Milling Slots End mills are designed to cut square slots. They will produce a slot to within two onethousandths of an inch in one pass. If greater accuracy is required, use an end mill a little smaller than the desired slot. Micro-sides cutters in a milling machine (like.010' diameter) to cut tiny slots into something like a plate of brass or steel without the cutter just instantly breaking off. You will find that there are plenty of end mills made in sizes like that, but they are all carbide from what I see. Damien is correct. Most machine tool tables are made this way for a reason. I also cut all my T-slots this way, for the same reason. Usually I go about 1/32' deep with the endmill, and make sure it is at least 1/32' larger than the stud diameter to be used in the T-nut.
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Milling – how to make – spherical surfaces
If the workpiece is mounted on a rotary table and the head of the milling machine is tilted it is possible to machine various concave and convex surfaces using a boring head. It would be possible to do this using a fly cutter but it is easier to set up with a boring head.
In all of these cases it is essential that the axis of the rotary table is in the same plane as that of the spindle in the x/z plane. The axis of the sphere that the surface being cut is part of will be coaxial with the axis of the rotary table the movement of the cutter to the workpiece changes because the milling table is raised.
Cutting a convex surfaces
This method can be used to make any spherical surface up to a whole hemisphere. It is possible to mill even more of a sphere but by a different method which is covered later.
The geometry of a typical setup for a convex surface can be seen in fig. xxx.
Fig. geometry for machining a convex surface 1003
If
radius of the surface = r
radius of sphere = R
tilt of the vertical head = ?
diameter of the fly cutter =d
height of the surface =x
then:
sin ? =(d/2)/R
sin ? = x/D
If the axis of the boring head is co-planar with the axis of the rotating table then, if the boring head rotates in a circle each point on this circle is the same distance from the intersection of the axis of the boring head and the rotating table regardless of the rotation of the rotating table.
The surface from A through CB to C is part of a sphere whose radius is R.
The diameter of the sphere is solely determined by the diameter of the cutting circle, that is, the diameter of the boring head and the distance from the top (or bottom) of the cutting circle and the point where the axis if the boring head and the axis of the rotary table. This is determined by the angle of tilt of the vertical head.
Fig.
machining a convex surface – 8
A key feature of this is that the boring head is cutting the outside of a circle on each revolution of the head. The cutting edge is on the outside of the circle. Using the conventional boring tool this means the tool is round the “wrong” way. This will only cut with the boring head rotating in reverse. There is, of course, only one cutting edge but it is shown in both the top and bottom position.
If the tip of the cutter in the top position is not in line with the axis of the rotary table the surface produced is not part of a toroid as one might expect.
572 finished convex surface
Fig. finished surface 572
An example of a part of a hemisphere like this would be a smoke box door. This is part of a sphere. How this works is easily seen from the drawing.
It is worth turning (on the lathe) the workpiece so it is the right diameter and thickness. It is probably not worth trying to remove any more metal, for example, on the lathe.
It is not usually possible to clamp the workpiece directly to the rotary table. In the example shown the workpiece has some holes drilled and tapped into the back of it. These are used to mount the workpiece on a plate which is then clamped to the rotary table.
It might be necessary to have some sort of spacer between the work piece and the plate that is fitted to it and bolted to the milling table.
Milling a hemisphere
An example of a whole hemisphere would be a dome covering a regulator on a boiler. This is really just a special case of milling a convex surface.
Fig.449 – geometry for milling a hemisphere – 1004
If the height of the surface equals the radius, i.e., X = R, then the surface is a hemisphere. In this case ? will be 45º.
Fig. milling a hemisphere 695
see
The workpiece is centered on the rotary table
The head is tilted to 45°.
In this case it is not obvious where the cutter is relative to the axis of rotation of the workpiece. But it is possible for the workpiece to be moved towards the boring head till enough is cut away to see when the cutter is close to the axis of rotation.
Milling a convex surface – alignment
The boring head is set to the diameter D.
see setting the outside diameter of a boring head
The rotary table is mounted on the milling table. It is aligned with the axis of the vertical head.
see centering a rotary table
The x and y-axes are locked.
The plate with the workpiece is mounted on the rotary table. The workpiece is aligned with the spindle using a DTI on an arm device.
see centering a round shape
The vertical head is tilted to the required angle.
