Finding true focus with a ramp test

The beam is only at its smallest, and therefore its most powerful, at one exact distance below the lens. A focus tool sets that distance quickly and is the right everyday method.

But sometimes you need to find it rather than set it: the tool has gone missing, a nozzle has been changed, you are not sure the tool matches the lens that is actually fitted, or you are cutting thick material and want to know where to put the focus inside it.

How the ramp test works

Instead of moving the head up and down and cutting a series of test lines, you tilt the material and cut a single line across it. Every point along that line is at a different height, so the line samples the whole range at once. Where the cut is narrowest, you were at true focus.

1. Set up a ramp. Take a flat offcut. Acrylic works well because the cut mark is easy to read, but anything consistent will do. Prop one end up so it sits at a shallow angle across the bed. A gentle angle gives better resolution than a steep one.
2. Mark a known reference. Note where the low end sits relative to the bed, so you can convert a position along the ramp back into a height. A ruler taped alongside makes this much easier.
3. Draw a single straight line running up the slope, and cut it as a vector at a speed and power that marks the material clearly without cutting through.
4. Read the result. The mark will be wide at one end, narrow through the middle, and wide again at the other. The narrowest point is true focus.
5. Convert to a height. Measure how far along the ramp the narrowest point is, and use the ramp angle, or your reference marks, to work out the corresponding distance from the nozzle.

Read it under magnification

The difference between “narrow” and “narrowest” is easy to miss by eye. A loupe or a phone camera zoomed in makes the minimum easier to identify. A couple of millimetres of error here shows up in every job afterwards.

Focus is different for each lens

A ramp test measures the lens that is fitted right now. Change the lens and the answer changes with it; that is the whole point of having different focal lengths.

• Different lenses: what each focal length is for
• Determining focal length
• Focus distance with different lenses
• Lens compatibility by machine model

Make sure the focus tool matches the lens

A focus tool made for one focal length sets a consistent but incorrect height on another, and nothing in the result flags it as wrong. If results changed after a lens swap, check this first.

Focusing into thick material

For engraving and for thin material, focusing at the surface is correct.

For cutting thicker material it often is not. The beam is a cone: it converges to the focal point and diverges again below it. If you focus on the top surface, the beam is already spreading by the time it reaches the bottom of a thick sheet, so the cut gets progressively less efficient with depth and the kerf becomes noticeably V-shaped.

Focusing into the material, somewhere below the surface, spreads the useful part of the cone through the thickness instead. The result is usually a more even kerf and a cleaner cut on thick stock.

The threshold is roughly 3 mm for the 50.8 mm lens and 6 mm for the 101.6 mm lens. Above those thicknesses a focus taper becomes visible. To avoid it, set the focus height to one third into the material: 10 mm acrylic with a normal focus height of 7 mm then drops to 3.67 mm. Do not go below 1 mm between the material and the nozzle.

Getting a replacement focus tool

If yours has gone missing you can cut a new one on the machine: focus height tools, under Downloads → Cut files.

On a BRM machine

If you have autofocus (on the Slim and Pro) it measures to the material surface, which is correct for engraving and thin material. For thick-material cutting where you want the focus inside the sheet, you will still need to apply an offset on top of what autofocus gives you. You can use the supplied focus stair to set a different height by hand, or use the Z-height parameter in LightBurn to offset the layer from the focus height.

See how do I adjust the auto focus of my BRM Slim and Pro?

If focus has shifted without anyone changing it, something moved: a knock to the nozzle, a lens shaft that has slipped, or a head that is no longer square after a collision. Re-setting the focus treats the symptom. Find the cause: my laser no longer cuts all the way through.

Focus also sets your kerf. If you are cutting parts that have to fit, measure kerf after any focus change: kerf and kerf compensation.