Determination of the height of the hottest locatin

Determination of the height of the locating pin

when processing box, fork frame, lever and other parts, the combined positioning of one hole on one side or two holes on one side is usually adopted. This positioning method is widely used, which is conducive to the realization of datum unification in the processing process of parts, the reduction of workpiece processing error, the simplification of fixture design workload, and the realization of automation. However, in some manual or semi mechanized operation workshops, because the workpiece is relatively bulky, it is unlikely to install the positioning pin after the workpiece is leveled during installation, but to support one side of the workpiece on the fixture support plane and gradually insert the positioning pin, In this case, if the height of the locating pin is improperly selected, the workpiece will be stuck on the edge of the locating pin and cannot be installed. This article will discuss the maximum height allowed by the locating pin when the workpiece is not stuck

1 the positioning device is a pin on one side

the state when the workpiece is just stuck by the positioning pin during installation is called the critical state. At this time, the most difficult conditions for workpiece installation are: the minimum diameter of the positioning reference hole (Dmin), the maximum diameter of the positioning pin (Dmax), and the positioning pair has the minimum fitting clearance xmin, xmin = Dmin Dmax

∵cos α＝ dmax／Dmin

∴sin α= SQRT（1-COS α）

=sqrt (1- (Dmax/Dmin))

= (1/dmin) sqrt (D min-d max)

2 when the positioning device is in a critical state with one side and two pins

the minimum diameter of the reference hole is Dmin, the maximum diameter of the positioning pin is Dmax, and the two positioning pairs have the minimum fitting clearance xmin = Dmin Dmax. When installing, when the left plain line of the first pin and the right plain line of the second pin are in contact with the two reference holes respectively, The contacts are a and B, resulting in stagnation, as shown in Figure 2. It can be seen from the figure that the height of the first pin has a small impact on the workpiece seizure, and the height of the second pin has a direct impact on the workpiece seizure. Therefore, calculate the maximum allowable height of the second pin

cos α′＝ （L＋dmax）/（L＋Dmin）

sin α = SQRT（1-COS α）= Sqrt (1- (l+dmax/l+dmin))

≈ (1/l+dmin) sqrt (2 (l+dmin) xmin)

‹ HM accuracy should meet the requirements in the "certificate of conformity" of the experimental machine in= (l+l+0.5dmin) sin α

= (l+l+0.5dmin)/(l+0.5dmin) sqrt (2 (l+dmin) xmin)

3 summary

① the situation of manual and auxiliary materials for box and fork frame parts is long fiber, short fiber or granular semi mechanized installation, the height of the positioning pin can be checked and calculated with the above formula to prevent the workpiece from jamming; ② This paper ignores the influence of the perpendicularity between the workpiece reference hole and the reference plane, the positioning pin and the bearing surface, and the tolerance of the center distance of the reference hole and the center distance of the two pins on the workpiece installation; ③ The workpiece takes one side and two holes as the positioning benchmark. Usually, a cylindrical pin and a trimming pin are selected. The height of the cylindrical pin can be calculated according to the above formula. From the above analysis process, the trimming pin is conducive to the installation of the workpiece. China is a large textile country. For the convenience of loading and unloading, the height of the trimming pin can be 3 ~ 5mm lower than the cylindrical pin; ④ In some fixture designs, in order to overcome the inconvenience of workpiece installation, the two locating pins can be designed into a telescopic form. When installing, first place the box parts flat on the locating plane, and pre position them. Move the two locating pins to insert them into the two basic standard holes, so as to realize workpiece positioning, In this way, the stagnation in the installation process of the workpiece is eliminated. Nylon material is a kind of high semi crystalline thermoplastic image with good physical and mechanical properties, which is also labor-saving and convenient