- A vacuum chuck is a perfect solution
for holding large thin work pieces
that just can't be held in a vice.
More surface area means more holding power.
So what about smaller parts that aren't held as well?
Well, in this video, I'm gonna show you exactly
how to do just that, let's get started.
(bright upbeat music)
So let me start off by showing you the two parts
that need to be machined.
These are left and right component
pretty close in size and shape.
In fact, they're the same outside profile.
Just the inside features are different.
They measure roughly 2.75 by 1.5 inches.
This is important to know because in all vacuum workholding,
surface area is key.
Let me pull up my calculator to get some rough numbers
to see what kind of holding forces we can expect.
1.5 times 2.75 gives us the surface area of 4.125.
And we're gonna multiply that by 14,
which is the holding power in pounds per square inch.
That gives us, we'll call it 57 pounds.
Now imagine a column of air sitting on top of the part,
extending to outer space.
That column would weigh 57.75 pounds.
The way that vacuum works is the weight of the column of air
is actually trying to fill in the vacuum.
But the part is in the way.
That's how parts are held.
That's why surface area is so important.
So let me show you the finished design
that I came up with and we'll work backwards.
This is a 13 and a half inch, top plate,
which bolts on top of our equally sized chuck.
Now a few features I put in here,
I put in set screws on this backside
and there's four flanged screws
to help hold the remnant material.
We'll get into that in a second.
Now our customer had to make 500 of each one of these parts.
So creating a custom top plate is definitely the way to go.
Each cycle, we're gonna have 12 of the windowless part
and another 12 of the windowed part.
So let me show you a zoom in of the windowless part.
Now I created a gasket groove
that sits just inside the perimeter of the part.
I went with one 16th inch diameter gasket on this one
because we're trying to maximize the surface area
within the gasket pattern.
We typically recommend eighth inch,
but 16th inch just gets us that little extra area,
which is key in vacuum workholding.
So this has an area of 3.52 square inches,
3.52 times 14.
This part will be held with 49 pounds of holding force.
This area over here has a surface area
of almost exactly 2.5 inches.
And again, multiplying that by 14,
this part will be held with 35 pounds.
So that's good, but it's not great.
Cutting forces can greatly exceed that.
And you're not gonna get any lifting from an end mill,
as much as you're gonna get side loads,
which slide the part off of the gasket pattern,
breaking the seal, and then you lose your part.
That's why nesting all these parts together
is a little trick to maximize the holding power
because when we're machining one part,
we get the benefit of the surrounding parts surface area
to hold that in place.
Let's take a look at the cam side of it.
So here's what it's gonna look like
when all 24 parts are finished on the chuck,
let me walk through some of the tool paths
to show you every tip and trick.
So in yellow here, here is the stock
and these parts nested really well
in a 12 inch square piece of material.
Now, ideally the parts would nest
with very little scrap on the outside edges.
The way that the math worked out,
we had a little extra material shown here along the edges,
which if we had more,
I would have created a gasket pattern
to hold the remnant piece.
But we're right on the edge there.
I opted to go with these flanged head screws
just to hold the material down
and also to give it a work stop.
So this first tool path is interesting.
The basic strategy is to ramp it down,
to nearly full depth,
but we're gonna leave about 20 thousands of material
at the bottom of the cut so that
when we rough machine these parts,
they're still interconnected
until we take our final finish pass at full depth.
So you can see this red line ramps down.
And it profiles it.
Now, you noticed I'm only doing these four corner pieces.
Let me show you why.
When I simulate this part right here,
the cutter here is surrounded by a part
above it and below it,
which essentially gives it 360 degrees of support.
As we step through.
When we get to this point right here.
Now there's material supported by the vacuum
to the left and right of the cutter,
but not in front of it.
Only behind it.
Here's where parts can become an issue
when you approach the outside corner of your work piece.
So we've machine along this edge, no problems,
but when we get here,
the cutting forces are only held
by this back section material here.
There's no material in this quadrant,
this quadrant or this quadrant.
So we only have 25% support for this cut.
When you're cutting along the side, you have 50% support.
And the best one is when you're on an inside corner
with adjacent parts,
you have a hundred percent support around the cutter.
In this tool path, I use 6,000 RPMs
and a feed rate of 25 inches a minute
with an eighth inch cutter.
That's really conservative,
but I value process reliability.
When working with multiple parts,
you don't want to scrap an entire sheet.
In this next tool path, we're taking the same cut.
Again, each of the parts, except for the corner ones,
again, leaving 20 thousands of the material at the bottom.
And this feed rate, I went with 80 inches a minute.
No problem since most of these parts are fully supported.
In this toolpath,
we're working with the holes that are drilled through.
Now, in vacuum applications,
you don't want to drill through if that through feature
is inside your gasket pattern.
Once you break through, loss of vacuum, parts not held.
So instead of using a drill,
I'm just interpolated each one of these
as if they were bores and leaving about 10 thousands
of material at the bottom,
because then they can easily be chamfered out by hand
when we're done with the parts
and this toolpath takes care of the holes
for the inside parts
and I've patterned them so that we don't have
to keep picking the same redundant geometry.
This next toolpath pockets out the window at full depth.
And I also added a finishing pass radially
just to make this dimension nice and tight.
And finally, we are going to duplicate all the tool paths
of the roughing, but again,
we're just gonna machine it at full Z depth.
In fact, we're going 10 thousands below the part,
just so we know we have a clean break.
So we finish at full depth
so that that light cut is last.
Because once you break these parts individually,
they're on their own.
They're not being supported by the surface area
of their neighbors.
So the lightest cut should always be taken last.
Let's go out to the shop and cut these parts
and see how they turn out.
(bright upbeat music)
So the trick to holding smaller work pieces
is all about maximizing surface area
by using smaller diameter gasket
and using neighboring parts for added stability.
Keep that in mind and thanks for watching.
(bright upbeat music)