Lately there have been a lot of really interesting HSM topics on PracticalMachinist forums.
In one of them a guy who owns his own resharpening business posted a video of his endmill milling a block of D2 hardened to over 60 RC. The forum topic is located here First try on D2 62Rc(video)
Here is his post so you know what we are talking about:
In an effort to perfect our speeds and feeds while hardmilling, this is the first try. Its not right yet, but far from a failure. I apologize for the language at the end, but I do not edit my videos. The endmill was a reground garr VRX at .353 diameter. Parameters were 750 sfm, .018 radial, .300 axial and .004 ipt. The next run will be at 650 sfm, .006 ipt using a mist sprayer. Also, any small areas will be blocked off to be ran at lower speeds to allow cooling time for the cutter. Just a note for anyone using a Mag Fadal, The E-stop button is not quick enough, use feed hold. The endmill was badly worn on the corners, but not broken, and will be resharpened and used again.
In the ensuing discussion i posted my own take on how and why HSM works
HSM works in many ways.
1) Reduced cutting time per edge per revolution allows it to cool down more. 2) Chip thinning allows to increase chipload (advancement per tooth per revolution) 3) Increased depth of cut combined with shallow radial positively affects deflection. Tool bends less as it is more rigid towards the tool holder. 4) Higher cutting speed actually reduces cutting forces as heat generated in the cutting zone makes it easier to shear off a layer of metal. Yet because the time of contact is so small, most of the heat is carried away with the chip. 5) Higher RPM also allows to get rid of hot chips faster thus further reducing heat transferred to the tool. 6) Higher feedrate actually reduces relative cutting speed. 7) At high axial engagements more than one flute is in contact with the workpiece at different points along the axis of the tool. This too helps combat vibrations and chatter. 8) You are using more of the tool than just its tip, so technically you can do more work with one tool before it gets dull. 9) lastly it looks cool as hell and is very impressive. Whenever we know visitors or bosses are coming we try to make sure some HSM is going on even if application does not merit that I am not sure if the air that is moved by the endmill is doing much, but i suspect he didn't mean exactly that.
If you are working in mold-making, prototyping or even in a job shop you have had to use unusual form tooling before in your life.
Form tooling is often used to machine undercuts and other features on regular 3 axis machines that would otherwise require a multi axis machining centre or are not machinable o at all.
The classical example of a form tool is a tear-drop ball mil, also known as a "lollipop". It has a tip with a certain diameter and a much smaller shank that produces enough clearance to machine undercuts on straight walls. It can also be used to regular surface finishing and 2d milling.
Another example is a T-slot cutter that is used to produce key-ways and t- slots
The main thing to consider when machining with reduced shank end mils is deflection and torque.
While deflection is especially dangerous for long tools, torque becomes much more important for tools with severely reduced shank.
Torque required to break a tool is directly proportional to the diameter of its shank.
And when shank diameter is much smaller than the tip diameter it does not matter how short that weak portion is: unless you compensate for it you will snap the tool.
The first thing that crosses the mind in many such cases is "I gotta run this tool very slow". It may take forever, but in many cases job gets somewhat done.
Contrary to that many experienced machinists have been proponents of different approach. Instead of reducing feed rate to the point of rubbing and below, it is much more productive to reduce cutter engagement if possible and leave feed rate settings largely unchanged.
Trying to keep proper chip load is even more important when machining work-hardenable materials like stainless steel and titanium. In those cases rubbing is not just unproductive, it leads to a very premature, in many cases instantaneous tool failure.
Just how much of a cut is possible to take in each particular case is the black magic that separates beginners from seasoned pros.
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