Fixture plate clamps

[Larry$]

I made a few more clamps for the fixture plate. I had a piece of unknown 1/4" scrap steel. Milled it square and cut it into strips on the cold saw. Drilled & tapped M6 & a starting hole for milling the slot. First slot went fine taking about .060" passes with a 4 flute HSS end mill. Next piece end about 1/2" on the first pass and the end mill went up in smoke. Changed out to a new 4 flute carbide end mill & it struggled to cut. I was using cutting oil. The next 3 parts all cut fine. Same piece of steel! I concluded there was a small area in the steel that for what ever reason was very hard. Is that a reasonable conclusion? The photo is just to demonstrate the assortment of clamps I've tried making.