Solutions To The 30 Most Common Problems In CNC

There are often some problems encountered in CNC machining. Mastering these 30 points, I believe it will be helpful to your machining work.

  1. The effect on cutting temperature: cutting speed, feed rate, and back-to-cut amount.

Influence on cutting force: back-cutting amount, feed rate, cutting speed.

The impact on tool durability: cutting speed, feed rate, back-to-tool amount.

  1. When the amount of back-grabbing is doubled, the cutting force doubles.

When the feed rate is doubled, the cutting force increases by approximately 70%.

When the cutting speed doubles, the cutting force gradually decreases.

In other words, if G99 is used, the cutting speed becomes larger, and the cutting force will not change much.

  1. The cutting force can be judged according to the discharge of iron filings and whether the cutting temperature is within the normal range.
  2. When the measured actual value X and the drawing diameter Y are greater than 0.8, when the concave arc of the car is greater than 0.8, the turning tool with a secondary deflection angle of 52 degrees (that is, the turning tool with a 35-degree lead angle and 93 degrees we commonly use ) The R from the car may wipe the knife at the starting position.
  3. The temperature represented by the color of iron filings

White less than 200 degrees

Yellow 220~240 degrees

Dark blue 290 degrees

Blue 320~350 degrees

Purple black is greater than 500 degrees

Red is greater than 800 degrees

  1. FUNAC OI mtc generally defaults to G commands

G69: Not so clear

G21: metric size input

G25: Spindle speed fluctuation detection disconnected

G80: Canned cycle cancel

G54: Coordinate system default

G18: ZX plane selection

G96 (G97): Constant linear speed control

G99: Feed per revolution

G40: Tool nose compensation cancel (G41 G42)

G22: Storage stroke detection is on

G67: Macro program modal call cancel

G64: Not so clear

G13.1: Polar coordinate interpolation mode cancel

low volume machining

  1. The external thread is generally 1.3P, and the internal thread is 1.08P.
  2. Thread speed S1200/pitch * safety factor (usually 0.8).
  3. Manual tool nose R compensation formula: chamfering from bottom to top: Z=R*(1-tan(a/2)) X=R(1-tan(a/2))*tan(a). Change the chamfering from top to bottom and change the minus to plus.
  4. When the feed increases by 0.05, the speed decreases by 50 to 80 revolutions. This is because reducing the speed means that the tool wear decreases, and the cutting force increases more slowly, so as to compensate for the increase in the cutting force and the temperature due to the increase in the feed. The impact.
  5. Cutting speed and cutting force are very important to the impact of cutting tools. Excessive cutting force is the main reason for tool collapse.

The relationship between cutting speed and cutting force: the faster the cutting speed, the feed will not change, and the cutting force will decrease slowly. At the same time, the faster the cutting speed will make the tool wear faster, the cutting force will become larger and the temperature will increase The higher the cutting force and internal stress, the tool will collapse when the cutting force and internal stress are too great for the blade to bear (of course, there are also reasons for the stress and hardness drop caused by temperature changes).

  1. The following points should be paid special attention to during CNC machining:

(1) For the current economic CNC lathes in my country, ordinary three-phase asynchronous motors are used to achieve stepless speed change through frequency converters. If there is no mechanical deceleration, the spindle output torque is often insufficient at low speeds. If the cutting load is too large, it is easy to get bored. However, some machine tools have gears to solve this problem.

(2) As far as possible, the tool can complete the processing of a part or a work shift. In the finishing of large parts, special attention should be paid to avoid changing the tool in the middle to ensure that the tool can be processed in one time.

(3) When using CNC turning to turn the thread, a higher speed should be used as much as possible to achieve high-quality and efficient production.

(4) Use G96 as much as possible.

(5) The basic concept of high-speed machining is to make the feed exceed the thermal conduction speed, so that the cutting heat is discharged with the iron filings to isolate the cutting heat from the workpiece, and to ensure that the workpiece does not heat up or does not heat up. Therefore, high-speed processing is selected very high The cutting speed is matched with the high feed and a smaller back-grab is selected.

(6) Pay attention to the compensation of tool nose R.

