Deep drilling

[Speedy Builder5Participant @speedybuilder5]

I need to drill some deep holes in Al Alloy (I don’t know the specs, but it machines really nice).  The holes are 3.5mm diameter by 75mm deep.  I can drill from each side. Once drilled, I need to transfer the position of the holes and  tap into a second component.

What is the best way to prevent wander.

Does the the drill speed faster or slower help ?

Bob

[renardiere7Participant @renardiere7]

Mill or Lathe?

[Speedy Builder5Participant @speedybuilder5]

mill/drill, lathe.

[JasonBModerator @jasonb]

I’m not a fan of marking out so would locate the three holes on the face that will mate to the tapped part by finding edges and then use the millshandwheels or DRO to position. Start with a spot drill (ctr drill if you want). Then a stub drill which is nice and stiff to get teh hole started true. Then jobber length and probably have to finish with a long series drill. just drill from that one end.

Somewhere around 2000rpm, paraffin or WD40, peck drill and clear the swarf often. Should not need to say sharp well ground drills.

I also don’t much care for spotting through hole positions so again would us eedge finders and then set the position of the tapped holes with the mill.

[DiogenesParticipant @diogenes]

Use the correct speed for a 3.5 drill in Ali, failing that the highest available.

start with a stub drill, then a jobber, and or a long series if required.

clear the chips often – very often, as you get deeper (peck 1D and then clear) and let the drill do the cutting.

Use bright drills if you have them and use paraffin / diesel / wd40 for lubricant

[HuubParticipant @huub]

In short, prevent the drill from bending, start at the exact center of the hole drill from both sides if possible.

That results in:

To much tool pressure will bend the drill so very modest tool pressure.
Good chip evacuation is an absolute must. So peck drilling is needed.
Start using a center drill.
After that use a smaller size drill (3 mm) the shortest you can get.
Drill the hole to just over half the depth, each time using a longer drill.
Bring the hole to diameter using the same procedure
Finish the hole using a reamer preferably from one side only.

An end mill is often more rigid than a drill. Start drilling using a drill and bring the hole to diameter using an end mill is also possible. 4 Flute end mills have a larger core diameter than 2 flute end mills and are more rigid. So the 4 flute end mill is better.

Whatever you do, it is quit a challenge.

 

[Speedy Builder5]

On 23 July 2026 at 14:22 Speedy Builder5 Said:

I need to drill some deep holes in Al Alloy (I don’t know the specs, but it machines really nice).  The holes are 3.5mm diameter by 75mm deep.  I can drill from each side. Once drilled, I need to transfer the position of the holes and  tap into a second component.

What is the best way to prevent wander.

Does the the drill speed faster or slower help ?

Bob

Use a new long series drill of known decent make.

Centre drill the location or use a spotting drill.

About 2000 rpm with minimal feed, peck it every 2-3 mm with WD40 as lube, clear the flutes with a small brush, blow out the hole with comp air, more WD40 down the hole, then back down the hole again with the drill, rinse and repeat, be patient, don’t try to rush it.

Try the speed out on an offcut if possible first, if the metal welds to the drill, cut the speed down.

 

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