CSS Formula

[Rogue_Machinist]

So Ive been trying to clean up some of my programs and start using CSS on certain jobs that really benefit from it. However I cant find my actual formula for CSS RPM values. Can someone please either link me or explain to me how the values are calculated. Sorry

 

[nihilistic]

G96 G-Code: Constant Surface Speed CNC Programming

www.cnccookbook.com

 

[CarbideBob]

You are confused on how to calculate surface footage vs dia?
One thing in CSS is that there is always a "clamp" or limit to how fast things can spin so it stops at some point.
This can be programmed or a limit of the spindle max speed.

900 feet per minute is cruising in the metal cutting world. What is this in MPH?
If I am doing 60 MPH in my car, open the door and stick my toolholder in the pavement what is the cutting SFM?

 

[Rogue_Machinist]

You are confused on how to calculate surface footage vs dia?
One thing in CSS is that there is always a "clamp" or limit to how fast things can spin so it stops at some point.
This can be programmed or a limit of the spindle max speed.

900 feet per minute is cruising in the metal cutting world. What is this in MPH?
If I am doing 60 MPH in my car, open the door and stick my toolholder in the pavement what is the cutting SFM?

I guess what im getting at is a program I have at my shop Is 3" stainless steel. And at the finish diameter says its suppose to be 436rpm. But when I program it how I think its suppose to be programmed I only get 290 RPM or even less sometimes.

 

[RC Mech]

Even 290 rpm is fast for 304 stainless turning at 3” diameter.

 

[DanielG]

If I am doing 60 MPH in my car, open the door and stick my toolholder in the pavement what is the cutting SFM?

5280

 

[bigjon61]

Even 290 rpm is fast for 304 stainless turning at 3” diameter.

For who? That's only 225 SFM.

Formula you need for RPM IS (SFM x 3.82)/DIA. 436 RPM is about right for 3" in 304.

 

[RC Mech]

For who? That's only 225 SFM.

Formula you need for RPM IS (SFM x 3.82)/DIA. 436 RPM is about right for 3" in 304.

3.82, you’re absolutely right. Funny math on my part!

 

[Cole2534]

5280

May need to up your coolant concentration.

 

[Rogue_Machinist]

Even 290 rpm is fast for 304 stainless turning at 3” diameter.

573RPM is what my book shows for 304/304L Stainless steel at 3" diameter. But even when I set that in my calculations my machine will give me like 379. Its not where it needs to be. Thats what im saying.

 

[DanielG]

May need to up your coolant concentration.

Liquid nitrogen!

 

[Cole2534]

573RPM is what my book shows for 304/304L Stainless steel at 3" diameter. But even when I set that in my calculations my machine will give me like 379. Its not where it needs to be. Thats what im saying.

573rpm at 3" = 450sfm, pretty fast for 304, IME.

379rpm @ 3" = ~300sfm, more reasonable, IME.

What control? Does it have material specific cutting conditions?

 

[Rogue_Machinist]

573rpm at 3" = 450sfm, pretty fast for 304, IME.

379rpm @ 3" = ~300sfm, more reasonable, IME.

What control? Does it have material specific cutting conditions?

Fanuc control. And no it doesnt have specific cutting conditions. And were talking carbide tooling not HSS. And yes the machine doesnt seem to like CSS.

 

[angelw]

So Ive been trying to clean up some of my programs and start using CSS on certain jobs that really benefit from it. However I cant find my actual formula for CSS RPM values. Can someone please either link me or explain to me how the values are calculated. Sorry

If you're going to use CSS mode in your program, you don't have to calculate a RPM, you only have to specify the Surface Metres per Minute (SMM) or the Surface Feet per Minute (SFM) for the material being machined. However, to answer your question specifically:

For SMM to RPM (for a particular Diameter)
RPM = SMM x 1000 / (Diameter x Pi)
This can be simplified to:
RPM = SMM x 318.31 / Diameter (318.31 is the constant)

For SFM to RPM (for a particular Diameter)
RPM = SFM x 12 / (Diameter x Pi)
This can be simplified to:
RPM = SFM x 3.82 / Diameter (3.82 is the constant)

Regards,

Bill

 

[sinha]

Read the following if interested in more details ...

