G96, G97 and How To Calculate Surface Speeds
Basic Turning, in the early days of CNC Turning G96 was one of the things that really made a massive difference.
It meant that instead of having to turn a part at a fixed speed and feed, the part could be programmed in G96 which was a constant surface speed.
Where diameters changed, particularly when facing, it made a massive improvement to tool life and surface finish as well as speeding up the whole process.
G97 Speed In RPM
In Basic Turning when you program G97 your machine will start the chuck up at a speed in RPM. So if you program.
G97 S1500 M3
Your chuck will start revolving clockwise at 1500 rpm.
G97 for Drilling Tapping and Screwcutting
When drilling a hole you are on the centreline of the machine so you just want plain old simple RPM.
When tapping or spot drilling it’s the same.
Screw Threading (G76) can only be done in G97
G96 however means meters per minute. This is a surface speed.
G96 S200 M3
Your machine would start up at a surface speed of 200 meters a minute. Now your RPM would depend on where on the diameter the tool was positioned.
If the tool was positioned at a 100mm diameter it would be as if the tool were able to run around this diameter at a speed 200 meters a minute.
It’s a bit like being on a running machine if you ran at 200 meters a minute and placed various diameters under your feet the large ones would turn at slow rpm and the small ones would turn at high rpm. (Just like the hamsters above)
That’s why on a manual lathe it is hard to face a large diameter without changing speed half way.
You know when you face a part on a CNC Lathe and you hear that change in pitch? It’s the spindle increasing in RPM as it gets closer to the center of the part.
When it gets to the center your spindle is flat out so the G50 becomes crucial.
The G50 restricts the speed of the machine.
G50 S2000 machine will go no higher than 2000 rpm.
Did you have a pet hamster as a child?
I know it’s a random question but bear with me there is a point to this.
Well maybe you still have a Hamster and that’s not a problem.
Anyway I did and his name was Harold Wilson (British Prime Minister at the time).
Well I bought my hamster loads of different wheels to play on just like the one above.
My hamster suffered with depression on account of being stuck in a cage all day and not having a girlfriend oh and he had a lot of credit card debts too.
These wheels varied in diameter from about 6 inches to a massive 2 foot one. They kept him happy all night. He was so tired he slept all day.
Harold could only run so fast but I noticed when he was on the small 6 inch wheel it absolutely whizzed around. Now on the big two foot diameter one it took him ages just to get it to spin around once.
Harold Had G96
A CNC machine in G96 will give a lovely finish because the surface speed always remains the same.
So even though Harold ran at 200 metres a minute (this is fuckin lightening speed for a hamster)
The wheels ran at different RPM depending on what diameter they were.
Harold Was a Clever Bastard
Oh by the way Harold had a tail (unlike other hamsters) and a maths qualification.
He knew that if he multiplied the diameter of the wheel by .00312 it would give him the circumference of whatever wheel he was running on in meters.
200 mm wheel (.00312 x 200 = .6864)
All he now needed to do was divide this answer into the speed he was running at and he would know how many RPM his wheel was revolving at.
If he was running at 200 meters a minute not only would he be fuckin knakered but the wheel would be running at 291 rpm
200 / .6864 = 291
Basic Turning Manual Machining
Using a manual machine you have to compromise. At the outside your speed is too fast and when you get to the center you are too slow.
On a CNC lathe we would normally program in mm per revolution as well because the speed is changing all the time so we need our feed to be locked into the speed.
With a machining center our cutter is always revolving at the same speed so the feed can be constant in mm per minute.
Someone out there will be thinking “what happens in G96 when you get to the center of the part”. Well the spindle will be flat out!
See how surface speeds are translated to speeds in RPM. There are many converters online that you can use for this and I do recommend their use. It will also mean you don’t have to watch my tedious video.
When I train people at the CNC Training Centre my emphasis is on understanding not memorizing. I usually start by saying “please don’t remember all the things I am telling you”.
In the early days training students in Basic Turning I remember them saying to me the next day that they had G codes floating around in their head from the lessons the day before.
What I really mean is that the most important thing is to understand what the machine can do and the concepts of programming and Basic Turning.
You could say “I know there is a G code that makes the machine run in RPM” so all you need is a list of G codes.
If you can be bothered to work through the simple maths above. It will help you to fully understand how G96 is works.
The ones you use every day you will remember whether you want to or not.
G96 Whoopee It’s amazing
So use G96 for everything.
- Threading (Screw cutting)
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