Mark Twain is to have once said, "You have to know the true definition of a word before you can start misusing it."
We have no idea where this started, but the word "Harmonic" has been used in place of "Chatter", or more specifically, regenerative waviness. Perhaps because chatter is audible and someone wanted a more "scientific" sounding word. It is used in a phrase like this:
"Our __________ breaks up harmonics!"
Harmonic Frequencies are defined as integers of a fundamental frequency. In the case of milling, its fundamental frequency is the rate at which it vibrates that matches the tooth impact frequency. If the endmill's teeth are impacting 500 times a second (500 Hz), and it is vibrating back and forth 500 times per second (also 500 Hz), then 500 Hz is its fundamental frequency. If the endmill vibrates at 1000 Hz, it is still stable because it is going back and forth twice between tooth impacts. The same for 1500 Hz (3X between impacts), 2000 (4X), 2500 (5X) and so on. Those are "Harmonic Frequencies".
The frequencies that cause chatter are "non-harmonic". The timing of the tooth impacts would be off and create unequal waviness on the part surface. Each succeeding tooth impact will make that regenerative waviness worse and eventually chatter will develop.
Here is an example of two tap-tests in the same toolholder in the same spindle. The endmills are two different lengths. As you can see by the amplitude of the graphs the longer endmill deflected more than the shorter one. That means after the first tooth causes each endmill to deflect, the longer one will have to spin at a slower speed than the shorter in order for the timing of the each's second tooth impact to match.
We are most often focused on tap-testing new milling tools in new machines. The circle of life for machine tools is that once their financing is paid off and they are fully depreciated they are often relegated to second operations being replaced with newer and faster technology. But, not always. With older machines, the question is:
“Can it make this part?”
Tap-testing the program’s milling tools in the spindle of an existing older machine answers that question, at least for the milling portion. The instantly produced Dashboard tells you how fast and how deep the tools can cut in the spindle at its current condition. It also predicts surface finish and accuracy.
You may still need to verify it’s positioning accuracy with something like a ballbar test, but that old machine may still have what it takes to do the job.
Also, never miss the chance to use a Big Lebowski reference.
Tony Schmitz recently posted that our friend, Dr Scott Smith was the recipient of the William T. Ennor Manufacturing Technology Award from the American Society of Mechanical Engineers. In case you are wondering about his chops, he was there at the birth of high speed machining. Kudos to another machining pioneer and great guy Jerry Halley.
Tap testing milling tools at University of North Carolina at Charlotte William States Lee College of Engineering. Thanks to Ryan Copenhaver, Tony Schmitz and Scott Smith.
You have to pay attention to the screw torque applied to side lock end mill holders.
We tap-tested endmills in 1/2" and 3/4" toolholders, each at four torque settings. We used the manufacturer's recommendation, 25% less torque, 25% and 50% higher torque. We saw no difference in over-torquing (they are hidden behind the red lobe diagrams of the recommended torque. But, under torquing, showed in light blue, made a drastic difference in performance. FYI - recommended torques vary by manufacturer. In this case we used 24 ft/lbs for 1/2" and 36 ft/lbs for the 3/4".
Our takeaway is to err on the high side when tightening the screws.
For loading the endmill, which we already knew would work, we would add a drill stop or shaft collar to the endmill to control the stick out length. But, first we had to remove the endmill, so off we went, rotating the toolholder while heating the end.
Although the heat gun was producing over 450 degrees, the small end of the taper would not rise above 110. For some reason, the heat was transferring to other parts of the assembly. The carbide endmill read over 200 degrees as did the CAT 40 shank. The coefficient of thermal expansion of the carbide is almost half that of the toolholder material, but the hot air gun was heating the carbide more than the toolholder end. In other words the carbide was expanding about the same rate as the toolholder, so it was not going to come out no long how long we heated it.
So, the takeaway was, while we could certainly load the endmill into an empty holder by heating the bore first with the hot air gun and then moving to the outside diameter, we could not use the same technique to remove the endmill.
I like to show when experiments don’t work out.
Recently, we traveled to a customer to tap-test milling tools. One of the assemblies was a new ½” shrink fit, but they had not yet received their shrink machine. At the shopping center next to my hotel was a Harbor Freight store and they had a 1500 watt heat gun on sale for $15. We used the gun to first heat the bore on the toolholder and then the outside of the taper end. Within a minute or two the carbide endmill dropped right in. We let it cool and it was ready to test. It gave us an idea.
What if we could make a low priced rig to load and unload shrink fit holders using this cheap heat gun? So, we went to Home Depot and bought a wire chicken BBQ stand. It was designed to hold a beer can (diameter was a little to big for a CAT40), so we bought a PVC pipe adapter to reduce the hole size but also to provide a little more height for removing the endmill. The thinking was that if we inverted the tool once it was heated to the right temperature the endmill would just fall out. No need for a temperature sensor, but we bought a $9 non-contact infrared thermometer gun from Rural King anyway. We also bought a small plastic turntable to rotate the tool. Total investment was less than $40.
By design, the inside of the spindle taper is harder than that of the toolholder’s shank. Therefore any “uniqueness” (not imperfections) in the spindle’s taper, no matter how slight, will be transferred to the toolholder. If you move that toolholder to another spindle, any of the new high spots on its taper will cold-weld to the new spindle surface. When the tool is removed that contact area will break off and appear as discoloration on the taper that is often mistaken for corrosion. That’s what we call fretting. It also leaves damage to the spindle taper that will transfer to the next toolholder that is loaded.
It’s a transmittable disease!
