CNC Machining of Cavity Panels
A cavity panel is a plate with pockets, ribs and a thin outer skin. This page explains how that geometry behaves under cutting load, which wall and rib sizes survive machining, and when 5-axis access beats a 3-axis setup. Read it before you release the model, not after the first scrapped plate.
What a cavity panel actually is
A cavity panel is one piece of stock with material removed from one or both faces, leaving a floor, a perimeter rim and an internal rib pattern. Electronics housings, robot arm links, EV battery tray covers, antenna plates and medical instrument enclosures all use this shape. The part looks simple in CAD. It is not simple to cut.
The reason is stiffness. A solid plate resists cutting force because material runs continuous from edge to edge. Once you pocket it, the remaining material is thin and unsupported. Every pass removes stock that was bracing the part, so the plate gets weaker as the job runs.
A 300 × 200 mm aluminum plate at 12 mm thick is rigid. Pocket 8 mm out of the middle and the floor becomes a 4 mm membrane. Push a 16 mm end mill through it at 3,000 rpm and the floor lifts, chatters and springs back. The finished pocket measures shallow in the middle and deep at the corners.
So the whole job is a balance. You want the lightest part that still holds tolerance and does not sing during the finish pass. Everything below is about finding that balance before the first tool touches metal.
How pockets change the load path
In a solid plate, cutting force travels from the tool tip outward through continuous material to the vise. Pocketing breaks that path. Force now has to bend around the pocket floor and travel along the ribs.
Ribs work in two ways. A rib standing 20 mm tall resists bending far better than a 4 mm floor of the same width, because stiffness rises with the cube of height. A 5 mm × 20 mm rib is roughly 100 times stiffer in bending than a 5 mm × 4 mm floor section.
That is why deep, narrow ribs let you remove a lot of mass. The catch is tool access. A rib 20 mm tall and 5 mm wide needs a cutter no larger than 4 mm to clear both sides, and a 4 mm cutter can only reach about 12 mm deep before it deflects.
Rib spacing matters too. Keep the gap between ribs at 4–6 times the cutter diameter. Tighter than that and the tool cannot clear chips. Wider and the floor between ribs starts to drum on the finish pass.
Wall and floor thickness limits
There is no single minimum thickness. The number depends on material, part size, how the part is held, and whether the panel is open on one side or both.
For aluminum panels under 300 mm long, a 1.5 mm floor and 1.5 mm rib are practical at ±0.05 mm. Below 1.0 mm the floor deflects under its own clamping load and the finish pass cuts air in places.
Stainless and titanium move the line the other way. A 316L panel at 1.5 mm will chatter unless you slow the finish pass to a light radial stepover, often 0.2 mm or less. That doubles cycle time.
Double-sided pocketing is the hard case. Once both faces are cut, the part has no neutral axis left. Use a sacrificial tab or leave a carrier frame and cut it free at the end.
If the design needs a 0.8 mm floor, ask whether the panel should be machined at all. A formed sheet metal part or a molded housing may hold that geometry better and cheaper.
3-axis and 5-axis access compared
Most cavity panels start as a 3-axis job. You face the stock, rough the pockets from the top, flip the part, and cut the back. Two setups, two datums, and one flip that adds error.
5-axis helps in three specific cases. Pockets with drafted or curved side walls. Ribs that are not perpendicular to the plate face. Holes and bosses that sit on more than one plane.
With a Ø400 mm rotary table and simultaneous motion, a 5-axis center can cut a drafted rib in one continuous pass and keep the tool normal to the surface. Tool overhang drops, so you can use a shorter, stiffer cutter on the same feature.
5-axis is not automatically faster. Programming takes longer and the machine runs slower on simple flat pockets. On a panel that is all flat geometry, a 3-axis setup with two good fixtures usually wins.
Our 16 simultaneous 5-axis machining centers handle panels up to 4,000 × 400 × 150 mm. Smaller plates run on the 750 × 1,150 × 550 mm and 600 × 600 × 600 mm travels.
