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Robert LaLonde
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I've been machining some 4140HT the last several weeks (milling) and its all over the place. I may never get an instinctive feel for it. I actually wear out cutters in a measureable time period. That being said it cuts ok if speeds and feed are right. I tend to use HSM and chip thinning strategies as my main CNC machines are fast, but not super rigid. With the smallest cutters I have to be very careful as even a tiny mistake in a corner on load and engagement will snap them right off. It does work harden a little, but that only seems to be a problem with the smallest cutters taking the lightest finishing cuts. 1/16 and larger ball mills seem to be able to cut faster than you might think. Unfortunately my current project is some very detailed embossing die/mold assemblies. I had hoped to finish with some 1/64 ball mills, but I just can't seem to find a happy strategy for those. I get a couple hours at very low feeds and pretty high rpm out of 1/32 ball mills. I had to go with all altin coated, and flood coolant. Bare carbide would shatter and without flood even with altin coating I had to drop the surface speed so low it was out of the power band of my spindles. When I say flood I mean flood. Two nozzles with valves wide open pointed at the cut area from two different angles. I would absolutely not run mist. I don't think even the altin coating would help mitigate the thermal shock with mist. On my big mill I was able to take quite aggressive cuts with the bigger end mills. If you have any problems with your mill this is where it will show up though. A little binding, a little more backlash, etc. Anything isn't right and you will get chatter. It is hard, and the mill has to be able to apply very steady force to the work piece.
You know what. Ignore this whole post. As I read it, its obvious I still don't have a handle on 4140HT other than to say some cuts work well and look good. Usually either the big heavy aggressive ones with bigger cutters or the fast deep light ones with cutters big enough to handle it. Everything else seems to be a balancing act. Fine detail work can be a struggle, but some of my details are pretty small. That and none of this post applies to turning, but... I will be doing some turning with 4140QT before the current project is finished to make some spacers between the embossing plate and the pressure plate.
You know what. Ignore this whole post. As I read it, its obvious I still don't have a handle on 4140HT other than to say some cuts work well and look good. Usually either the big heavy aggressive ones with bigger cutters or the fast deep light ones with cutters big enough to handle it. Everything else seems to be a balancing act. Fine detail work can be a struggle, but some of my details are pretty small. That and none of this post applies to turning, but... I will be doing some turning with 4140QT before the current project is finished to make some spacers between the embossing plate and the pressure plate.
