View Full Version : Endmill runout

01-25-2005, 01:50 AM
Wondering what everyone thinks is an acceptable amount or runout for a 1/4" dia endmill. Is there a standard way to measure runout on an endmill? Does one just check the highpoints on the flutes? the bottom of the shaft before the flutes start? etc.

I ebay'd some endmills for next to nothing but alot of them seem to have a fair bit of runout. Like a hundredth measured an inch from the end of the collet on the shaft. Is it a mistake to assume that the shaft should be straight an circular, after all as long as the flutes are in line with the part of the shaft thats in the collet what does the accuracy of where im measuring matter? Anyways, I'm just a newbie to this and hopefully some of this will make sense to someone more experienced.

Also, I checked the runout on on the collets, its not perfect but its decent enough that its not the cause of all that wobble in the endmills.

01-25-2005, 02:33 AM
Usually this is much worse on resharpend endmills.
Makes it harder to measure the dia. a endmill will cut.
It also shortens the lifespan of the endmill because only
one of the flutes will see most of the cutting and become
dull much sooner.

01-25-2005, 08:07 AM
The best method is to make test cuts and measure the cut geometry.
High precision parts should always be cut with a little stock and measured and then "sneak" up on the finished size very carefully and slowly with repeated measuring.

01-25-2005, 08:53 AM
I would say that .001 is definately too much runout, I have not measured my own personal runout on my machine, something I should do sometime. But I think it should be well under that.


01-25-2005, 09:33 AM
I wouldn't call .100 run-out I would call it bent :D

If you must use that cutter you can rotate the cutter in the collet 90 degrees at a time and re-check the runout in an attempt to average out the error.

01-25-2005, 11:51 AM
So I've had a few different responses but no one seems to agree on just what a hundredth is. A hundredth is what I said, and hopefully a hundredth is still .01", but maybe thats changed since the last time i checked? I didnt get the memo or am I incorrect here?

01-25-2005, 12:11 PM
My bad, one of those hurry up and read/post issues, well there you go, you can always say you were right at least once :D

01-25-2005, 12:30 PM
Are you sure the runout isn't from the collet?

01-25-2005, 09:14 PM
The runout of the collet measured at the collet is ~.0005 or less.

So since there is this wobble/runout in the collet the further away from the collet the more the perceived runout of the endmill will be. But how can this be calculated? My assumption is that the wobble in the collet that causes the runout must have a point of intersection with the virtual line that represents the perfect spindle axis, no runout. The position of this virtual point of intersection could be used to calculate the angle difference between the ideal and actual runouts. This angle could be used to determine expected runout at any distance from the collet for a perfectly straight endmill. However this is a terribly theoretical approach and not particularly useful in real life as far as I can see. So does anyone have a better/easier way to figure it out?

Addendum, its possible that the virtual point discussed above can be either infront of (between the collet and table) or behind (in the spindle) and determining that might be a good start.

Second Addendum, upon re-reading I might be full of it. let me know.

01-25-2005, 09:45 PM
There may be two possibilities here,
1) If the collet or spindle or both are canted then I would think the amount is relative to distance down, that is if you measure 1" down from collet and the runout is say .010 (1 hundredth of an inch ):D if you go two inches the mount is simply doubled, 3" tripled etc,
2)if it is not canted and the spindle is simply running out of concentric then it seems this runout will not change with length.


01-25-2005, 10:20 PM
Check the runout with a gage pin in the collet and you know were the runout is coming from.

01-25-2005, 11:35 PM
If you have a V-block, and a dial indicator, you can roll the shank of the endmill in the V, while you indicate the highest spots on the flutes. This will tell you if the grind is eccentric relative to the shank.

More than likely, it might be the state of your chuck or spindle taper that needs checking over.

01-26-2005, 10:41 AM
Hu and Ken have covered all the angles in this post.

01-27-2005, 10:35 PM
Who care you got them next to nothing exept you spend a lot of time and come out with nothing, hey now you can throw them away and it will still cost you nothing. :cheers:

01-28-2005, 12:26 AM
Well as bad as the runout is or isnt, I've managed to use them fairly successfully for a few pieces so far and I cant really complain. Even though they may have a slight wobble, that i cant detect in operation or in the finish quality. They run like the dickens, 4 flute carbide yumminess.