3/4 and 5/8 endmills won't cut but 9/16 will.

[EOsteam]

Here is the setup. These are the frames for a lead truck on a 4-8-4 LE old Northern. They are 3/4" thick mild steel. The two frames are bolted together on a 1/2" piece of aluminum plate. The plate is clamped to the mill table and is further clamped for rigidity on the frames. I first tried mill the frames using a 5/8" endmill but it just seemed to rub and wouldn't cut. The same results were obtained with a 3/4" endmill. I switched to a 9/16" endmill and even though I had to mill in two passes (extended the endmill in the collet for the full depth on the second pass), it cut well and made blue chips. I made the setup as rigid as possible and used a collet to hold the endmills minimizing excess length of the endmill out of the collet. Why would the 5/8" and the 3/4" endmills not cut? I have had this problem before and jumping down to a 9/16" endmill always works. I calculate the RPM's using the formula RPM= 3.82 x 100/diameter of endmill

Any and all guidance is most appreciated.

[GlennW]

If they are HSS end mills you may be spinning them way too fast.

Blue chips are not good with HSS tooling.

If the parts were flame cut the "skin" on them may be hardened.

[EOsteam]

I'm not sure how they were cut but the cuts were very accurate without much material left for final dimension. I was using 500 RPM's for the 3/4", 600 for the 7/8" and 700 for the 9/16". The endmills are HSS. Next time should I try knocking the RPM's down a bit? Should the color of the chips match the parent material when using HSS?

Thank you.

[GlennW]

I'd say to slow them down until the chips match the parent metal or to a light straw color at the most and see how that does.

Upon enlarging the image the parts do look to be flame cut. That purplish/black "crust" on the edges is very hard and abrasive. You need to cut through that rather than just skimming it with the end mill.

[EOsteam]

Thank you Glenn. I will need to face this same issue when I get to the main frames someday.

[GlennW]

You're welcome!

Let us know how it works.

[EOsteam]

I finished cutting the pocket for one of the journals but had to resort back to the 9/16" endmill. I tried a fresh 11/16" endmill and cut the speed back to 375 RPM's but the dang thing started to bind up the whole assemble and started to lift the cradle. I'm starting to wonder if these endmills might be the problem. They came from Grizzly about 7 or 8 years ago. The edges are no longer smooth and when I run my fingernail down the cutting edge, it feels pitted. Two of the endmills I used today were from this batch.

I'm going to place an order tonight for a few more endmills. Does anyone have recommendations for quality manufacturers without killing the budget?

[Harold_V]

Post a picture or two of the end mills in question. I expect you've destroyed the periphery from over speed, or they may simply be dull from being used.

Are you familiar with cutting geometry of end mills? That goes a long ways towards helping you identify a cutter that is suited for work, or one that is in need of being sharpened. It's considered poor practice to use a dull one, as once the edge is slightly rounded (typical dulling), it cascades quickly and can destroy the end mill.

As Glenn said, it's important that you get below the slag (or cut line) of flame cut materials. If the torch was not set exactly right, the slag can be quite hard, as can be the area adjacent to the cut. It helps to NOT climb mill in such a circumstance, although I expect that you probably are not climb milling.

Harold

[revrnd]

Would using a die grinder to "clean up" the surface help to remove the slag/scale?

[BadDog]

That appears to me more in the realm of angle grinder, but yes, getting the flame cut edge off by some other means can make things much easier. However, since you have said there is little extra to machine, I would almost certainly avoid that in favor of just get under the problem edge while conventional milling. That should eliminate most of your problems. I also generally use carbide EMs for such things.

[warmstrong1955]

Yeah.....what BadDog said....

I machine a lot of flame cut parts, and try to get off as much of the scale as possible prior to machining. Even the mill scale on hot-rolled steel can wreck a good HSS end-mill. Can also be tough on carbide, but not near so much, or near so fast, as what it can do to HSS.

I also use carbide for such things.

Bill

[ctwo]

How much is enough to get under the crust?