Computer Numerical Control (CNC) machining is one of the most commonly used production techniques worldwide due to its high precision. One of the key reasons for its success is the relative motion between the CNC machined object and the tool. We can classify these movements as cutting and feed movements and measure them using cutting speed and feed rate. What is cutting speed and how does it differ from feed rate? How do these machining parameters contribute to the success of a manufacturing project? This article will answer all these questions and more.
Difference between cutting speed and feed
To facilitate understanding of these two concepts, let's consider a simple analogy to a car moving at a linear speed of 60 km/h, and the wheels rotating at a speed of 500 rpm. The wheel's diameter and its rotation make the car move on paved roads. But when you describe the speed of the vehicle, you explain it in kilometers per hour. Cutting speed can be compared to the car's linear speed, which depends on the diameter and number of wheel rotations. It measures the linear distance that the tool moves relative to the workpiece over a specified time. Cutting speed is measured in millimeters per minute (mm/min), meters per minute (m/min), or feet per minute (ft/min). Feed, on the other hand, can be compared to the rotation of the car's wheels. It is simply the distance that the tool covers during one rotation of the part. We measure it in inches per revolution (in/rev) or millimeters per revolution (mm/rev). Still using the car example, a wheel rotating at higher speeds may consume more power and wear out faster than a wheel rotating at lower speeds. This wear is caused by friction and heat between the tire and the road surface. Similarly, spindle speed affects tool life, cutting temperature, and energy consumption. Feed also affects tool life and energy consumption during machining, but their interaction is usually overlooked compared to cutting forces. However, feed has a greater impact on machining time and the finish of the machined part surface. This is important because the selection of cutting parameters affects the final quality of the product. The course of the cutting process is different at low cutting speeds and at high ones. That's why the selection of machining parameters is so important.
Choosing the Optimal Cutting Speed
To determine the optimal cutting speed for a given machining project, one must consider the hardness of the object being machined and the strength of the tool. Hardness defines the material's resistance to deformations caused by abrasion, dents, or scratches. Harder materials require special attention during machining, as they can easily shorten the tool's lifespan. Generally speaking, the harder the material, the lower the cutting speed should be. For example, materials such as titanium require lower cutting speeds than steel. The strength of the cutting tool plays an important role in the permissible cutting speed for machining operations. For example, when machining with tools made of high-strength materials, such as diamond and boron nitride, high speeds can be used, while high-speed steel tools require lower ones.
Thinning of chips and optimal feeds
Chip thinning is a production defect that occurs during the machining of an object with a cutting width less than half the diameter of the tool. This reduces the chip load (the amount of material removed during one rotation of the cutting tool), resulting in longer execution times. One way to reduce the impact of thinner chips is to machine the workpiece with large feeds. This helps to increase productivity and tool life. Now that you understand the difference between feed and cutting speed, you will agree that these two machining parameters are important in CNC machining. However, even if you choose the ideal cutting speed and feed, the success of the project depends on the workshop you are working with. Chips affect the proper cutting depth.
Increasing cutting speed based on the hardness of the material being machined
The hardness of the cutting tool material also has a significant impact on the recommended cutting speed. The harder the drill, the higher the cutting speed. The softer the drill, the slower the recommended cutting speed.
-Carbon steel
-High-speed steel
-Sintered carbide
Increasing cutting speed depending on the hardness of the cutting tool
Cutting speeds for types of materials:
- Low carbon steel 40-140
- Medium carbon steel 70-120
- High carbon steel 65-100
- Free machining steel 100-150
- Stainless steel, C1 302, 304 60
- Stainless steel, C1 310, 316 70
- Stainless steel, C1 410 100
- Stainless steel, C1 416 140
- Stainless steel, C1 17-4, pH 50
- Alloy steel, SAE 4130, 4140 70
- Alloy steel, SAE 4030 90
- Tool steel 40-70
- Cast iron- regular 80-120
- Hard cast iron 5-30
- Gray cast iron 50-80
- Aluminum alloys 300-400
- Nickel alloy, Monel 400 40-60
- Nickel alloy, Monel K500 30-60
- Nickel alloys, Inconel 5-10
- Cobalt-based alloys 5-10
- Titanium alloy 20-60
- Unalloyed titanium 35-55
- Copper 100-500
- Ordinary bronze 90-150
- Hard bronze 30-70
- Zircon 70-90
- Brass and aluminum 200-350
- Non-metallic materials not containing silicon 100-300
- Non-metallic materials containing silicon 30-70
Spindle speed (rotational speed of the spindle)
After determining the SFM for a given material and tool, the spindle rotational speed can be calculated, as this value depends on the cutting speed and the diameter of the tool:
RPM = (CS x 4) / D
Where:
RPM = Revolutions per minute.
CS = Cutting speed in SFM.
D = Diameter of the tool in inches.
Milling feed
The feed rate of a tool can be defined as the distance in inches per minute at which the work moves towards the mill. On milling machines, the feed rate is independent of the spindle speed. This is a great solution for faster feeds and for larger, slow-working tools.
Feed per tooth
Feed per tooth is the amount of material that each tooth of the tool should remove when it rotates and moves towards the workpiece. When the machining moves towards the tool, each tooth of the tool moves equally, producing chips of equal thickness. The thickness of the chip or feed per edge and the number of teeth in the tool are the basis for determining the feed speed. The ideal cutting speed and feed are measured in inches per minute (IPM) and are calculated according to the following formula:
IPM = F x N x RPM
Where:
IPM = inches per minute
F = feed per tooth
N = number of teeth
RPM = revolutions per minute
For example:
The feed rates for end mills used in vertical milling machines range from 001 to 002 feed per tooth for very small diameter mills, on steel work material to 010 feed per tooth for large mills in aluminum work pieces. Because the cutting speed for soft steel is 90, the number of revolutions per minute for a high-speed, two-flute 3/8″ mill is:
RPM = CS x 4 / D = 90 x 4 / (3/8) = 360 /.375 = 960 RPM
To calculate the feed, we will choose .002 inches per tooth
IPM = F x N x RPM = .002 x 2 x 960 = 3.84 IPM
Machine Feed
The movement of the machine that causes the cutting tool to cut into or along the surface of the workpiece is called the feed. During metal cutting, feeds are usually measured in thousandths of an inch. Feed is represented in a slightly different way in different types of machines. Drill presses with motorized feed are designed to move the drill by a specified value with each spindle rotation. If the feed is set to 0.006″, the machine will move by 0.006″ per spindle rotation. This is expressed in inches per revolution (IPR).
Threading Procedure
Tap guides are an integral part of the process of creating useful straight threads. When using a lathe or milling machine, the tap is already straight and centered. Be careful when manually setting taps, as a tap guide at a 90° angle is much more accurate than the human eye. It is very important to use oil during drilling and tapping. Thanks to it, the drill does not squeak, the cut is smoother, chips are removed, and the drill and material do not overheat.
Drilling
Spot drilling prevents overheating and breaking of the drill during drilling or tapping. It involves drilling through part of the piece, then retracting the drill to remove chips and allow the piece to cool down. A common practice is to turn the handle a full turn, then return half a turn. After each withdrawal of the drill or tap, as many chips as possible should be removed and the surface between the drill, or tap, and the workpiece should be oiled.