Drilled holes not smooth

[Jake M]

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

If it's random...... It's not something that is unchanging... I wouldn't jump to the tool first.

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.

I question this here. Using a 120 spot drill for a "larger than 120" drill bit forces it to start it's cut on the corners. At 4.2mm (five thirty twos, or number 19,20 ish in the wire gauge drills for inch folks reading), the drill isn't that rigid. When it starts on the two outer flutes, it's gonna wobble, and it isn't rigid enough to recover from that.

When you choose different materials, different RPMs, different feed rates (or feed pressure), it WILL change that characteristeric, for better or for worse. How you "hit" the spot drill mark can also make a difference. Ease in and let it "bore" it's own center while you hope it still guides some, (usually better on smaller drills, but being a touchy feely intuition thing.... Anything goes). Or the "plow through it quick and get the center of the drill to the center of the spot as fast as you can appoach. (usually better on larger drills, but again, anything goes).

So those are my thoughts. I wouldn't say I know the answer, as I havn't seen in person or used your drill. But if it's random, it's "probably" something you are doing, vs the drill being good or bad from day to day. Or your spindle bearings loosening and tightening from day to day. The spot drill at the wrong angle WILL aggrivate that situation. (Although it often works... I'm as guilty as anybody).

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.

The 6mm drill bit (15/64 ish, or number A,B ish in wire gauge) is a much more robust drill, with (roughly) twice as much cross sectional area. You can get away with a lot more. They're both still small, it's not a free for all with either, but you can get away with more.

Were it me, having this issue, I'd validate my suspicion (again, that's all it is), by taking the 4.2mm drill, with no spot drill, perhaps a center pop just large enough to capture the split point, but no more, and I'd just plow. By that I mean not "how strong are you", or "how fast is your power feed", but bear down until it makes a good continuous chip, and keep up with it. Keep that chip as long as you can keep it. Then examine the hole. If that solves the problem, you have a solution. Fix the spot drill thing. If that does nothing, it's gotta be something else, right? What else has changed from "good holes" to "poor holes"? Even if my first thought is wrong, I still say that. If it's random, and "sometimes" that the problem occurs.... It's a variable that's changing in between uses.

 

[great white]

That looks like its the material, not the drill. But, hard to tell anything with any certainty when looking at a 2d pic on an iphone…

 

[markba633csi]

Agreed- I have my doubts about the material too- that scarring in the hole doesn't look like anything a drill would do
I also see what looks like cracking or fracture lines-? Hmmm

 

[compact8]

It could be camera distortion but the point of the smaller drill looks to be off center ..... The spot drill point should be a bigger angle than the drill point....

The drill is not grinded off center. When the hole is good ( on aluminium ), the hole diameter is slightly smaller ( by 0.02 mm or 8 tenths of a thou ) than the stated diameter of the drill. If it's off center, the hole should be always bigger.

I know the angle of the spot drill should be bigger than that of the drill bit but I just could not find such spot drills ( 140 deg ) locally. I have also tried to find 118 deg drill bits but the majority available here are 130 ~ 135 degrees. I do have 140 deg short carbide drills and I have tried to use them as spot drills but because of the profile of the drill tip, the divot drilled out has a flat center of diameter 2 mm and tests have indicated that the resulting hole position wanders.

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.

Just tried it and it WORKS !!!

I tried using a 6mm 120 deg carbide spotting drill instead of 3mm. The drill is the same ( 4.2 mm 130 deg ). It chattered a bit when drilling out the cone but the chattering stopped after that. Tried three holes on mild steel, all have normal finish. The most important of all, the resulting hole position error is within 0.01 mm or 4 tenths of a thou.

 

[Parlo]

If you stone the corners of the drill bit to a small radius it helps with the finish and can bring the size of the hole down a little if cutting oversize.

 

[fcs]

There's a lot of good info for beginners in this thread. About appropriate size center drills, size of pilot drills, speeds for different materials, etc. It would be great if someone could put it all together in a sticky in this forum. (Or maybe there is one?) I'm just a beginner but my interest (making fishing rod ferrules from different materials on a hobby lathe) involve a lot of drilling and I suspect I'm doing a whole lot of things wrong or poorly.