What is the appropriate screw engagement length? This seemingly simple question has many scenarios to consider.
Let's first take a look at the relevant standards specified in the manual.
The remaining length of the internal and external threads in regular threads, the remaining drilling depth, and the length of the bolt protruding from the end of the nut
| Line step | Thread diameter | Remaining length | Final length | |||
| Thick yarn | Thin yarn | Internal thread | Drilling | External thread | ||
| P | d | me 1 | me 2 | me 3 | The | |
| 0.5 | 3 | 5 | 1 | 4 | two | 1~2 |
| 0.7 | 4 | – | 1.5 | 5 | 2.5 | 2~3 |
| 0.75 | – | 6 | 6 | |||
| 0.8 | 5 | |||||
The table above gives the remaining length of common internal and external threads, remaining depth of drilled holes, and final length of bolt protruding nuts.
If we design the depth of threaded holes (referring to the depth of the threaded part), we generally make the hole depth 1.5-2 times the thread diameter.
For example, how deep should we design the M10 screw hole? You can design it to be 15-20mm deep, but of course this depends on whether your space is sufficient. For thicker parts, a depth of 20 mm can be used, while for thinner parts, the depth can be reduced accordingly.
Greater depth has some advantages, such as increasing the range of screw specifications that can be used.
For example, if the thread is only 10mm deep, the screw may only be available in one length specification. If it is 20 mm, two or three screw lengths may be available. This is just a summary technique in actual assembly and may not always be accurate.
If engaging bolts and nuts, it is recommended that the bolt protrudes from the nut by at least 2 to 3 teeth, which is usually about 3 to 4 mm.
Note that this is at least – the bolt should protrude from the nut as much as possible when it is assembled to be most suitable.
Does the material affect the thread depth requirement?
The answer is definitely yes. Try to think about it: if a soft material is engaged by only 3 teeth, will it slip easily? In fact, the softer the material, the deeper the thread depth should be considered in the design. The screw insertion length also varies depending on the material.
For example, for steel, the thread length of an M3 screw is designed to be 4 mm; for gray cast iron, the thread length of an M3 screw is designed to be 6 mm; for aluminum, the thread length of an M3 screw is designed to be 8 mm.
Of course, in my opinion, if there is enough space, we can design the thread length of M3 screw as 8mm, which is easy to remember.
