What Goes Around
I have been doing some thinking about the controlling of rotary devices I could use to power the milling and cutting bits on my various rotary cutting machines. This is everything from the tiny dental size bits in my air turbine powered hand pieces to the X3 machine mill and woodworking routers. This is a huge range. The speed range varies from the hundreds of thousands of rpm down to a few thousand or a few hundred.
What jumps out at me is that no size fits all. Every tool has its purpose and limitations. That is the way life is. The thought that I could find the one combination that can do everything is erroneous. The best tool is the one designed to do a particular job. Multi-purpose is always a compromise of some sort.
I could ramble in many directions, but what I am discussing here is the cutting speed and rotation power required for my rotary tools. The extremely small but high speed air turbines have negligible power ratings. The term horse power is not even considered. It is very easy to stall this rotation with overload. The “touch” required is very light to get the best use of the tool. “Let the tool do the work” is nowhere better demonstrated. “Wow!” it does do a fine job at its intended purpose when used correctly. We probably all know what a dentist “drill” sounds (feels) like as it takes a heavy load (but drill is a misnomer.)
The slowest speeds are used in metal milling machines with larger diameter tools. For this bench top hobbyist discussion I will consider “large” to be ¾” or more. The larger tools can take bigger “bites”. So therefore more power is desirable or required.
Three factors are important in small tool bits, surface feet per minute (SFM) at the cutting surface, material load per tooth (chip load) and tool strength. What it all sums up to is “Let the tool do the work”. The challenge I find is to get all three of these details in balance. Some of the factors have two or more other factors, so the process is very detailed.
In the end there are three primary control settings to get right. Those are rotations per minute, feed rate and depth/width of cut. Note that I do not include motor power as a primary concern.
Rotation controls the surface feet per minute in relation to tool diameter. The general rule is the smaller tool the faster it needs to go around (spin).
Feed rate is determined by the number of teeth on the tool and how much material the tooth can “carve out” in each revolution.
Depth and width are determined by available power, chip load and tool strength. Here is where power first shows some consideration.
When doing rotary tool work by hand held or hand feed, the good operator usually develops a feel or skill in making instantaneous adjustments to many of the variables, especially load. The same is true for any work done by hand, touch or feel. For some people hand work is the definition of a craftsman, but that is another road to explore.
The challenge in CNC operation is that these dozens of combinations must be considered then set into exact parameters. I can’t feel in my hands the answer to, “what if I go a bit faster or deeper?” When using a CNC controlled machine, I have to calculate the starting point, and then be willing to test the results. In my case the ArtSoft MACH3 program I use to run the machinery lets me change some factors like feed rate and RPM on the fly. I don’t have automatic control on RPM (yet) but I can push or decrease feed rate. Everything else requires a code instruction edit.
I’ll give you a hint. Small bits are very easy to break well before their theoretical chip load when “pushed”, especially the carbide.
The last factor I consider (finally) is the rotation power required. Above I said depth and width of cut determine power required in the spindle. That is true as long as I don’t violate any of the other factors. It also determines the power required in the axis drive(s) (directional feed) which is another thought trail.
Beyond tool strength, machine rigidity (strength) is also a limiting factor to how hard a tool bit can be pushed. A mistake I have seen is the “over tooling” of our micro milling machines while expecting heavy duty cutting operation.
Putting a one and one half horsepower motor on a Taig micro mill to power a ¾” end mill at a ½” depth of cut is ludicrous. Better to buy a Tormach machine built to do that. Yeah, extreme example but it makes the point.
The HB2 overhead gantry router I am constructing is shown with a .9 hp and a 1.75 horsepower router motor grafted onto its head. The big router is probably plenty for anything this machine can comfortably handle. Trying to cut ¾” plywood in one pass seems it would be extreme duty. I am sure there are some 3 hp or higher routers installed on this class of machine to do this task. My whole thought is to consider the entire design and purpose when it comes to, “How big of a spindle drive do I need?”
My plan is to experiment and design several spindle power options which depend on purpose. Small spindles are not expensive or hard to change. Small bits can’t withstand high power but need high speed. Maybe I will need 3 Hp someday. That’s the enjoyment I get with playing with my own designs. I don’t have to follow the rules, but rules are a good place to get started…
I use my avocation to play so I can enjoy the trip to life’s destination. 🙂