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.

Thanks.

Terry wrote:

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.

Thanks.



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.

fred

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.

fred

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

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.

Thanks.

Terry wrote:
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.

Thanks.

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.

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?

Terry wrote:


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.

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

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.

Thanks.

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.

--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube, then
they come up with this striped stuff.

1-800-3MHelps

Pete Stanaitis
------------------

Grant Erwin wrote:

Terry wrote:


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.


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