Tapping aluminium

[blackrose]

Any hints or advice about tapping aluminium please? 

 

I want to drill and tap some holes in some 6mm thick aluminium solar panel brackets I've made. I looked up the hole size and it's supposed to be 6.75mm for an M8 thread but I've only got a 6.5mm drill. I'm thinking that because aluminium is softer than steel I'll be able to get the M8 tap through easily, or is that not the case?

 

Also I've only got one M8 tap and it looks like the final one. I don't have a taper or middle. I'd like to get the job done tomorrow morning before the wind and rain set in, unless I have to get the right size drill and more taps in which case I'll have to wait. The panels are just sitting on the brackets which I've stuck to the roof with Stixall, so I need to get them attached as soon as possible before the wind picks up again.

Edited October 18, 2022 by blackrose

[Tracy D'arth]

You will struggle with a plug tap, it won't start straight very easily. The drill size will make it worse.

I would grind a taper on the tap making sure that you have a lead angle and that all flutes are equal. Clear the hole well with the drill, a bit of a wiggle to open it slightly. I use 7mm drill for 8mm threads, makes it a bit easier.

A tip I was told years ago was to use paraffin as a lube on aluminium.

[blackrose]

Thanks. I have a 7mm drill. 

 

Would you expect the threads to last in aluminium with a stainless bolt if it's not screwed in/out that often, or will soft aluminium threads wear out quickly?

[BEngo]

It will wear, but the biggest risk is over tightening  which will strip the thread in soft aluminium much more easily than in steel.  M8 is reasonably coarse, so nog badly prone, à but don't use a drill/electric screwdriver, do it with a hand screwdriver or small spanner.

 

If the thread does get a bit slack with use, araldite some studding (allthread) into the hole, and put a full nut,  wing nut or plastic knob with M8 hole on the studding.

 

N

[blackrose]

Ok thanks both.

[dmr]

I think I would stick with your 6.5mm drill rather than the 7, but wait till you can get the taper, it will be too risky with the plug tap, you might well end up cross threaded and as Ali is soft it will all go wrong.

[blackrose]

Ok I'll get hold of an 8mm taper

[Eeyore]

Yep, paraffin to lubricate, and some of https://shop4fasteners.co.uk/duralac-jointing-compound.html  to prevent any corrosion issues further down the road.

[Bob Chamberlain]

Stainless steel and aluminium can have a weird reaction together believe it or not!  That's why not a good idea on old Land-Rovers (I know Birmabright is a 'special' aluminium alloy) but you might be better with brass screws into the ally for doing term stuff?

[Ronaldo47]

Thete is normally no single "right" drill size for tapping metal. There are tables setting out the diameter of drill for the depth of thread. Here is the range of drill sizes vs. %depth for ISO -M8 from my copy of Engineering's "Guide to World Screw Threads" book:

 

6.5mm  98%

6.6mm  91%

6.7mm  85%

6.8mm  78%

6.9mm  72%

7.0mm  65%

 

For harder metals like cast iron, larger sizes are preferred:  smaller sizes can be used with softer metals.  So your 6.5mm drill should be fine on aluminium as long as you use a taper tap. Your recommended 6.75mm is right in the middle of the range, and is a compromise diameter that should be ok for tapping any metal.

 

Taps specially designed for rapidly tapping aluminium have spiral flutes that allow the removed metal to emerge as single piece spirals rather than chips. With straight flutes, periodic reversal to remove swarf is usually necessary as aluminium can otherwise clog the flutes and damage the profile of the tapped thread being cut by tearing the metal rather than cutting it  

 

 

 

Edited October 19, 2022 by Ronaldo47
typos

[BEngo]

That is all true, but neither a spiral flute, nor a spiral point tap is of much use in  hand tapping.  They are designed for use in a tapping  machine, and have little or no lead.  This makes them blloddy difficult to start, by hand, let alone start them square.

 

N

 

[Ronaldo47]

True enough: I really only mentioned spiral taps  to highlight that, when hand tapping aluminium, it is inadvisable to try using brute force to tap thick metal in a single pass, especially  when using a small diameter hole.

[cuthound]

If @Blackrose has concerns over stripping the thread Inthe aluminium brackets he could always drill oversize and use a Helicoil should his thread strip.