  1. Some commonly used forms:

Workpiece material machinability classification table

Common thread cutting times and back-cutting scale

Common geometric calculation formula

Inch to millimeter conversion table

  1. Vibration and tool breakage often occur during grooving. The fundamental reason for all of this is increased cutting force and insufficient tool rigidity. The shorter the tool extension length, the smaller the clearance angle, and the larger the blade area, the better the rigidity. The greater the cutting force, but the larger the width of the groove tool, the cutting force it can bear will increase accordingly, but its cutting force will also increase. On the contrary, the smaller the groove tool, the smaller the force it can bear. Its cutting force is also small. image
  2. Reasons for vibration during car trough:

(1) The extended length of the tool is too long, resulting in a decrease in rigidity.

(2) The feed rate is too slow, which will cause the unit cutting force to become larger and cause a large vibration. The formula is: P=F/back-cutting amount*f P is the unit cutting force F is the cutting force, and the speed is too fast. Will shake the knife.

(3) The machine tool is not rigid enough, which means that the tool can withstand the cutting force, but the machine tool cannot withstand it. To put it bluntly, the machine tool does not move. Generally, new machines do not have this kind of problem. The machine with this kind of problem is either old. Either the machine killer is often encountered.

Cylindrical Grinding

  1. When I was driving a cargo, I found that the size was okay at the beginning, but after a few hours, it was found that the size has changed and the size is unstable. The reason may be that the cutting force is all new because the tools are new at the beginning. It is not very large, but after turning for a period of time, the tool wears out and the cutting force becomes larger, which causes the workpiece to shift on the chuck, so the size is old and unstable.
  2. When using G71, the value of P and Q cannot exceed the sequence number of the entire program, otherwise an alarm will occur: G71~G73 command format is incorrect, at least in FUANC.
  3. There are two formats of subroutines in FANUC system:

(1) The first three digits of P000 0000 refer to the number of cycles, and the last four digits are the program number.

(2) The first four digits of P0000L000 are the program number, and the last three digits of L are the number of cycles.

  1. The starting point of the arc remains unchanged, and the end point is offset by a mm in the Z direction, and the bottom diameter of the arc is offset by a/2.
  2. When drilling deep holes, the drill does not grind the cutting grooves to facilitate drill chip removal.
  3. If the tool holder is used for drilling holes, the drill bit can be rotated to change the hole diameter.
  4. When drilling stainless steel center holes or stainless steel holes, the drill bit or center drill center must be small, otherwise it will not move. Do not grind the grooves when drilling with cobalt drills to prevent the drill from annealing during drilling.
  5. According to the process, there are generally three types of blanking: one material is one, two goods are one, and the whole bar is one.
  6. When there is an ellipse when threading, it may be that the material is loose. Use a tooth knife to cut a few more times.
  7. In some systems where macro programs can be input, macro programs can be used to replace subprogram cycles, which can save program numbers and avoid a lot of trouble.
  8. If a drill bit is used for reaming, but the hole jumps a lot, a flat-bottom drill can be used for reaming at this time, but the twist drill must be short to increase the rigidity.
  9. If the drill bit is used to make holes directly on the drilling machine, the hole diameter may be deviated, but if the hole size is generally not run on the drilling machine, for example, if a 10mm drill bit is used to expand the hole on the drilling machine, the expanded hole diameter is generally It is about 3 wire tolerance.
  10. In the small hole (through hole) of the car, try to make the crumbs continuous curling and then discharge from the tail.

Key points of rolling crumbs:

(1) The position of the knife should be properly raised.

(2) Appropriate blade inclination, cutting amount and feed rate, remember that the knife should not be too low, otherwise it will be easy to break chips. If the auxiliary deflection angle of the knife is large, the tool bar will not be caught even if the chip breaks, if the auxiliary deflection angle is too small , After chip breaking, the chips will get stuck in the tool bar and easily cause danger.


  1. The larger the cross section of the knife bar in the hole, the less likely it is to vibrate the knife, and a strong rubber band can be attached to the knife bar, because the strong rubber band can play a certain role in absorbing vibration.
  2. When turning the copper hole, the tip R of the knife can be appropriately larger (R0.4~R0.8), especially when the taper is under the turning, the iron parts may be nothing, and the copper parts will be very jammed.

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