RE: [CAD_CAM_EDM_DRO] Looking for a decent milling tutorial

Some aircraft manufacturers rout aluminum skin pieces sitting on ¼ thick MDF.



From: CAD_CAM_EDM_DRO@yahoogroups.com [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Leslie Newell
Sent: Sunday, January 27, 2008 8:27 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Looking for a decent milling tutorial



Hi Danny,

> I have a 2D part that needs to be cut out through the entire depth of the aluminum. How do I keep the mill from striking the XY rails underneath? Stick plywood underneath it or what?
>

Yes. This is called a 'spoil board' and is quite a common technique. I
use MDF (an artificial board commonly available from DIY stores) because
it is cheap and very flat. If you are using flood coolant you can only
use it once because it swells as it absorbs the water. Depending on the
thickness of the material you may be able to hold it in a vice. I made a
set of mild steel jaws for my vice and machined little steps in the tops
of the jaws. This makes it very easy to hold smaller parts without
having to mess around with parallels.

> Say the part's a circle (it's not, let's just assume) cut out of a square. Obviously the part's going to become loose & bendy partway through and eventually free itself. What's the best practice? Is there a software feature to leave some corners connected at first and then work them down last or what?
>

For this 'holding tabs' work well. You don't cut all the way through at
a number of points around the part. Afterwards you can cut these tabs
and free the part. Depending on the part these tabs could be so thin
that you simply snap the part out of the sheet. This is a pain to
program by hand but Cam software such as SheetCam <www.sheetcam.com>
makes it easy.

If you are cutting thin sheet then double sided tape can work well. Just
stick the sheet to your spoil board and cut it out. This trick doesn't
work very well if the part you are cutting out is very small. 'Carpet
tape' is cheap and very strong. It is however a pain to remove
afterwards. There are tapes available that are specifically designed for
this job. Unfortunately they are often difficult to obtain in small
quantities.

> Say my clamps are 1" tall and I'm using a short mill on a piece of 1/8" aluminum. The collet will strike the clamps if it drops down far enough and gets near the sides. What's the best way to address the problem? I could always put a block of wood underneath it to bring it higher up in the clamp, but say I'm engraving the surface. If the part's not precisely flat the engraving will be shallow on one side and deep on the other and wood is hardly flat. What practice would address this problem?
>

Clamps are always a problem. As I mentioned earlier, MDF is pretty flat
and parallel. I find it is accurate enough for most jobs. By the way,
WD40 or AC90 make excellent cutting fluids for aluminum. Some harder
grades of aluminum cut quite well dry but softer grades stick to the
cutter if you don't use a lubricant. Once it starts to stick it rapidly
builds up and breaks the cutter.

> I see that many end mills are not good for plunging into metal. What are the limits of plunging? I mean I can do a pocket without a starter pilot hole, can't I? Is that plunging restriction only applicable when there's not much lateral movement, like less than half the width of the mill or something?
>

Normally 2 flute cutters plunge quite well but cutters with more flutes
either won't plunge at all or have to be plunged with extreme care. This
is because there is no room to clear the chips from the center of the
cutter. One solution to this problem is to ramp the cutter into the
work. This means that you move the cutter down at an angle so it cuts a
ramp. Once at depth you back up and remove the ramp. If you are cutting
a fairly small hole the ramp will probably end up as a spiral. This
technique works particularly well in aluminum. Most CAM packages will
be able to do this. I find and angle of around 15 degrees works well. If
you are using a particularly gummy grade of aluminum then you may need
a shallower angle.

Les





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