Positioning reference and fixtures and the use of commonly used gauges

1, the concept of positioning benchmark

The datum is the point, line, and surface on which the part is used to determine the location of other points, lines, and faces. The reference used for positioning is called the positioning reference. Positioning is the process of determining the correct position of a part. Two center holes are provided on the outer cylindrical grinding shaft parts. Usually, the shaft adopts two top clamps, and its positioning reference is a central axis formed by two central holes, and the workpiece is rotationally formed into a cylindrical surface. cnc machining part

2, the center hole

The general cylindrical grinding process is considered on the general shaft parts, and the design center hole is added to the part drawing as the positioning reference. There are two criteria for common center holes. The A-type center hole is a 60° cone which is the working part of the center hole. It is supported by a top 60° cone to set the center and to withstand the grinding force and the gravity of the workpiece. The small cylindrical bore on the front face of the 60° cone stores lubricant to reduce friction between the tip and the center hole during grinding. B-type central hole with 120° protection cone, which protects 60° conical edges from bumps, is common in workpieces with high precision and long processing steps.stamping part

3. Technical requirements for the center hole

(1) The roundness tolerance of the 60° cone is 0.001 mm.

(2) The 60° conical surface shall be inspected by the gauge coloring method, and the contact surface shall be greater than 85%.

(3) The coaxiality tolerance of the center hole at both ends is 0.01mm.

(4) The surface roughness of the conical surface is Ra 0.4 μm or less, and there are no defects such as burrs or bumps.

To meet the requirements for the center hole, the center hole can be repaired in the following ways:

1) Grinding the center hole with oil stone and rubber grinding wheel

2) Grinding the center hole with a cast iron tip

3) Grinding the center hole with a shaped inner grinding wheel

4) Extrusion of the center hole with a quadrangular cemented carbide tip

5) Grinding the center hole with a center hole grinder

4, top

The top handle is a Morse cone, and the tip size is expressed in Morse taper, such as the Morse No. 3 tip. The top is a universal fixture that is widely used in cylindrical grinding.

5, various mandrels

The mandrel is a special fixture for clamping the set of parts to meet the precision requirements of the external grinding of the part. plastic part

6, vernier caliper readings

The vernier caliper consists of a measuring claw, a ruler body, a vernier depth gauge, and a fastening screw.

7, micrometer reading

The micrometer consists of a ruler, an anvil, a micrometer screw, a locking device, a fixed sleeve, a differential cylinder, and a force measuring device. The measuring surface of the micrometer should be cleaned and the zero of the micrometer should be checked before use. Pay attention to the correct measurement posture when measuring.