Holding tolerance on a thin floor
A thin floor moves during the cut and again after unclamping. Rough the pockets leaving 0.5 mm on the floor, let the part rest, then take a light finish pass. The rest period matters more than the cutter choice.
Measure the floor with the part still on the fixture. If you only inspect after it comes off the vise, you are measuring springback plus machining error and cannot tell which one is which.
For a ±0.005 mm callout on a pocket depth, we rough, semi-finish, then finish with a 0.1–0.2 mm radial stepover and a sharp, coated end mill. Heat is the enemy. Air blast beats flood coolant on thin aluminum floors.
Clamping pressure is a real variable. Vacuum fixturing spreads the load evenly across the floor and avoids the local dimple a vise jaw leaves. On panels under 2 mm, vacuum or a low-melt fixture is the safer route.
Every panel we ship is inspected before it leaves. Raw material is checked on arrival, dimensions are monitored in process, and final inspection reports are available on request.
Which setup for which cavity panel
Match the geometry to the machine before you quote the job.
| Panel feature | Best setup | Why |
|---|---|---|
| Flat pockets, single face | 3-axis | Fewest setups, fastest cycle |
| Pockets on both faces | 3-axis, two fixtures | Flip adds one datum error only |
| Drafted or curved pocket walls | 5-axis | Tool stays normal, no stepping |
| Ribs not square to the face | 5-axis | One pass instead of several |
| Floor under 1.5 mm aluminum | 3-axis + vacuum | Even clamping load, less spring |
| Ø400 mm round features | 5-axis with rotary table | Cut in one continuous rotation |
| Panel over 2,000 mm long | 3-axis gantry travel | Fits 4,000 mm bed |
| Prototype, 1–5 pieces | 3-axis + soft jaws | No fixture cost, fast turn |
The short answer
If the panel is flat, single-sided and thicker than 2 mm, machine it on 3-axis and spend the money on a good fixture. If it has drafted ribs, curved pockets or features on more than one plane, go to 5-axis.
Common questions about cavity panels
What is the thinnest floor you can machine in aluminum?
Around 1.0 mm is the practical floor for a small panel under 300 mm long, and it needs vacuum fixturing and a light finish pass. Below that the floor deflects while it is held, so the measured thickness varies across the pocket.
If the design truly needs 0.8 mm, a formed or molded part will usually hold that geometry with less risk.
Why do my pockets measure shallow in the middle?
The floor is lifting under cutting force. A long end mill pushes down at the tip and the floor springs back after the tooth passes. The center of a large pocket has the least support, so it springs the most.
Reduce radial stepover, shorten tool overhang and check that the part is not being pulled up by the vacuum or vise.
Do cavity panels always need 5-axis machining?
No. Most flat, single-sided panels are cut faster on a 3-axis machine with a solid fixture. 5-axis pays off when the pocket walls are drafted or curved, or when features sit on several planes and would otherwise need extra setups.
A second setup adds a datum shift. On a tight tolerance that shift is often the largest single error in the part.
How does clamping affect a thin floor?
A vise jaw puts load on a narrow band, which dimples the floor locally. Vacuum spreads the same force over the whole face and leaves the pocket flat.
On panels under 2 mm we usually machine with the part on a vacuum plate or a low-melt fixture and check flatness before unclamping.
Can ribs be machined with sharp internal corners?
No. Every cutter has a corner radius. A 6 mm end mill leaves a 3 mm radius in the corner of the pocket. If the drawing calls for a sharp corner, that detail has to be added by EDM or changed in the design.
Tell us the corner radius you can accept. We will pick the largest cutter that fits the rib gap.
What material is easiest for a thin cavity panel?
6061-T6 aluminum is the most forgiving. It cuts fast, holds a thin floor reasonably well and is easy to source in plate. 7075 is stiffer but more prone to stress movement after pocketing.
Stainless 316L and titanium hold shape once cut but demand slow finishing passes, which raises cycle time and cost.
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