What is high speed machining

[camminc]

The method used to create a stability chart is simple. All that is required is the correct equipment and someone to perform the operation. The system I use is calle The MetalMax System. It takes only a few minutes and can be used on any of your machine tools with any tool. Once done, personnel assigned to perform this duty, inputs all pertaining information into a database for programmers, tool crib and presetting to use. Now programming, tool crib and presetting simply look at the tools in the database, all the information they need is there. A stability chart showing what RPM to use for highest depth of cut and to stay out of chatter, gage length, feed rates, cutter description, number of flutes along with much more information if wanted. If their looking for a ½ diameter - two flute ball end mill all they need to do is query the database for this style of cutter of the machine they are going to use and it will show all cutters in the database with those parameters. (The cutters in the database are called Stackup) The programmer simply chooses the Stackup assembly he wants to use and puts that Stackup assembly tool number on the setup sheet for presetting to assemble. He programs this Stackup to the known values, rpm, depth of cut, feed rate per material as the stability chart and database record information show.

The method I use to collect a stability chart is: I have a portable computer on a cart with wheels. The dynamic process optimization equipment (MetalMax) I use is all attached to this computer so I simply roll it out to the machine I want to get a reading from. The cutter is in the machine, I attach the accelerometer to the tip of the tool with some wax, turn the computer on or it is already running from a batter backup, tap the tool with the impact hammer and get the reading. It’s done. I look at the stability chart, tell the operator what rpm to run, make sure the depth of cut is not over limit. I push my cart back to the tool crib, enter a few fields of information and that record is finished. I then go back out to the machine, make sure everything is running okay and if anything is changed or modified - I simply record that information into the Stackup record I just created for use later.

Now many shops have already input cutter Stackup information into a spreadsheet. We can take this information and import it into our database. A database is much easier to use, better searches, more information, easy to view and creates detailed reports. My last customer had 1,953 Stackup’s in their spreadsheet, but they did not have any dynamic information – stability chart, machine information, etc. It was a mess to look at and search with no report functions. Once their information is in the database all they need to do is set up a picked Stackup and perform the dynamic process to get a stability chart and see if that Stackup is running at maximum. Odds are, it is not and can then be maximized further. The database automatically calculates spindle power, material, depth of cut to give proper parameters that will not exceed power limits, sfm, cutting stiffness, material specific power, etc.

[trubleshtr]

Thanks for replying, sounds like a job for someone "in the know". We have outside companies doing our V.B. analysis,I think it would be intresting to do a stability test at our place, definetly room for improvement there. I think I'll leave it to the pro's..........

[Scott_bob]

Our CNC control retrofit is now complete!

It's very impressive to see an 8 year old DC Servo Drive CNC machine out perform a brand new Digital AC Servo Drive machine right next to it...

I love the Rubicon (Numeryx) controls automatic feed rate motion algorithms that uses geometric look ahead, and jerk factoring to smooth out the motion.
There is no point going fast if the control cannot maintain accurate positioning and smooth dynamic feed control, speeding up when it can, and slowing when it needs to, automatically. It's really incredible. We set "one" feedrate and the control does the rest. When geometry has sharp "inside corners", feed is reduced, or with small radii. When sharp "outside corners" are detected, feed is increased, as well as when cutting straight. I have simplifed the dynamics here, maybe someday there will be some video posted. Right now we gotta make some parts...

By the way, the feed rate is constantly changing depending on the upcoming geometry, so there would be no point in determining a single feedrate that may represent an ideal "low chatter" frequency.

[camminc]

To trubleshtr:

I understand what you are saying that vibration analysis "sounds like a job for someone "in the know".

It sounds like a lot but it really isn't that difficult. You or anyone else can understand it and use it. I published an article from the beginning. June of 2004 at:

http://www.moldmakingtechnology.com/

Register yourself at that site to view articles. My article is not out yet, it will be out in June called " Chatter Myths".

It should help you in HSM or Maximized Machining.

