frankly 3 other machinists with 10+ experiences at my work agreed with her.

This is why I'm asking your guys opinion, because i learned and believed that g50 should be max rpm that machine can handle yet slow enough so the part won't fly out nor vibrate to cause chatters.

Gillby · Feb 15, 2019

During my shift I ran over 150pcs with same inserts at g50 s2500 g96 s400.
Which reduced cycle time by 25% , and proved that g50 s2500 runs totally fine.
But on the next day, all the g50s were back down to s1000. And the 18years experience machinist hates me now..
I just wanted to prove myself.

angelw · Feb 15, 2019

Gillby said:
During my shift I ran over 150pcs with same inserts at g50 s2500 g96 s400.
Which reduced cycle time by 25% , and proved that g50 s2500 runs totally fine.
But on the next day, all the g50s were back down to s1000. And the 18years experience machinist hates me now..
I just wanted to prove myself.

Hello Gillby,
Carbide Tooling can be worn out just as quick running it too slow as too high. Too slow often leads to Built Up Edge and damage to the cutting edge when the build up of material breaks away. At a 1000RPM clamp (G50 S1000), the surface speed at 0.5" is approx 130Ft/Min (40M/Min), which is the high end for HSS with Stainless Steel (type of stainless dependent).

Regards,

Bill

Philabuster · Feb 15, 2019

Gillby said:
During my shift I ran over 150pcs with same inserts at g50 s2500 g96 s400.
Which reduced cycle time by 25% , and proved that g50 s2500 runs totally fine.
But on the next day, all the g50s were back down to s1000. And the 18years experience machinist hates me now..
I just wanted to prove myself.

Just so you know, the direction you are going by pissing off the senior machinist may have dire consequences depending on the attitude of your boss. I should know, I pissed off some of my co-workers on a daily basis by running circles around them productivity wise.

Anyway, that situation happened about 20 years ago and I'm still here. I learned a lot since then and now try to take a tactful approach when dealing with difficult people vs just proving them wrong.

I agree G50 S1000 is painfully slow for that particular part. G50 S2500 is more like it.

Cwalster · Feb 15, 2019

My experience is that running super slow will mask a lot of issues. Running that slow, I can't see that part chattering ever. At the higher speed, it might chatter occasionally based on the material batch - an easy feed override fix, but something that won't work 100% of the time. Running at the proper feeds and speeds works 99% of the time with a huge productivity increase, making it the better option for the company, but from the perspective of the operator, that 1% means she needs to pay attention. And yes, shes probably demolishing inserts at that slow a speed.

Depends on the company culture, but there are companies who simply don't care about cycle time and just want the green light on as much as possible, and deviating at all from the norm is seen as heresy. I think your in that situation. Tread lightly.

DouglasJRizzo · Feb 15, 2019

Generally I set my G50 to a safe limit regarding work holding and work piece. By what you're describing, the machine may be trying to max out regardless because of the small diameters. I'd go for a G50 S4000 MAX and Set the G96 to around S300 depending on cutting tool grades.

706jim · Feb 15, 2019

We have a Victor lathe with an 8" Kitigawa chuck equipped with bolt on hard jaws. The machine can go to 6000rpm. I use a G50 limit of 4250 mostly because of safety. At 6000 rpm, all you can hear is wind noise form the chuck jaws. I figure if one of them let go at that speed, it would go right through the sheet metal enclosure.

And wonder if this has happened to anyone reading this.

litlerob1 · Feb 15, 2019

Gillby said:
During my shift I ran over 150pcs with same inserts at g50 s2500 g96 s400.
Which reduced cycle time by 25% , and proved that g50 s2500 runs totally fine.
But on the next day, all the g50s were back down to s1000. And the 18years experience machinist hates me now..
I just wanted to prove myself.

Ewwwww, I can relate!!

In retrospect, I would say watch your ass, it's not worth the headache to have coworkers that have a grudge. BUT I am a masochist in a manner of speaking, I don't really give a shit if people want to be babies. Bottom line is my throughput is always higher, people hate it because by nature most people are lazy. I learned to not talk about it, keep my head down, play a little dumb (askance) and run Machinery the way it's meant to be run.

IME it never cost me my job, but it did cost me some sociality at work.

R

Appropriate G50 for cnc lathe?

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