Constant RPM vs. Constant Surface Speed (CSS)​

It has been observed that the life of a cutting tool (insert) is maximum when the cutting speed is maintained in a narrow range for a particular combination of the workpiece material and the tool material. All tool manufacturers specify this range in their catalogues (in m/min and ft/min). Therefore, for a particular workpiece-tool combination, one has to select the RPM so as to ensure the desired cutting speed. Let

V = Cutting speed

ω = Angular velocity of the spindle

n = Angular frequency (Revolution per second) of spindle

N = RPM (Revolution per minute) of spindle

D = Turning diameter

Then,

V = ωD/2 = πnD = πND/60

If D is in mm then

V = πND/60 mm/sec = πND/1000 m/min = ND/318 m/min

If D is in inch then

V = πND/60 inch/sec = πND/12 ft/min = ND/3.8 ft/min

In practice, the rounded expressions ND/300 and ND/4 are used. Therefore, if D is in mm then the cutting speed is ND/300 m/min, and if D is in inch then the cutting speed is ND/4 ft/min. Thus, the desirable RPM can be calculated as

N = 300V/D where V is in m/min and D is in mm, or

N = 4V/D where V is in ft/min and D is in inch.

In a facing operation, in CSS mode, the required RPM to maintain the same cutting speed while machining at the center of the workpiece would be infinite (because D = 0). In such a case, the RPM increases up to the maximum possible RPM on a particular machine. It is also possible to restrict the maximum RPM to a lower value than what is available on the machine through a program code. This might be desirable for reducing the vibration caused by high spindle speeds.

A drawback of the CSS mode is that it causes excessive load on the spindle in the case of a large and sudden change in the radial position of the tool, as the spindle is subjected to high acceleration or deceleration. The worst of this situation arises when the tool is sent to its X-home position in a rapid move. This may cause damage to the spindle motor. Therefore, whenever there is a large and sudden change in the radial position of the tool, the CSS mode should be temporarily cancelled.

G97 selects constant RPM mode and G96 selects CSS mode. G50 restricts the maximum RPM. Except in a threading operation, where G97 must be used, G96 should be used in all turning operations.

 

[DDoug]

If I am doing 60 MPH in my car, open the door and stick my toolholder in the pavement what is the cutting SFM?

Routinely see (even on this morning's commute) sparks coming from the road plow's skids.

Amazing how long the sparks stay glowing in the snow (Coolant) several feet behind the plow truck.

I found one a few years back, it was 2" thick steel, approx 6"wide and 24" long and had carbide brazed into 1/2" wide slots.

 

[sinha]

Read the following if interested in more details ...

Constant RPM vs. Constant Surface Speed (CSS)​

It has been observed that the life of a cutting tool (insert) is maximum when the cutting speed is maintained in a narrow range for a particular combination of the workpiece material and the tool material. All tool manufacturers specify this range in their catalogues (in m/min and ft/min). Therefore, for a particular workpiece-tool combination, one has to select the RPM so as to ensure the desired cutting speed. Let

V = Cutting speed

ω = Angular velocity of the spindle

n = Angular frequency (Revolution per second) of spindle

N = RPM (Revolution per minute) of spindle

D = Turning diameter

Then,

V = ωD/2 = πnD = πND/60

If D is in mm then

V = πND/60 mm/sec = πND/1000 m/min = ND/318 m/min

If D is in inch then

V = πND/60 inch/sec = πND/12 ft/min = ND/3.8 ft/min

In practice, the rounded expressions ND/300 and ND/4 are used. Therefore, if D is in mm then the cutting speed is ND/300 m/min, and if D is in inch then the cutting speed is ND/4 ft/min. Thus, the desirable RPM can be calculated as

N = 300V/D where V is in m/min and D is in mm, or

N = 4V/D where V is in ft/min and D is in inch.