Thanks, Randy

[camminc]

To Scott_bob

Your input of: "By the way, the feed rate is constantly changing depending on the upcoming geometry, so there would be no point in determining a single feedrate that may represent an ideal "low chatter" frequency."

Dear Scott _bob. There is no “low chatter frequency” as you speak of, that is called process dampening. Your about 90 years behind times.

In the real world there is a natural frequency and flexibility - period. These readings can be taken to determine a constant feedrate to maximize machining operatations.

[Scott_bob]

camminc,

Well I suppose you could tell me all about it...
It's always interesting to me when someone goes on the attack when their ideas are challenged.
No harm, no foul...

What do you say to the question:
If a cutting tools feed rate is constantly changing, then what is the optimum feed rate for that tool?

Is this what you mean by "constant feedrate"?

[camminc]

Scott-bob:

I don't mean to attach you, I just get tired of saying the same thing over and over. You know what I have written in this forum and it is the truth. I don't play games. I have fought antiquated ways of everyone for too long.

Bottom line: A cutting tools feed rate is constantly changing due to programming. Chip load is chip load and that is how they program it. Programming changes feedrates (Chip load) because of depth of cut, or it will chatter. (Because the cutter exceeds stability and they don't know this with out data given to them) If you know the maximum depth of cut (Stability) for a cutter then programming will not have to change feedrate. In another words, if you know a 1" cutter in an assembly with a specific gage length, in this holder, in this machine, at a certain rpm, can go 1" deep with a chip load of .010, you can program it to do that all day long, full depth of cut of 1", even in corners. But if you exceed the depth of cut or change the rpm it will fail. Example of this cutter being in the 1x multiplier of NF and Flexibility.

Problem being: Programming has no idea of the optimal depth of cut for a cutter, (Stability) if they did then chatter would not occur. They simply program a tool for a chip load, that is it. But when that cutter reaches to high a depth of cut it begins to chatter or forced vibration takes over. Such as in a corner, etc. The chip load has nothing to do with it, the cutter simply overloads the machine tool dynamics causing chatter or forced vibration.

To maximize one must know the frequency of the cutter at the tool tip, period. It will tell you exactly what rpm to run to maximize. Flexibility will tell you depth of cut. If you can't reach the optimum rpm, then you go to the next best thing, the 2x, 3x, etc. The 1x is the optimum rpm to maximize that cutter on that machine due to measurements taken. It will give you the highest depth of cut, feedrate will stay upon the chipload required of that cutter at that rpm. It might be 40,000 rpm, but you can't reach that rpm, you divide that by 2, to get the 2x, or 3 to get the 3x. Each time you go to a higher multiplier you give up depth of cut.

Natural Frequency X 60 / number of teeth. This is the equation to know, to maximized rpm.

Example:

1000 hz natural frequency x 60 x 2 teeth = 30,000 rpm = 1x multiplier, maximized condition.

/ x 2 = 15,000 rpm the 2x multiplier (Less depth of cut then the 1x)
/ x 3 = 10,000 rpm the 3 x multiplier (Less depth of cut then the 2x)

The trick is: Dynamics of that cutter of the machine tool, do not exceed those parameter. Rpm will allow you to reach depths of cut that you could not imagine before.

Simply put: If you know the deepest depth of cut of a cutter in full diameter and run it at the desired chip load it will cut without chatter or forced vibration.

The way to know this is with knowing the Natural frequency and flexibility of that cutter, then program it. Limits are not exceeded, chatter, forced vibration do not occur. Maximizing. Controllers play a part but if you go an impact test it will give you parameters. If those parameters do not transpire then it is the controller, cpu, etc.

Dynamic response at the tool tip reveals machining capability of that tool, period.

I have seen things happen. Take a Marwin 40,000 rpm machine. Tolo use to have them then sold out to BF Goodrich. They still have the same problems because they won't listen.

