The two halves of a laser job
Vector cutting and raster engraving are different operations with different time behaviour, and mixing them up is the usual source of a wrong estimate. Cutting is a length problem: the head follows a path, so the time is the total path length divided by the feed rate, multiplied by the number of passes. Engraving is an area problem with a hidden multiplier. The head sweeps back and forth in lines, and the number of lines it has to make is the height of the engraved area times the lines per inch. So the time is the area times the line density divided by the speed, and doubling the density doubles the time for a result most people cannot see from more than a foot away.
That relationship is worth internalising. A part with twelve square inches of fill at three hundred lines per inch and four hundred inches per minute takes nine minutes of raster. The same part at one hundred and fifty lines per inch takes four and a half. Nothing else on the job responds to a single setting like that.
Why the machine beats the formula
The arithmetic above assumes the head moves at the set speed the whole time. It does not. It accelerates from a stop, decelerates into every corner, and on small features it never reaches the programmed feed rate at all — a half inch square cut at forty inches per minute will not run at forty inches per minute anywhere except briefly in the middle of each side. Add the rapid moves between separate shapes, the sorting order the software chooses, and any pauses for air assist or z moves, and the real time on a detailed part can be well over what a length divided by a speed suggests.
The other variable is the material. Feed rate for a clean cut depends on the material, its thickness, its moisture content, the colour of the dye in it, the tube power on that particular day, the state of the optics and where you set the focus. Two sheets of nominally identical plywood from different batches can want different settings. Everyone who runs a laser has a settings sheet built from test cuts on their own machine, and that is the only source for the speed you should type into the field above.
Handling is usually the surprise
On a short run of small parts, the beam is often the minority of the elapsed time. Loading a sheet, focusing, framing the job, running a test, unloading, picking parts out of the cut sheet, peeling masking off both faces and cleaning the edges all happen at human speed. This calculator separates them so the split is visible, because the answer to a slow job is usually to nest more parts per load rather than to push the feed rate. Reloading five times when you could have reloaded twice costs three setups.
Masking deserves a specific mention. Applying and removing transfer masking on both faces of an engraved acrylic part can genuinely take longer than the engrave. It is the right choice when the alternative is smoke staining you cannot clean off, but it is a real cost that belongs in the handling figure and not in a footnote.
Materials, fumes and fire
Two things about laser cutting are not negotiable and they have nothing to do with efficiency.
The first is what you put in the machine. PVC and flexible vinyl contain chlorine, and lasering them releases chlorine compounds that are harmful to breathe and that corrode the machine — the rails, the optics and the electronics — from the inside out. This is not a subtle long term effect; shops that have done it by accident describe finding the damage. Other materials release their own hazards, some of them serious, and a great many of the sheet goods sold for other purposes have adhesives, coatings or fire retardants in them that nobody has characterised. The rule that follows is simple: know what the material is before it goes in, from a source that actually states the composition, and run proper extraction. This page is not going to give you a list of approved materials, because a list reads as permission and the responsibility for identifying a material sits with the person feeding it in.
The second is fire. A laser cutter is a device that applies enough energy to burn through material, in a box, over a bed that collects combustible offcuts, and a flare-up that goes unnoticed for two minutes becomes a fire. Unattended machines are a well documented cause of workshop fires. Stay in the room with it, keep the bed clean, keep something to put a fire out within reach, and do not start a long job as you leave.
Related
Work out how many parts fit the sheet with the sheet nesting and yield calculator before you set the parts per load here. For other cutting processes, see plasma cutting cost and machining speeds and feeds. If the laser is cutting sign letters, size them with the sign letter height calculator. For pricing the rest of the bench time, see shop labour estimate and 3D print quote.
Questions people ask
How do I find the cut path length for my design?
Your CAD or laser software will report it, usually as a total path or perimeter length for the selected vector layer. In vector editors it may appear as a path length measurement or you may need a plug-in. If you have no tool at hand, approximate it: sum the perimeter of the outline and the perimeter of every interior hole. Perimeters are easy to underestimate because interior detail adds up quickly, so if the part has a lot of small holes, measure rather than guess. Remember to count each pass separately, which the passes field handles for you.
Why is the machine estimate always longer than this one?
Because the machine plans the actual toolpath and this does not. It knows the acceleration and deceleration profile, so it knows the head never reaches the set feed rate on short segments and slows into every corner. It also knows the rapid moves between shapes and the order it will cut them in. On a design made of large simple shapes the two numbers land close. On a design full of small features the machine figure can be a good deal higher, and the machine is the one that is right. Use this to quote before the file exists and the machine to confirm once it does.
What speed should I enter?
Whatever is on your own settings sheet for that material at that thickness on that machine. There is no universal number. Tube power, optics condition, focus, air assist, material batch, moisture content and even the dye colour in the sheet all move it. Shops build a settings chart by test cutting a grid of speed and power combinations on a scrap and writing down what worked. If you do not have one for the material in front of you, make one before you run a job on it, not during.
Does lowering the LPI hurt the engraving?
Less than people expect, and it depends on the material and the viewing distance. Raster line density controls how finely the sweeps overlap. Above the point where the lines merge visually, extra density buys you nothing but runtime, and where that point sits depends on the beam spot size, the material and how close anyone gets to the finished piece. On photographic engraving into a hard surface the density matters. On a logo in wood that will hang on a wall, halving it is often invisible and halves the time. Run a test tile at three densities on your material once and you will never guess again.
Can I put any sheet material in a laser?
No, and treating it as a materials question rather than a safety question is how machines get destroyed and people get hurt. PVC and vinyl release chlorine compounds that are toxic to breathe and corrosive to the machine, and they must never go in. Beyond that, many sheet goods carry adhesives, coatings, retardants or fillers that nobody has tested in a laser, and unknown composition means unknown fumes. Identify the material from a source that states what it is, run the extraction the machine was designed to have, and if you cannot establish what something is, do not cut it. Add to that the fire risk from any material at all, and the standing rule is that the machine does not run without a person in the room.