Guy with 30 years experience told me to run endmills slower and I refused for 6 months

Kept chipping carbide on a 4140 job at 10k RPM until I dropped to 6500 and now they last 3 times longer. Anyone else stubbornly ignore advice from the old heads until you finally eat crow?

3 comments

3 Comments

felixb258h ago

Old heads been running that stuff since before carbide inserts were common, they know how the steel actually behaves. Ran into the same thing with 4340, thought I could just crank the feed and speed like aluminum. Chip evacuation is the real killer at high RPM in hard stuff, the tool rubs and gets micro cracks fast. Dropping to 6500 lets the flutes actually clear the chips instead of burning them into the cut. SFM matters way more than people think, especially with coated carbide where heat management is everything.

beth_butler757h ago

Huh, I dunno. Feels like people overthink this stuff sometimes. I've run 4340 at 10k with a coated carbide endmill, flood coolant, and never saw any micro cracks or chip evacuation problems. Maybe your tool holder or DOC was wrong. 6500 sounds slow for a modern machine unless you're on a Bridgeport with a drill chuck. I'm not saying the old heads are wrong, but sometimes "this works better" just means "I got used to it this way.

jennifer_west7h ago

Old heads definitely have a way of making you feel like a kid who just learned to tie his shoes again. Had a similar thing with a 303 stainless job a few years back, where a retired guy told me to back off the feed per tooth to like 0.001" on a quarter inch endmill. Told myself no way, just cranked the feeds and speeds like I was cutting aluminum, and ended up with a beautiful mess of welded chips and a dead tool in about 30 minutes. Finally tried his number and the finish was smooth as butter and tool life tripled (you know, like you saw). Sometimes we just need to admit that real world data from decades of doing it beats a theoretical chip load calculation every time.

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