Post By exkenna
Balanced Tools G2.5 vs. G6.3
Most machine tool builders recommend that tooling turning over 7500 RPM should be balanced to ISO G2.5. I just bought some hydraulic tool holders which are balanced by the manufactuer to G6.3. Will these tool holders be ok in my machine running 10,000. What is the difference beween G2.5 and G6.3? These are the most common balance ratings I have been seening and wanted to know if something balanced to G6.3 is equal to something balanced at G2.5. Thanks for any and all input. Have one on me.
G6.3 is a looser spec than G2.5. Hydraulic holders in general are harder to keep balanced. I would recommend to keep the rpms a little lower or use a different holder.
With the hydraulic holders, the max we are running is 9900 RPMs. Being that they are balanced at G6.3 @ 15,000 they seem like they would be fine since we are not getting close to 15k. Is this ok?
No because your machine specs out G2.5. Both the G spec and the rpms must be within machine tool builder specs. More than likely you wont have any issues, just its not recommended. Balance specs and tolerances work on exponential growth so G6.3 if very far from G2.5 as far as tolerance.
As Mari says, with G levels, the forces raise (by memory) by the square.
So, running at any rpm, if a holder balanced to G2.5 gave a vibration output of say 3 units, a holder balanced to G6.3 would give an output of 9 units, ie 3 x worse.
All our collet chucks are rated at 2.5, but we use Schunk hydraulic holders. We get lots more tool life out of these compared to ER chucks, as tools run within nothing. They are lovely and shiny and ground/polished all over, and because of the guaranteed low run-out, I assumed they would be well balanced.
But I just checked and they are rated at 6.3.......
It means that its within that range, you use it to find your weight allowance. So although it may not make 2.5, it could have been right on the edge of it. Hard to know unless it comes with some more data as to what the last reading was on the plane of correction. As to how much it matters, it would probably vary a bit with how heavy a spindle the machine has?
A balance grade of "G" means nothing without an RPM. A holder with a balance grade of G2.5 at 10,000 rpm is also G6.3 at 25,200 rpm.
Most toolholder companies can offer a higher balance grade if you like. Generally speaking, it is easier to balance a heavier tool than a lighter tool. Most toolholder companies offer only static balancing...there are a few that offer dynamic balancing, but I would only be concerned about that on long toolholders.
What we really care about is the total balance of the entire rotating assembly. A short, unbalanced ER11 toolholder will have a negligible affect on a CAT50...whereas an unbalanced 3" facemill will have a HUGE affect on a BT30 spindle. Having said that, most spindle manufacturers are a bit unrealistic in regards to their toolholder balance requirements...just the drawbar and the pull stud induces imbalance.
IMO, run your hydraulic holders at 15k and don't worry if they are not enormous. If it bothers you, you can send them back to the manufacturer and ask them to balance to a higher grade, but I doubt you are going to see any benefit.
Originally Posted by fpworks
"G" is a value in a formula that allows you to calculate "allowable imbalance" for a given tool holder over a range of RPM
Here's the actual formula:
U= G X 9549 X W/RPM
U= Allowable unbalance (in gram millimeters)
G= The "G" value you want to use, in this case 2.5
9549= the constant
W= weight of toolholder assembly
Let's say the tool holder assembly weighs 2.25 Kg
U= 2.5(the 2.5 "G' figure we chose) times 9549, times 2.25 (the tool holder weight in Kg) divided by 7500 RPM.
U= 2.5 X 9549 X 2.25 / 7500 = 7.16 gram millimeter
7.16 gram millimeter is your max allowable imbalance to keep G2.5 at 7500 RPM.
So let's solve for G6.3 at 7500
U=6.3 X 9549 X 2.25 / 7500 = 18.04 gram millimeter.
According to ISO-1940 unbalance becomes a factor above 8000 RPM.
At 18,000 RPM things get interesting.
2.5 X 9549 X 2.25 / 18,000 = 2.98 gram millimeter
6.3 X 9549 X 2.25 / 18,000 = 7.52 gram millimeter
Guess what? There's a 59% reduction in allowable unbalance at 18,000.
And Fpworks is correct: A toolholder rated G2.5 at 7500 rpm will have about the same amount of allowable unbalance at G6.3 and 18,000.
Now, once you add a retention knobs, collet nut, and cutting tool it starts all over because the tool really should be dynamically balanced in 2 planes.
Curt, do you have all that in your head? If yes, how do you get thru the doorway!!! Anyway TMI for many, wealth of knowledge for others.
Thanks for the education, Steve
Originally Posted by exkenna
Balance grade is defined as a unitless G value WITH an RPM. Note that RPM is a variable in the equation, so you have to specify your variable.
Imbalance is defined as a mass [of imbalance] and distance from center. It is without respect to total mass OR speed.