Boring through unknown steel with a Nomad

CNC Machines Nomad
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So I managed to break off an M4 thread tap in a chunk of 7075 Aluminium and I’d like to get it out.

This is my first time milling steel, let alone boring but I set up a bore operation in HSMWorks:

  • 1/8", 0.5" LOC, 2-flute uncoated C3D endmill (102? 112?)
  • 5mm hole diameter
  • 0.05mm pitch
  • 200mm/min feed
  • 6k RPM

I’ve so far got to ~6mm deep and it’s quite loud but it is cutting; a good chunk of the thread tap has been removed.

Does anyone have any suggestions on how I could improve this?

Before anyone asks, I have no clue what kind of steel this is. It came in this set, which seems to be Chinese.

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The 0.05mm pitch looks good, 6k might be a little fast depending on type of steel. Also I would recommend a 4 flute endmill, a 2 flute can work but it’s going to wear out very fast.

Are you running coolant because cutting through a hard tap its going to get hot really quick.

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How low would you go on RPM?

I do have some 4-flute endmills I could use but they’re a bit nicer and I’d rather not sacrifice them. The 2-flute endmills I’m using are ones I doubt I’d use for anything else so I’m fine with breaking them.

The only coolant I have is airblast. Not much more I can do without making a mess. This is a fairly hefty chunk of Aluminium though, it should act like a heatsink, shouldn’t it? I also felt the endmill after cutting and it wasn’t hot at all.

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lowest I would be around 4000.

If you do have 2 flute endmills your are willing to sacrifice you can use those, you will just have to feed them slower then you would the 4 flute.

Air blast should be fine, on hardened tool steel airblast actually works better then flood coolant.

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5

Why use a tool?

Use chemistry and simply dissolve the tap and leave the aluminum behind undisturbed.

Get some alum (aluminum potassium sulfate), mix it up (directions are easy to find), keep warm/apply heat, and wait, then clean things up.

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Damn, that stuff sounds amazing. I never knew it existed.

Unfortunately it doesn’t seem to be easy to get a hold of here. I read that in the US you can buy it in the supermarket but here in Switzerland it only seems to be available from specialty shops.

I find it hilarious that I can buy it from a cosmetic store though.

I’ll try to continue milling it out, otherwise I’ll try to buy Alum and give it at least a few hours and see how it goes.

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There’s a chemical for the opposite direction as well — lye (often found in drain cleaners) will take alu. galling right off an endmill.

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Never thought chemistry could be this useful…

And noodle and pretzel-making… The latter makes it a lot easier to get a hold of around these parts (drain cleaner is actually some kind of restricted substance here, you can only buy it from a pharmacy).

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My landlady used to use lye for stripping furniture when she wasn’t selling vintage clothing from her antique shop — dropped in to pay my rent once and almost ran into Johnny Cash and his wife.

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Okay, I managed to bore the rest of the tap out:

  • 2-flute 1/8" endmill
  • 4k RPM
  • ~91mm/min feed rate
  • 5mm bore diameter
  • 0.05mm pitch

The Nomad was not happy with the task it had been given:

  • It vibrated like crazy. Chips were dancing the whole time.
  • The spindle stalled a few times (but started back up).

But this time the endmill wasn’t chipped, which is nice.

Since I still need an M4 thread here, I’m going to bore out the hole to 6mm and then put a thread insert there. I think that’s what I’m going to try to do for most of my threading needs, as it’s easier to fabricate and produces more durable threads.

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The thread insert is a good way to go.

In the past I’ve deliberately drilled out aluminium engine blocks and heads on racing kart engines to put helicoils in them to get a decent grip. After the first few rebuilds the threads tend to get weak.

Edit - Also, if you don’t have one already, get a spring loaded tap guide to use in a drill press to hold the tap vertical whilst you’re working, I find that very useful on the small taps.

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For aluminum I default to thread forming taps:

On my old Bridgeport I’d drill, chamfer, and then with a drop of gear oil on the tap, hand feed straight in at 60 rpm.

Note, the drilled hole is larger than normal for a forming tap.

Bob

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