In a facing operation, in CSS mode, the required RPM to maintain the same cutting speed while machining at the center of the workpiece would be infinite (because D = 0). In such a case, the RPM increases up to the maximum possible RPM on a particular machine. It is also possible to restrict the maximum RPM to a lower value than what is available on the machine through a program code. This might be desirable for reducing the vibration caused by high spindle speeds.

A drawback of the CSS mode is that it causes excessive load on the spindle in the case of a large and sudden change in the radial position of the tool, as the spindle is subjected to high acceleration or deceleration. The worst of this situation arises when the tool is sent to its X-home position in a rapid move. This may cause damage to the spindle motor. Therefore, whenever there is a large and sudden change in the radial position of the tool, the CSS mode should be temporarily cancelled.

G97 selects constant RPM mode and G96 selects CSS mode. G50 restricts the maximum RPM. Except in a threading operation, where G97 must be used, G96 should be used in all turning operations.

It is a part of a book which I am presently writing for absolute beginners in the CNC-lathe area. Though I will be skipping complex things, basic principles will be properly explained. Essentially, this will be the first step in learning CNC-lathe programming, tooling and machining, through self-study alone.
Once the book is complete, which will take several months, I will write another book for milling machines - again, basics only.

 

[guythatbrews]

Nissan did you ever say what constant surface speed you programmed? I don't think you did and this is an essential part of your question.

Please what is the programmed css? And what rpm does the control show at what diameter?

 

[Rogue_Machinist]

Nissan did you ever say what constant surface speed you programmed? I don't think you did and this is an essential part of your question.

Please what is the programmed css? And what rpm does the control show at what diameter?

Here is what my cam produced for me. This was using the formula specified.

N300(FINISH TURN)
G00 G97 X10. Z10. T0303 S550 M03
G50 S550
G96 S1000
G00 X6. Z1.
G01 X1.512 Z.1 F.05 M08
G01 Z-0.0005 F0.005
X4.921 Z-0.4572
Z-2.0
X5.1
M09
G00 X10. Z10.
T0300
M01

Now here is the thing. For this Material 304SS at 1.512" is 1137RPM@450SFM. But control is giving me like 600. Which is weird. Hence why I asked is there a formula for what values do I need to input in my initial g97 line as opposed to the G50/G96 lines.

 

[guythatbrews]

Here is what my cam produced for me. This was using the formula specified.

N300(FINISH TURN)
G00 G97 X10. Z10. T0303 S550 M03
G50 S550
G96 S1000
G00 X6. Z1.
G01 X1.512 Z.1 F.05 M08
G01 Z-0.0005 F0.005
X4.921 Z-0.4572
Z-2.0
X5.1
M09
G00 X10. Z10.
T0300
M01

Now here is the thing. For this Material 304SS at 1.512" is 1137RPM@450SFM. But control is giving me like 600. Which is weird. Hence why I asked is there a formula for what values do I need to input in my initial g97 line as opposed to the G50/G96 lines.

The G50 S550 forces the max RPM to 550 RPM. You say control is giving you 600 RPM at 1.512 dia. Are you sure it is not 550 RPM? That G50 line is important for safety so the machine doesn't max out rpm and throw stuff out of the chuck.

The next line,G96S1000, sets your CSS to 1000 surface feet/minute, way to fast. As programmed, anything under 6.945 diameter will run 550 RPM, since you've set the max allowable RPM at 550. Over 6.945 the RPM will slow to achieve a constant surface speed of 1000.

It's common to start the spindle in G97 at the RPM corresponding to the SFM and diameter at the start of the cut. Then invoke G96 for the cut. When the cut is finished G97 again at the RPM corresponding to the ending diameter of the cut. This way the spindle doesn't have to ramp up and down every time you go to your safe tool change position. That RPM value is figured with the formula:

RPM = 3.82*SFM/diameter

To find the 6.495 mentioned above rearrange the equation:

diameter = (3.82*SFM)/RPM

Thus

diameter = (3.82*1000)/550
diameter = 6.945