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CNC Drilling Steps, and Methods to Improve the Drilling Precision

CNC drilling steps and methods to improve drilling precision are as follows:

1. The basic concept of CNC drilling

In general, drilling is a way of drilling holes on the product surface with a drill. In general, when drilling the product on the drilling press, the drill bit should  complete two movements simultaneously:

(1) Main motion, that is, the rotary motion of the drill bit around the axis (cutting motion);

(2) Secondary motion, that is, the straight motion of the bit along the axis towards the work piece (feed movement).

When CNC drilling, because of the defects in the structure of the drill bit, it will leave traces on the processed place of the product and affect the processing quality of the work piece. The machining accuracy is generally below IT10 level, and the surface roughness is about RA12.5μm, which belongs to the rough machining class.

2. Operation process of CNC drilling

(1) Marking: before drilling, first understand the drawing requirements, according to the basic standard requirements of drilling, use tools to draw the center line of the hole position, and the center line must be clear and accurate, and the thinner the better. After drawing the line, use vernier calipers or steel plate ruler for measurement.

(2) Checking the square or circle: after drawing the line and qualified, check the square or circle with the center line of the hole as the symmetrical center should be drawn as the check line when testing the drilling, so as to check and correct the orientation of the drilling when drilling.

(3) Proofing and eye punching: after drawing out the corresponding check box or check circle, the sample should be carefully made and eye punching. First hit a small point, in different directions of the center line measurement, to see if the punch is really hit at the intersection of the center line of the cross, and then the sample punch force hit right, round, big, in order to accurately centering the knife.

(4) Clamping: wipe the machine surface, fixture surface and work piece datum surface with a rag, and then clamp the work piece. Clamping is smooth and firm according to the requirements, and it is convenient to inquire and measure at any time. Need to pay attention to the work piece clamping method, in order to prevent the work piece deformation due to clamping.

(5) Test drilling: before the formal drilling must first test drilling: drill the drill bit edge alignment hole center sample drilling a shallow hole, and then visually test the shallow hole orientation is correct, but also need to continue to correct the deviation, let the shallow hole and check the circle coaxial. If the violation is small, you can force the work piece to go in the opposite direction of the violation together with the drilling, and reach the gradual proofreading.

(6) Drilling: machine drilling is generally manual feed operation, when the test drilling azimuth accuracy requirements, it can be carried out. When manual feeding, the feeding force should not cause the bit to bend, to avoid the axis of the hole skew.

3. CNC drilling method with higher precision

(1) Grinding the drill is the beginning of everything

Before drilling, the corresponding bit should be selected for grinding. In addition to ensuring the accuracy of the vertical Angle, back Angle and transverse edge bevel Angle of the grinding bit, the length of the two main cutting edges is flat and symmetrical with the center line of the bit, and the surface of the two main and rear cutting edges is smooth. In order to facilitate centering and reduce the roughness of the small hole wall, the transverse and main cutting edges should be properly polished (it is best to grind on the grinder first, and then finish on the asphalting stone).

(2) Accurate line is the basis

Make accurate line with height ruler. First of all, the standard accuracy should be ensured before accurate cnc machining, by marking the needle angle and the work piece between the plane of the marking 40 ~ 60 degrees (along the direction of the marking), making the line clear and uniform. Pay attention to the choice of marking datum surface, datum surface to be processed accurately, to ensure its own flatness and verticality with the adjacent surface. After the cross line of the hole position is drawn, in order to ensure that it is easy to find the right point during the drilling, the central punch is used to rush out the central point on the cross line (it is required that the punch point should be small and the orientation should be accurate).

(3) Correct clamping is the key point

Under normal circumstances, if the accuracy of the  hole diameter less than 6mm is not high,  pliers to clamp workpiece can be used to drilling; About the hole of 6 ~ 10mm, if the work piece rule is acceptable, can be used to clamp, but should make the work piece surface and the drilling machine spindle vertical. When drilling the hole with a larger diameter, the flat jaw pliers must be fixed with a bolt plate; When the diameter of drilling is more than 10mm and the work piece is larger, the method of pressing plate clamping is used for drilling.

(4) Accurate finding is the key

At the end of work piece clamping, do not rush to drop the drill, but first to alignment. There is static positivity and dynamic positivity. The so-called static alignment, refers to the alignment before the launch of the drilling machine, so that the intersection of the center line of the drilling machine spindle and the cross line of the work piece is right. This method is safe and convenient for beginners, and it is easier to grasp. But because of the failure to consider such uncertainties as the swing of the drilling machine spindle, drilling accuracy is low. Dynamic finding is the alignment of the drilling machine after the launch, in the alignment together with some uncertain factors are taken into account, the accuracy is relatively high.

(5) Careful detection is important

The test can accurately and timely find the accuracy of the hole, so that necessary measures can be taken to compensate. To drill the hole with higher precision, we generally adopt the drilling, reaming, reaming processing technology. After the first step to drill holes with a caliper testing center of the bottom hole to the datum of fault offset, checked by actual measured conversion direction of the center of the bottom hole and ambition, if the fault is greater than 0.10 mm, can be in reaming, appropriate increase bit apex angle, weaken the self-centering effect, to borrow the positive direction appropriate push artifacts, gradually increasing the diameter of the drill point method to compensate. If the fault amount is greater than 0.10mm, the mixed round file can be used to trim the two sidewalls of the bottom hole, and the trimming part should be connected with the smooth transition of the bottom hole arc.

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