There''s a lot of bad reviews about this product but these seem to be by people who don''t seem to understand how it works rather than the product itself. This is ultimately a failure of the manufacturer to provide any instructions either with the product or online about the correct and safe use of this tool.
Firstly, its worth understanding how the tool is designed to work as this will allow you to set it up correctly to get a effect desired. Looking at the tool you''ll notice that the two cutting blades are actually different from one another. One blade will cuts through the material at the actual circumference of the hole required (the outer cutter) whilst the other cuts through the material at a slightly smaller circumference (the inner cutter) thereby allowing the outer cutter to continue to penetrate the material beyond the depth of the tip of the blade where it is affixed to the blade holder. This applies equally to the inner cutter so they work in tandem to facilitate each other passing through the work-piece.
It is essential therefore to set up the inner and outer cutters appropriately - I used a vernier caliper for accuracy e.g. for a 100mm diameter hole you would use the caliper to set an outer cutter distance of 54mm from one side of the 8mm pilot drill to the outer side of the outer cutter (i.e. for a cutting radius 50mm it''s 100/2 = 50mm + 8/2 = 54mm). For the inner cutter use the caliper to set an inner cutter distance of 54mm from one side of the 8mm pilot drill to the outer side of the inner cutter blade carrier - NOT the inner cutter blade itself!
It''s also worth noting that it''s better to pilot drill your hole centre with an 8mm drill before attempting to use this tool and replace the supplied 8mm pilot drill bit with piece of 8mm plain bar (I just turned the supplied drill bit round so the smooth shank would insert into the pre-drilled pilot hole to guide the tool through the material. If you don''t do this using this tool in a hand drill for anything other than plasterboard or the like the cutters may catch and pivot the drill bit into the pilot hole thereby opening the pilot hole up and causing further catches etc, etc.
Next, using an appropriate speed and drill - cut slowly perpendicular to the material let the cutting bits "nibble" away at the material. Remember the fact that you''re using this tool probably means you''re cutting large holes and the bigger the diameter the higher speed the cutting bits will be travelling at. A 3000 RPM drill speed equates to a cutter speed of in excess of 1.35km/h - if the bit catches and stops suddenly then that energy has to go somewhere... choose a drill that has low torque or even a torque clutch driver.
One thing even I don''t understand is the purpose of the inclusion of the 6mm diameter point supplied with the tool - when inserted in the tool this is too short to even touch the work-piece surface!