M3 blind thread in ali about 4mm thick. How?

I need to repair a CNC machined part that has four M2.5 tapped holes that have stripped and corroded threads. At most the (hard anodised) ali is 4.5mm thick and the tapped holes have to be blind to maintain the sealed watertight nature of the housing.

I imagine this would be best done by thread milling but I wonder if I am likely to succeed using conventional taps by grinding off the lead of the tap/s in stages?

Helicoil or thread insert, back to M2.5 would be preferable but its still a very shallow hole.

Ian P

I know you say it has to be watertight but could you tap a through hole and then use a small threaded plug sealed from the other side or maybe plug it with an epoxy resin of some sort?

I'd thread mill it.

Before that was an option I threaded some small flat bottomed holes less than 1D in depth by gradually grinding the end off a tap until I had near full thread to full depth, it's a long, boring and fairly exacting job and it will cost you the tap but it's entirely do-able.

There is quite a bit of mechanism and moving linkages behind the holes and its all been threadlocked after it was setup and adjusted so I would rather not dismantle it. Threadmilling would be good but its beyond my capabilities so I will go down the sacrificial tap route.

Fortunately whilst the housing is all curves and 'organically' shaped and would be difficult to mount on a machine. these four holes are on one face and are accessible.

Ian P

Following the helicoil idea - drill out to say 4mm. Use a 4mm end mill or make a D bit to get a flat bottom to the hole. Then make a plug tapped through at the original M2.5 and loctite in place.

I've done several M3 blind holes 6mm deep in 6082T over the last few days.

Drill 4mm deep (use a collar!). If you take just the very tip off the M3 bottoming tap you shoudl eb fine, and not even ruin the tap. I doubt that it is worth altering the first and second taps.

Do use lubricant and proceed very gently.

Use a spiral flute tap. If it has a point from the manufacturing process grind it off. That should get you with a pitch or so of the bottom of the blind hole.

In addition you should locate the designer and smack him round the head for creating such a poor design in the first place.

Andrew

Good suggestion about using a spiral point tap, I do have one M3 with the point already ground off. As it happens Roger's idea of fitting a parallel plug has some merit as the part that these screws hold in place is plastic and only has very small lugs so drilling them out to M3 clear will seriously weaken them.

The designer will be somewhere in China, the product is a good design and well made except for these small fixings (M2.5 SS capheads) in combination with sea water. The ali casing is all hard anodised but it only takes a minute pit or a scratch to create perfect conditions for galvanic corrosion to wreak havoc.

Ian P

An off the wall suggestion, that I've not tried myself, and may not work with such a small diameter hole.

Several Model Eng, and car shows have an exhibitor who demonstrates alloy soldering using a low melting point alloy filler rod.

Part of his sales spiel relates to "casting" a new thread into a stripped hole. Essentially melt the filler rod into a hole, insert the relevant sized bolt (maybe pre-heated) into the molten alloy and allow to cool. Unscrew bolt and leave behind, one newly formed threaded hole.

Bill

Edited By peak4 on 15/06/2018 00:30:36

Edited By peak4 on 15/06/2018 00:30:26

1.5 x dia helicoils are available, so do the helicoil route. If it is non load bearing then 1.5 x Dia. thread would be fine.

As Andrew said spiral FLUTE will lift the swarf out of the hole and they also tend to have less of a lead in taper than spiral POINT ones.

Plunging the holes with a 2 or 3-flute 2.5mm milling cutter will also give you a bit more thradable depth.

The holes are already flat bottomed, also as its very difficult to measure the wall thickness on three of the four holes I am reluctant to live dangerously by deepening them. I am also not keen on standard Helicoil inserts into aluminium that is going to be used in seawater. I know that there are inserts manufactured in other materials (Titanium, bronze, inconel?) but I imagine they would be hard to obtain.

If I had the patience (or CNC kit) I would make a solid insert with 'ears' or lugs which engaged with undercuts in the recess, bit like a bayonet lamp. Installed with epoxy or a locking compound.

Ian PO

In such circumstances, provided there is space behind, how about a blind 'Rivnut'?

Regards,

Richard.

A helicoil creates the challenge of tapping an even larger diameter hole than using an M3 tap, so the basic problem doesn't go away.