Countersinking radial holes

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strud
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Countersinking radial holes

Postby strud » Mon May 11, 2020 10:11 am

As part of my 4" PoC build, I'm testing out different build processes and trying to improve on the way I do things.

Something I've been wanting to do is countersink the radial holes in the tank and couplers etc to reduce the stuff 'hanging out in the breeze' so to speak.

Problem is the countersinking tools I have are probably good for steel but maybe not so great for aluminium.

Secondly when you countersink holes, the tolerancing for the hole positions gets a lot tighter.

Anyway, I had the tooling for the mill to do the job (rotary table, active centre etc) and once that was all setup, I proceeded.

Image

1. Spot holes (20deg pitch) with still centre drill (don't want any wandering)
2. Drill through at 4mm with stub drill
3. Countersink to depth such that screw head is just shy if surface

So I had purchased some single flute bits supposedly for the job, but they tended to smear the aluminium more than cut it :( :evil:

I dug out an old tool (the single cross hole type) and it worked much better although some smearing rather than cutting was still occurring, even with lubricant.

Result with 'better' countersinking bit (still not cutting creat)

Image

Anyway, I had to go a bit deeper than I had expected, but turned out well after I put it back into the lathe to skim off the 'smeared out' lip around the edges.

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Installed onto forward end motor bulkhead

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Showing the screw heads flush compared with the low profile cap screws

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So there are too many steps in this process, far too time consuming. 'Tedious' is a good word to describe it.....

Anyway it seems there are combination tools out there that are effectively a centre drill with the correct angle that can act as a single pass tool.
Something like this: Image

strud
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Joined: Thu Oct 02, 2008 7:37 am
Location: Central Tablelands, NSW

Re: Countersinking radial holes

Postby strud » Wed May 13, 2020 6:38 am

So I ended up just buying some good quality countersink tools from P&N and what a surprise, they worked really well.....

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No smearing or cleaning up on the lathe afterwards etc.... happy.

Will keep my eye out for a suitable combo tool to save time and hassle, but nothing found thus far.

Jim-K
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Re: Countersinking radial holes

Postby Jim-K » Sat May 16, 2020 11:55 pm

What are the loads you are looking to react with the fasteners?

From memory, the remaining cylindrical bits of the holes do the work rather than the conical bits. Do your fasteners work hard, or could you work with smaller ones? You might get away with counterbores rather than countersinks?

Jim K

strud
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Re: Countersinking radial holes

Postby strud » Mon May 18, 2020 7:47 pm

Hi Jim-k

The only real significant loads will be during recovery and the highest likely would be drogue deployment at apogee.

This is a little hard to determine but some simulations have this at about 250m/s/s*8kg = 2000N but attempts will be made for this to be very much attenuated by using a reefing ring and line energy dissipation methods.

Even so, the fasteners used (12 x M4 grade 12.9 bolts in front end of coupler, 18 on the interface with motor forward closure) will far exceed this loading. The main reason to use this arrangement is to eliminate clearance/slop and to provide high stiffness such that the airframe stiffness is not compromised by any of the interfacing connections.

To answer the other part of your question, the countersunk holes provide a tapered interface hence remove any clearances ie higher overall stiffness.

ogivemeahome
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Re: Countersinking radial holes

Postby ogivemeahome » Wed May 27, 2020 8:06 pm

If memory serves, (it's been a while) the "single angled hole" type c'sink tool works well if the job is not clamped - use low speed and watch it all orbit the table - not so good for accuracy. On aluminium especially, you need a sharp edge (freshly sharpened on a machine - hand sharpening is rarely accurate enough for true concentricity). Three flutes are better than two or four. Increasing the rake angle helps (but not for plastic or wood).
A smooth sea never made a skilled mariner. (English Proverb)


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