These machines have high rpm but they also have very small holders / cutter assemblies. (Smaller assembly higher frequency) Programmers program them at 40,000 rpm and it chatters, when they maybe should be programming them at 36,000 or 28,000 rpm. They have no idea. To run a machine tool at maximum rpm does not give you maximization, due to dynamics. You have to know the natural frequency at the tool tip to know the dynamics of that machine tool to cut, period.

It is simple, you take readings with an impact test, you put these reading in a database, it gives you the maximum doc for that cutter on that machine, send it to the programmer, he sends it to presetting to set up and away it goes. The database makes tracking simple for tool crib, presetting and programmers. Reports, etc.

You do this for about 2 years and you have 1,000 tools ready to go, maximizing operations of all machines. Once a reading is taken of a machine tool it will continue later, if not then the machine tool dynamics have change alerting you of maintance of that machine tool. (Another quaility of impact testing called "Modal Analysis" very usefull) All it takes is someone to do it and someone to make it happen. Otherwise, it is a guessing game, production is sacrificed, profits and redundant procedures occur making for inefficiency.

Thanks, Randy

[Scott_bob]

CAMMINC,

I respect your commitment.
But, IMO your insight is a bit myopic (tunnel vision). I don't want to be too direct but the process variables from your posts reflect old school methods.

High Speed Machining (HSM) would not use the "maximum depth of cut" that you mention.
This is one of the fundamental realities of HSM. I agree that there is significant benefit to finding the sweet spot for a cutter assy but, if the program is not going to be cutting at the maximum depth of cut, then it may not chatter anyway right?

The point of HSM is to maximize metal removal rate by very high feeds with lighter cuts. Obviously, not too light of a cut else the process will not yield faster results.

Another observation I have is that you refer to the feed rate changes as if that is a result of the programmer making those changes:

"Bottom line: A cutting tools feed rate is constantly changing due to programming. Chip load is chip load and that is how they program it. Programming changes feedrates (Chip load) because of depth of cut, or it will chatter."

A good HSM control will compensate the feed rate depending on the approaching sharp corner, or small radii, or density of the point data. A smart control looks ahead and buffers many steps ahead, thus it is able to decelerate before sharp corners, this is called geometric look ahead. This type of control adjusts the feed rate depending on the algorithms that are used to handle high speed motion. The CNC program needs just one feed rate say a maximum (perhaps determined by your impact testing) and surface finish requirements, then the "control does the rest".

There are two other problems that are often ignored, namely acceleration and servo lag, which affects high-speed contouring more than most people think.

1) Acceleration and deceleration are features of the servomotors. If a program tries to accel/decel faster than the servos can handle gouging or overshooting occurs. This is not chatter; it is the result of inadequate control...

2) Servo lag, 2 factors control the smoothness of the motion (and its inaccuracy as well): the servo lag and the jerk factor. Both are doing similar things but in a completely different way.
Servo lag is an inherent feature of the servo loop and jerk factor is part of the motion generation by the control. The servo lag is exponential; when step acceleration is sent to the servo the result is a little rounded. The jerk factor converts the rectangular acceleration to trapezoidal or triangular.

Not all controls have jerk factor, and some have exponential one, which is easier to calculate but introduces greater errors. Without jerk factor the machine jerks upon accel/decel. This is noticeable with short jogs.

Both servo-lag and jerk-factor are time delays and they make the servomotors move a little behind the program. For a linear move that's not a problem, because all axes are behind equally (if the servo gain is equal), but for an arc there is a decrease in the radius:

It can be seen that the contribution of the jerk-factor is much less than that of the servo-lag.

Conclusion.
Not all HS are created equal...

