I bought a second-hand dimple die a couple years back and it didn’t seem amazingly cheap, so I decided to use CAD to draw my own DWG/DXF files.
There is something about Dimple Die holes that just shouts ‘Race Car!’
Dimple die or Hole swage tools add that ‘trick’ look to any fabricated Steel or Aluminium project. They not only lighten a panel, with careful choice of size and location, they actually add rigidity. With rigidity, the lack of flexing can add strength and long term could prevent stress fractures.
The hole saves weight and the dimple maintains or adds rigidity.
That’s right, they lighten a panel whilst adding strength!
If you are adding loads of bracing to a chassis, you are adding weight. With that extra weight comes extra stress. Carefully placed dimple die panels can add that extra bracing without adding loads of extra mass.
I learnt when making my A pillar that using thicker metal is not always the answer. I’d made it out of 18awg flat steel and it lacked strength and rigidity. When I added thicker steel the flexing simply moved. Adding thicker steel adds something every car maker hates: weight. To my surprise, I added a loads of rigidity with the addition of some super light dimple die panels.
If I were to re-make the A pillar, I’d add some raised corrugation on the bead roller AND internal dimple die plates. With the added strength, I could reduce the amount of 3mm plate I’d added. Obviously I’m not going to add dimpled holes to everything, however in certain situations they could work great.
Hydraulic Hole Punch
I have a Slug Buster hydraulic hole punch, a cheap but handy piece of kit. I bought this for less than it cost for one Dimple Die.
It cuts the following sized holes:
The pull rods use M20 x 1.5 and M10 x 1.0 threads.
The slug buster uses a stamping technique called coining. The metal is sheared without loads of heat being transferred.
I used my new hand made die to to make some strengthening plates:
The reason I use a hydraulic hole press, is the hole usually has a super clean cut edge and can be done in seconds. I could also use Q-max cutters, but they often fetch more money than their hydraulic cousins. Using drill type cutters, I find the holes never look so crisp, round or sharp and have that nasty tendency of wandering. In contrast, with a sharp hydraulic cutter, drill cutters need the hole thoroughly deburring.
Inspecting the hole’s edges
What I didn’t want was any cracking around the hole edge. Imperfections could turn into stress fractures or tears whilst the hole flare was added. The deeper the flare the more likely there is to be cracking. Therefore, for harder steel, slightly oversize the initial hole.
A flared hole with a tear or fracture is weaker than a panel without a dimple.
My hydraulic punch takes only one pump to flare a hole so the pressure applied is fairly consistent. Without a press, you have to wrench a bolt up and the pressure would be inconsistent and ultimately lower.
Although I’m using these Dimple Dies in conjunction with a hydraulic hole punch, you could also use them in a traditional hydraulic press, fly press or even carefully in a large vice. Theoretically, you could put a grade 8 steel nut and bolt through the middle and just wrench them tight. Using a vice or a nut and bolt will probably produce a less than perfect result, which could; if not closely inspected; add weakness.
Force required to Punch a Hole in Sheet Metal
Punching Force = Π x D x t x σS
D = Hole Diameter = 1.5″
t = Sheet Thickness = 0.0478″ (18awg)
σS = Sheer Strength = 46,050psi (Carbon Steel)
Force (18awg) = 3.1416 x 1.5 x 0.0478 x 46,050 = 10,373lbs (5.2 US Tons)
My 8 ton hydraulic punch was perfectly adequate. This highlights how much force is necessary when a hydraulic press isn’t available.
Dimple Die Tips
I did a lot of research on when and why dimple die panels are good and when they are bad.
A carefully flared hole, produced with no signs of stress, distortion or fracture will be stronger than a simple hole.
However, the following rules must be applied.
Flared holes should not be placed in high stress areas as they can reduce the overall strength.
In a high stress area, a seamless panel with no holes will be stronger due to the lack of potential stress concentrations or fracture points.
Ideally, flared holes should be located in low stress / flex areas, such ‘web components’. By removing mass and by flaring the hole, the area will benefit from additional strength whilst having reduced weight.
Without FEA, or years of experience, knowing the optimal hole position isn’t simple. Therefore keeping flared holes to conservative areas would increase strength and safety whilst also maintaining minimal weight.
Additional Mass / Weight in components can lead to extra stress being placed on leveraged components. Try imagining a heavy hammer head on a cracked handle.
