[slashmaster]

Is it when you use huge servos instead of steppers? Or when the machine cost more than $100,000 or what?

[davereagan]

Only well above 1000 ipm, it starts to be worth measuring in G's. I was once standing in front of a small Enshu machine at a show and the table went sideways as fast as you could drop it. It's quite a thing to see. Even half a G is 11,520 ipm/second, so you're at 2000 ipm in ~.2 seconds. Japanese machines are the only ones I know of who talk about G's.

[Jeff-Birt]

'G' is a measure of acceleration in gravitational units, (http://en.wikipedia.org/wiki/Acceler...due_to_gravity), so it will be given as units/time/time. 1 g = 9.8 m/s/s which just says that the velocity increases by 9.8 m/s every second.

For instance these are acceleration:
mm/s/s (millimeters per second per second)
in/sec/sec (inches per second per second)


Measurements given as units/time are velocity (speed).

For instance:
mm/min (millimeters per minute)
IPM (inches per minute)


The point is that 'G' and IPM measure totally different things, so there is no choice between them. Once your machine is up to its rapid velocity (speed), you are not accelerating any longer so there are no 'G' forces present. 'G' only measures how fast you speed up or slow down.

Hope that helps.

[Geof]

Is it when you use huge servos instead of steppers? Or when the machine cost more than $100,000 or what?

It is when you want to impress people using a term that is not particularly helpful.

[garagefela]

Useless info in my opinion. In other words you can get up to your maximum rapid speed of 10 IPM in the smallest fraction of a second and in doing so pull 100 g's.

Rapidly get up to your crawling rapid speed.

Big whoopy doo

M

[escott76]

Useless info in my opinion. In other words you can get up to your maximum rapid speed of 10 IPM in the smallest fraction of a second and in doing so pull 100 g's.

Rapidly get up to your crawling rapid speed.

Big whoopy doo

M

Or if you are actually concerned how productive your machining center is, it is useful info.
The ability of a machine to get to speed makes a big difference when making lots of small moves in complex contouring. It's all well and good to set a reasonable feed rate, but if you never hit it because acceleration can't get you there, then it's useless to try and cut material at that rate.
It may not be a priority to someone just doing things as a hobby who can afford what time it takes, but to call it useless info is pretty short sighted.

[davereagan]

The gap between an Enshu that has 1G accelaration and 2376 ipm rapids and the homemade cnc stepper motor machines is bigger than the gap between a model T and a Corvette Z06. The reason the faster machines talk about acceleration is to differentiate themselves from another machine with the same or higher rapids that does not get hole to hole as fast. These guys think 3 seconds chip to chip is a slow tool change. I remember thinking how silly that was until I ran some cycle times calculations with hundreds or thousands of tool changes in a day. It adds up and people pay a lot of money for Brother machines for their super fast tool changes, rapids, and spindle reversals. Their 30 taper machine can reverse from tapping at 4000 rpm so fast, you could miss it if you weren't watching closely.

[Geof]

...These guys think 3 seconds chip to chip is a slow tool change. I remember thinking how silly that was until I ran some cycle times calculations with hundreds or thousands of tool changes in a day.....

This is absolutely correct and many times the wrong approach in my experience. When you are doing thousands of tool changes per day you are making many parts and they are probably going to be smallish; compared to the capacity of most machines. Loading multiple parts in a slower machine will produce tool change times per part in the fractions of a second with a lower upfront cost and probably a longer machine life. But this is the mundane approach and doesn't impress people.

[davereagan]

Awesome machines:

[ame="http://www.youtube.com/watch?v=L5aC5Kpvibc&feature=PlayList&p=302033874C7 5849B&index=27&playnext=3&playnext_from=PL"]YouTube - Watch This Fast CNC Machine[/ame]


[ame="http://www.youtube.com/watch?v=ZgQja-UETVc&feature=PlayList&p=302033874C75849B&index=26 &playnext=2&playnext_from=PL"]YouTube - Brother TC-32C Cutting[/ame]

[ame="http://www.youtube.com/watch?v=4XYakTeQahA&feature=PlayList&p=302033874C7 5849B&playnext=1&playnext_from=PL&index=25"]YouTube - CNC Brother TC32 B QT-Fast tool Change[/ame]

[Andre' B]

Useless info in my opinion. In other words you can get up to your maximum rapid speed of 10 IPM in the smallest fraction of a second and in doing so pull 100 g's.

Where do the find a machine tool that can do 100 g's
The highest accelerations that I have ever read about were on the order of 5 or 6 g and those are micro mills with travels around 4 inches and resolution & accuracy that is hard to believe.

In more normal machines, the typical older mill is under 0.5g, some newer mills get just over 1g.

With certain types of programs, hard milling using high speed tool paths, just that little jump can take a big chunk out of the run time.

Acceleration also has a big effect on how well a machine can follow a programmed path, all machines have to slow down to make the corners without rounding them, the higher the acc. the less impact on the average speed.
Along with that is wear on the tool in the corners, since the spindle does not slow down in the corners like the feed rate the feed per flute drops. So the cutter is rubbing more then cutting, bad in general real bad when hard milling.

[Caprirs]

As correctly pointed out, G and ipm are different "rates". Second and first derivatives of position for you calculus geeks.

There is certainly marketing hype to the speed of some machines. However, shop owners who can afford such equipment can do the math on how cost effective that speed is. If you were machining engine blocks and cylinder heads for Ford or Toyota (millions per year for a decade), multiple part fixturing would not be practical. Thus, every aspect of machine performance becomes critical. In such an application, reducing the effective tool change time by putting more engine blocks in the machine cannot be done. So the tool changer has to get its job done faster. Similarly on a lathe, getting the turret far enough away from the part to allow the tools to change, then back into the cut is crucial as well.

In other applicaitons, the acc/dec of a machine has very obvious consequences on peck drilling of deep holes and gets painfully obvious on parts with lots of holes. If you've ever made a batch of parts with hundreds of drilled and tapped holes, you likely know how long all that peck drilling takes. In many instances, more aggressive pecks just load up the drill so faster in/out of the hole can be a big time saver. That is accomplished by higher rapids and higher acc/dec.

I have run the same part on two different machines using the same tools and programs. One machine takes 45 minutes and the other 55 minutes. The machine with the longer cycle has 700ipm rapids, while the shorter cycle machine has 400ipm. The difference really occurs when the peck drilling starts. The 700ipm machine never gets close to those rated rapids while pecking whereas the 400ipm mill hits them.

When buying machines, it is crucial that the machine fit the application.