Key factors Determining Success of High Speed Machining (HSM)
As a developer of a very successful line of speed and feed calculators I sometimes get questions like : "I calculated speeds and feeds for a conventional toolpath. Got 5.5 cubic inches MRR(Material Removal Rate). And then I calculated S&F for the same endmill with HSM parameters turned on and got almost the same amount of MRR! What is even the point in using HSM parameters?" -they ask.
I would like to clear some things up for my friends.
In this article I will explain exactly WHY HSM machining is better and HOW to achieve better productivity and tool life.
For starters here are the main features of a HSM-capable cutter:
As usual there are several components of HSM that need to be present in order for it to work to its fullest. These are:
a) Machine
b) Tool
c) Workpiece geometry
d) Workpiece material
I intentionally did not number these as each one of those is equally important.
Ways in which High Speed Machining (HSM ) works
Lately there have been a lot of really interesting HSM topics on PracticalMachinist forums.
In one of them a guy who owns his own resharpening business posted a video of his endmill milling a block of D2 hardened to over 60 RC.
The forum topic is located here First try on D2 62Rc(video)
Here is his post so you know what we are talking about:
The next run will be at 650 sfm, .006 ipt using a mist sprayer. Also, any small areas will be blocked off to be ran at lower speeds to allow cooling time for the cutter. Just a note for anyone using a Mag Fadal, The E-stop button is not quick enough, use feed hold. The endmill was badly worn on the corners, but not broken, and will be resharpened and used again.
In the ensuing discussion i posted my own take on how and why HSM works
1) Reduced cutting time per edge per revolution allows it to cool down more.
2) Chip thinning allows to increase chipload (advancement per tooth per revolution)
3) Increased depth of cut combined with shallow radial positively affects deflection. Tool bends less as it is more rigid towards the tool holder.
4) Higher cutting speed actually reduces cutting forces as heat generated in the cutting zone makes it easier to shear off a layer of metal. Yet because the time of contact is so small, most of the heat is carried away with the chip.
5) Higher RPM also allows to get rid of hot chips faster thus further reducing heat transferred to the tool.
6) Higher feedrate actually reduces relative cutting speed.
7) At high axial engagements more than one flute is in contact with the workpiece at different points along the axis of the tool. This too helps combat vibrations and chatter.
8) You are using more of the tool than just its tip, so technically you can do more work with one tool before it gets dull.
9) lastly it looks cool as hell and is very impressive. Whenever we know visitors or bosses are coming we try to make sure some HSM is going on
even if application does not merit thatI am not sure if the air that is moved by the endmill is doing much, but i suspect he didn't mean exactly that.
Numbers Behind High Speed Machining (HSM)
HSM or High Speed Machining is becoming more and more popular each day.
Many of us have seen those youtube videos where endmlls remove large amounts of material at high speeds/feeds.
While definitions of HSM may vary between tool manufacturers and even individual shops, the physics behind it remain the same.
In this article i would like to explore flat endmills.
HSM is not about ramping up your speed/feed overrides to 200% and puling out your smartphone to record another youtube-worth video.
What is HSM?
HSM is a complex of programming, machining and tooling techniques aimed at radical increase of productivity.
Programming
The cornerstone of HSM is low radial and high axial engagement of an endmill with the workpiece.
There are many CAD/CAM systems that allow you to create HSM tool-paths. Mastercam's Dynamic milling and SurfCAM's Truemill are some of them.
When radial cutter engagement with the material is smaller than the radius of the tool an interesting thing happens.
Chip load- the distance the tool advances per cutter revolution per tooth- does not equal the actual chip thickness anymore.
Chip thinning mainly happens at radial engagements below 30% of the diameter.
| 100% |
1.0 |
|
| 50% | 1.0 | |
| 30% | 1.091 | |
| 25% | 1.212
|
|
| 20% | 1.641 | |
| 15% | 2.1 | |
| 10% | 4.375 | |
| 5% | 6.882 |
In order to get compensated chipload you need to multiply recommended by manufacturer chipload by the chip thinning factor.
Usual Radial Engagement for HSM toolpaths however is between 5 and 15%.
Axial depth of cut varies depending on geometry, but Read More
HSMAdvisor Screenshot
Calculating Tool Engagement Angle, Radial Depth of Cut
Here i will show you how to calculate Tool Engagement Angle using tool diameter and Width Of Cut (radial deopth of cut)
Lets first draw a pretty image that shows us everything we need.
Where:
- r: Radius of the cutter = Diamater /2
- a: TEA - Enagagement angle we are trying to find here
- WOC: Width of cut or RADIAL Depth of Cut
- r2: The difference between r and WOC, r=r2+WOC
Below we develop 2 formulas that allow us to find TEA and WOC
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