Holding Down and Machining Curved Material

Advice on rigging up clamps or vacuum jigs to hold down arched sheet material for CNC machining. March 4, 2009

I have a situation whereby I need to do volumes of a simple chair back. See the image below. I am unable to use the vacuum feature (because of the curve in the material). I wanted to know of any alternate suggestions for holding the board in place. The material used is 8mm chipboard. Has anyone had a similar job or experience? How were you able to resolve this issue?

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Forum Responses
(CNC Forum)
From contributor A:
You could easily make a fixture that will match the arc and supply vacuum with a hose or directly from the table. You have to make sure your machine will have enough stroke in Z-axis to rout the shape. A three axis router will not make a perpendicular edge on the ends. If you need the edge perpendicular around the profile you will need to use a 4 or 5-axis machine.

From contributor I:
You could build a vertical fixture and then cut the shape with an aggregate set to 90, but that would not give you great feed speed. Also, depending on what software you are using, getting the toolpath correct could be troublesome. Looks like a fun project with the right machine, a nightmare with the wrong machine/software. Been there and done that more than twice.

From contributor M:
If you have a serious volume of these I would use a dedicated modified spoilboard. Take a piece of melamine and put it on your table. Drill a few 1" holes in it where you want to place the part and a groove for a gasket. Make a curved buck that matches the back of the chair back on one side like AK suggested. Make it flat on the other side and use MDF.

Laminate the two faces that make up the top and bottom of the buck so you don't leak air and screw the buck to the dedicated melamine deck board from the back. Then route holes through the buck matching the ones in the table, a gasket groove slightly smaller than the buck for the curved piece to sit on and some vacuum channels on the face of it. This will suck the part onto the buck when the vacuum is turned on.

Be careful with height. Any angles you need can be had either with dedicated tooling that has an angle built into it, or milling the edges with a lollipop cutter, (though this will take more time). It might be easier to do the edges by hand later. Once you get it working well for one part, you could build maybe three or seven more and put one or two in each vacuum zone.

From contributor J:
To contributor M: What is a curved buck?

From contributor R:
The photo seems to show a laminated back, and I'm guessing you need to machine the top and bottom to shape. The problem with a curved vacuum fixture is that if all the parts are not perfectly the same curve, holding power might be compromised.

I'd be looking at a curved base but use clamps. Clamp the back edge while cutting the front, and a second station next to it that holds it for milling the bottom edge. If you'd like, I can send you a slideshow file that shows a similar fixture I built for cutting cabinet door stiles and rails.

From contributor M:
A buck is a form, in this example a shaped block of MDF with a hole or two to let the vacuum through. It would be the shape of the curve on one side and flat on the other. The vacuum would be as strong there as anywhere else as long as you have good seals.

Think of it as a shaped pod with gaskets on both sides. It is a little trouble to make, but not too bad, and if you are doing lots of repeats of the same thing it's worth it.

Contributor R is right in that if you have a lot of inconsistency in your parts it can cause problems, but I'll bet you are making the blanks yourself, right? Even if not, whoever is using it is using a consistent process.

Clamps are a possibility here, but it's a lot of time to make two runs out of what could be one, and if the blanks are that inconsistent clamping itself may cause part movement.
I think that some form of vacuum fixture will do this for you, whether it is one as I describe, or some other similar application.

From contributor R:
To contributor M: It does not have to take any more time to mill the parts in two operations. A two place fixture, one holding the part to mill the front, and the other to mill the back can mill both sides in a single program run. Building two of these fixtures allows the operator to be unloading/loading a fixture while the other is being machined.

I've worked with way too many laminated parts to rely on them all being exactly alike. Yes, you can use thicker gaskets to keep leaking to a minimum, but vibration is always an issue for a clean cut, and the thicker gaskets tend to allow more vibration.

From contributor M:
To contributor R:
Pendulum processing is a great way to crank it out. Don't get me wrong, I am not saying clamps won't work well, they will. I am also saying that properly gasketing parts works well under the right conditions. Maybe it is best for this application and volume, maybe not.

I owned a Multicam MG for six years. To pendulum with clamps is something I have done many times. I have also pendulumed with continuous cycle and working the vacuum valves manually.

If I had 200 parts to do I would likely keep it very simple. If I had several thousand I would be looking harder at cycle time and volume, especially if it were an ongoing contract. If your blanks are reasonably consistent a vacuum jig will work pretty well. If they are not, you make an excellent point.

In the end it is you on your shop floor looking at your material and making your judgment. You know your machine, experience level and equipment. Either way, let us know what you decide and how it works out.

From contributor R:
Right again, there is always more than one way to do it and get it right. One of the aspects of my business is to try to tailor which is best for the existing shop environment and culture. I used to sell install and train on MultiCam machines several years back, the MG is a workhorse.