How to cut out a part from larger stock without cutting a "slot", and why do this?

I’ve read prior posts (from Will Adams, for example) that it’s best not to cut a slot the same width as the cutter because the entire front of the cutter (half of its circumference) is in contact with the stock. I’ve noticed when doing this that there is a lot of vibration and noise that I don’t experience in other cutting scenarios.

One of the suggested remedies when cutting a part out of larger stock is to create an outside offset at least 10% larger than cutter diameter and then cut a pocket down to tab depth, after which an outside contour of the part can be cut. When doing this the first pass of the pocket cut at any depth is a slot the width of the cutter, followed by another pass that is only a small fraction of cutter width. There is a lot more noise and vibration with that first pass, then the second pass is smooth and relatively quiet. Because the first pass is essentially a slot, how is this approach of using a pocket a better method than just cutting the outside contour?

I’m very new at this and am not challenging the advice given, but am looking to better understand the thought process behind it.

Thank you in advance for your guidance.




Thank you for the quick reply, Will. I previously read those posts and used the techniques you described when cutting out a part yesterday. It was very easy to set up. It seems there is no way to avoid cutting a slot the diameter of the cutter with the first pass at each depth in the pocket. Is the rationale behind your recommended approach to essentially create a roughing pass followed by a finishing pass that should exhibit less chatter and result in a smoother finished surface?

Correct, one can’t avoid a slot at the first and last pass — that said, usually one doesn’t get into trouble until one gets past half the endmill diameter in terms of depth.

I see. What kind of trouble does one get into when exceeding half the endmill diameter in depth?

My depth of my slotting passes yesterday were 40% of endmill diameter. Not that noise is necessarily the best indicator, but I didn’t like the way it sounded and will probably take shallower passes next time.

In my experience, excessively high tooling engagement leads to lost steps and the machine getting pulled off-course.


Thank you very much for sharing this information.

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