Drilling Accurate Holes in Aluminium – CNC or Pillar Drill???

In this video I test drilling smaller diameter holes in aluminium using two different type of bits on the CNC machine and the pillar drill. So in the last video where I cut out a test segment of the CNC machine I’m working on, I noticed drilling with a carbide single flute bit wasnt producing accurate holes. I decided to buy a set of cobalt stub drill bits the exact sizes I needed to use as an alternative.
The cutting angle is 135 degrees. I thought these were Europa ones but they are osborn sheffield. They are quite sharp. I just did a test on the pillar drill and it drilled pretty bang on. It’s really nice. And I’ve bought a few different sizes. Essentially I’ve bought all the exact sizes I need for the CNC machine I’m working on.

My thinking was I could either drill the holes out on the CNC machine or mark spots up on the CNC and drill the holes out on the pillar drill. I tired to drill holes using those stub bits with pecking and a step down from 0.5 to 0.1, with the spindle running as low as 900rpm but every time the bit plunged, no matter how little they were cutting into the aluminium it would always shudder.

Despite having made my CNC machine from acetal it is pretty accurate when cutting in softer material, but what’s unusual is if I cut on the outside of the vector in aluminium the parts were coming out really well but drilling holes would not produce the same consistency.

That’s got more wobble to it than the one I did on the pillar drill. And that’s actually 7.3mm and this is 7.2.

I also tried to use the larger stub drill bits to mark out two spots 10mm apart on the CNC machine which I could then drill out on the pillar drill. This is the 4.2 mm stub drill bit I’m going to be using. I’m going to use the 4.2mm holes to tap a M5 thread. And of cource if I was actually going to do this on the CNC plates I’m making I would check the bed first with a dial gauge indicate. Make sure it’s nice and level.

I drilled the holes out using a 4.2mm bit and the distance between them should have been 5.8mm… I got 5.9

I decided to go back to the single flute bits and this time use a ramping tool path where the bits moves into the cut with an angle instead of plunging straight down and this produced much better results. The bit wasn’t shuddering as it did when I rushed to cut the test segments.
However the holes holes were a lot smaller than they needed to be. Now I could drill the holes out to the correct size on the pillar drill but when I did a bit of research into this – I was concerned that the bit might not centre itself correctly into the holes. I was always taught that when you drill into metal you should work your way up to the size you need. But in a lot of forums and placed I’ve read online it was suggested you should use a spot drill first, and then use the exact size you want as the drill stays more true from the centre. Anyway I would be interested to get peoples opinions as I’m a bit out on this. I personally think if the hole is 0.1-0.2mm smaller than it needs to be and I drill it out to the correct size with the stub bits on the pillar drill – as long as I’ve correctly tramed the pillar drill base (and pocket 83 has a great video on how to do this) the margin of error should not be notable on the final CNC plates.

Anyway back to ramping – what I gathered for drilling holes this way, is I should use cutting bits which are the step-over size smaller than the hole I am trying to create. So for example If I need a 4.2mm hole, and my step-over for aluminium is 20% of the bit diameter, I should use a bit that is less than 3.15mm wide so I could use a 1/8 of an inch bit.

I also created a hole with two separate tool-paths were I first area cleared an opening with ramping, and then cut along the inside of the vector. What I noticed, was while the second tool path was cutting I could hear it catching more aluminium at the front of the hole, closest to the camera. A combination of factors had resulted in some deflection during the first pass as the bit was removing a lot more material, which the bit could reach the second time around.

That feels a bit better.

I then noticed something else. When I placed the eccentric spacer into the hole I created. It would pinch at a certain point while I turned it.

It feels like it’s a little bit tighter in one position when I’m turning it. Its quite fluid there but when I get to this angle it tightens up. So maybe this isn’t a perfect circle?

I thought my hole might have cut out with an ellipse but on inspection with the callipers it turned to be the spacer itself was ever so slightly elliptical.

7.0 that way and 7.2 that way. So it’s not my hole. 7.17. 7.14. So the hole I drilled on the CNC machine with the single flute carbide bit, and the hole that I finished up on the pillar drill are consistent. They are pretty round and a lot more round than this section here of the eccentric spacer.

Anyway I still haven’t made my mind up as to how to start cutting out the holes in the new plates, but I am swaying toward using bits that are around 20% smaller than the holes I want to create with a ramping tool path. But if you have any suggestions I am curious to know.

Anyway thanks again for watching and please let me know if you have any suggestions. don’t forget to sacrifice a thumb to the algorithm gods and I’ll catch you in the next one.

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