The smaller the drill, the higher the PSI required to achieve this type of stream. 5) Coolant PSI, Type, and FiltrationĪs was mentioned earlier, the coolant should be coming out of the drill as a steady straight stream and not just a dribble or waterfall effect. A more conservative recommendation would be to drill to a depth of 3x - 4x the diameter, then experiment to see if you can get away with a shorter hole for you part, thus improving cycle time. Many manufacturers will specify the depth of the guide hole to be only 1.5x - 2x the diameter, but this may be insufficient if you are using small diameter drills or have tight tolerances that need to be met. Remember that gun drill diameter tolerances are +0 / -.000X ", therefore its recommended to drill guide holes with a clearance of the drill diameter +0.0005" to +0.001" depending on how critical the specifications of the final hole are. If the guide hole is not staight, too loose, or not deep enough it will cause problems. If you are using a gun drilling machine they have a highly aligned guide bushing at the part face, but if you are gun drilling on a CNC lathe then you have to create that guide hole first. 4) Guide Hole Dimensions When Using A Lathe If you need to squeeze more time out of the cycle and are willing to maybe give up tool life, then experiment by incrementally increasing the SFM from the recommended numbers. Now that's not to say its always the best for every application. The companies that do just deep hole drilling for a living know that they get the most balanced performance by staying around the recommended speeds and feeds from the gun drill manufacturer - that is best hole quality and tool life. One of the worst things you can do is not get a good straight stream of coolant out the gun drill and have the chips bind up in the hole and start breaking tools.Īs an experienced machinist or engineer this may seem obvious, but I can't tell you how many times I've seen people running the SFM out of the sweet spot for that gun drill. If the machine has good high pressure coolant then maybe a two hole or kidney configuration is best. Selecting a single round coolant hole may be good for a machine with questionable coolant pressure as it will create a stronger stream at the tip.
The easiest way to determine which configuration is best, is to consult your gun drill manufacturer and have them guide you based on your hole requirements.Īs in most all metal cutting applications, it is important that you are able to evacuate chips from the cutting area quickly and also cool the tool. Nose grind will determine how the chip is formed and the relief will influence hole finish and coolant flow. In that case you are better off using an N-4 nose grind and R4 relief. This is fine for steel, inconel, and stainless steel but what if you are using aluminum or brass. Most gun drill manufacturers will supply a general purpose N-8 nose grind and R1 relief unless specified otherwise. Let's go thru 5 common mistakes that are made when selecting and using a gun drill.
THE GUN DRILLING HANDBOOK PDF HOW TO
Most machinists and engineers are familiar with twist flute drills as they are certainly more common than gun drills, so its not unexpected that many don't understand how to select the best drill for their application. Selecting and using a gun drill for deep hole drilling can sometimes be confusing.
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