Tool Balancing - When do you use it? Is it worth it?
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  1. #1
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    Default Tool Balancing - When do you use it? Is it worth it?

    I'm interested in hearing your opinions on tool balancing:
    Where do you use it?
    At what spindle RPM's does it become beneficial, necessary?
    When do you not use it, such as under certain spindle RPM's where benefit does not justify cost?

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    I am really curious for some good answers to this question. When I worked for an MTB, we said it wasn't really critical for <20k spindles.

    However, one day at a previous job I was doing some light milling on a vertical with a 12,000RPM spindle. Finish cuts were using a .500 endmill in a prebalanced ER32 of good quality. I was bored, and decided to balance a tool/holder assembly with the system used for our 30k spindle machines. Didn't expect much.

    To my surprise, the difference was HUGE. Massive reduction in chatter on all of the finishing passes. I couldn't really feel the difference, but it was clearly visible by the increased quality of surface finish. Was that a fringe case? I don't know, but it haunts me when I think about all of the extra wear and tear I might be causing by not balancing my tools.

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    I would think that all smaller tools like milling cutters, slitting saws, etc., are already perfectly balanced. The only tools I consider balancing are large flycutters. I have made two of those out of solid steel disks (about 5" and 7") with two symmetrically opposite slots for inserts (one insert cutting and the other dummy), this way it always stays balanced, though I never run those at at speed high enough that can create a problem even when using one insert. The same is true of boring heads that are by their nature unbalanced, but not run at high speeds.
    As for grinding tools (in jig or cylindrical grinder) the small stones (under 1" OD) will be almost perfect after truing and larger ones might need balancing after truing.

    flycutter.jpg

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    This has always been haunting me as well, we don't have a balancer, nor will we ever. I run all MST and Haimer pre-balanced shrink fit holders up to 25K. I cannot imagine shrinking in a premium 6mm ball endmill and having the balance being thrown off enough to notice any difference. The surface finish that comes off the Hermle is amazing, but I suppose there could be improvement if they were balanced, I just cannot justify the 40ish grand they cost. I would say if your running in the 36K and up rpm's we would balance, under that, in my opinion, it's a sales pitch, I would never dream of balancing a tool that is run at less that 15K rpm's. Our machine tool builder claims the more runout on the balance, the more spindle heat you create, I could see that being true, but to what extent? If your off one G at 40K rpm's you gain 20 degrees, 10 degrees? I don't know, and does a few degrees matter with spindle life?. I do know however, that getting a balancer and running an old noodled out cnc will not get you the results your looking for. I will add, before they did our test cut, they sort of made a "thing" about not having a balancer. They took all the test cut tools back to their shop and balanced them, and just like expected, they were well within the running recommendations and needed not attention. My advice, save your money on the tool balancer, buy the best shrink fit toolholders you can afford and run top of the line tooling.

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    Yes,
    balance is important. And tricky. Holders or cutting tools that you would think can't have any imbalance do. A perfectly symetrical tool holder with all surfaces perfectly ground and concentric will still have an imbalance. Material itself can have an imbalance. This is why we balance all of our tool holders.

    Best bang for the buck is to have your face mills balanced with the tool holder as an assembly. Once balanced like this the face mill and tool holder should never be seperated. Most of the time face mills are used for a good portion of your program and with the high rotating mass balance is important. I would be more concerned about a 3" face mill spinning at 8,000 rpms than an ER16 collet chuck spinning at 18,000 rpms.

    As far as balancing higher than G2.5 @ 20,000 rpms. Not much value in it. At that fine of a balance ( like G2.5 @ 25,000 rpms) just reclamping the tool holder and it sitting 20 millionths different in the taper will dismiss the difference between a 20,000 rpm and a 25,000 rpm balance. So now imagine putting in a drill or an endmill.

    Best thing you can do is use quality tool holders that are balanced. Any tools that you are spinning at high rpms use a tool holder that hase a small nose diameter and shortest gae length permissable for your application. Now if you are milling make sure nose diameter is big enough so give you the proper rigidity you need. But just because an a CAT40-er32 collet chuck is balanced doesn't mean it is smart to spin it at 15,000 rpms and use it to drill a 3/16 hole. Use a CAT40-er11 or er16 collet chuck that has much less mass and a smaller nose diameter.


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