CNC | Summary of Drilling Steps and Methods to Improve Drilling Accuracy!

1, Basic concepts of drilling
In general, drilling refers to a machining method where a drill bit is used to process holes on the product surface. Generally speaking, when drilling products on a drilling machine, the drill bit should synchronously complete two movements:
① Main motion, that is, the rotational motion of the drill bit around the axis (cutting motion);
② Secondary motion refers to the linear motion of the drill bit towards the workpiece along the axis direction (feed motion).
When drilling, due to the structural defects of the drill bit, it will leave marks on the processed areas of the product, affecting the machining quality of the workpiece. The machining accuracy is generally below IT10 level, and the surface roughness is Ra12.5 μ Around m, it belongs to the rough machining category.
2, The operation process of drilling
1 Dashing
Before drilling, the first thing to understand is the requirements of the drawing. According to the basic standards of drilling, use tools to draw the centerline of the hole position. The centerline must be clear and accurate, and the finer the better. After drawing the line, use a vernier caliper or steel plate ruler for measurement.
2 inspection squares or inspection circles
After drawing the line and passing the inspection, a inspection square or inspection circle with the hole centerline as the symmetrical center should be drawn as the inspection line during the trial drilling, so as to check and correct the drilling direction during drilling.
3. Sample punching
After drawing out the corresponding inspection squares or inspection circles, careful sampling and punching should be carried out. First, make a small point and measure multiple times in different directions of the cross centerline to see if the punch is indeed hitting the intersection of the cross centerline. Then, apply force to make the sample punch straight, round, and large, in order to accurately center the cutting.
4 clamps
Clean the machine tool table, fixture surface, and workpiece reference surface with a cloth, then clamp the workpiece and ensure it is level, firm, and easy to check and measure at any time according to the requirements. Pay attention to the clamping method of the workpiece to prevent deformation due to clamping.
5 Trial drilling
Before formal drilling, it is necessary to conduct a trial drilling: align the drill bit's horizontal edge with the center sample of the hole, punch a shallow hole, and then visually inspect whether the orientation of the shallow hole is correct. Continuous correction is also needed to ensure that the shallow hole is coaxial with the inspection circle. If the violation is minor, the workpiece can be pushed towards the opposite direction of the violation with the same force during the drilling process, achieving gradual calibration.
6 Boreholes
Machine drilling generally relies on manual feed operation, and drilling can be carried out once the accuracy of the drilling direction is required. During manual feeding, the feeding force shall not bend the drill bit to avoid the deviation of the hole axis.
3, Methods with higher drilling accuracy
Grinding the drill bit well is the beginning of everything
Before drilling, the corresponding drill bit should be selected for sharpening. In addition to ensuring accurate top angle, back angle, and lateral edge inclination, the length of the two main cutting edges is equal and symmetrical with the centerline of the drill bit, and the two main back cutting surfaces are smooth. In order to facilitate centering and reduce the roughness of the hole wall, appropriate grinding should also be done on the lateral and main cutting edges (preferably rough grinding on a grinder first, and then fine grinding on an oilstone).
Accurate marking is the foundation
When accurately marking with a height ruler, it is important to maintain accuracy and accuracy. When marking, the angle between the marking needle and the marking plane of the workpiece should form an angle of 40-60 degrees (along the marking direction), so that the drawn lines are clear and uniform.
Pay attention to the selection of the marking reference plane, the reference plane should be processed accurately, and its flatness and perpendicularity with adjacent surfaces should be ensured. After drawing the cross line of the hole position, in order to ensure easy alignment during drilling, use a center punch to punch out the center point on the cross line (the punching point should be small and the orientation should be accurate).
3. Correct clamping is crucial
Normally, for holes with a diameter less than 6mm, if the accuracy is not high, the workpiece can be clamped with pliers for drilling; For holes with a diameter of 6-10mm, if the workpiece is regular and flat, it can be clamped with flat pliers, but the surface of the workpiece should be perpendicular to the spindle of the drilling machine. When drilling holes with larger diameters, the flat pliers must be fixed with bolt pressure plates; When drilling larger workpieces with a diameter of 10mm or more, use the method of clamping with a pressure plate for drilling.
Accurate alignment is key
After the workpiece clamping is completed, do not rush to drop the drill, but prioritize alignment.
There are two types of alignment: static alignment and dynamic alignment. The so-called static alignment refers to aligning the center line of the drilling machine spindle with the intersection point of the workpiece cross line before the drilling machine is initiated. This method is safe and convenient for beginners, and is relatively easy to grasp. However, due to the lack of consideration of uncertain factors such as the swing of the drilling machine spindle, the drilling accuracy is relatively low. Dynamic alignment is carried out after the drilling machine is initiated, taking into account some uncertain factors during the alignment process, resulting in relatively high accuracy.
5 Careful inspections cannot be omitted
Testing can accurately and timely detect the accuracy of holes, in order to take necessary measures for compensation.
For holes with high drilling accuracy, we generally use drilling, reaming, and reaming processing techniques. After drilling the small hole in the first step, use a caliper to detect the offset from the center of the bottom hole to the reference plane. After actual measurement, the orientation between the bottom hole and the center of the aspiration can be calculated. If the offset is not greater than 0.10mm, the top angle of the drill bit can be appropriately increased and the automatic centering effect can be weakened when expanding the hole. The workpiece can be appropriately pushed in the positive direction, and the drill tip diameter can be gradually increased for compensation. If the amount of error is greater than 0.10mm, a mixed round file can be used to repair the walls on both sides of the bottom hole. The repaired part should be connected to the smooth transition of the bottom hole arc.