Milling out a simple pocket

I am making some aluminum lens holders for an old 1940's camera. The original was basically a stamped piece of .054 alluminum approx 4"x4" with the side walls being about .2" high. Then a hole was punched out for the lens.

I have made a few of these so far on my cnc mill and they have worked out great. But since I am still learning about cnc machining, I had a question that I have not been able to figure out.

When I am making these parts, I am starting with a peice of .200 thick aluminum. I first pocket out the center, then cut out the hole, and then finish off by cutting out the outside edge. The only small imperfection (which doesn't effect performance, but annoys me) is when I am pocketing the final depth, I always end up with a ridge on the bottom surface which is the thickness of the cutter away from the sidewall.

Basically what is happening is when it is cutting that last .002 off the bottom (my finsihing pass), it is also cutting a small amount on the side walls. It looks as if the side cutting action is effecting the bottom cutting action. I am 'chip cutting' so I don't know if this is what is causing it. But it seems to always cut about .001 deeper along the side walls than the rest of the pocket. The rest of the pocket is perfectly smooth.

I have tried pocketing from in to out and out to in. And have tried using a finish pass, and without a finish pass, but I always get that little extra depth when cutting along the side wall.

Anyway, if anyone has any pointers please let me know. I hope I was somewhat clear in my explanation.


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What's happening is the cutter is lifting the material when it cuts the walls of the pocket and that results in a shallow slot around the periphery. So, what you do is do a separate finish pass on the walls but stay .001 up from the bottom. Then take a finish pass on the floor to depth but stay .001 off the walls.


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Yeah. Terry, how thin is the material in the bottom of the pocket? Maybe holding down the blank with double sided tape would help with this. Randy

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Randy Replogle

The material is only .054 at the bottom of the pocket, and its aluminum, so it makes sense that the cutter is easily lifting up the material. And I also noticed that the shallow slot it leaves is actually a little deeper along the two sides without clamps. The two sides I am clamping down on have less of a slot.

I won't be able to clamp down all four sides since my table is only 3" wide and the material I am cutting is just over 4". But I could use some carpet tape to hold the peice down. I don't know if the foam double sided tape would work since it has give to it. I could also build a larger table to clamp to my existing table which would allow me to clamp all four sides.

I think using the separate finish passes along with the tape should do the trick. I'll experiment on the next one I make.


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I've used a reverse spiral fluted right hand 2-flute end mill... the end mills flutes cut to the normal direction but the flutes spiral to the other direction.... so when you're cutting side walls the force pushes down on the part. Jack

p.s. its been 10 years since I've seen one, but they make them.

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Sounds like you're using a spiral flute cutter and that will dig down at the edge. Cutting the edge first to about .001' or so less and then cutting the rest of the hole leaving a little rounded edge is the best thing. Also using a dtraight flute cutter will tend to keep the mill from digging down. As to the caps, why not make them thread on? That would be a lot more classy to me. Even more spectacular would be to have the m snap on and off with internal threading.

-- Why isn't there an Ozone Hole at the NORTH Pole?

Reply to
Bob May

Don't use foam tape. Find the right 3M tape. It works great. Sorry, can't give you the source, someone else must be able to. (mine's out of the box and I just know where to find it at my local store) - GWE

Reply to
Grant Erwin

Another idea is to use super glue to mount the part to a plate. You can later release the bond with a little heat and clean it with a little acetone. Just dont use so much heat as to warp your part.

Reply to
Roger Shoaf

Pete Stanaitis


Grant Erw> Terry wrote:

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Pete & sheri

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