We are carrying some of these new tool path strategies a little far. Last week I was asked to tap test a 3/8" 3 flute endmill with a 4" length of cut. They wanted to cut at full depth in aluminum. That is 10.66X the diameter! It was like measuring a wet noodle. There were no stable lobes at any speed at any axial engagement. Just because someone makes a tool doesn't mean it will work. About 40% of the carbide was removed from a cylinder to grind the flutes, so this endmill was the equivalent to a 0.225" rod as illustrated above. You can imagine how much that would flex. Even if we could find a speed to cut at, the wall would not be straight. So what to do? If the diameter of the tool could not be changed, you could go to a relieved shank with a shorter flute length and make more passes. If you select the right speed with minimum displacement (SLE) you would not get water-lining if that is a concern. If you still wanted to go in one pass, then increase the diameter (and stiffness) of the cutter. At 3/4" you would cut the L:D ratio in half. Adding more teeth would also increase the stiffness.
The top panel of the computer screen shown in this video shows the force of the tapping hammer's impact. The lower panel shows the tool's (or anvil's) response to that force as captured by an accelerometer attached to the opposite side of the hammer strike. It deflects, rebounds and then vibrates back to rest. After tap-testing tens of thousands of milling tools we have yet to see one with a response that was a flat line. They are all flexible and will deflect and vibrate from the forces of the tooth impacts.
No physical object can be perfectly rigid. It's physics.
I just watched a fascinating biography of Ted Williams on PBS's American Masters. Having admired him since I wore a Ted Williams model baseball glove from Sears in Little League, I actually discovered some new information about him. I learned that he was Hispanic (his mother was from Tijuana) though he never acknowledged it publicly. His legendary eyesight was, in fact, 20/15. Really good but not superhuman. They also discussed his amazing book, the Science of Hitting, and this famous strike zone graphic. Not unlike our Stability Lobe Diagrams for milling, his chart shows improved batting averages if you swing at balls located in specific areas in the strike zone. Sweet spots apply to batting...and milling.
Sooooo, I have received a few messages that think this slow motion video is atypical. Their endmills certainly don't behave this way because they have slow motion videos of their tools and they don't see them vibrate. Standard video is recorded at a rate of 24 frames per second. Unless you buy a very, very expensive special purpose camera, most slow motion video is captured at 60, 120 and sometimes even 240 frames per second. Well, if you have a vibration frequency of say 600 Hz or back and forth cycles per second (12,000 RPM x 3 teeth/60 seconds = 600) then most slow motion video frame rates wouldn't be fast enough to pick that up. The endmill will look like it is not vibrating at all. This video was shot at 54,000 frames per second! No tool is 100% rigid. They all deflect and vibrate. Some less. Some more. Shoot them at 54,000 FPS and it would be clear. As we said, this is a very hard concept for people to accept.
Here is another incredible video posted by Dr. Gabor Stepan of Budapest University, shot at 30,000 frames per second. This view shows the waviness caused by each tooth of the vibrating endmill. If the timing is right, the next tooth will regenerate an identical wave keeping the chip thickness constant. If the timing is off the chip thickness varies and will lead to chatter.
Here is a video posted by Dr. Gabor Stepan of Budapest University. It was shot at 54,000 frames per second, clearly showing the endmill deflecting from the force of a tooth impacting and snapping back as that tooth leaves the workpiece. If the timing is off, the next tooth could be over-cutting or under-cutting. Or, if you pick the right speed it could be cutting just right.
This is a concept that we have a hard time conveying. No milling tool assembly is 100% stiff. With the sideways force of a tooth impacting the workpiece it deflects and when the tooth leaves the cut it swings back. It doesn't matter how great your tool's geometry, substrate, coating, concentricity, balance or tool path is. If you don't get the timing of the "swing" and tooth impacts just right, you really ain't got a thing. So says the Duke.
We are the first to admit we are a "One Trick Pony".
We do one thing. We can tell you, with precision, what speed a milling tool wants to run in a machine from what is called a tap-test. We can also tell you the depth of cut limits, but axial depth is usually pre-determined by the part program and the tool. Same with the radial engagement, but if we tell you that your trochoidal cut can take 20% instead of 10% we just doubled your productivity.
But, ah, SPEED.
Knowing the best speed means you can push your tools to their thermal and chip load limits with each tooth cutting equally to maximize tool life. Speaking of teeth, the right number of teeth will give you the best speed. The best speed will maximize metal removal rates, improve surface finish or accuracy (FYI-the best speed for each might be different). Knowing the best speed for multiple tools will help you choose the right tool for your application. Knowing the best speed will slash your setup times. Knowing the best speed will allow you to quote more competitively and win more work.
These two basketball players represent two milling tools. The one on the right is stiffer therefore the tool point (ball) deflects less and returns to his hand (workpiece) sooner than the one on the left. The frequency of the dribble, or the number of times it hits their hands in one minute, is lower on the left than the right. For a milling tool, the waiting for the ball to return to the hand is the tooth impacts. To get the tooth impact timing right (to prevent chatter) you either must slow the spindle speed down or reduce the number of teeth on the less stiff tool.
