Hi,
When cutting a profile with an 1/8" end mills (either straight or spiral) I get these lines which coincide with each pass. I have tried reducing the plunge rate (use 30in/min now) and run the cut at between 20 and 40 in/min. Still get them all the time.
Any assistance would be appreciated.
Thanks
Steve
Are they even around the entire perimeter?
Yes they are.
Horizontal lines around the perimeter of a cut are usually caused by spindle/collet/bit run-out.
Can you explain what that means? Or do you have any good references for understanding / learning about it?
Can you explain what you mean by “it’s the wood”? What is wrong / different about the wood?
Softwoods can compress while cutting
Why would compression while cutting cause horizontal lines at the bit edge?
Not entirely unlikely that the very edge of the bit isnt sharp and the wood grain is “teared” vs “cut”
@HaldorLonningdal - I have changed to new bits and it still happens.
@PhilJohnson - It happens on all sides of the pieces. Will try your suggestion. Thanks.
An out of true bit, for any reason, can cause minute vibrations which add to bit deflection and tear out, and more so with different wood types, speed and feed settings. Some woods are more prone to this as Phil posted. For the OP it may be a combination of issues.
Phil, link to hone please.
Thanks,
John
Thanks. I thought might be a powered hone. I have one similar to yours I’ll try, if I can find it??? 
I believe a slight tilt in the Z axis could cause this. Something like this diagram (I exaggerated the tilt to make the rings more obvious)
What if the bit was in the collet in a way that was off tilted. Would it get rotated around and create a depth-per-pass ring on both sides?
An unsharp bit (even ever so slightly) will at the very edge compress the wood fibres and fail to cut these fibres completely, the CNC complete the pass, dig into next level and carves again. Now the last cut edge is cleared but the partial cut fibres will be torn off instead of cut off. So what you are left with is a frayed edge/step.
Sharpening of the tool may help. Harder wood also, dryer wood vs wet wood will make a difference. There are many factors at play.
Visually the “defect” is pronounced but dimensionally it may be spot on.
So he did already mention earlier that he has tried other bits with the same result, making me think it might be a looseness / alignment of the bit in the collet. New diagram
Ok, will check the collet as well. Thanks
If all else fails, take a look at this video it shows a technique to keep the lines on the edge from ever happening…I do this on some materials that are more susceptible to the edge lines.
@ErikJenkins 
Thanks very much. Never noticed that option before.
