Toolholder Balancing

[underthetire]

I agree with rocky. A little story from last Friday. I work on my days off at my real job to help an independent service guy in this area. He is actually my best friend. We were balancing a horizontal spindle for 12k rpm. Got the balance very close, .1 gram. Everything looked good, but we left the transducer for the balancer on. Customer put a different holder in and we ran it up. Sounded different. Ran a balance test again, now we were out .5 grams. Both holders were "factory balanced to 20k". Both looked ok, both had the same brand end mill. Put the other holder in and back to .1. Went up to another shop, put a gauge pin in a balanced holder, used there balancer, balanced the tool, and now holding at .2 gram. So as you can see, there was big a big difference even with the same brand tooling . None of the results were terrible, but there is a difference. This was a standard cat 40 btw.

[BKGUY]

Are you talking about buying balanced holders or balancing each tool assembly individually in a tool balancing machine? You need to be careful when buying balanced holders. Some holders you buy, come with a balancing certificate where they were, supposedly, balanced at the factory. Also remember that the "G" tolerancing scale for balance, is very old. It was developed back in the '40s. Most of the balancing that is discussed for tooling, when it comes to G2.5@20,000 RPM, or G1.0@xxxRPM, is mostly smoke and mirrors. A balancing tolerance of G2.5 is for machine spindles. Really, anything within G6.3 is fine for a tool assembly. I do agree though that the better the balance, the better a tool will run.
When tool holders are balanced at the factory, they are balanced without the nut installed (in the case of a collet chuck). So when you install a tool and put the nut on, the balance has changed dramatically. The real way to check balance is by measuring the gram/millimeters of imbalance that is present. Which a good tool balancing machine will allow you to do. You can buy holders with balancing screws that allow you to balance an entire tool assembly on a balancing machine. See link below for an example of holders with balancing screws.

http://www.bigkaiser.com/pdfs/mega-er-grip.pdf

[Brakeman Bob]

I used to believe that G6.3 was good enough, then I caused over 8,000 GBP worth of damage running a HSK heat-shrink holder certified as G6.3 @ 25,000 rpm up at 21,000 rpm. It pulled so hard on the drawbar it broke the expanding collet that pulls the tool holder back into the taper. Didn't take long either; I was warmng the spindle up and put the empty holder in the spindle and was going up in 2,000 rpm increments, 3 min a time (starting at 1,000 rpm) - I wrote a program just for the purpose. At 19,000 everything was fine, then the machine upped the rev's and BANG! It scared the life out of me....

The spindle had to go back to Switzerland for repair and the report said plainly that unbalanced tooling had been the cause. A very uncomfortable time for me. After that I have all my tools balanced to G2.5 at full rev's plus one third (i.e on a 16,000 rpm machine the tools are balanced at 24,000).

BKGUY is right about about grams force per millimetre being the only true measure of out of balance forces but I find up to about 40,000 rpm G2.5 works OK. I tried having tools balanced to G1.0 but it meant rebalancing the tool assembly every time a cutter was changed, even ones sold as "fully balanced".