CNC Knowledge: Just 6 steps to completely solve common tapping and problems

Thread shape (standard metric thread)

1. From the picture we know:

P means pitch is the distance from the tooth to the tip or bottom of the tooth.

The usual method of expression is M3×0.5, where 0.5 is the pitch and the unit is mm.

(M means metric), 3 means nominal size and the size is the maximum diameter of the thread or the maximum diameter (bottom) of the internal thread.

2. Generally speaking, if the threads are regular, the large diameter (or root diameter) should conform to the nominal dimension of the thread, and the medium diameter (the same as the internal thread) and the small diameter ( the internal diameter of the internal thread) must conform to the numbers on the specifications as shown in the table below (metric standard coarse thread).

If it is a thin wire, the gap becomes smaller and its large, medium and small diameter changes accordingly.

3. British threads are the same except that the pitch is expressed as the number of threads in 1 inch, such as: 1/4-20UNC, i.e. the 1/4″ major diameter external thread (6.35 mm) has 20 threads per inch.

UNC (UNTRUE WIRE)

C means coarse teeth;

F means fine teeth;

EF means extremely fine teeth;

C, F and EF each have different heights.

4. How to use thread gauge (divided into spiral studs and rings)

A. Understanding the structure

1) The T (pass) end is generally longer than the Z (stop) end;

2) The one with a gap in the middle of the British system is the through end;

3) It is usually made of hardened (tempered) steel material, which is very fragile and will break if dropped on the ground;

4) It will wear out after long-term use. Generally, it needs to be re-inspected after more than 10,000 times.

B. How to use

Only those that have passed quality assurance inspection or have a qualified label (affixed to the box) can be used.

For long-term use, it should be used about 10,000 times (can be estimated), then subjected to inspection and qualified before use.

Use the gloved thumb and index finger to gently pinch and twist, avoid using strong force, then the T (open) end is applied to the end to pass through, and the Z (stop) end should enter 1 2-toothed, and it is qualified if it no longer enters, never use strong force, after using the thread gauge, the thread gauge (sampling ring or column) should be wiped with a cloth soft and clean, coated with anti-rust oil and put back in the box.

C. The correct tapping method

1) Choose the appropriate tap

There are one, two and three thread taps. Generally we use the third tap, unless the board is very thick, it is divided into one, two and three taps. Generally, a machine tap (one time only) can be used. .

2) The form of wiretapping can be divided into

Plain Silk Faucet

Spiral tapping: more expensive, but has good chip evacuation and high efficiency;

Point tap: more expensive, but with good chip evacuation and high efficiency;

Chipless tapping: Use extrusion to press thin materials (usually less than 3 M/M) into screw shapes, so that the holes are larger than those made by ordinary tapping and threaded tapping, for example : M3-0.5 Ordinary and spiral threads generally drill 2.6 holes. in iron die, but chipless tapping can drill 2.78 to 2.8 holes.

3) If it is a thick plate (above 3M/M) before tapping, the plate hole burrs should be deburred with a drill, but it should not be chamfered, otherwise the thickness of the plate will be reduced due to chamfering, resulting in fewer screw threads.

4) When tapping, the tap should be perpendicular to the workpiece.

5) When tapping, iron (aluminum) shavings on the surface should be removed. The removal method is to use a brush (toothbrush) or high pressure air cleaning.

6) When tapping, you should apply clean engine oil instead of dirty engine oil containing iron filings or other impurities.

7) The diameter of the hole before tapping must be correct and can generally be obtained by inspection, and the thickness and material of the plate all affect the diameter of the hole.

5. The first part must pass the thread gauge inspection. If it fails, it may be due to the following reasons:

1) The tap is not qualified (worn or defective)

2) Qualified hole diameter (before tapping). If the hole is too small, the difficulty of tapping will be increased due to high friction, and the tap will wear out quickly. On the contrary, if the hole is too big, it will wear out quickly. will be easy to press in, but the thread The quality is very poor because the teeth The small diameter will become larger and the bonding force with respect to the screw and nut will be insufficient and the thread will slip easily. Taking M3 × 0.5 as an example, the correct hole (general iron material) should be between 2.50 and 2.65. If it is too big, it will be difficult to get good quality thread.

3) The tap is not perpendicular to the workpiece.

4) If the faucet is not clean, iron filings will get stuck in it, causing the teeth to enlarge.

5) The iron die contains iron filings or impurities that have not been removed, causing the thread gauge to fail inspection.

6) The tap is not oiled and the friction force is too great, causing the teeth to break.

7) The tapping machine is defective and the shaft shakes, causing the teeth to become larger and unskilled.

8) Use a screw gauge to inspect every 20 parts or so during the manufacturing process (the frequency depends on the qualification status. If the qualification rate is high, the inspection cycle can be lengthened, otherwise it will be shortened. )

Common problems when tapping with ordinary taps

1. Main problems that often occur when tapping:

1) The faucet is broken;

2) Chipped tap teeth;

3) The faucet wears out too quickly;

4) The pitch diameter of the thread is too large;

5) The pitch diameter of the thread is too small;

6) The thread surface roughness value is too high.

2. Causes

1) When the tap breaks and the bottom hole of the thread is processed, the diameter of the bottom hole is too small and the chip removal is not good, causing chip blockage when the thread cannot be tapped , the depth of the hole is not good; enough ; when tapping the thread, the cutting speed is too high and too fast; the tap used for tapping is different from the bottom hole diameter of the thread is not axial, the selection of tap sharpening parameters is inappropriate, and the hardness of the workpiece is unstable; the tap is used too long and is excessively worn.

