CSS Formula

[sinha]

RPM override switch possibly not at 100% ?

 

[Rogue_Machinist]

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

I think Im understand the software now. I need to put the SFM in the CSS and not an RPM.

 

[Mtndew]

the machine doesnt seem to like CSS.

HUH?
What machine?

 

[Rogue_Machinist]

HUH?
What machine?

DMC DL30 Turning Center.

 

[moonlight machine]

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.

I had a old rust bucket Studebaker Lark, You could get an amazing spark show by rubbing the concrete road divider at about 60 MPH at night. Good for about a 250' spark shower. Instantly removes tailgaters!

 

[Rogue_Machinist]

I finally got CSS to work on my machine. Numbers from my SFM formula were off by 100 RPM. But it works. Damn. I feel dumb.

 

[sinha]

Is spindle override at 100%?

 

[EmGo]

Numbers from my SFM formula ...

I'm kinda curious about these formulas ? All I ever used was π * D = Circumference. That and 12" = 1 ft .... What else do you need ?

 

[sinha]

Details in post#15

 

[Rogue_Machinist]

Is spindle override at 100%?

Yeah it always was. But for some reason the data I was input per the SFM formula would never give me the correct RPM at the control. So whatever data I get for SFM if I increase the value by 100RPM it nails the speeds at control side.

 

[sinha]

So, the difference is of 100 rpm, irrespective of CSS value?

 

[CarbideBob]

When doing SFM numbers on the fly in the field and with no calculator I think 4 inch.
1 inch part is 1/4 of that so 4 times faster. 40 inch part is ten times so 10 times slower than the SFM I want.
This gets you close.

 

[Rogue_Machinist]

So, the difference is of 100 rpm, irrespective of CSS value?

Yep.

 

[gcodeguy]

Question. If you use MDI to run the spindle at S500, what actual speed is the control showing it to be?

228 SFM (S290 at 3 inch dia.) is kind of slow. However, There is a bit more to it than just finding SFM in a generic manual. Rather than pick SFM from a 60 year old guide, use the insert company's catalog to find out their recommendations for the grade being used.

I've run a few 304 SS jobs. Not fond of it. Have run a ton of 316 SS. Maybe that's why I prefer 316 SS. I've run 316 SS as low as 250 SFM...maybe slower...roughing. I've run it as high as S1300 SFM to finish. Same insert was worthless rough turning. I've run a cermet at S1000 SFM to finish 316 SS. 250 piece order, 20 finish, one corner did the whole job.

All that to say that the grade of the insert, and how it is being used, plays a major roll in deciding SFM.

As pointed out elsewhere, some of us use G97 to start the spindle at the RPM it will be running for the first X-dimension. Then program the G96 after the first movement block. I am not a fan of the spindle constantly ramping up and down between operations. If there is a huge difference between RPMs from one operation to the next, I use G97 to insert an intermediate RPM when going to the index position. I may increase (or decrease) the RPM during the tool index and then program the actual RPM for the SFM being used on the rapid move to the part.

This saves significant cycle time on one of our older lathes. If you drill at S168 and then start the spindle up at S2500 for the next operation, it won't move until the spindle reaches S2500. Using intermediate RPMs going to the index position, and approaching the part (and changing RPM during index) saves time as the spindle will slow down or speed up when RPM is given on a axis move.

 

[EmGo]

Details in post#15

If you insist on making a mountain out of a molehill, yeah. Otherwise 22/7 x diameter gives you the circumference and 12" = one foot, so surface speed in feet per minute is pretty much third grade arithmetic ... might be harder in metric tho, I wouldn't know about that.

 

[sinha]

Is it a mountain?

 

[EmGo]

Is it a mountain?

Yes, it's silly. Circumference of a circle is pi D. Surface speed really is a third-grade arithmetic problem. Post #15 was like eighteen column inches. That's nonsense.

 

[sinha]

Circumference and CSS are not the same.

 

[EmGo]

Circumference and CSS are not the same.

Maybe in Japan. In other parts of the world you take the circumference, divide the inches by twelve to get feet, then divide again to get the relevant rpm for surface speed in feet per minute. Give it a G96 and a number. Viola, there is your constant surface speed. Not that difficult.

It wasn't magic before Fanuc got ahold of the idea. They could make tying your shoes into a seventeen page presentation.

 

[sinha]

Can you please explain your method with a numerical example.