Direction of Flare
In high stress areas, where flared holes are necessary, the flare should protrude to the side that bends outwards under load. The aim is to minimize the lose of strength due to material removal.
Here’s some very interesting articles:
Mark out the centre line where you want the holes. Now measure the overall width (not the hole size) of the desired hole cutter. Divide that width by two, to get the radius and mark a second parallel line. Clamp some 1 inch angle iron down with the vertical edge on that 2nd line. Now your bolt clearance holes only have to be roughly accurate as with the cutter firmly butted up to the angle iron the edges will always be in line.
If you have over-sized the centre hole, make sure you accurately centre the dimple die. If the dimple die is off-centre it can tip or place uneven pressure around the flare. This can cause the surrounding metal to distort. The more pressure applied, the less distortion there will be.
Inspect for Sheet Metal Coining
Coining is a form of precision stamping in which a workpiece is subjected to a sufficiently high stress to induce plastic flow on the surface of the material.’
The hydraulic hole punch uses coining to create the hole (desirable). What I was worried about, was placing so much force onto the dimple die that it also induced coining (undesirable). Coining on the flared edges could leave fracture points or stretching in the surrounding metal.
If insufficient force was applied, the surrounding metal could be left uneven. Luckily, I think due to the consistent force of the hydraulic punch, warping was minimal.
Therefore when I flared the holes, I closely viewed the die whilst sensing the punch pressure. I wanted to make sure that both edge surfaces made contact, but I didn’t want to leave a circular impression.
If you cut several dimple die holes into one piece, you can find that the panel distorts. The same applies if you are pressing thick metal with a weak press.
Always ensure that the dimple die is 100% central to the hole. This can be tricky if you have an over-sized central hole. If the dimple die is not centralized, a variable amount of stretching is applied around the hole. With uneven pressures the die can also tilt .
When pressing, make sure the two edge surfaces of the die come in firm contact with the metal, but not too much to start ‘coining’.
If there is distortion, flip the fabricated piece over and place the female die centrally over the hole. Place the fabricated piece and the female die on a flat surface and press down firmly to flatten the metal. Do not press so hard that you begin stretching the metal or ‘coining’.
Dimple Die CAD
I’m building my car body from 18awg steel so hopefully the dies don’t need to be super tough. I had some EN8 (1040) steel left over from when I made my English Wheel, which is fairly hard steel and should be good for dimple dies. I’ve seen others made of aluminium and heat treated 4130 chrome moly.
The Slug Buster uses a M20 x 1.0 pull rod, so I could thread the dies to match, however I don’t have a tap that size, so I just put a clearance hole through the middle. I will then use half a Slug Buster cutter as a nut.
This guy has done it too:
AutoCad DWG Files
I’ll post the rest here as soon as I finish them.
The chamfers are at 45º although some prefer 33º.
These sites are worth a read:
OD female = OD male + 2*material thickness*tan(flare angle)
18awg = 1.0237mm
16awg = 1.2908mm
32mm + 2*(1.0237mm)*tan(45 degrees) = 32 + 2*1.0237*1 = 34.5816
I also scaled the distance to the 45º angle by ≈ 1.24
Making Dimple Dies
On my 60 year old lathe, the first set of cutters took 3 hours to make. I can now see why they cost so much to buy. Ignoring time and electricity, my 1st pair cost me less than £8. They weren’t so cheap once I had them heat treated; in fact they worked out £5 cheaper than off the shelf items. For a lathe novice, like me, getting the internal bores right was rather difficult. I’m happy to say though, they work very well.
With the DWG/DXF files I contacted a local CNC machine shop. They quoted £108 the pair (without heat treatment). That’s over twice what it would cost to buy off the shelf heat treated items.
It’s not economical to get a CNC shop to make the dies made from DWG/DXF files unless you are buying in bulk. Having made one, I’d buy them.
If you want to make your own dies then use the above files, but I’d recommend heat treating them too. Although ‘simple looking’, for a lathe novice, they aren’t the quickest to make. A full set would take me a weekend to make.
To get the promised ‘added strength’, you need to take a lot of care. That care is not just in the fabrication but also at the planning stage.
Realistically, you also need hydraulic hole cutters and associated presses. Doing things on the cheap or adding crude flared holes to every panel would ultimately reduce strength and potentially add fracture points.