The three cases of copy paper cost exactly the same as the box of carbide inserts shown in the lower left. The cases weigh a total of 150 pounds giving them a value of $0.15 per ounce. The box of inserts weighs 1.6 ounces giving them a value of $232 per ounce. By comparison, the price of silver today is $15.43 per ounce. Yet, we order and ship the the mission critical and "precious metal" cutting tools the same way we do the paper. They ship on the same trucks, are delivered to the same docks on the same packing slips.
Perhaps a different tier of service is in order with specialized distribution centers, more secure and traceable shipping, packaging and accounting. Treat it like jewelry.
WHAT IF YOUR CUTTING TOOL SUPPLIER HAD VISIBILITY INTO KEY PERFORMANCE INDICATORS OF YOUR SHOP?
They would know beyond when a tool was vended, recording exactly when an endmill was changed or an insert was indexed and the part count when it happened. They would know when there were surges in production, when scrap rates increased or when a machine was down for service and adjust inventory levels (in your vending machines and their safety stocks) in real time.
If they ran out of a certain endmill they could tell you to lower the surface speed to next slower stable speed (according to the stability lobe diagram or Dashboard on file) to increase the expected tool life enough for your current inventory to last until it could be replenished.
If the preferred endmill manufacturer had a production interruption of their own and ran out of inventory, you or your tool supplier could tap-test a substitute and generate a Dashboard to determine the stable speed it will run.
Production never stops (maybe just slows down a little) because you don’t have a Mission Critical Consumable cutting tool.
How often is production stopped because of an out of stock cutting tool?
As tool designs become more specialized, the sources of supply become more fragmented and further away from the point of consumption (e-commerce, drop-shipping) the chances of this happening become greater.
Let’s divide MRO into three distinct categories:
1. MISSION CRITICAL CONSUMABLES are the cutting tools, abrasives, coolants and other supplies that are essential for production and will cause it to be stopped if not available. They also cannot be easily substituted.
2. MISSION CRITICAL DURABLES are primarily the toolholders, cutting tool bodies or other devices that hold mission critical consumables. They are purchased once and should last indefinitely. If they need to be replaced, it is probably because of a catastrophic event, which will buy a little time to get another one.
3. EVERYTHING ELSE are the tools and supplies that are price but not time sensitive. They can also be easily substituted from local sources. Examples would be AA batteries or HSS jobber drills. They may be supplied on a contract, but if you ran out you could easily find equivalents locally.
Those items that fall into the first category must have a different supply strategy than the rest to prevent production interruptions.
We are often asked about the workpiece material and how it affects the stability lobe diagram. I like to use a basketball analogy. If I drop a ball from eye level on a concrete floor it will bounce fairly high. If I drop the same ball from the same height on a carpeted floor the ball will not bounce as high. The carpet absorbs some of the energy of the dropping ball. Think of the floor as the workpiece material of a milling operation. Let’s use a multi-material tool, like a conventional 4 flute 30-degree helix carbide endmill in a toolholder. In steel, a tooth of the endmill impacts the workpiece and causes the tool to deflect. In a soft material like aluminum, some of the tooth impact’s energy is absorbed and it deflects less. Now this is where it gets a little counter-intuitive. The speeds (lower axis) of the stable lobes DO NOT CHANGE with the material. After all, the beam (tool/toolholder/spindle) didn’t change. However, the height of the lobes (left axis) are REDUCED as the material gets harder. Why? Like the ball on concrete, the greater the height of the bounce the longer it takes for next tooth to present itself to the workpiece, therefore the force, or depth of cut, has to be reduced to reduce the bounce.
These illustrations are not entirely accurate (an endmill cuts radially and tooth by tooth) but are meant to convey how chatter in milling develops. The first image shows the "not 100% rigid" endmill leaving a wavy surface as it cuts. The second image assumes, that for the next pass, the back and forth frequency of the endmill vibration matches the first pass. The chip thickness is consistent and the cut is stable. The third image is of frequencies that don't match. The chip varies from too thin to too thick. This variation is what causes chatter and is amplifying. If we had a fourth image the variation would be increasing, the chatter gets worse and leads to failure. Think of the film of the Tacoma Narrows "Galloping Gertie" bridge.
The first image is the CAD/CAM view of a milling cutter. A perfect cylinder, 100% rigid, no imbalance, no runout and NO TEETH, therefore no deflection or vibration. Unfortunately, it doesn't work quite like that. The second image shows the effect of one tooth impact. the tool deflects, rebounds and then returns to the centerline. That is one cycle, expressed as Hertz (Hz), of vibration. The number of cycles per second, or Hz, is the tool point's frequency. The third image shows the results of the tool point's flexibility. Teeth can arrive at the workpiece too early, too late or on-time. The spindle speed has to be tuned to the tool's back and forth frequency to get the timing of these tooth impacts just right.
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