Making a rounded slot with drill bushing?

pianoman8t8 · Dec 6, 2020

I need to put a blind slot into a hole, about 8" deep. See photos for the general idea. Would start with the block (aluminum) that already has a hole, and my thought was to make a sort of drill bushing out of steel, make the slot in the steel bushing via ball endmill, start the slot in the aluminum with an endmill to at least get 2-3x "diameter" slot depth, then proceed with drilling the rest of the way down using the bushing/guide to prevent the drill from walking "away" into the thru-hole. Obviously the guide/bushing would be held in place via some method. My intial thought was sinker EDM, but I don't have one, and even if I did it's time consuming. My 2nd thought was shaper, but there's no provision for a relief of any kind at the bottom nor a way to access it. (block part has been hugely simplified to just show the idea of the guide/bushing).

Do you think this will work?

rblalexander · Dec 6, 2020

And one of the constraints is that you MUST start with the part having the pre-existing hole?

Id consider sacrificial aluminium plugs for each part. Youd be able to reuse then a few times each.

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pianoman8t8 · Dec 6, 2020

rblalexander said:
And one of the constraints is that you MUST start with the part having the pre-existing hole?

Yes, at this point. It's a modification to an already existing part. The slot depth, radially from hole centerline, also can't be allowed to walk "inward" towards existing thru hole by more than, say, 0.010. An aluminum sacrificial slug may work, but I'm suspecting drill walk will become a problem at almost 35xD...

thermite · Dec 7, 2020

pianoman8t8 said:
My intial thought was sinker EDM, but I don't have one, and even if I did it's time consuming.

You probably don't have a womb, either, but "time consuming" or not, humans exist all these long years because of partnering with those who DO have.

Same thing, sinker EDM.

You don't HAVE to "have one". Just arrange the use of it.

If you DO have a shaper or slotter?

Do the slot short of the blind end.

THEN dive-down and finish it with a shorter hardened steel "foot" as the guide to control the last quarter inch or so.

No more "35 D" in the equation as far as CUTTING.

Only a longish reach in free air.

"Page Two":

Given it is a (section-of) a ROUND slot, not rectangular? A clever mill-hand could generate that slot, top down OR bottom up. Just not with a conventional endmill.

I'd want to start at the blind end's bulkhead and back my tooling and chips alike OUT of the blind end towards daylight, coolant-assisted chip-clearing traveling with it. And it would clear best on a horizontal. Less of an argument with gravity.

I'll leave-off that one to the grown-ups....

Page Three;

This isn't sane, economically. The operation is too costly.

Scrap the f**ker for the metal. "Product Engineer" the part.
Eg; make it economically sound to manufacture..

Then have what you REALLY need precision cast, forged... extruded...

..or machined, but with the "blind" end open, then closed up later.

It's only shiney-wood, after all. "How hard can it be?"

Strostkovy · Dec 7, 2020

Are you trying to replace keystock with ball bearings for smoother axial travel?

jscpm · Dec 7, 2020

You don't need a bushing. Use a sacrificial plug.

Turn a piece of aluminum of the same type as the work to fit snugly in the hole and drill and counterbore the blind hole. Then remove the sacrificial plug.

Mtndew · Dec 7, 2020

pianoman8t8 said:
Do you think this will work?

Yes,I've done this many times. Just make 100% certain that your bushing/plug cannot move or rotate or you're gonna have a bad time.

gbent · Dec 7, 2020

I believe I'd try the sacrificial plug. But for starters I'd drill the hole undersize an inch or so deep and bore to size to give a good starting location. I might even be tempted to drill in sections and ream each section before drilling the next.

thermite · Dec 7, 2020

gbent said:
I believe I'd try the sacrificial plug. But for starters I'd drill the hole undersize an inch or so deep and bore to size to give a good starting location. I might even be tempted to drill in sections and ream each section before drilling the next.

Well NOW yer onto a way to doo NEW ones at reasonable cost.

Place the small 35 d hole that becomes the "slot" first - full bearing of material to all sides of it.

THEN drill & bore to intersect it "just so".

Already-existing parts - and probably "too damned many" OF them - are the PITA.

At prevailing US labour & machine costs, fully burdened?

It really COULD be cheaper to scrap and start-over?

Turning a through-bore into a blind-end just ain't all that difficult, either.

