How to use CNC punch correctly?

May 27, 2024 Roberto Magalhães

Numerical control punch is a type of equipment that automatically cuts the sheet metal according to the pre-programmed processing program.

The reasonable use method should not only ensure the processing of qualified parts that meet the design requirements, but also allow the reasonable application of the CNC punch and give full play to its performance.

In the process of using CNC punching, each company has its own methods to better maintain the equipment.

The following are the reasonable use methods of CNC punch summarized by our company according to many years of experience in sheet metal processing.

Operation Specification

The processing of sheet metal parts is inseparable from NC punching technology, and the correct operation specification of NC punching can not only make the processing smooth, but also maximize the utilization efficiency of the machine tool and significantly extend the service life.

The following are our operating specifications:

(1) Before using the CNC punch, remove objects that hinder safety or work from the work area.

(2) Close the air compressor vent valve and turn on the air compressor power supply.

(3) Clean the table, turn on the main motor power supply, check for abnormal sound and abnormal state in the empty machine running state, and check the air pressure

(4) Enter the drawing creation program and confirm whether the tool device required for this drawing is appropriate.

(5) When preparing the tool,

1. Select the most suitable tool size.

2. Fix the mold.

3. Determine the bottom clearance of the die.

When the plate thickness is less than 1.2t and the gap is 0.2mm;
When the plate thickness is 15t ~ 1.6, the gap is 0.3mm;
When the plate thickness is 2.0T, the gap is 0.4mm;
When the plate thickness is 2.3t, the gap is 0.5mm;
When the plate thickness is 3.0T, the gap is 0.6mm;
When the plate thickness is 4.0t, the gap is 0.8mm;
When the plate thickness is 4.5t, the gap is 0.9mm;

If the material becomes aluminum or copper, the gap must be added one level.

If there is no tool used above, it should not be punched in principle. If it is urgent, the next level pattern can be used.

4. Measure the length of the upper die and determine the tool loading position (standard length: 74 mm, oblique knife: 77 mm).

5. Determine the bottom thickness of the die, standard 30 is 18mm, 60?

The thickness is 20mm, and insufficient thickness must be supplemented with gaskets.

6. Select the correct intermediate ring. 31？ (including) the following concave, 32? Above plan.

7. Select an appropriate stripping plate (based on the assumption that it is larger than 3 mm from the tool).

After the die combination is completed, check whether the vertical direction of the upper and lower dies and the stripping plate is correct, and calibrate the tool on the tool gauge.

(6) Apply the tool die according to the tool program.

(7) Prepare appropriate materials.

(8) Start running the program sprint.

(9) After punching the first sample, measure whether the dimensions of each part are the same as the drawing, whether the burr is normal, and whether the process needs to be changed.

In this case, the program must be changed first (if there are still materials and products in continuous production later, the qualified parts must be retained and the material number must be marked in the appropriate position as a sample for comparison in future production).

(10) Before production, it is necessary to consider how to place the perforated materials to ensure the quality and facilitate the use of the next process;

Pay attention to the storage conditions of the bottom material box at any time in production (not too much, inconvenient for handling);

When the mold is lubricated, remove debris from the table at any time (to avoid scratching the work object).

(11) Pay attention to whether the cutting tool leaks or breaks during production; Whether the burr meets the requirements; Whether the bed surface is clean; If the processed objects are scratched, pay attention to sudden collisions, removal plate falling, etc.

(12) In case of the above situation, immediately press the emergency stop switch.

(13) If there is any code displayed on the computer screen, it must be eliminated according to the operating manual for each model.

Otherwise, maintenance must be carried out according to the general method, and production will be restarted after the obstacles are removed.

(14) During production, the molds (knives), materials and other data required for the next work project must be prepared so that after work is completed, production can continue in the shortest time (i.e. reduce the time machine idle and improve grain movement rate).

(15) When the part is delivered to the next workstation after completion, the correct quantity and care must be provided.

(16) After the machine tool stops working, the following items must be completed:

Remove the trash can;
Retract the tool into the tool magazine;
Adjust the bed surface to the middle position;
The air compressor is empty;
Perform level I maintenance (daily);
Clean the surrounding environment;
Perform level 1 maintenance (daily):

1. Check the machine's lubrication point for lubricating oil. Whether to add;

2. Clean and lubricate the x and y axes.

3. Clean debris from the upper and lower C-axis molds.

4. Clean debris from gears and racks of X and Y axis motors.

5. Check whether the electrical control panel is loose and operates normally.

6. Open the electrical panel door to check whether the electrical switch is normal.

Operational skills

Ensure the best tools i.e. release

Die clearance refers to the sum of clearance on both sides when the punch enters the lower die. It is related to the plate thickness, material and stamping process.

Proper die clearance can ensure good punching quality, reduce burr and collapse, keep the plate flat, effectively prevent material carryover, and extend the service life of the die.

By checking the stamping residue, we can determine if the die gap is appropriate.

If the gap is too large, the residue will have rough, wavy fracture surfaces and small shiny surfaces.

The larger the gap, the greater the angle between the fracture surface and the shiny surface.

Crimp and fracture will be formed during puncture, and even a thin edge protrusion will appear.

On the contrary, if the gap is too small, there will be small angle fracture surface and large shiny surface.

When local stamping such as grooving, step punching and shearing is performed, the lateral force will deflect the punch and cause very small one-sided clearance.

