The basic size of the diameter of the inner hole of the drill sleeve (also known as the pilot hole) should be the maximum limit size of the tool used, and the base shaft system should be used for clearance fit. When drilling or reaming, take F7 or F8, take G7 for rough reaming, and take G6 for fine reaming. If the drill sleeve guides the guide column part of the tool, it can be selected according to the corresponding matching of the base hole system, such as H7/f7, H7/g6 or H6/g5, etc.
Guide length H
The guide length H of the drill sleeve has a great influence on the guiding effect of the tool. When H is large, the tool is not easy to be deflected in the drill sleeve, but it will speed up the wear of the tool and the drill sleeve; if H is too small, the guiding performance of drilling will be reduced. not good. Usually the ratio of the guide length H to its aperture is: H/d=1~2.5. When the machining accuracy is required to be high or the hole diameter to be processed is small, due to the poor rigidity of the drill bit used, the H/d value can be larger. When the distance tolerance of holes is ±0.05mm, H/d=2.5~3.5 can be taken.
Chip clearance h
The chip clearance h refers to the space between the bottom of the drill sleeve and the surface of the workpiece. If h is too small, the chips will be difficult to discharge, the machined surface will be damaged, and the drill may even be broken. If h is too large, the deflection of the drill will increase, affecting the positional accuracy of the machined hole. Generally, when processing iron castings, h=(0.3~0.7)d; when processing steel pieces, h=(0.7~1.5)d; where d is the diameter of the drill bit used. For holes requiring high positional accuracy or drilling on inclined planes, the h value can be made as small as possible, or even zero.
