I wish I could keep my mouth shut, but I just have to weigh in on the drilling and the friction! [
] When we drill a hole in a soft material (like wood, hard rubber, bakelite, etc) the fibers do one of three things depending on a number of factors; pull from the hole wall, crush against the hole wall or shear off at the hole wall. Which happens depends on how sharp the drill is, the drill geometry, the density and condition of the material, the natural oils in the wood, the pressure employed and the RPM of the drill. The deal is the drills we use don't have keen enough edges, or are soon dull from all the silicas, minerals etc in our wood. In reality we all drill with relatively dull tools - even the brand new ones! This means fibers are crushed or pulled when the cutting edge goes around the hole. This leaves the fibers in the hole to rub on the drill. More speed, more friction, thus more heat ONLY if you don't feed fast enough (rubbin' is not racin' in machining - it's heatin'!). And drilling is funny... consider the the closer to the point of the drill we go the lower the surface footage is for any given RPM, so the cutting condition at the outer diameter of the tool is worlds different than the condition near the point.
Consider also pushing any slightly dull drill down causes a vector force that also pushes the material to the side, especially at the corners. Heat is a normal by-product of any cutting operation. The question is do we take the heat out in the cut material (ideal situation) or does the heat transfer into the drill. One or the other is going to happen. I like to take the heat out in the chip, regardless of whether I'm cutting Stainless, Inconel, Titanium or Hastelloy... or wood. So in wood this means FEED fast using FAST RPM and retract just before the flutes pack with chips. The rule is fewer revs while buried in the material reduces the heat. So what's fast? In machining references for manual drilling a 1/4" HSS drill in hard wood it's 1800 RPM.
Boring end grain material with a standard brad point drill with outlying spurs can be a problem because the spurs are not needed. The spurs drag in end grain. You can modify the drill by grinding the spurs off following the existing back clearance. Then, regrind the cutting edge with a 7 to 15 degree angle from the O.D. of the drill, tilting downwards to the point, again following the existing back clearance. This is called an acme cut. Now the brad point enters the material followed by the outlining O.D. (where the spurs were), which sizes the hole without dragging. You will still require a 4" or up twist length to clear the chips (if you like pushing straight through). Use 1700 to 1725 RPM in hard wood. One note - the modified drill will not work as well boring cross grain.
For a 7mm (0.291 inches)HSS drill, it's generally published by the drill manufacturers, and adopted by pro's, to use 1800RPM for drilling soft wood and 1000RPM for hard wood. But this is recommended assuming cross-grain drilling, not end-grain. End grain needs to be drilled at higher RPMS and faster feeds. Check out the link below, it shows an RPM table based on drill diameter. These are good starting numbers, if you add 30% for end-grain drilling.
http://www.newwoodworker.com/ref/drilspds.html
Specialty tool manufacturers catering to those of us with high-speed spindle CNC machine access recommend the following geometry for wood drilling. I've used both and really like the one on the right - it cuts fast, on-center and leaves a nice straight hole in a single plunge in end grain. But that's at 18,000 RPM and 300 inches per minute feed in hard maple. My drill press at home won't do that. [
] But we can still apply the geometry - it works well at less than 2000 RPM too.
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