[jonathanA]

I just wondered if he might be better just using 6mm into 6mm ally rather than 8mm - I'm thinking more threads in the thinish ally. 

[BEngo]

A standard M8 nut should be 6.5 mm thick, and has a bit of thread run out either size.  Given that the idea is for the nut to be the weakest part, I would not expect Blackrose to end up with anything less strong than a nut and bolt, provided  he is tapping a well formeda thread  into 6mm plate.

 

M6 would certainly be less strong than M8.

 

N

[Ronaldo47]

I recall reading that the coarser UNC threads (or even Whitworth) were preferred to the relatively finer standard Metric threads for tapped holes in  Aluminium. The camshaft cover of my Hillman Imp's engine was secured by studs with a 1/4- 20 UNC thread on one end  for screwing into tapped holes in the Aluminium cylinder block, and 1/4-28 UNF threaded at the other end for a steel hex nut.  The official Rootes workshop manual has a section on repairing stripped threads using Helicoil inserts, and the kit if parts that were needed, not that I had the need to use it.  

 

We used to use Helicoil inserts with aluminium alloys for military stuff at work in the 1970's, but they will be unlikely to be an economic option for occasional use by the DIY-er. As well as the inserts themselves, you need special taps and insertion tools for each screw size.  

[dmr]

UNC is a bit out offaashion these days. 😀

Fitting Helicoils used to be specialist stuff, and somebody always knew a man who could do it for you.  Nowadays kits are available on eBay etc for not much money at all, don't know how good they are, have not needed one yet, but it looks relatively easy.

Although Helicoils are sometimes seen as thread repairs, as you say, they are also used for strengthening new threads. The famous VW air cooled crankcase is a good example.

 

On thread pitch and material thickness, "4 threads" is a rule of thumb and works for me, but maybe a bit more in Ali might be wise.

[agg221]

When I am repairing stripped threads on cast iron, which seems to be an all too common occurrence (not because I have stripped them but because they have already been stripped and bodged and I am trying to sort them out properly) I tend to drill an oversize hole to the next common size up (so for M8 that would be M10 etc) and either use a bolt or thread a piece of bar part way, and screw it in hard enough to jam where the thread ends, or use some Loctite, and saw off flush. If the original hole was central I pre-drill and tap the internal thread to the original size as it makes sawing and filing it off in situ easier, but if not I leave it solid so I can correctly dot the centre (if that's too far off then I might need to go bigger than one size up, so say M12). Unlike helicoils, this gives a solid thread which is far less prone to damage, but if it needs to be unscrewed and replaced it can be. It is also much cheaper than buying various helicoil kits. If the material is a bit thin but with space behind, I would do the same with a top-hat shaped insert, either a turned one or a cut off set screw. That way it won't pull through.

 

One other point on tap drill sizes - I find if I go for the smaller drill sizes for near full thread engagement then on softer, more ductile metals it tends to flow a bit both up and down the thread, rather than just cut. That leaves a slight upstand of material so things won't sit flush to it without either filing off or countersinking. I tend to use a countersink, rotated by hand, to just clean off the lead in.

 

Alec

[Ronaldo47]

A good point. Now I come to think of it, the tapped bores  in my cylinder head were counterbored about 1/8" . The counterboring certainly  helped with inserting the studs. I guess it might also have reduced sny tendancy for the soft metal around the bore to swell up when a stud was tightened, although they were only torqued to 6 lb-ft. 

[ditchcrawler]

11 hours ago, dmr said:

UNC is a bit out offaashion these days. 😀

Fitting Helicoils used to be specialist stuff, and somebody always knew a man who could do it for you.  Nowadays kits are available on eBay etc for not much money at all, don't know how good they are, have not needed one yet, but it looks relatively easy.

Although Helicoils are sometimes seen as thread repairs, as you say, they are also used for strengthening new threads. The famous VW air cooled crankcase is a good example.

 

On thread pitch and material thickness, "4 threads" is a rule of thumb and works for me, but maybe a bit more in Ali might be wise.

I used one a couple of years ago when the 5mm thread failed holding my cabin side step. It all went very well. drill, tap and a little tool to wind the springy bit in.. I bought rivnuts as well in case it didn't hold, but so far all good. https://www.ebay.co.uk/itm/264319793601