Cast iron chewed up end mill...?

[Hukshawn]

My vise I epoxied that I mentioned in another thread, I was milling it down and smooth with a 3/4" 4 flute hss cobalt 8% end mill. Fairly new. The finish started getting really crummy after a while. I stopped and looked at the end mill and noticed the cutting tips are all ground and chewed... What gives? The epoxy or the cast iron?

 

[benmychree]

I would be looking at excessive cutting speed; with cast iron, perhaps 80 ft/min. would be a starting point. I would not think hard spots would be a problem, the material seemed to drill quite freely ----

 

[Hukshawn]

3/4 end mill running about 600 rpm, I was not feeding fast, but not super slow either.

 

[HBilly1022]

I had a similar problem milling some mild steel, using a 3/4" HSS end mill running at 475RPM. I might get a couple of square inches milled with a freshly sharpened end mill and the it would start grinding and making a horrible finish. I ended up slowing the RPM down to 250 and adding some wipe on lube (magic tap) and the problem was solved. From my reading (I'm new to this stuff too) the speed should be even slower for CI, like benmychree said.

 

[4GSR]

A 3/4" end mill cutting cast iron should be running some where around 150-250 RPM. 600 RPM is way too fast for cast iron. You want to run cast iron at around 40-50 FPM or less. Very abrasive stuff! Use dry air from a mister to blow chips from cutter and aid in cooling. DO not cut wet!

 

[Hukshawn]

I wasn't cutting wet.
150-200 rpm, lol I would have been there all day. I wound up switching to an inserted carbide tool to clean up my mess. Which I likely should have done in the first place.

I really need to get a speeds chart...

 

[benmychree]

Eighty feet per minute for a 3/4" diameter tool would be 400 RPM;

I had a similar problem milling some mild steel, using a 3/4" HSS end mill running at 475RPM. I might get a couple of square inches milled with a freshly sharpened end mill and the it would start grinding and making a horrible finish. I ended up slowing the RPM down to 250 and adding some wipe on lube (magic tap) and the problem was solved. From my reading (I'm new to this stuff too) the speed should be even slower for CI, like benmychree said.

Steel should never be cut dry with HSS tools; I use Magic Tap cutting fluid for all my machining needs, it is good stuff!
250 RPM would be a cutting speed of 50 FPM. My 50 year old Kennametal feed and speed slide rule calculator does not read any slower than that speed. The tables printed on it suggest 100 to 400 FPM, but that is for carbide tooling, and depending on the material hardness, of course.

 

[benmychree]

I wasn't cutting wet.
150-200 rpm, lol I would have been there all day. I wound up switching to an inserted carbide tool to clean up my mess. Which I likely should have done in the first place.

I really need to get a speeds chart...

With cutting cast iron, you should not need to use a lubricant, and with CI, it just makes a big mess and is not necessary; the use of a insert face mill was a good move, like you said, it should have been the first choice. Live and learn!

 

[Downunder Bob]

Cast iron varies a lot in quality. Really good cast iron machines very nicely and with a clean finish. Cheap nasty CI often has slag inclusions and even pockets of other metals, and or sand, and can be a real pain to machine.

Always remove the outer cast skin of CI with a grinder if possible before machining, if you have to machine the skin do it at half speed or even less.

I have had to reduce speeds to half or even slower with HSS on some CI. Carbide is much better
As others have said machine CI dry, perhaps a little bit of air to help with chip removal and cooling.

 

[benmychree]

I have dealt with three foundries over the last 50 years or so; the first is long out of business and turned out poor work with slag and inclusions, the second did excellent work in all ways but got too expensive; the third likes to give rough dirty castings, but metal quality is very good; I've just given them two more items to cast with instructions to use a fine facing sand to improve finish; to their credit, they work cheap!