The x axis is unlocked.
The milling table is moved along the x axis till the tip of the boring head at its highest point is over the middle of the workpiece.
The x axis is locked.
The spindle is turned on. The workpiece is raised. The cutter will first touch the workpiece at a point Z at one place on the edge of the workpiece. But if the workpiece is raised a small amount and then rotated a thin ring will be cut all the way round the edge.
After each full turn of the workpiece it is raised again and the ring gets wider.
Finally the width of the ring covers the whole radius of the workpiece.
It will be noticed that the diameter of the boring head was significantly larger than the diameter necessary but it does show the nature of successive cuts.
All of the cutting is done with the x and y movements locked. The workpiece is raised till it is being cut. While this is happening the workpiece is rotated till is has done a complete circle.
The cut will form a ring at the edge of the workpiece. Each time the workpiece is raised this ring will get wider. The cutting is complete when the cutter finally cuts to the middle of the workpiece.
103 Odd case
An odd case occurs if the diameter of the boring head passes over the axis of the rotary table.
In this case what is cut is still part of a sphere but it is the surface between heights A and B. This is easy to see if one considers that height A is the highest the cutter ever reaches and B is the lowest.
Fig. Odd case – 1057
It might seem that if the vertical head is less tilted the shaped formed would be the surface round from A to B. But it is easy to see that if the head is vertical it does not cut the surface round from A to B.
What happens is that the surface cut is the surface between the vertical height of A and the vertical height of B.
Cutting a concave surface
This is in many respects similar to the concave case. It is set to the pointer touches at A and B. The difference is that the position of the cutter is used it starts from position A dash and b dash
Fig milling a concave surface – geometry
632 machining a concave surface
Fig. Milling a concave surface 632
21.02.2020 by Andreas VellingCNC machining is a highly utilised subtractive manufacturing technology. Computer numerical control systems offer less need for manpower and higher levels of automation.
One of these automated fabrication methods is CNC milling. It is a process where rotary cutters remove material, which makes it the opposite of CNC turning.
The milling centres do not just perform the cutting automatically, but also the changing of tools. During the average process of creating a finished product from a block of metal, for example, various tools are used.
So let’s see what milling tools are used on the machines and what are the purposes of each.
What Are the Types of Milling Cutters?
The most common types of milling cutters are:
- End mill
- Face mill
- Ball cutter
- Slab mill
- Side-and-face cutter
- Involute gear cutter
- Fly cutter
- Hollow mill
- Shell mill
- Roughing end mill
- Dovetail cutter
- Wood ruff cutter
First, we should start with one of the primary questions.
What is the difference between end milling and face milling?
Steel T Slot Milling Table
These are two of the most prevalent milling operations, each using different types of cutters – the and mill and the face mill. The difference between end milling and face milling is that an end mill uses both the end and the sides of the cutter, whereas face milling is used for horizontal cutting.
End mill
These tools usually have a flat bottom but not always. Round and radiused cutters are also available. End mills are similar to drills in the sense that they can cut axially. But the advantage for milling lies with the possibility of lateral cutting.
Face mill
Face mills cannot cut axially. Instead, the cutting edges are always located on the sides of the cutting head. The cutting teeth are replaceable carbide inserts.
This makes the lifetime of a tool longer while maintaining a good cutting quality.
Ball cutter
Ball cutters, also known as ball mills, have a hemispherical cutting tip. The objective is to maintain a corner radius for perpendicular faces.
Slab mill
Slab mills are not that common with modern machining centres. Rather, they are still used with manual milling machines to quickly machine large surfaces. That is also why slab milling is often called surface milling.
The slab itself spins in a horizontal position between the spindle and the support.
Side-and-face cutter
A predecessor for the end mill. Side-and-face cutters have teeth around the circumference as well as on one side. This makes the functionality very similar to end mills but their popularity has waned over the years with the advancement of other technologies.
Involute gear cutter
There is a special cutting tool for milling involute gears. There are different cutters available to produce gears within a certain number of teeth.
Fly cutter
These tools have the same function as face mills. They consist of a central body that holds either one or two tool bits (double-end fly cutters).
Face mills are better for high quality cutting. Fly cutters are just cheaper and the cutting bits are often made at the shop by a machinist rather than bought from stores.
