So, I’ve been a delinquent, and not posted anything for a while, but I think something has come up in the shop that is worthy of writing about.
We have a recurring job in the shop that is extremely dependent on metal removal rate(MRR, or cubes. It’s all about the cubes). So what machining strategies are the best? A High Efficiency technique where we bury the tool axially and take smaller step overs radially then feed the living snot out of it? Or go more traditional in conservative Z steps and wider axial tool loads? What cutters do we choose? Tool Holding? Work Holding?
Step one – the setup. It’s quite rigid. 2 to 3 vises depending on the configuration of the part. It’s not going anywhere.
Step two – Now come all the decisions. My first thought was to rough with a corn cobb roughing style cutter. I programmed it using the manufacturers recommended settings, which I found worked well. I wanted more though. With this job it’s really all about the cubes.
What is possible with a Haas VF-3 with a 10,000 RPM spindle and 30 hp geared head? The job is 6061 Aluminum. The roughing tool is a Ø¾ roughing end mill with a 1″ DOC with a reduced neck. With our initial parameters we were hovering around 39 cubic inches of material being removed a minute. After playing a little bit with speeds and feeds we got that sucker to just a hair under 60. This is fair I believe for a tool of that size on this machine.
I wanted to know if this is the limit or if there is something out there that can do better with my machine. So I put out the feelers to a local supplier who graciously brought in a couple tools on a guaranteed trial. So basically, we buy them, and if they don’t work, they go back. So their parameters claimed something silly like 124 cubic inches a minute. Under their instructions I programmed the tool path to their recommended settings.
At the machine, being cautious, I started at 50% feed over ride. I started running it, and it did not cut 20 linear inches before it broke! It lost 2 of 3 teeth, and done. Email to my sales guy who emails his product guy and well there was a mistake in their calculation software.
Another crucial factor that was not taken into consideration was the spindle power. Nor were any considerations given to torque. Their parameters were 10,000 RPM and 345IPM. CRAZY! but then I put their info into a HP calculator, and it’s a 50hp cut!!! I took their data and tweaked it. I was able to get up to 345ipm feed, material removal though was well under 124, but it was still higher then my initial cutter, but why was my cycle time longer?
Answer – the tool is now spending significantly more time in the air. With my old school roughing tool, during that cycle, the tool spends more time in the material cutting and less time travel in non cutting moves. Those moves don’t make you money as a shop owner. Another thing with this new cutter didn’t last. The rougher, we are still using the first one we started with. It’s cut close to 80 parts and removed yards of material. The new style one designed for HEM? It didn’t last 8 parts before the edges burned up. This cutter was also almost double the price. The killer for us though was the cycle time. Too much non cutting time, and that is the end.
So is HEM the way? Answer – It depends. Sometimes you need to be open to the old way as well. It has it’s advantages. We are still on the look out for something that will push the limits of our equipment and get cycle times lower. Some times, though, it does not work out.
See you soon. Hopefully the interval is shorter then this last one!