Machining 304 stainless steel

[compact8]

Stainless steel is well known to be difficult to machine especially 304 but what I experienced today surprised me. I was trying to drill some 3 mm diameter holes on a piece of 304 stainless steel plate. As usual, I started with a carbide spoting drill before going to a 3 mm HSS drill which is said to be of "high cobalt" type and good for drilling stainless steel. What happened was it wouldn't go in. Then I switched to a new drill of the same type, no joy. The strangest thing is when I tried to drill a piece of scrap material cut off from the workpiece earlier WITHOUT spot drilling it first, the HSS drill went in with normal effort. The chip coming out were relatively long curls indicating that the drill cut very well.

My theory is that the use of spoting drill work hardened the surface of the material to the extent that HSS drill cannot cut into it. The work was finally done with the use of a carbide drill but are there any other ways to get it done ?

 

[davidpbest]

I do a lot of M4 tapping with 304 (3.3mm pilot hole), and agree that your spot drill probably work hardened the material. One quick jab down into the material with the spotting drill will help. Don’t let either the spotting drill or the twist drill bit dwell in one spot. The cutters should either be chewing away, or not in contact. Also, a 140-degree spot and 135-degree twist drill bit will also help, as will Anchor Lube.

 

[pontiac428]

If it's possible, set up a drill stop so that when you bang into your material (similar to penetrating case hard) you can maintain your feed rate until the hole is complete (peck if needed, but power through if it works). I find machining 304 to be an on/off operation, no rubbing or slowing. Same with your spot drill, bop it down to depth and retract swiftly. 304 was invented by the devil to sew doubt among machinists.