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Gun drill is a very common tooling in deep hole processing, and it’s internal coolant and external chip removing. For now, the min. drilling hole diameter of gun drill is 1mm, and the drilling depth and drilling hole diameter’s draw ratio is bigger than 100 times, and at large it can reach to 250 times. The drilling hole precision is from IT7--IT11, and the roughness is from Ra0.4--Ra3.2.
For one time drilling, it can get a good hole precision and low surface roughness, it usually use to process the precision shallow hole and special deep hole.
Also there are some attentions of gun drills when deep hole drilling : 
1) The clamping of workpiece must be reliable,and make it in the same axis with machine’s center. Also you can drill a positioning hole or make a chamfer in the surface of the workpiece to position the workpiece.
2) When processing the long workpiece, the workpiece and gun drill should use the steady center rest , and also there should be 1 or 3 movable supporter to hold the drill shank.
3) The center hole in the workpiece must be smaller than the hole diameter of gun drills. If it’s difficult to reduce the hole diameter of center hole, use the special guide bush.
4) When start drilling, there must be a guide bush using along with the drill bit and the inside hole diameter of guide bush’s precision must be IT6. When the workpiece is short or the requirement of hole’s linearity are not so strict, we can also choose to drill a center hole to guide the drilling. 
5) The guide bush belong to wearing parts, when the wearing capacity of guide bush’s inside hole diameter is bigger than 0.02mm, replace the guide bush, and better use the carbide guide bush.
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