Design for Laser Cutting: Toolpaths July 21 2016, 0 Comments

Design for laser cutting is a pleasant introduction to CAD for a number of reasons. There are many software packages that can be used to generate the correct type of design files including some that may be familiar - such as Adobe Illustrator or Inkscape - from other design practice. Modern desktop laser cutters tend to use vector files containing paths that are simple to generate and edit with lots of tutorials available online. It is possible to be designing and making physical things within hours.

When designing for laser cutting one tends not to take the width of the laser beam into account - meaning that a relatively complicated step used in more traditional CNC machining - toolpath generation - can be ignored. The toolpath of a job is the path through space that the tip of a cutting tool follows to produce the desired workpiece.

Imagine a CNC milling machine: it uses cutting tools similar to drill bits to cut away material. These tools have a diameter that must be taken into account when calculating the toolpath. Say we want to cut out a circle of 50mm diameter from a sheet of MDF using a 6mm milling bit. If we draw a 50mm circle in our design software and export that as our toolpath the milling machine will follow the toolpath and cut out a 44mm circle.

If a 6mm milling bit follows a path it will cut out a 6mm groove along said path. This results in 3mm extra cut from the inside and outside of the path. See Figure 1 below.

Figure 1 - cutting with a 6mm milling bit
 

By generating a toolpath we can resolve this problem and cut out a workpiece of desired size. See Figure 2.

Figure 2 - following a toolpath with a 6mm milling bit

 

Milling bits need to be substantial enough to cut through material - it is possible to buy bits with a very small diameter but these have drawbacks: they are delicate, break easily and cannot cut material very quickly. Laser cutters cut sheet material with a focused laser beam rather than a milling bit. This beam can be very small (high quality machines down to 0.1mm or smaller, cheaper machines up to 0.5mm or larger).

Figure 3 - cutting with a focussed laser beam
 

Comparing Figure 1 and Figure 3 highlights the difference between the outputs from a laser cutter and milling machine without extra toolpath generation. For much of the design work we currently do (cases for electronics projects, jigs, tool holders, tools for our workshop, signs etc) we do not need 100% accuracy to within thousandths of a mm. If we were building engines or rockets with very high tolerances we would need a much higher level of accuracy, generating toolpaths from design files and we would hope to be using very expensive tools and machines!

The width of the laser beam in a relatively low cost desktop machine is not negligible - in fact we can use it to our advantage as shown in Figure 4 below.

 

Figure 4 - using the width of the laser beam to our advantage
 

The above is often overlooked when designing for laser cutting and simply taken for granted. Some may argue that not generating toolpaths is poor engineering. Ignoring the above information and not wanting to understand may indeed be poor engineering practice but this is up to the individual maker, engineer or designer to decide.

For our needs we can continue to use our design files as toolpaths and produce good usable results for both rapid prototyping and manufacturing physical things. If we wanted to mill out the above parts using a CNC milling machine we would have to use a more complicated work flow with additional steps - something we will cover in later posts.

 

References + further reading

- laser information: http://www.parallax-tech.com/faq.htm

- toolpath definition: https://www.wordnik.com/words/toolpath