What is counter milling? Up and down milling. Characteristics of up milling

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What is counter milling? Up and down milling. Characteristics of up milling

Although most CNC operators are in the habit of using up milling, there are times when up milling is preferable. Before we look at each of them, let's define the difference.

Down milling – when the direction of feed and edge movement at the cut point coincides. This method provides the best surface cleanliness. Below is a diagram illustrating the difference between serving and counter-serving.

Arrows show workpiece movement

Keep in mind that in this illustration it is the workpiece that is moving, not the spindle. On some machines, such as a gantry router, the spindle moves, so the marks may change.

Try experimenting on your machine with cutting in both directions and you will see that down milling gives a smoother surface (this is in most cases. However, there are also situations where up milling gives best result). Note that depending on which way you are routing, you will need to ensure that the part will not move under loads applied in that direction.

Characteristics of up milling:

The chip width starts at zero and increases as the cutter finishes cutting;
The cutting process creates upward forces that tend to lift the workpiece during milling;
Up milling requires more energy than down milling;
The quality of the machined surface becomes worse because the chips are lifted by the chip flutes and fall in front of the cutting tool. As a result, most of the chips are cut again. In this situation, supplying coolant to the cutting zone can help;
The tool wears out faster than during down milling;
Counter is preferable for processing rough surfaces;
The tool deflects parallel to the feed direction

Features of down milling:

The chip width starts at maximum and decreases;
The chips fall behind the cutting tool, thereby reducing their repeated cutting;
Less tool wear – service life is extended by 50%;
Improved surface finish due to less secondary cutting;
Less power required;
Feed cutting exerts a downward force on the part, which simplifies the clamping requirements. Downward force can also help reduce vibration when installing the machine on thin floors;
Feed milling reduces the hardening of the part;
However, it may cause chipping when milling hot-rolled materials due to the hardened layer on the surface.
Tool deflection occurs perpendicular to the feed, so it can increase or decrease the width of the cut and affect accuracy.

Backlash and down milling

There is a further problem with climb milling, which is that the tool may pick up gear backlash if the cutting forces are high enough. The problem is that during the cutting process the work table will drag due to the forces on the cutter. And if there is play in the transmission, this will lead to a displacement of the table with the workpiece by the amount of play. And, if the amount of play is sufficient, and the cutting tool is working with sufficient power, this will cause vibration, which can lead to tool breakage and possibly even injury to the operator due to flying fragments. Therefore, many workshops prohibit down-milling on all machines with known backlash. Some machines are equipped with gears with free play, such as ball screws with double nuts.

One way to look at this issue is to approach it from a tooth feeding perspective. This is an indicator of how much material each tooth has cutting tool trying to cut. Typical values for finishing range from 2-4 hundred parts per tooth. For roughing, this value can increase to several tens. In the worst case, down milling can hook the bed and jerk the part to the full amount of play at the moment when the tooth cuts the part. Therefore, by the time the next tooth cuts in, the feed will increase by the amount of backlash. Let's assume that the rough feed per revolution is 6 hundred parts and there is a backlash of 4 hundred parts. In the worst case, the feed per tooth may suddenly increase to 0.1 mm. This, of course, is not the end of the world, but it is already a decent burden. Now let's assume that you have an older machine with a backlash of 0.3 mm and a feed per tooth of 8 acres. If play occurs, the next tooth will start cutting chips of 0.38 mm instead of 0.08. It with high probability indicates tool failure.

It is necessary to consider whether the cutting force is sufficient to remove the backlash. Much will depend on the precision machining scenario of your machine. If you have light table On low friction ball guides, it can be easily grasped by the tool. If you have a lot of iron on the table and you are working with the adjusting wedges tightened, the possibility of seizing will be less. There are ways to calculate cutting force, new in general approach requires the use of smaller end mills, shallower depths of cut, lower feeds and more low speed spindle rotation - all this reduces the cutting force and the likelihood of seizure and backlash.

By the way, CNC machines generally should not have noticeable backlash, so this applies more to manual machines.

Under certain conditions, climb milling creates a negative cutting geometry.

