What causes runout?

(Brent Halbersma) #1

Can someone explain to me what causes runout? Is it possible to fix? If not I heard that you can account for it in your design. Can someone explain this preocess? Thanks all!

(William Adams) #2

There’s runout, which is caused by the collet being off center, and there’s tool deflection which is how much the endmill flexes as it spins.

The Precise Bits folks have a good bit on the former: http://www.precisebits.com/tutorials/spindle_runout.htm

Bob Warfield at CNC Cookbook has a bit out the latter: http://s3.cnccookbook.com/CCCNCMillFeedsSpeedsDeflect.htm

Our suggestion is that you cut a slot: http://docs.carbide3d.com/shapeoko-faq/how-to-calibrate-the-machine-for-belt-stretch/ and use that to determine the effective endmill diameter — or better still, do a diamond-circle-square test: https://www.shapeoko.com/wiki/index.php/Calibration_and_Squaring_the_Machine#Diamond_Circle_Square_Test

(mikep) #3

This is complex. When people are referring to runout, they’re generally talking about how much the tip of the tool moves. This is caused by a lot of things, as @WillAdams says, the collet can be off center (poorly made, dirt in it or the nut), but it just keeps going up the chain - the bearings/bushings in the spindle, flexibility in the tool while it’s chattering, I’m sure there are others. Don’t confuse this with backlash, which is inaccuracy in the mechanical movement of the mill due to spacing in the mechanics such that the machine can actually move. That’s a different creature.

How to account for runout in your design isn’t totally straightforward, and depends on exactly what is causing it. If you’re running, say .002" of runout (you have to measure it), any cut by that tool is going to essentially be .002" over the nominal size of the end mill. So you adjust the size of the tool to match. The problem with that is that the runout at the tip of the tool is usually larger than the runout at the top of the tool, and that means measuring both and creating a fairly complicated “tool” in your CAM software. If the runout is not always stable (ie. sometimes it’s .005" and sometimes it’s .001" measured at the same part of the tool - it wobbles) then you have a whole lot of problems.

How important all this is depends on what you’re trying to do. I generally ignore it, and instead focus on preventing chatter, which seems to have a bigger impact on my part quality.

(Phil Gorsuch) #4

Hmmm - this feels like a leading question - are you having some sort of problem?

(Brent Halbersma) #5

Thank you for your response, Will. This helps. I’ll review this information and let you know if I have further questions!

(Brent Halbersma) #6

Thanks Mike, very detailed reapsonse. This really helps as I am completely new to running a CNC. I am worried that the runout will cause the inlay that I am working on to not fit consistently. The smallest bit in have been using is 0.0157”. I use endmills to create the pockets and the inlay.

(mikep) #7

Given it’s a nomad, the spindle if pretty good, I don’t think it’s likely you’ll have a noticeable problem, I suggest trying it before worrying too much.

(Brent Halbersma) #8

I’ll give it a try thank you!

(system) #9

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