Dedicating new toolholders to a single spindle keeps the taper conformity contained and reduces fretting, extending the life of the holders and spindle.
Don't store toolholders like those in the photo above. Use stands, shelves or carts that keep the tapers separated and from being damaged. Wood shelving with holes cut in with a hole saw work great.
To finish the thought on retention knobs, I wouldn't recommend using impact wrenches, but you do need to control the torque when installing them. There are very low priced click-type torque wrenches available (I know they are cheaply made, but his isn't surgery). Buy one for each different knob socket you use. Set each to the right torque and lock it down, tape the handle, use torque marking compound, anything to keep it from being changed. While at it epoxy the socket to the wrench. Single purpose and preset = LEAN.
Why? Even expensive torque wrenches are hard to read so constantly changing the setting for different knobs has potential for error.
The amount of torque is a moving target with different values recommended by different manufacturers. I would lean more to the light side to ensure there is not taper distortion. I have never seen a pull stud back out, but if you are still concerned, add a thread locker like Loctite.
How long does a retention knob last? Why risk it? They are low priced so decide on an interval, lets say one year of use, and change them out. Some clever distributor out there might send out an email reminder to replace it, like an oil change, one year after they sell a retention knob.
Years ago, we were working with a VERY LARGE company on optimizing their tooling. They were having severe fretting on the tapers, One thing we noticed is that their pull studs could not be screwed into the toolholders by hand. This is a tell tale sign. We brought in a ring gage to prove that the knob threads were out of spec. But, it could also have been the toolholder, so inspecting incoming holders and pull studs with a thread and ring gage is a good practice.
Additionally, the guy in the tool crib (I hope he has retired and is not on LinkedIn) welded a 4 foot cheater bar to the retention knob socket. To remove a knob he installed, we had to heat the shank in a shrinker. Despite what we told him, he wouldn't even consider using a torque wrench. So, the next time we came in, we brought with us a NASCAR logo'd pneumatic impact wrench with a retention knob socket, but in between we installed what is pictured below, a torque limiting adaptor (they are known as torque sticks). They are made with different shaft OD's and will twist when its designated torque rating is reached. The tool crib guy wailed away on the knobs with his new gun and the problem was fixed.
"Variable Processes produce Variable Results" My favorite quote from Dr. Scott Smith of UNC Charlotte. If you want to create a truly lean, repeatable and error free process you must eliminate variability. Single purpose and preset. One product that can benefit from this is the very popular ER collet chuck. An ER spring collet is indeed a spring. If you drop one it will bounce. Like any spring, its properties change with how tight you compress it. If you under-tighten the collet the tool may slip or pull out. If you over-tighten the collet it will twist and distort the shank angle creating runout. Dynamically, the stiffness and damping properties of the tool assembly change with the collet torque, thereby changing the location of stable speed lobes. Yet, you rarely see users torque collet chucks. Recently when searching the internet, I came across this product from a company called the iSWISS CORPORATION. These are preset torque wrenches, one for each ER collet nut type (spanner and hex). They are inexpensive and with nothing to adjust and no socket to change, all variability, all chance for error have been removed. Find them here: https://lnkd.in/dSAVNc2
Here is a map of AMT member machine tool distributor locations in North America. It is also from a few years ago, so there may be some changes and omissions, but note how this map closely mimics the milling consumption heat map.
This might be interesting. This heat map showed where milling is done in the United States. It was from a few years ago when we compiled data on machining center sales and milling tool consumption.
This is purely theoretical, but considering that carbide is 3 times stiffer than steel, to match a 3/4" shank carbide endmill, a toolholder would have to be 2.25" in diameter (if there was a through hole it would have to be 3"!). Of course there is a lot more going on. The mechanical leverage of the long endmill may put too much stress on the toolholder connection and the cost of a long solid carbide endmill may be prohibitive. But it does make you think about tool and toolholder choices. To increase stability, a longer carbide endmill, in place of a long steel extension, may be a viable alternative.
Trochoidal milling tool paths are all the rage. Light radial width of cut at full axial depth of cut. It works so well, let's make the endmills with longer and longer flute lengths.
Problem is the most flexible part of a tooling system (endmill, toolholder and spindle) are the flutes of the endmill. On a 1/2" 4 flute endmill, about 70% of the mass is removed to grind the flutes, so its stiffness is equivalent to a 0.350" rod. As the chart below shows, the a tool's performance degrades as the flute length increases.
It is as simple as this: The more flexible a tool is, the more it deflects on a tooth impact and the longer it takes for the next tooth get back to workpiece. That means you have to slow the spindle speed down to wait for that next tooth. To reduce the deflection you have to reduce the force. With trochoidal, you do that by reducing the radial width of cut and your metal removal rate suffers.
Too much of a good thing is sometimes too much.
There are two companies, Briney Tooling and BADAXE Tooling Solutions, that provide side lock toolholders that address the runout issue (if you know of others, please comment below). Figures 1 and 2 show a conventional side lock. The tool is inserted into the holder and then the screw pushes it to one side of the bore. The difference between the bore size and the shank diameter create runout.
The Briney and BadAxe solution is to offset the bore as shown in Figure 3 and when the screw is engaged the tool is brought close to the centerline (Figure 4). How close is dependent on the amount of the offset, the shank diameter and the bore size.
How well does it work? We tested a 3/4" toolholder with ten different 3/4" carbide blanks, all within the H6 tolerance, and the results are shown in the chart below.
FULL DISCLOSURE: At my prior company I was involved in the original design and testing of what later became the Bad Axe Tool.
Bad Axe Tooling Solutions
Sharing information about high performance milling technologies, the result of 30 years of research.