Up to this point, you probably thought that you should use climb milling everywhere you can. After all, this approach creates best quality machined surface, requires less energy and is less susceptible to cutting tool deflection. And operators working in manual mode say that you should not use a passing one, because it is dangerous when working on a machine with backlash. In fact, the truth is somewhere in the middle. The following rules of thumb can be noted:

When milling to a depth of half the diameter of the cutter or less, it is better to use a downcut (provided that your machine has low backlash and it is safe);
When milling to a depth of ¾ of the cutter diameter, the milling method does not matter;
When milling to a depth of ¾ - 1 cutter diameter, counter-cut is better.

Tool deflection and cutting accuracy when milling at feed and against feed

How does feed milling direction affect tool deflection and accuracy?

The following figure shows small arrows (called vectors) that indicate the direction of tool deflection as the cutter moves along the tool path:

The arrows show where the cutting force tries to deflect the cutter. Climb cutting at the top, climb milling at the bottom

Note that the deflection force vector is more parallel to the cut in up milling (although the arrows are longer and show more high strength cutting). When downfeed milling, the force vector is almost perpendicular to the cut. If your cutter is deflected by 3 acres, wouldn't it be preferable to point it along the feed? An alternative may also be to remove or deepen the cutter into the cutting line (changes in removal per pass). Conversely, the lengths of the vectors with the oncoming one are greater than with the passing one. This indicates that the cutting forces are more powerful and the tool is more likely to deflect.

Try using climb milling for roughing because it will allow you to work faster and the effect of tool deflection does not significantly affect accuracy and is irrelevant - a subsequent finishing pass will ensure accuracy. You can rough work much faster because the cutting force is lower and the thick-thin chip profile transfers heat to the chip. Chips carry away heat, which is especially important for processing hard materials such as stainless steel. This ensures a better surface finish if you can afford a repeat finishing pass.

Climb milling for finishing

This contradicts common sense, according to the majority of machine operators. All other things being equal, they are right, but there are nuances.

The problem is that the deviation also affects the surface finish. If the cutting force vector is almost parallel to the feed direction, you can consider that the part of the vector that pushes it "away from parallel" is very small. Therefore, the tool will have a slight tendency to deflect and cause “waves”.

Please note that this can be especially important when working with thin walls where they are very thin!

Therefore, it is important to switch to up milling for finishing, if deviation is generally unacceptable to you. At the very least, too much depth of cut should be avoided when climb milling to avoid deviations. To keep deviations to a minimum, use no more than 30% of the cutting tool diameter for up milling and 5% for down milling.

Proper deflection control can help you avoid the need for additional milling to clean the surface.

Up milling for micro-machining

For the same reasons, and especially considering that deviation has a much worse effect on micro-milling, it is worth choosing the up-cut type instead of the down-cut type for processing

Let's understand the difference in the quality of milling when the cutting edges are directed in the opposite direction and in the same direction relative to the material.

During up milling, the cutter tooth is loaded smoothly while removing chips from the material being processed, due to which the tool life is exhausted much longer compared to down milling.

This milling method will also help extend the service life of the cutter in the case of a superficially compacted workpiece, since chip removal begins from the loose subcortical layer of the material.

Depending on the direction of rotation of the cutter, there are 2 types of milling: uphill and downhill.

Down-cut milling is a milling method in which the supply of material coincides with the direction of rotation of the cutter, Fig. 1.

Rice. 1. Up milling - A. Down milling - B.

If the direction of milling and feed are in opposite directions, then we are dealing with counter milling

Both methods have both disadvantages and advantages.

During down milling, the cutter tooth at the moment of entering the workpiece is loaded abruptly, and an impact occurs on the surface of the material being processed. As a consequence, we get increased tool wear. This effect manifests itself most significantly when processing surface-compacted material and surface processing with straight-cut cylindrical cutters.

During down milling, the surface being processed is compacted. This, of course, cannot be clearly attributed to advantages or disadvantages. Compaction is due to the fact that plastic deformations of the cut layer of material occur directly in the processing zone, and due to the fact that the pressure forces of the cutter and the reaction of the workpiece in the processing zone are directed in the opposite direction, the layer of workpiece material is crushed (that is, compacted).

Due to the fact that the cutter presses on the workpiece during operation, pressing it against the supporting surface and thereby increasing the rigidity of the interface, the processing accuracy is higher compared to counter milling.

