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I’ve been working on an electric Dirtsurfer project that has many parts to be made from folded sheet aluminium. screenshot

Obviously the sheet metal needs holes in it for bolts. The sheet aluminium will be either CNC laser cut or CNC water jet cut.

I’ve got three options:

  1. Add holes to my design at the exact dimensions that I need them to be. I am placing myself at the mercy of the accuracy of the CNC cutter to ensure I have no slop which might let the bolts move. But no drilling!

  2. Leave the holes out of the design. I measure the cut patterns to find the right places for the holes and drill them myself. Using a drill will give me a clean hole that I can fully trust the dimensions of. This is what I’ve done in the past, but I haven’t been accurate enough with my placement.

  3. Undersize the holes in my design, and drill them out to the correct size later. I don’t have to measure anything, but still get a clean accurate drill hole.

Any practical advice from someone who’s done this before?

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  • $\begingroup$ Option 1 is the best way typically. $\endgroup$
    – Jonathan R Swift
    Jul 6, 2018 at 10:56
  • $\begingroup$ 2. You should build a jig if you need more than 1. $\endgroup$
    – joojaa
    Jul 6, 2018 at 12:50
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    $\begingroup$ Is the CNC a company or an amateur? $\endgroup$
    – bukwyrm
    Jul 6, 2018 at 15:34
  • $\begingroup$ It’s a company in my city that offers CNC cutting $\endgroup$
    – Hannesh
    Jul 6, 2018 at 20:20

4 Answers 4

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This answer only refers to laser cutting

Your main problem will not be the accuracy of the CNC machine, which will be way better than what you can achieve when measuring yourself.

The main problem with clean edges is the hole size, if they are too small, say under 6mm diameter (this also depends on the thickness of the metal), the initial puncturing for each hole will leave some residue that will cause unclean edges of the holes. The laser basically drills a small hole and then afterwards cuts out the shape of the actual hole.

For small holes, I would suggest an adaption of your 3rd idea. Some CNC machines can basically do punch marking, marking the center of the hole with a single short pulse. You dont have a lot of residue because very little material is removed, but the hole is marked accurately and you can afterwards drill exactly how you need it.

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    $\begingroup$ Additionally, sizing the holes to be only a small amount short of the correct size will allow for easier drilling with the large (final) size bit. If the hole is to be 6 mm as in the example, a 5 mm hole means easier drilling than if the placed hole was 2 mm. Of course, check with the laser cutting vendor for confirmation. $\endgroup$
    – fred_dot_u
    Jul 6, 2018 at 12:31
  • $\begingroup$ The waterjet can do the holes down to about a mm. but you will need sacraficial material between and around. The smaller the hole the less accurate it will be on the other side. $\endgroup$
    – joojaa
    Jul 6, 2018 at 12:49
  • $\begingroup$ It will depend much on the thickness of the metal sheet, the thinner it is, the less molten material will splatter around until the laser is through and the process gas can push it out below $\endgroup$
    – OpticalResonator
    Jul 6, 2018 at 12:55
  • $\begingroup$ Great answer, thanks! I will ask the CNC guys about punch marking. $\endgroup$
    – Hannesh
    Jul 6, 2018 at 20:16
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Similar to OpticalResonator's answer, I would suggest using the laser/waterjet cutter to mark the hole locations (although you can always mark the holes manually after cutting). You should probably use a punch press to cut the holes, if one is available in your machine shop. Then it's just a matter of selecting the exact die size for the hole you want to make.

We have a small, manually-operated punch press in the machine shop that I work in. Check with your machinist to see if you have something similar. A drill isn't the best tool for cutting holes in thin sheets, but it will get the job done. A punch press is intended for punching holes in sheet metals, but bear in mind that there is an upper limit to the thickness of material that you can cut with one.

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  • $\begingroup$ I should’ve added this to the question, but the sheet is between 3mm and 5mm thick. I will keep this in mind for thinner sheets though! $\endgroup$
    – Hannesh
    Jul 6, 2018 at 20:17
  • $\begingroup$ @Hannesh depending on the rating of the punch press available to you, they can do 0.25'' stock (~6 mm). You just need to check with your machinist first. $\endgroup$
    – ConjuringFrictionForces
    Jul 6, 2018 at 22:58
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TLDR: Ask someone and do tests.

I've worked with hobby level CNC mills (480 watt spindle) and CNC laser cutters (80-100 watt), and I've found that after doing a couple of test cuts, I know what I can and can't do. As ConjuringFrictionForces mentions, talk to your machinists to see what you're likely to get for a result.

You're likely going to have to do a couple (several) prototypes, so looking at the results, with measuring, will help you figure out what you need to do. It might be cutting a 6mm hole at 5mm and punching/drilling it out, or it might be cutting it at 6.5mm to allow room for the bolt to slip through easily during assembly. (BTW, you usually need some "slop" for bolts, otherwise they end up being screws. Besides, not all bolts are created equal, unfortunately.)

As much time as I've had to use my machines, I still rely on actual tests to see what result I'm going to get with a material and machine combo. Your machinists probably have a better idea of what their machines can do. I don't use my machines 8 hours a day, and I'm still learning what my home built CNC mill can do.

I've already up-voted the other 2 answers and a comment, so this is just my 2 cents with the experience I've earned the hard way.

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What I have done before from a very short period of time, I have used a CNC laser cutter to cut 12mm sheet of stainless steel, which I'm sure that you are going to use a thinner sheat metal. My design was something about 135 x 18 mm and the final result was completely nice. But you have to take in consideration that laser has its own cutting diameter "pierce" ( some thing near 0.2 mm - according to the machine power and the sheet metal thickness ) . Think of it as the tool used to cut metal in CNC router. Ensure that the size of this pierce is taken in consideration while cutting to get your desired hole diamter. Machine operators can handle this issue without any problem. You just have to manage this issue with them.

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