To be able to engrave "SLATE" you need a diamond drag bit, a ones off investment but with endless possibilities like engraving all sorts of oddball materials including granite, glass, metal, plexiglass ect
The cutters should be kept sharp. Chipping or overheating will occur with a dull cutter. Both will impart stress.
Step by Step Guide For Glazing - By Tim Merrill
This guide will take you through each step and discuss techniques, tools and materials. For a 3D model, I have chosen the Wolf Head from Vector Art 3D. This model was chosen for three reasons: Its final appearance is greatly improved by glazing (my opinion), the detail is enough to be a challenge, and it is one of the included 3D clip-art models with Vectric's Aspire.
"FLAT DEPTH" option
By selecting the FLAT DEPTH option you restricted the DEPTH OF CUT to the depth you desire
This will be the result without selecting the "FLAT DEPTH" option - You will cut thru the job!