Strange question, right? Almost nonsensical. Surface Speeds are surface speeds, and a difference of 700 SFM has got to result in a difference to the cutter–probably a BIG difference. Yet, paradoxically, there is reason to believe, backed by science, that 1000 SFM really can be just like 300 SFM in some cases during High Speed Machining.
This is just one of the interesting factoids uncovered in the High Speed Machining chapter of our Free Feeds and Speeds Tutorial. If you’ve never thumbed through that resource, you’re missing out on a wealth of free knowledge that can help you to be a Better CNC’er. It’s completely free and each article is a bite-sized nugget of goodness that will tell you all about some aspect of Feeds and Speeds or some other specialized machining condition such as Conventional vs Climb Milling and when each should be used.
I bring all of this up because I just spent all morning the other day revising the High Speed Machining and the Toolpath chapters. I do this whenever I discover new information relevant to one of our existing in-depth Cookbook’s. Our Blog Posts have a little shorter and more “newsworthy” sort of lifespan. But the Cookbooks are sacrosanct–they’re Evergreen Content that we want to keep up to date so they’re the best possible source of information on the Internet for the topics they cover, and they’re always free.
So, if you’re wondering about that aluminum, here’s the scoop:
Temperatures actually come down as cutting speed (surface speed and spindle rpm) increases…
This amazing chart is from Dr Herbert Schulz’s, “History of High Speed Machining.”
The dotted lines represent temperatures at various surface speeds. Note that all of the materials go steadily up and then eventually start dipping back down again as surface speed increases. Somehow, temperatures decrease beyond a certain spindle rpm!
This chart is in meters/minute, so multiply the values by about 3 to get to SFM. For aluminum, we have a pretty good dip by the time we’re hitting 1000 SFM, for example. In fact, it’s temperature is more equivalent to less than 300 SFM on the other side of the aluminum curve–that’s nothing for aluminum. Heck, if we have a fast enough spindle, there’s even room to run HSS faster and get lower temperatures (you’ll note various cutter materials critical temperatures are also marked off–stay below the line for your cutter!).
Steel and cast iron taper down more gently than aluminum, but the effect is still alive and well. Yes Virginia, there surely is some strange behavior when you start in with that HSM stuff!
The same research showed that cutting forces also come down, and that’s at least one reason why the temperatures drop, and why for HSM machining in the right rpm ranges, you can achieve high MRR’s with lower cutting forces.
For all this, and more, check out:
- High Speed Machining: Tool Engagement Angle, Trochoidal Milling, Peeling, and all that Jazz. You can even learn how much to slow down in corners if you can’t use HSM.
- Toolpath Considerations: Quick, is it better to plunge, ramp, or helix into a pocket? All that and much more in terms of telling you how to give your CAM software the very best option choices for your CNC job.
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