Note
Dimple Dies: Springback, Non-Springback, and Why Ripples Happen
What springback dies are actually for, why using them on ordinary sheet makes things worse, and the real cause of the ripples around a dimpled hole.
Dimpling is where a flush riveted skin is won or lost, and it is also where the tooling advice online is least reliable, because two different problems produce similar looking symptoms.
What a dimple die pair does
A male and a female die, matched, that form a conical depression around a drilled hole so a countersunk rivet sits flush with the surface. The included angle matches the rivet head, normally 100 degrees for standard aircraft flush rivets, with 120 degree pairs used for some pop rivet applications.
The critical point: dies come in matched pairs per rivet size. A 3/32 inch pair and a 1/8 inch pair are not interchangeable, and forming a 1/8 inch dimple with 3/32 inch dies produces a hole that looks nearly right and holds a rivet badly. See dimpling tools for sizes.
Springback dies, and the confusion around them
Aluminium formed past its yield point tries to return a little. Springback dies overform the dimple slightly, so that after the material relaxes the result is the shape you actually wanted.
Which sounds like something you always want, and it is not.
Springback dies are for material that resists holding a dimple. Harder alloys and thicker sheet, where a standard pair leaves a dimple that has relaxed too shallow to seat a rivet head flush.
On ordinary 2024-T3 in normal skin thicknesses they overwork the hole. The dimple ends up too deep, the material around it is stretched more than it needed to be, and you have made the ripple problem below worse in the name of solving a problem you did not have.
Practical rule: use non-springback dies unless you have a specific reason. Our standard 3/32 inch and 1/8 inch pairs are non-springback, at 34 and 36 EUR ex VAT, because that is what most work wants.
Why ripples happen
Ripples, oil canning, or a row of holes that looks like a piece of corrugated iron. Four causes, and they are usually confused with each other.
1. The hole was the wrong size
The most common cause by a distance. A hole slightly too large gives the die nothing to form against and the material displaces sideways instead of into the cone. A #40 hole for a 3/32 inch rivet and a #30 for 1/8 inch, drilled with a sharp bit. A worn bit produces an oversized, out of round hole, which is the same problem arriving quietly.
2. Too much force
A dimple needs enough force to form and no more. Hitting a C frame harder does not make a better dimple, it displaces material outward from the cone and that displaced material has to go somewhere. It goes into a ring around the hole, and when the holes are close together the rings meet and the whole row ripples.
3. Dies not aligned
If the male and female dies are not concentric, the dimple forms off centre. In a hand squeezer that shows up as a shallow side and a deep side. This is the strongest argument for a C frame on any run of holes: it holds alignment mechanically rather than depending on how well you are holding two things at once. At 299 EUR ex VAT it is a real expense and it is the tool that most visibly changes the finish on a wing skin.
4. Springback dies on material that did not need them
As above. Overformed dimples in soft sheet are a self-inflicted version of cause two.
Order of operations
Dimple after deburring and before priming. Deburring first, because a burr under a die transfers into the formed surface and becomes a stress point. Priming after, because dimpling a primed surface cracks the primer exactly where you least want a break in it.
Test on scrap of the same alloy and thickness first, every time the setup changes. Two minutes on an offcut against a skin you cannot unbend.
Close quarters, and the flange problem
Standard dies have a body diameter that fouls a flange or a rib. A close quarters set at 28 EUR has reduced bodies that reach where standard dies cannot. Cheap, and it turns an impossible hole into a normal one.
Keeping the dies
Dimple dies are matched pairs, small, and shiny, which makes them the single easiest thing in a sheet metal drawer to separate and lose. A mismatched pair produces exactly the symptoms above and nobody suspects the tooling, because the tooling is still all there, just not in pairs.
Which is why a die set is worth a foam pocket per pair rather than a box. A cut insert keeps pairs together and makes a missing die visible, and for a set of a dozen pairs that is the difference between a five second check and not checking.