Spindle RPM and Feed Rate Calculations
A precondition to obtain maximum machine performance, tool life and chipping efficiency is to balance the optimum chip removal, cutting speed and feed rate of a machine. The following article provides valuable information on how to maximize the machining performance.
Incorrect spindle speed is a common error in CNC machining. Each material and kind of machining requires an ideal tool profile and cutting speed. Larger tool diameters require slower cutting speeds. It is important to align spindle speed and feed rate for any given machining task to achieve the maximum quality, tool life and service life of the spindle. The spindle speed is hereby controlled by a frequency converter. All spindles are 3-phase asynchronous motors with an infinitely adjustable speed from 0 rpm to the maximum nominal speed.
The feed rate of the cutting tool must be balanced proportionately to the spindle speed. Changing one factor influences the other factors. A too slowly set feed rate reduces the tool life of the cutting tool and might cause overheating and thus result in burn marks on the work piece. As a result, not enough material is removed and the cutting point is insufficiently cooled. The feed rate chart below provides information on starting point values. We recommend setting the specified mean values for the first start-up. Contact your cutting tool supplier for individual advice for your specific application.
The chip load is a term used to describe the thickness of a chip removed perpendicular to the cutting direction, measured vertically to the cutting direction. The chip load is sometimes also referred to as "feed per tooth" and is calculated as follows:
The chip load is an important factor to determine the cutting speed and the feed rate. As far as wood is concerned, a chip load that is too low causes too much heat on the cutting point. The “dust-like” chips may cause burn marks on the surface in turn. A chip load that is too high is pushing the cutter through the material causing too much pressure. The high radial loads on the spindle bearing resulting from that can cause spindle failure, excessive tool wear, and tool break over time. As far as milling is concerned, there are generally two kinds of milling: synchronous milling and conventional milling. For wood working, synchronous milling produces a better milling quality as the generated chips are torn out to a lesser extent. Synchronous milling should be preferred as loads are lower and tool life is longer.
The chip load is one of many factors used to determine spindle size and machine parameters, such as:
- Speed range and the power requirement for the spindle
- Determination of tool load depending on cutting tool model
- Calculation of torque for available application
We recommend contacting a PDS applications technician in case of new spindle applications to support you in establishing the baseline cutting values. Our employees will be glad to answer your technical questions on the phone or by email. The chart below provides information on chip loads for common milling tools. These are “not to exceed” starting values for first time setups. Actual values may vary as a result of various machine factors, of course. These factors include: rigidity, power, tool clamping, spindle fixing, work piece clamping, and others.