Cam software system. Dynamic toolpath
The most exciting aspects of CAM programs are the new efficient ways to mill material – whittling it away rather than making brute force cuts. Previously we mostly moved a milling cutter from point A to B – in a straight or curved line. Then, as machining centers became fast enough to perform high-speed machining (HSM), we had to invent new toolpaths to take advantage of their speed.
High-speed machining can be defined as taking shallower depths of cuts at a higher federates (600+ in/min) and spindle speeds (10 – 15,000 rpm or higher). The first problem to be solved was to make sure the toolpath had no right angle turns, because the extreme feeds created staggering physical loads at hard corners. They could literally shake a fast machine apart. So one definition of a high-speed toolpath (HST) is “rounding all corners, and having smooth lead-ins and lead-outs on surfaces”.
Protecting Cutters. The next two issues to overcome were the worst-case scenarios for shocking or stressing cutters.
Spiral Entry. Cutter shock and stress was significantly reduced by incorporating an elegant spiral motion while plunging, rather than caveman hammering down like a drill press.
Trochodial Motion. Trochodial motion was the greatest change from straight-in cutting to advancing in a circular motion, taking a little bite/shave of the material with each cutter arc into and out of the work. At high feed rates, one cannot expect the cutter to survive when buried: that first cut in a pocket after the plunge is the worst for the cutter, as it is deep in the cut, and the trapped chips have nowhere to escape and are recut. Trochodial motion solved this; now with a spiral plunge and then moving immediately into little circular contact with the work, feed rates can remain high yet not break the cutter.
Maximum axial engagement - whittling away tall, thin chips using the longest end mill side teeth possible. The chips are thin but of constant volume as they trace the part contour (thus differing from trochodial, where they vary). This engages the best cutter geometry, as opposed to the previous method of mostly cutting with the end teeth in successive down layers, where the end of the cutter does most of the work. With dynamic toolpaths, cutters last longer, since they use better cutter side tooth geometry, and heat goes out with the sheared chip. Cooler cutters, better fi nishes, longer tool life, faster removal—yet lighter machines can perform them.
References: Gen [1-3].
Assessing questions:
What the difference between machining centers and turning centers?
Main purposes of using by shaping and planing.
3. What is the features of broaching?
4. What kind of machines do you know for sawing?
5. CAM software system.
Lecture 9. Tasks and composition software, management post processing for CNC, stages of software development, software features
As with trochoids, for dynamic milling we use 10–15% radial engagement (side tooth contact). We back away from deep radial engagement (end tooth contact) to around 10 to 15 % of the cutter’s diameter (Fig. 9.1).
Chip Thinning
Looking up a feed per tooth (chip load) for a given cutter and setup, you’ll find numbers that represent feeding a cutter such as this with a large radial engagement (Fig. 9.2). For example, the feed rate for this cutter is based on 0.006 inch per tooth per revolution. Each tooth advances 0.006 inch into the work.
Figure 9.1 – This cutter has a high axial and low radial engagement.
When we back away to a small radial engagement, as we’ve been describing, the chip is made thinner. This means that the feed rate should be increased to keep the chip load constant. That upward adjustment maximizes cycle times, but it’s also necessary to correct the thin chips to keep heat leaving with the chip and not overheating the cutter. This is true for standard milling and especially so for HSM, where heat removal is crucial.
Figure 9.2 – Less radial engagement requires increasing feed rates to keep chips thick enough to carry away heat and for efficient cut rates.
The dynamic toolpath constantly adapts to the remaining material, with a constant chip load and volume. Each successive cut will be made up of the following components:
• Large axial engagement
• No sharp corners
- Spiral entry
- Smooth entry/exit
• Climb cutting always
• Micro-lift rapid return reengage
The following are only possible through highly intelligent CAM software:
• Cycle times reduced by 30 percent or more compared to traditional cutting techniques
- Machines only where material exists
- Consistent chip load allows for maximum feed rate/material removal
- Accounts for chip thinning, keeping feeds maximized
- Better finishes due to constant climb milling
- Better accuracy due to light cutter loads – less deflection of work and cutter
• Improved tool life
- Full flute utilization for longer tool life
- Side teeth are better geometry for cutting/shearing the chip compared to end teeth
- Deformation heat leaves with the chip; far less cutter heat
• Economical
- Smaller tools can be used with similar results – fitting into tighter spots, too
- Less carbide needed, since cuts are not brute force
- Smaller, lighter duty machines can accomplish more work
- Fewer spindle loads
- Less demand on setups due to reduced forces.