Be careful with claims of fast spindles and mill times. True, faster rpm's can result in faster milling times however they REQURE faster milling feed rates which in reality, only POTENTIALLY yield faster milling times.
However rpm is not really the issue or the whole story in determining milling times as spindle torque, machine construction, milling strategies and machining parameters are of more significant importance.
Its also important to note that RPM, power and torque ratings should only be considered valid if the rating stated is the CONTINUAOUS power and torque rating at a give rpm and NOT the maximum rating (as is the common practice with dental milling machines).
Power and torque are usually lower at high rpm and when cutting faster (SFM) as required, more torque is required from the spindle motor and if not available will stall the machine. This is particularly important when machining PMMA, model materials and metal.
More importantly, the machine weight, frame type/material, part fixturing etc. need to be solid, vibration and heat absorbing. If they are not, milling times would also not decrease as the final restorations would exhibit margin problems (chipping, open, etc. with possibly thicker margin reinforcement),finish surface quality issues and excess tool wear. These undesirable by-products of pushing the machine beyond that which its construction is capable of producing (in terms of quality of finished product) would result in slowing the machine down by decreasing machining feed rates (and necessarily spindle speed) to achieve acceptable final restoration quality.
Short high feed rate moves generate a lot of inertia and corresponding vibration. Most, if not all small-framed, lightweight machines will exhibit excessive vibration during a machining cycle. This can be verified by placing your hand, a coin or cup of water on the machine while it is machining. All those vibrations you see and feel are being transferred to the cutting tool and the part being machined resulting in less than optimal restoration quality and tool life.
These points are discussed on our website as well as in our "
Questions to Ask BEFORE you Buy a CNC Dental Mill" check list which you can review
HERE.
This list abbreviates some of the key issues discussed in our "
Guide to understanding the key construction characteristics of a quality CNC dental mill" whitepaper which provides an in-depth understanding of the features, functions and benefits of key construction elements.
The full whitepaper is available for your review
HERE.
We did some head-to-head testing on some of the more popular "2nd and 3rd generation machines" and this is what we saw based on actual machining of the same crown on the latest machine configuration with the latest tooling and machining templates as provided by the distributor. No names just images. Your mileage of course may vary, however these tests and images illustrate the point.
http://dentallabnetwork.com/forums/media/albums/crown-comparisons.26/
