Drilled holes not smooth

[compact8]

I have been getting this problem randomly and never know the reasons. Today I got bad holes again. some details :

1) 3 mm 120 deg carbide spot drill, 4.2 mm 130 deg M42 HSS drill, good holes on 6061 and 7075 aluminum, no chattering
2) same as above but the material of the workpiece is mild steel, some chattering, bad holes as shown in the photo. Have tried 6 holes, all bad.
3) 6 mm 120 deg carbide spot drill, 6 mm 130 deg M35 HSS drill, good holes on all materials mentioned above. No chattering

Oil used in all cases.

What might be the reasons ? Is the 4.2 mm drill to be blamed ? It's new.

 

[pontiac428]

Can you post pics of your drills? Are they relieved for chip clearance? What diameter and speed? What cutting fluid are you using? Are you peck drilling to clear chips? What material are you working when you say mild steel, CRS or...?

And there's this:

Drilled holes not smooth

Which is true in all cases, as is the nature of drills. But they should be smoother than that.

 

[markba633csi]

Looks like weird material to me- as if it has inclusions in it

 

[compact8]

Can you post pics of your drills? Are they relieved for chip clearance? What diameter and speed? What cutting fluid are you using? Are you peck drilling to clear chips? What material are you working when you say mild steel, CRS or...?

The picture is attached. left : 4.2mm ( good holes on Aluminium and bad on mild steel ). right : 6mm ( good holes in all materials ). Both are split-points
Speed is 680 RPM in all cases.
Not sure what kind of mild steel. It can be cut quite easily. The chips are long and continuous.
Cutting fluid is Rocol.


Have tried pecking and it made no differences. All the holes are quite shallow , no more than 5 mm deep so chip evacuation should not be an issue.

 

[erikmannie]

Compared to other materials, I get a poor surface finish on mild steel.

Have you tried step drilling? Maybe use the shortest increments as possible (limited by your collection of drill sizes) as you near your target hole size.

 

[projectnut]

The drills in the picture appear to have parabolic flutes rather than standard flutes. As such they may be pulling themselves into the material too fast and causing a rough finish. This style drill is primarily used for deep holes in softer materials like aluminum or plastic. The steeper flutes allow for better evacuation of chips and better coolant flow, but they also tend to suck the drill into the material if the down feed isn't properly controlled.

 

[Flyinfool]

It could be camera distortion but the point of the smaller drill looks to be off center.

AL is softer than steel so it is easier to get a good hole.

WHen spot drilling you still want the point of the drill to contact the work before the outer edges of the flutes. The spot drill point should be a bigger angle than the drill point. this helps center the drill bit on the same center as the spot. You also need to spot to a slightly bigger or at least same size as the drill so that the drill can not hit anything that will deflect it. Do not spot drill to where you are starting to make a straight sided hole unless the spot drill is bigger than the final hole size. The drill will rattle all over trying to pick up the hole. A 3mm spot is not enough for a 4.2mm drill.

 

[mattthemuppet2]

You can't use the same speed/ feed on steel as you do on alu. That speed seems fine in alu, maybe a little slow if anything, but faster than I would start in steel. Try half the speed and drilling without spotting. If you have a shorter drill, try that too

 

[Nutfarmer]

The smaller drill looks miss ground where the web was thinned. Try a new drill same size or regrind and see what happens.

 

[aliva]

To me it looks like the grind is off. Could just be the way the pic was taken but I would regrind, and don't worry about regrinding the spit point, just go with 118.