End mills are cylindrical milling cutters with a shank that has cutting edges on both the periphery and end face. They are used for a wide range of applications such as shoulder milling, slotting and contoured surface milling.
Compared to other tools, end mils are used with a relatively long tool overhang, and this can easily lead to unstable machining with vibration. This makes tool selection, setting and cutting conditions difficult to determine.
Previous solid type end mills were made from high-speed steel. However, with today's rapid development of coating technology and tool substrate materials, solid carbide coated end mills and indexable insert end mills have become popular and are used over a wide application area.
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Up and down (climb) cut milling
Depths of cut
Depths of cut when end milling include the axial depth of cut (ap) and the width of cut (ae). Examples of ap and ae when shoulder milling with a square end mill are as shown in the image below, while for slotting, ae is equivalent to the diameter D1
When shoulder milling with a ball nose end mill, the pick feed, (pf) is often used instead of ae.
Pick feed and cusp height
When shoulder milling with a ball nose end mill, the pick feed, (pf) is often used instead of ae. As shown in the image to the right, when machining a level surface, ae is equal to pf, but when machining an angle, the depth of cut along the angle, ae', is equal to pf. Larger amounts of ap and ae will increase machining efficiency, however the load on the tool and machine will increase and can cause vibration and tool damage.
The theoretical surface roughness when machining with a ball nose or corner radius type end mill is determined by the cusp height, (h). The formula shows the relationship between the pick feed and the cusp height for ball nose or corner radius type end mills.
If the pick feed is increased, machining time will shorten but the surface roughness will deteriorate. Conversely, if the pick feed is decreased, the surface roughness will improve, but the machining time will increase.
ap = Axial depth of cut
ae = With of cut
pt = Pick feed
h = Cusp height
r =Ball nose or corner radius
ap = Depth of cut in the direction perpendicular to the angled face
a = Inclination angle
Up and down (climb) cut milling: Description
Up cutting and down (climb) cutting are two methods with which to machine a workpiece. It is determined by the direction the workpiece travels in relation to the cutter as shown in the top image.
Down cut milling and up cut milling are the same as the methods of a face milling cutter shown in the bottom image. However, machining methods with solid end mills tend to differ from face milling in that the axial depth of cut tends to be larger and as a consequence the width of cut has to be decreased to avoid vibration.
Up cut milling
Down cut milling
Setting position of a
face milling cutter
- Backlash in the table feed mechanism can be negated, older type machines can be used.
- The surface finish appears smooth and shiny, caused by the cutting edge rubbing.
- Suitable for workpieces with outer scale.
- Due to the initial rubbing of the cutting edge, flank wear is quickly generated and tool life is short.
- High cutting resistance.
- Burrs are easily formed on the workpiece.
-For materials with a work hardening property, tool life is further shortened.
- Less flank wear and longer tool life.
- Low cutting resistance.
- Suitable for machining materials with work hardening properties.
- A device to remove table feed backlash is necessary.
- For workpieces with outer scale, the cutting edge is easily damaged.
Up and down (climb) cut milling: Slotting
During slotting, when cutting edge AB (step 1 in the animation to the right) begins up cut milling, A'C the opposite of cutting edge A'B' is down cut (climb) milling, therefore the end mill bends towards the up cut milling side (step 2), causing the end mill to dig in to the workpiece. As the cutting position moves from point A of cutting edge AB towards B, the length of cutting edge A'B' actually engaged in down cutting becomes shorter, decreasing the amount of deflection. Thus, a slot tends to lean towards the up cut milling side.
Up and down (climb) cut milling: Usage
Up cut milling is used when using a machine with backlash in its table feed device, or when machining workpieces with surface scale such as a casting. Workpieces with surface scale are usually up cut to prevent the cutting edge from directly cutting the cast surface and being damaged.
The image to the right shows a deteriorating condition of an end mill when shoulder milling. For down cut milling, the end mill bends away from the workpiece due to cutting resistance, and this bending causes deflection on the machined surface.
For up cut milling, the end mill deflects towards the workpiece due to the cutting resistance, creating a hollow undulation on the machined surface. The amount of deflection is largest just before the position where the end mill exits the workpiece because it is in the hollow that has already been created.