During up milling, the cutter tooth is loaded smoothly while removing chips from the material being processed, due to which the tool life is exhausted much longer compared to down milling. This milling method will also help extend the service life of the cutter in the case of a superficially compacted workpiece, since chip removal begins from the loose subcortical layer of the material. In the region of the dense crust (area “A” is highlighted in Fig. 1 b), the separation of the material occurs largely due to tensile and bending forces. These types of loads require much less force to destroy the material, in contrast to the crushing that we have in the case of down milling.

With the counter milling method, the change in the density of the processed layer of material occurs to a lesser extent. However, in this case, it is possible for the tooth to slip along the surface of the workpiece, which will lead to strengthening of the depressed layer and a subsequent increase in the load required for processing.

With the counter method of processing material, the cutter tends to pull out a layer of material from the workpiece during operation. In this case, the thickness of the cut chips is not constant. Due to the elastic deformations caused by this, vibration occurs and, as a result, the quality of the processed surface decreases.

So, taking into account the mentioned advantages and disadvantages of the milling methods under consideration, we can conclude that down milling more suitable for:

Finishing;

In cases where a thin layer is removed per pass;

Processing of surface-uncompacted materials.

Up milling more suitable for:

Rough processing of material;

Processing of surface compacted materials.

Figure 101

In up milling, the feed direction of the workpiece does not coincide with the main movement. If it happens along the way, it coincides. Advantages of up milling:

If there is a hard crust on the workpiece, the cutter tooth cuts it from below, and does not hit or chip;

There is no picking up of the workpiece by cutting forces, during which S z sharply increases by the amount of the gap in the screw-nut pair of the feed chain, so you can work even on a worn-out machine.

Flaws:

The cutter tooth does not immediately cut in, but slips (a = 0) and therefore hardens the cutting surface and wears itself out;

The chips remain on the front surface and, when cutting in, chip the cutter tooth.

With climb milling, the opposite is true, so on a new machine it is better to use the climb milling method, since the quality of processing is higher.

5.8 Pulling

Surfaces of various configurations are stretched, both internal and external.

Cutting speed when broaching – 2-15 m/min.

Processing accuracy 6-9 quality, roughness Ra 0.63...2.5 microns.

5.8.1 Broach design

If the length of the broach does not exceed 15 diameters and the broach works under compression, then it is called broaching.

R
Figure 102

R
Figure 103

1 – shank;

2 – neck;

4 – cutting part;

5 – calibrating part;

6 – rear end of the broach.

The working part of the broach is made of steels R9, R18, R9F5, HVG (lowest ability to deform).

5
.8.2 Geometric parameters

Figure 104

5.8.3 Cutting mode elements

V p – along the broaching axis,

S z – feed per tooth, difference in height of adjacent teeth of the cutting part,

a – equals feed per tooth S z,

b - depends on the shape and design of the broach, which is determined by the surface being processed,

There is no rise on the calibrating part to improve the roughness class.

α=2…4 0 on the cutting part of the broach, α=1…2 0 on the calibrating part.

5.8.4 Pulling patterns

Profile.

Figure 105

The result is the best quality and precision of processing. Rarely used due to the complexity of manufacturing broach teeth.

Generator room.

Figure 106

The accuracy and roughness class are lower. The method is widely used when there is no high requirements to the details.

Progressive (group).

It is carried out using a generator or profile circuit.

The allowance between teeth of the same height in a group is divided by width. Cutting forces are reduced and durability increases.

Figure 107

5.8.5 Wear and durability of broaches

There is minor wear on the front surface. The back surface of the broaches wears out predominantly. For broaching, a technological wear criterion is assigned, since broaching is a dimensional tool. The amount of wear is up to 0.2-0.3 mm, then the broach is sharpened. Cutting temperature is the main factor influencing wear, since at idle the broach is completely cooled and cutting speeds are low. The thickness of the cut layer is very small. This is the main wear factor.

S
z =0.02-0.2 µm.

Figure 108

The cutting process is possible when a>ρ.

Durability from 120 to 600 min.

ρ – radius of rounding of the cutting edge.

Broaching is used only in large-scale and mass production and, as an exception, in repair shops.

In Fig. Figure 21 shows an example of processing with an end mill. Insert teeth - cutters 4 - are installed in the body of the end mill 5. Each cutter removes an allowance determined by the feed s z and the depth of cut t. The teeth of the cutter cut the allowance along a curved path. Depending on the location of the workpiece relative to the cutter, the cutting conditions change.

