0.5" focus range not achievable

When I ordered my Glowforge it boasted a 0.5" focal range and still does https://glowforge.com/tech-specs. Not long ago the focus range was limited to 0.433" in the GFUI. Recently the limit was changed to 0.5", however my experiments show it never moves past 0.417". I also have it on good authority that there isn’t room inside the head for it to move more than about 0.47".

The laser beam going into the lens is very close to parallel, so the beam focuses at the focal length of the lens, about 50mm, below it. This means to cover 0.5" range it would need to move 0.5". It is driven by an 18 degree per step motor, a nine tooth pinion and a 0.5 mod rack. This makes each half step 9 / 360 * 9 * 0.5 * PI = 0.3534mm or 0.0139". Testing each depth in 0.0139" increments reveals it moves two half steps at a time up to 30 half steps and then stops. So although you can enter 0.5" it is exactly the same as entering 0.417".

Why is it advertised with 0.5" when it isn’t physically possible?

Why does it allow 0.5" to be entered when the software limit is 0.417"?

Why does it always move in multiples of two half steps making the resolution only 0.7mm whereas auto focus boasts a resolution of 0.1mm?

When people complain that PG does not cut through they are told to make sure it is absolutely flat and that small amounts of warp will have a big effect. If the focus is only to the nearest 0.7mm it doesn’t make sense it should be much flatter than that.


I’m glad we have you back here, digging into the details and asking big questions.


I’ve posted some additional details here (in accordance with GF guidance on information of this type):


Thanks for writing. The specification says “Lens moves internally up and down inside the head to focus on materials up to 0.5” (13mm) thick”. I can confirm the tech specs are correct - your Glowforge will focus on material from 0" to 0.5" thick, and will not focus on material outside that thickness range.

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