Do the math first. THEN make chip, more wisely.

pianoman8t8 · Dec 7, 2020

thermite said:
You probably don't have a womb, either, but "time consuming" or not, humans exist all these long years because of partnering with those who DO have.

Once I saw the thermite had a reply, I knew it'd be a good one.

It's a 1-off pc, prototype. If it goes full production, that will no longer be my problem.

I've done plugs before for similar situations, but not for 35xD...

Even if generated with a not-so-standard cutter, from bottom up ideally, would still need a bushing/guide of some sort I would think, as well as an existing hole in the same exact location. Which brings us back to square 1 I believe.

Maybe a sinker EDM is really the way to go, would need to be able to clear about 20" in height though. A foot or so for part height, plus another 8" or so for electrode length. Unless it's burned along the X axis? I'm not fluent enough in EDM to know if that's a feasible procedure or not.

pianoman8t8 · Dec 7, 2020

Strostkovy said:
Are you trying to replace keystock with ball bearings for smoother axial travel?

No. It's just a slot (with a country mile of clearance) to keep other moving components within the bore aligned.

pianoman8t8 · Dec 7, 2020

Mtndew said:
Yes,I've done this many times. Just make 100% certain that your bushing/plug cannot move or rotate or you're gonna have a bad time.

Thank you, a response to the question asked. Yes, the guide/bushing would be clamped in place as to not move.

pianoman8t8 · Dec 7, 2020

gbent said:
I believe I'd try the sacrificial plug. But for starters I'd drill the hole undersize an inch or so deep and bore to size to give a good starting location. I might even be tempted to drill in sections and ream each section before drilling the next.

I would do this same procedure if I went with the plug, but I wouldn't bother reaming in sections. Seems it would take away side support for the drill? Plus at some point, you'd have no good way to re-centerdrill and line bore.

thermite · Dec 8, 2020

pianoman8t8 said:
Once I saw the thermite had a reply, I knew it'd be a good one.

It's a 1-off pc, prototype. If it goes full production, that will no longer be my problem.

KISS method, then. Mess with a plug as guide on a 35d in soft Aluminum? Could EASILY scrap two outta the first three attempts. Why wuddja?

Do it "though bore" instead.

Because it is waaaay faster to do..

- Use the shaper to cut the slot. Or a broach.

Plug and TiG the bore to create the "blind" bottom. It ain't Rocket Science.

Cheap, fast, and ready for functional evaluation much sooner!

If it fails, they'll try sumthin' else and not a lot of time will have been wasted.

If does what it is meant to do, it will then be up to "Product Engineering" - who should then go to volume production with an ignorant RFQ for a near-as-dammit "no" machining needed precision casting.

TONS of stuff is cast that way every dam' day, nowadays.

Most of it is easily an order of magnitude more complex, if not two orders.

Scottl · Dec 8, 2020

I think the idea is OK up to the part of starting with an end mill but in my experience a standard drill will flex if used with one side cutting empty air. I think your best bet might be a single point drill that is ground so the outer rim cuts first, similar to a boring bar. I'm far from an expert on drill geometry so the other guys will have to guide you there.

And speaking of guidance, make sure your bushing has a collar with a fully round hole to keep the shank of the drill on track.

Strostkovy · Dec 8, 2020

pianoman8t8 said:
No. It's just a slot (with a country mile of clearance) to keep other moving components within the bore aligned.

Will a regular keyway work? Way easier to cut.

Bill D · Dec 8, 2020

Rotary broach, either a small round one with a slug in the big hole or a custom one with a tab sticking out to cut the slot.
Bill D

TeachMePlease · Dec 8, 2020

Bill D said:
Rotary broach, either a small round one with a slug in the big hole or a custom one with a tab sticking out to cut the slot.
Bill D

If we're talking about the same kind of rotary broach, I don't think it's going to stay straight at 35X D.

Bill D · Dec 8, 2020

You are right. I think rotary broaching is only good to something like 1.5 x diameter. I missed how deep his hole will be.

pianoman8t8 · Dec 21, 2020

Just a follow up, initially tried with a long twist drill, which unsurprisingly walked into the 4140prehard bushing (30-35HRC) by roughly 0.010" to 0.015" over about 3". Switched to a gun drill, and that went slicker than sh*t with the same, new bushing. Nice and straight, about 8.5" deep. It did walk into the bushing a bit, but only about 0.003" over 8.5", which is very acceptable for this particular application.

Making a rounded slot with drill bushing?

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