There are some secrets to deep hole drilling, and at over 5 times the diameter deep, that is deep hole drilling.
The biggest problem you face is keeping the hole straight, the deeper you drill the more a hole wants to lead off, or rather the more pronounced lead off becomes.
You need to look at what causes lead off.
#1-the drill starting at an angle, either because the drill was not held parallel to the part or because it "walked" off center when it started. So step 1 is making sure your workpiece is held parallel to the spindle. If you are using a standard drill, you will definitely want to center drill the hole.
#2 - An improperly sharpened drill will cause one side to load heavier than the other, the uneven load causing the drill to pull off center. Solution, make sure your drill is properly sharpened and of high quality, the longer the drill, the more important this becomes!
#3- Chip pack, causes inadequate cooling as well as again a "cutting force" issue. Unload the pack often and use plenty of coolant/oil.
#4 - Improper feed and or speed, get your operating parameters correct!
Assuming a good tight machine and good tools, making a deep hole straight is not a problem. The start is crucial, get started crooked and the deeper you go the worse it gets!
How many holes do you need and how accurate? Just one - fractional tolerance, just drill it, take your time and don't push it! A few holes with high tolerance, now you would want to look at a jig bore or jig reamer, a good solid tool to finish the hole with.
High production on alot of holes - solid body Inserted drill, your choice of quality tooling manufacturer, I'd call Komet!
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