Hollow mill
Hollow mills are basically the opposite of face mills. Here, the workpiece is fed into the inner part of the mill to produce a cylindrical outcome.
Roughing end mill
As the name says, these are pretty much end mills with a slight difference. The roughing end mill has jagged teeth. These make the cutting process faster than with a regular end mill.
The cut bits of metal are smaller than usual and therefore easier to clear. Multiple teeth come into contact with the workpiece at the same time. This reduces chatter and vibration, which could otherwise be larger because of the jagged teeth.
Woodruff cutter
Woodruff, or keyseat/keyway cutters are used to cut keyslots into parts, for example shafts. The cutting tools have teeth perpendicular to the outside diameter to produce suitable slots for woodruff keys.
Thread mill
The name of this tool says everything you need to know about its purpose. Thread mills are used for producing tapped holes.
Threading operations are usually carried out on drilling equipment. Using a thread mill, though, is more stable and has less limitations regarding the environment.
What Materials Are Used for Cutting Tools?
As you could see, there are a lot of different machine tools available for wide range of purposes. The same applies to the materials used to make these tools.
Let’s dig deeper to look at the most common materials for milling bits.
Carbon steel
The cheapest of the bunch. And this is exactly why it still finds use. As carbon steel is not very durable, it is only suitable for low-speed operations.
Carbon steel loses its hardness at 200° C. This is the reason for lower speeds – to keep the heating effect low.
High-speed steel
High-speed steel, a grade of tool steels, has a few alloying elements added to it to provide better response to heat and wear than a regular carbon steel. While the life cycle of such a tool goes up, so does the cost.
Loses its hardness at 600° C. Therefore, higher milling speeds are suitable for these tool steels.
Cemented carbides
This material is harder than high-speed steel but the toughness qualities are not that impressive. The higher hardness provides better protection against wear but lower toughness levels make it a little more susceptible to cracking and chipping.
The upper temperature of use is at 900° C.
Cutting ceramic
Cutting ceramics are even harder than cemented carbides but lose in the toughness aspect. Both aluminium oxide and silicon nitride are used to produce these tools with varying properties.
Cutting ceramic tools are prone to cracking when used on hard materials and with high temperatures. Therefore, they are not really suitable for machining steels, for example. Otherwise a short tool life is to be expected.
Selecting the Right Machine Tool
As is the norm in manufacturing, the choice of method or tool comes down to a balance between speed, cost and quality. The cost depends on both the price of the tool, the wear machining results in, and the time it takes (speed) to produce the parts.
Choosing the material of the tool
Regular carbon steels are usually out of the option pool because of their limited capabilities. HSS (high-speed steel) is therefore the most inexpensive one to get the job done. At the same time, its rate of wear means that in the long run, there are better options.
Cobalt-bearing HSS, for example, are suitable for even quicker milling. This makes them sufficiently adequate for most jobs.
Cemented carbide is another step towards high performance milling because of the aforementioned properties of such milling machine tools. In the long run, they are a more cost-efficient choice while the up-front costs are higher.
Diameter
This is quite simple. A tool with a large diameter is able to mill the part quicker. Limitations apply based on the geometry of the final part.
For example, if certain inside radii are necessary, the tool cannot deviate from them. At the same time, you can use a large tool for milling away the bulk of it and apply a smaller one to finish the inside corners.
Tool coating
There are some different coatings available to protect the tools from wear. For example, a titanium nitride coating increases the tool’s lifespan but also the cost of it.
Such a coating reduces stickiness of the cutting material which can be a problem with aluminium. Therefore, less lubricant is necessary during the cutting process.
Number of flutes
Flutes are the channels on a milling bit. More flutes allow a higher feed rate because less material is removed.
At the same time, this increases the overall diameter of the milling cutter. This leaves less room for swarf.
Angle of helix
The helix angle, along with the rotation speed of the spindle, determines the cutting speed or feed rate. A steeper angle is suitable for softer materials and metals.
Choosing the right milling cutters for your job needs an understanding of the materials, parameters and definitely some experience. The final outcome depends on these choices and a machinist must understand what material cutters are suitable for cutting different mediums.
A good choice leads to high feed rates and therefore shorter cutting times as well as lower costs.
How To Cut Slots With Milling Machine Tools
When choosing a CNC machining service, make sure that they have all the necessary tools to make your parts.