Sometimes the edge offset is too large, which will scratch the lower die and cause rapid wear of the upper and lower dies.

When the die is stamped with the best gap, the residue fracture surface and the bright surface have the same angle and coincide with each other, which can minimize the cutting force and reduce the punching burr.

Timely grinding can effectively extend the service life of the die

If the part exhibits excessive burrs or abnormal noise during stamping, the die may be passivated.

Check the punch and lower the die. When edge wear produces an arc with a radius of about 0.10 mm, grinding is necessary.

Practice shows that frequent micro-grinding, rather than grinding when necessary, will not only maintain good part quality and reduce cutting force, but also extend the life of the die by more than two times.

In addition to knowing when to grind the die, it is particularly important to master the correct grinding method.

The die rectification procedures are as follows:

1) Sintered alumina grinding wheel with hardness of D ~ J and abrasive particle size of 46 ~ 60 is used.

It is better to select the suitable grinding wheel for high-speed steel grinding.

2) During grinding, hold the punch vertically in the V-groove or magnetic chuck holder of the surface grinder.

The grinding amount each time is 0.03 ~ 0.05mm. Repeat grinding until the punch is sharp.

The maximum grinding amount is generally 0.1 ~ 0.3mm.

3) When the grinding force is large or the die is close to the grinding wheel, adding coolant can prevent the die from overheating, cracking or annealing.

High-quality, multi-purpose refrigerants must be selected in accordance with the manufacturer's requirements

4) The downward feed of the grinding wheel is 0.03 ~ 0.08mm, the cross feed is 0.13 ~ 0.25mm, and the cross feed rate is 25 ~ 3.8m/min.

5) After sanding, polish the edge with an oil stone to remove burrs and sand a fillet with a radius of 0.03 ~ 0.05mm to prevent the edge from cracking.

6) Demagnetize the punch and spray lubricating oil to prevent rust.

Methods for eliminating and reducing adhesion

Due to the pressure and heat during stamping, the fine particles of sheet metal will stick to the surface of the punch, resulting in poor punch quality.

The adhesive can be removed by sanding with fine oil stone, and the sanding direction should be the same as the direction of punch movement, to avoid more adhesive.

Do not polish with thick gauze, to prevent the surface of the punch from becoming rougher and sticking more easily.

Reasonable die clearance, good stamping process, and necessary sheet metal lubrication will reduce the generation of sticky materials.

To prevent overheating, lubrication is usually used, which will reduce friction.

If lubrication is not possible or waste recovery occurs, follow the following methods:

Alternatively, several punches of the same size are used to punch in turn so that it can have a long cooling time before being reused.

Stop using overheated die.

Through programming die change control, stop your repeated work for a long time or reduce your stamping frequency.

Measures to avoid deformation of the material when drilling many holes

If a plate is punched with many holes, the plate cannot be held flat due to the accumulation of punching stress.

During each punch, the material around the hole deforms downward, resulting in tensile stress on the upper surface and compressive stress on the lower surface.

For a small number of holes, the effect is not obvious, but when the number of holes increases, tensile and compressive stresses accumulate somewhere until the material deforms.

One way to eliminate aspect ratio deformation is to first drill every other hole and then go back to drill the remaining holes.

Although this also produces stress, it will relieve the stress build-up during sequential stamping in the same direction and compensate the stresses of the front and rear hole groups so as to prevent deformation of the sheet metal.

Try to avoid drilling very narrow strips

When the die is used to cut the plate with a width smaller than the thickness of the plate, the punch will be bent and deformed due to the action of lateral force, causing the gap on one side to be very small or the wear to be intensified.

In severe cases, the lower die will be scratched and the upper and lower dies will be damaged at the same time.

It is recommended not to use narrow strips with a step width less than 2.5 times the sheet thickness.

When cutting very narrow strips, the sheet will tend to fold into the opening of the lower die instead of being completely cut, or even snap against the side of the die.

If the above situation cannot be avoided, it is recommended to use a fully guided die with the return plate supporting the punch.

Punch surface hardening and its scope of application

Although heat treatment and surface coating can improve the surface characteristics of the punch, it is not a general method to solve stamping problems and extend the service life of the die.

Generally speaking, coating improves the surface hardness of the punch and improves side lubrication, but when stamping large tonnage and hard materials, these advantages disappear after about 1000 stamps.

Hardened punches can be used in the following cases:

Drilling soft or viscous materials (such as aluminum);
Diluted abrasive materials (such as epoxy glass sheets);
Diluted hard materials (such as stainless steel);
Frequent pitch punch.

Abnormal lubrication:

Surface hardening generally adopts titanium plating, nitriding and other methods, and the thickness of the surface hardening layer is 12~60μm and is part of the punch die, not just the coating.

Hardened molds can be sharpened in the usual way.

Surface hardening can reduce die wear when punching stainless steel plates, but cannot extend its service life.

Proper lubrication, timely grinding and operation in accordance with regulations are effective methods.

Maintaining the position of the punch die in case of low neutrality

If the position of the punch die is not neutral, resulting in rapid passivation of the die and poor machining quality of the part, the following points can be revised:

Check the machine tool level and readjust if necessary;
Check and lubricate the die hole and guide ring key on the rotary table, and repair in time in case of damage;
Clean the bottom die base of the rotary table to facilitate accurate installation of the lower die, check the wear of its key or keyway and replace it if necessary;
Use a special chuck to calibrate the die station and adjust it in time in case of deviation.