2) The cutting angle of the tap is too large; the cutting thickness of each tooth of the tap is too large; the quenching hardness of the tap is too high; the tap is used too long and is seriously worn.

3) The tap is too worn and the cutting speed is too high when tapping the thread; the tap grinding parameters are not selected appropriately; the cutting fluid is incorrectly selected and the hardness of the workpiece material is too high; burns occur when sharpening taps.

4) The pitch diameter of the thread is too large. The degree of precision of the pitch diameter of the tap is incorrectly selected; the cut selection is unreasonable; the cutting speed of the tapping is too high, the coaxiality of the tap and the bottom hole of the thread; the workpiece is poor; the tap grinding parameters are inappropriate; burrs are produced in the grinding tap and the length of the cutting cone of the tap is too short.

5) The pitch diameter of the thread is too small and the precision level of the pitch diameter of the tap is incorrectly selected; the tap grinding parameters are selected unreasonably, causing tap wear, and the cutting fluid is incorrectly selected;

6) The thread surface roughness is too large, the tap grinding parameters are inappropriate; the hardness of the part material is too low; the grinding quality of the tap is unreasonable; too high when tapping the thread; the tap is used too long. Lots of wear.

3.Resolution method

1) When breaking a tap, correctly select the diameter of the threaded bottom hole; sharpen the edge angle or use a spiral groove tap; the depth of the drilled bottom hole should meet the specified standards, appropriately reduce the cutting speed and select accordingly; the standard; correct the tap and bottom when tapping, ensure its coaxiality meets the requirements, and select a floating tapping chuck; increase the cutting angle of tapping and shorten the length of cutting cone, select; a safety chuck;

2) If the tap is chipped, reduce the cutting angle of the tap appropriately; appropriately increase the length of the cutting cone, reduce the hardness, and replace the tap in time;

3) If the tap wears too quickly, reduce the cutting speed appropriately; reduce the cutting angle of the tap and lengthen the length of the cutting cone; choose a cutting fluid with good lubricity, perform appropriate heat treatment on the workpiece and sharpen the tap correctly; .

4) The pitch diameter of the thread is too large. Select a pitch diameter of the tap with a reasonable level of precision; select a suitable cutting fluid and appropriately reduce the cutting speed of the tap and the bottom hole of the thread when tapping; use a floating chuck; appropriately reduce the cutting angle and clearance angle of the cutting cone; eliminate burrs produced by tap sharpening and increase the length of the cutting cone appropriately.

5) If the thread pitch diameter is too small, select a tap pitch diameter suitable for the precision level; appropriately increase the tap cutting angle and cutting cone angle, and replace excessively worn taps;

6) The thread surface roughness value is too large. Increase the cutting angle of the tap appropriately to reduce the cutting cone angle, carry out heat treatment to appropriately increase the hardness of the workpiece to ensure that the cutting surface of the tap has a roughness value bottom surface and choose. a cutting fluid with good lubricity; Reduce cutting speed appropriately; replace worn taps.

Methods and precautions for tapping threads with ordinary tapping machines

Due to the low efficiency of manual tapping and some quality problems, it is not suitable for mass production. Therefore, in actual mass production, machine tapping threads are mainly used. To ensure the quality of tapping parts, improve production efficiency and reduce production costs. However, during the process of tapping threads, machines and tools must also be used correctly. If machines and tools cannot be used properly, it will also affect the processing quality of threaded holes on the workpiece.

1. The radial runout of the drill spindle should generally be adjusted within 0.05mm. If the threaded hole tapping accuracy is high, the radial runout of the spindle should not be greater than 0.02mm. The workpiece clamping device conforms to the drill spindle. The verticality error of the center or center of the tap should not be greater than 0.05/100, and the coaxiality of the threaded bottom hole of the workpiece and the tap should generally not exceed 0.05mm.

2. When the tap is about to finish tapping the thread, the feed should be light and slow. This is to prevent the front end of the tap from interfering with the depth of the workpiece’s threaded bottom hole and damaging the tap.

3. When the threaded hole cannot be tapped or the threaded hole is deep, a tapping safety chuck should be used. The tapping cutting force that the safety chuck can withstand should be adjusted according to the tap size. be Adjust it appropriately to avoid breaking the cone or penetrating it.

4. During the tapping stroke of the cutting length of the tap, uniform and appropriate pressure should be applied to the power handle of the drill press to help the tap enter the bottom hole. This can avoid pulling down the incomplete thread due to tapping. first rounds. As you turn the spit, scrape the threads. When the calibration piece enters the workpiece, make sure the threads feed naturally to tap the threads to avoid thinning the tooth profile.

5. The cutting speed of tapping is very important. It is mainly determined by the cutting material, tap pitch diameter, pitch, threaded hole depth and other precisions, as well as the actual processing results on site. . Generally, when the depth of the threaded hole is between 10 and 30 mm and the workpiece is made of the following materials, the cutting speed is approximately as follows:

1) Steel v=6～15 m/min;

2) Quenched and tempered steel or harder steel v=5~10 m/min;

3) Stainless steel v=2～7m/min;

4) Cast iron v=8~10m/min.

Under the same conditions, the smallest tap diameter is used for relatively high speed, the largest tap diameter is used for relatively low speed, and the large pitch is used for low speed.

6. When tapping through threads, be careful that the calibrated part of the tap cannot be fully exposed, otherwise random deformation will occur when the tap is reversed and withdrawn.

7. When machine tapping, the selection and use of cutting fluid is very important for plastic materials, a sufficient amount of cutting fluid should generally be used if the surface roughness value of the thread. The hole on the coin is relatively high. When the temperature is low, rapeseed oil and molybdenum fluid 2 can be used, and soybean oil is also more effective.