[Scott_bob]

Coolant issue:

http://www.cnczone.com/forums/showth...1013#post41013

[Gunner]

Camminc and Scott_Bob,
Both of the issues you have brought up are important aspects in high speed machining. The idea behind high speed machining is to optimize the cutting condition to the machine, tooling, and control. To do this you must know the machine and spindle power rating, the vibration frequencies and limitations of your tools, and the limitations of the control. If you know these you can optimize the machining operation. This means you will achieve less power usage, maximum metal removal at lower cycle times, optimal tool life, and lower maintenance costs. Without all of these you can get pretty good using trial and error. Only problem is there isn't much error in high speed machining. If you have one it's usually costly.
True high speed machining isn't guess work. Using the the power graphs, a few formulas, and the frequency analysis chart you can determine the maximum (and most optimal) speed and metal removal rate for your operation. You can actually see the effect of depth of cut vs power and stay within your most optimal range. HSM reduces the amount of power needed to remove the same amount of material as a conventional machining operation thus reducing wear and tear on the machine and tooling. The control is important because of the acceleration and deceleration at the higher feeds and speeds. With an old controller you may never hit the the programmed feedrate the the HSM rates. You need fast "G" acc/dec and good look ahead control capabilities to be successful. Now you come into the drive part of the machine. You need the positioning accuracy to be able to handle the information from the control. This gives the advantage to linear motors/drives which can postion faster and are more accurate than the ball screws.
Briefly, here are a few more important issues in HSM.
*Tooling - The tooling must be balanced for high speed machining. An unbalanced tool will decrease accuracy and will increase vibration as RPM increases. It will also cause premature damage to spindle bearings. Max RPM ratings on tools must be followed and put into your HSM equation if alternate faster tooling can not be used. HSK style holders are a must when working with spindle speeds over 24K. They are more stable and rigid at high spindle speeds than the CAT type. The frequency analysis for each tool at the cutting edge would also be added to your HSM equation. Note, if you change a tool you would need to repeat the frequency analysis and make any required adjustments.
*Coolant - high pressure... thru the tool if possible. Dry methods should be explored but if coolant is used misting can be a problem and mist evacuation should be considered.
*Fixturing - conventional techniques and methods are OK for conventional parts. Picture framing is an option for thin wall parts where vibration or distortion are possible.

There is a lot more to this but I'm getting tired of writing. I've only highlighted some of the things I learned at the "Introduction to High Speed Machining" seminar I just attended at Tech Solve in Cincinnati, Ohio. I strongly recommend this to anyone interested in HSM. It lays it all out but it's up to you to take the next step. You both are right, each of your areas are important, but, high speed machining is a system with all areas being of equal importance. If one area is weak, you will not be able to acheive the optimum.

[Scott_bob]

Gunner,

On any CNC machine the control is the primary limiting factor for HSM. Most controls cannot deliver the high performance motion that is considered High Speed Machining (HSM). The limitations of our control prevented us from using this methodology for max metal removal rates. When we replaced the control, we were able to use this new process. Before the retrofit, we could not.

All the contributing technologies only contributed small improvements to our processes reduction in cycle time or improvement to quality.

I appreciate your input on the list of the variables...
If it's ok with you, I’d like to disagree on the equality of each variables effect on the optimum...

Sincerely,

[camminc]

The machine control is not the primary factor to HSM. You can't run a cutter at 25,000 rpm when it wants to run at 23,000 rpm, etc no matter what kind of controller you use. See these facts: Impact Testing

F18 E/F Fighter: MetalMAX impact testing has been in use on this program for over ten years. MetalMAX was a critical component in the cost reduction program that saved the US Navy F18 E/F fighter program over 1,000,000,000 (one billion dollars).

Aircraft Engine Mount: The American Helicopter Society article, “Productivity
Improvements through Collaboration” (June, 2002), credits MetalMAX technology for driving down the cost of a titanium aircraft part from approximately $43,000 to $28,000 per part, a savings to the program of $2,000,000 over the life of the contract. This is a 35% cost reduction made possible by the MetalMAX system. MLI earned the prestigious Pickney Award form the AHS in 2003 for this accomplishment.

Automobile Head: In rough milling the internal ports of a cast aluminum cylinder head, the MetalMAX system, in less than one hour, created dynamically optimized cutting parameters on three milling cutters. The cycle time was reduced 33%, from 1.5 hrs to 1.0 hrs. This single cost reduction and ROI justified the purchase of the MetalMAX system.