Buy the best quality you can afford. I can recommend "Norton" as I used these a lot in the past. Last year I was unable to obtain "Norton" from my supplier, in the sizes I required for a particular job, I decided to give "Marcrist" a try, what a revelation! Absolutely superb, speedy and efficient cutting, used either wet or dry. I've cut nearly a hundred holes with a "Marcrist" 117 x 165 CCU850X and still have well defined sharp diamond lands.
Core drills generally come in two forms: - 1) The conventional chuck type takes a hex arbour. 2) The threaded tool holder type takes a screw fit arbour. The former, allow you the flexibility of a standard drill. The later, higher torque of a direct drive at the expense of more laboured tool changing.
Of the chuck type the Makita 8406 is fine example, offerings from Dewalt and Bosch are better, but my personal favourite is the Milwaukee DD2-160XE, the Hilti DD120 and 130 being the firm favourites with the majority of my work mates.
Direct drive types are the professional choice however. The Marcrist DDM2 and DDM3 allow wet and dry cutting at fixed speeds. I have never felt the need for variable speeds when using one of these, I find them remarkably efficient when paired with the same make of core cutter, but a few of my colleagues prefer to use the Milwaukee or Hilti models.
What ever you choose to buy, don’t be tempted to use an SDS drill to drive a large core cutter. The SDS tool holder is very inefficient when used in the hammer stop mode, a large core drill will quickly stress and wear the tool holder and ultimately destroy a SDS if it does not have thermal protection.
If you suspect your drill of getting excessively hot, stop drilling, switch to the max speed range and run it flat out for least 5 minutes in free air to allow the motor fan to cool the windings.
