63.5mm Lense problem

[Terence Christian]

I wonder if any one can help. I purchased a 63.5mm lens for my machine (80W Reci) hoping it would cut thicker material quicker and easier than my 40mm lens. How ever i can't seem to get it to cut anywhere near as well as the shorter lense, even with the focus length set to 33.5mm as suggested by the suppliers.

For example i cut 6mm MDF with the 40mm at 12mm/s 75% power (Does this seem ok?)

Any advice would be appreciated.

[Dave Sheldrake]

even with the focus length set to 33.5mm as suggested by the suppliers.

Focal length is measured from the back of the lens (the curved surface) to the top of the workpiece, be aware though the lens can be +/- 1mm or so on the actual stated figures.

Longer focal length = more depth of field (it cuts straighter for longer) but lower power density (less actual cutting ability)

6mm MDF @ 12mm per second is pretty good for an 80 watt tube.

cheers

Dave

[Dan Hintz]

I purchased a 63.5mm lens... even with the focus length set to 33.5mm as suggested by the suppliers.

This is horrible advice. When cutting, it is rare that you will focus into the material more than a few mm's (acrylic and some closed-cell foams are two of those rare exceptions as they act as waveguides). Defocusing by 30mm means you have practically no real power density to make a cut and I'm surprised you're not doing more than just burning a fat black spot on the material.