Monthly Archives: February 2014

Laser Cutter Tips

In the course of making my 40-node Raspberry Pi cluster, I learned quite a lot about working with a CNC laser cutter. These are a few of the tips I have for making laser-cut parts that I used for this project.

Tip 1: Acrylic Prep

Before cutting parts, first prep a suitable piece of your acrylic.

Tip 1 - Add soap and water to your acrylic Tip 1 - Spread soap and water across the acrylic surface Tip 1 - Make sure surface is thoroughly coated

  1. Peel one side of your acrylic. It’s best to peel the blank side, so you can easily identify your material later.
  2. Apply soap to the peeled side, as shown in the pictures. You can use mostly water with a little soap. I’ve found that more soap is better for keeping vaporized acrylic from condensing back on the workpiece, but less soap is a little easier to rinse off. Find the balance that works best for you.
  3. Place the acrylic on the cutting bed with the paper side down.

Don’t prep all of your acrylic at once, as the paper/film tends to protect it a lot better from scratches than .soap does.

Tip 2: .svg Color Codes

I generally use the following guidelines to color code my .svg files:

  • Cyan: 1st pass – Lightly etch at minimum power
  • Red: 2nd pass – Interior holes – Cut all the way through
  • Blue: 3rd pass – Edges – Cut all the way through
  • Green: Alignment guides, etc. – do not cut (0 passes)
  • Black: Notes – do not cut (0 passes)

Black and green are interchangeable. So are blue and red, so long as your grid is reasonably level. With this in mind, I occasionally break my color code a little in some files.

Tip 3: Watch Your Job

The following video is one I shot to give an idea what the process of laser cutting looks like. Don’t watch it all, unless it just entertains you. However, keep in mind that you need to keep a close eye on your cutting jobs while they run to ensure that your cuts are good and to be sure your workpiece or laser cutter hasn’t caught fire. Yes, that does happen occasionally! Be prepared for it!

Tip 4: Dealing with Errors

You can see in the above video that I made an error in laying out my cut, causing some of the lines for one of my parts not to cut in my first run. (The first run ends at 12:34, at which point my second camera also stops recording.) I remedied this with a second run, where I set all of the already-cut lines to a single color, sent the cut job to the driver software again, and set that color to 0 (zero) passes. This is shown in the 4 screenshots included below. Use this technique if you happen to forget something and you don’t want to waste material. Just remember not to move your workpiece before you cut the remaining lines!

Laser Cut Error Step 1  Laser Cut Error Step 2 Laser Cut Error Step 3  Laser Cut Error Step 4

Tip 5: Wash and Dry

After your acrylic is cut, peel your parts, wash them, and dry them with lint-free cloth. Even the “lint-free” microfiber cloths you see in local stores can put out quite a lot of lint while you work. Experiment a little, in very good lighting, before choosing drying cloths for a big project. You’ll be more happy with the end result that way.

A technique I found useful was to pre-dry parts with a large high-quality terry cloth towel, then buff them off with a microfiber towel. This worked better than using microfiber alone, because the terry towel could hold a lot more water.

In the last pass of drying/buffing a part, reduce lint by passing over the part in one direction, as opposed to going back and forth over it.

Tip 6: Measuring Kerf

Many parts will simply work better if you adjust them for kerf (effective width of the laser beam as it cuts). The required adjustment will vary depending on your model of laser, the settings you’re presently using, how clean your laser is, and how well it’s currently focused. Consequently, I can’t adjust for this offset in my files. However, the following procedure has worked well for me so far:

  1. Cut six to eight 5 mm circles out of your chosen acrylic. If you cut eight, you can afford to lose two.
  2. Measure the width of these circles with a caliper. Record the list of results.
  3. Toss out the highest and lowest measurements.
  4. Average the remaining values. (You should get something a little less than 5 mm.)
  5. Subtract the average from 5 mm. (You should hopefully get a fractional value.)
  6. Divide by 2. (We only want to adjust by half of the beam width.)
  7. Adjust by 80% of this value.

As an example, suppose the average circle measured 4.80 mm. That would mean the width of the beam was 0.20 mm. Halving that value gives us a beam radius of 0.10 mm. 80% of that values is 0.08 mm, which is how much we should adjust by.

Do not simply adjust by the full radius of the beam, because some of the parts would be overly tight and a few of them simply would not fit. Even with my procedure, you may need to sand some of your tighter parts down a little if you’re working with little margin for error. An emery board is good for this if they’re close. A rotary tool with a small sanding drum is better if they are not.

Once you’ve found an offset that works well with your laser and settings, it will likely work well for a large number of cuts, so long as nobody dirties up your laser or ruins its calibration.

Tip 7: Adjusting Kerf

In Inkscape, I adjust for kerf with the following procedure, as I’ve found it to be very precise:

Step 1: Create a resizing rule – I generally use a horizontal and vertical line, each as long as the resizing width, connected at one point. But use what works best for you. It should have no fill and a very narrow stroke width, such as 0.002. The width you choose should depend on how well you can see a narrow stroke when you’re fully zoomed in.

Kerf Adjustment 1

Step 2: Select the exact poly line in question by using the “edit paths by nodes” tool. Reduce the stroke width to the same narrow poly line width you used above. Keep this poly line selected with the “edit paths by nodes” tool.

Kerf Adjustment 2

Step 3: From the “Path” menu, select “dynamic offset”.

Kerf Adjustment 3

Step 4: Near the newly-appeared white diamond, move the resizing rule (from step 1) in line with the poly line that needs adjustment. Check one last time to be sure you’re adjusting in the correct direction. Blue lines should generally be adjusted outward, while red lines should generally be adjusted inward.

Kerf Adjustment 4

Step 5: Drag the white diamond until the poly line has been fully resized. This can be a slow process, but don’t let go of the diamond until the poly line is where you want it. With practice, you can learn to get it where you want it in 2 or 3 “ticks” of the offset resizer. If you can’t seem to get it exactly where you want it, aim for resizing a little less instead of a little more. Also, don’t worry about the funny rounded corners – they won’t matter in the end.

Kerf Adjustment 5

Generally, you can get the resizing extremely close with this method. However, if you can find a way to get the inset and outset tools to work equally well for your purposes, I suggest using those instead.

Tip 8: Cut Extras of Small Parts

Small parts more easily get lost through the grid and during processing before they’re glued in place. But your current laser cutting job will often have small parts and some slightly larger cutout holes. When possible, place one or two extra of each small part in smaller cutout holes. The material in these holes would usually end up being waste material anyway, so the only real cost for making spares is the laser time.