Rice. 21. : 1 - machined surface, 2 - cutting surface, 3 - machined cutting surface. 4 — cutter (insert knife), 5 — cutter body; v - direction of rotation of the cutter, s z - feed per cutter tooth, t - depth of cut

Rice. 22. Different positions of the end mill relative to the workpiece:

a—symmetrical, b—above the center (counter milling); c—below the center (climb milling); 1 - cutter, 2 - workpiece; v - direction of rotation of the cutter, s - direction of feed

In Fig. Figure 22 shows different relative positions of the cutter and the workpiece. In Fig. 22, and the workpiece 2 is located symmetrically relative to the axis of the cutter 1. In this case, the cross section of the chip during the cutting process, although not constant, turns out to be approximately the same at the moment the cutter enters the metal and at the moment it exits. The direction of action of the cutting force in relation to the feed direction is also not constant, but remains close to 90°, especially if the cutter diameter is significantly larger than the width of the machined surface.

In the case when the part is not symmetrically located relative to the cutter (above the center), as shown in Fig. 22, b, cutting conditions change significantly. At the moment the cutter enters the metal, the cross-section of the chip turns out to be significantly smaller than when it exits. The movement of the cutter during the cutting process is always carried out towards the feed movement. Such cutting conditions are called up-milling.

If the workpiece is shifted relative to the cutter axis in the opposite direction (below the center), as shown in Fig. 22, c, then the cross-section of the chip at the moment the cutter enters the metal will become larger than when it exits, and the direction of movement of the cutter will be close to the direction of feed. Such milling conditions are called down milling.

When processing brittle metals, sometimes it is necessary to create conditions for a smooth exit of the cutter from the metal in order to prevent chipping of the edge of the workpiece. This will correspond to the climb milling method. However, with this method there is always a danger of arbitrary movement of the workpiece together with the table in the direction of movement of the cutting edge. This can happen if there are large gaps in the table movement mechanism. When the table moves arbitrarily, the cutting process occurs in jerks, the roughness of the machined surface increases and there is a danger of breakage of the cutter. Therefore, before setting the down milling mode, it is necessary to adjust the gaps in the table movement mechanism. For this purpose, the machine is equipped with appropriate devices.

In Fig. 23 shows up and down milling in relation to milling with a cylindrical cutter.

Rice. 23. Machining with a cylindrical cutter:

a - down milling, b - up milling; v—direction of rotation of the cutter, s p—upstream feed, s in—counter feed, s z—feed per cutter tooth, t—cutting depth, B—milling width

From Fig. 23, and you can see how the cross section of the chips changes from highest value to the smallest for down milling and from the smallest to the largest for up milling (Fig. 23, b).

Rice. 24. Scheme of the action of forces during up and down milling: a—climb milling, b—counter milling; R - cutting force, P x - horizontal component of the cutting force, P y - vertical component of the cutting force, P ok - circumferential force, P rad - radial force, s - feed direction, v - direction of rotation of the cutter, D - cutter diameter

In Fig. Figure 24 shows a diagram of the forces that arise during various milling methods. The cutting force R consists of the circumferential force P ok, the direction of which coincides with the direction of the cutting speed v, and the radial force P rad, the magnitude of which is proportional to the depth of cut. To compare cutting conditions, the position of the cutting edge is considered when it is at the same angle relative to the vertical (Fig. 24, a, b). In this case, the cross-section of the chips will be the same. The magnitudes of the cutting forces of the circumferential and radial forces will be the same, but the directions of the force vectors will be different.

Let us decompose the cutting force vector into two components P x and P y and compare their effect during down and up milling.

Horizontal component P x at down milling acts in the same direction as the feed, and the vertical component P y is directed downward, pressing the workpiece against the table.

At up milling the horizontal component P x is directed towards the feed, and the vertical component P y is directed upward, as if lifting the part from the table. The larger the allowance, the more pronounced the effect of this component is.

If during down milling the gaps in the threaded connection of the lead screw and the nut of the machine, with the help of which the table moves in the feed direction, are dangerous, then during up milling the danger is caused by the gaps in the table guides since the vertical component P y can lift the table along with the workpiece, and this will lead to oscillations (vibrations). The table feed mechanisms experience the greatest load during counter milling. For this case, the safety mechanisms of the machine are adjusted.