Drilling and tapping into aluminium

[eddystone]

Having decided to go for a screw in sail entry gate I'm going to have make a couple of threaded holes in the aluminium mast. I have zero experience of this and no general metalworking skills as evidenced by my attempts at drilling out rivets.

The Selden instructions say use a 4.2mm drill and an M5 tap. I have a few beginner questions;

1. For a clean hole in aluminium would you use an HSS, Tn or Cobalt bit?
2. If i used an undersized drill, i.e. 4.0mm, would i reduce my risk of oversizing the hole for the tap?
3. Is M5 normally 0.8 thread pitch or is it possible Selden use something coarser/finer?
4. I understand taps come in three types, taper, plug and bottoming; I know I don't need the last one but can I start a clean thread with the intermediate one alone?
5. How critical is the quality of the tap - does £5 or £6 sound about right?
6. What is the best type of T-handle to use, bearing in mind I probably won't use it again.

Sorry if this is a bit OTT but if I make a hash of it, I imagine I would need a pretty expensive mast repair (if that is possible).

 

[Motor_Sailor]

A lot of questions!

As for drill size, the general rule is you subtract the bolt thread pitch size from it's diameter to get the pilot drill size. So M5 is 5mm less the pitch (.8mm) equals 4.2mm which is what they recommend. A decent toool shop (or Axminster Tools online) will let you have a 4.2mm drill.

But with thin walled aluminium you could join all the other boat owner bodgers and use a 4mm drill, then start with a taper tap and work it backwards and forwards until it cuts the thread. Then wind it all the way through so you're using the full thread at the top of the tap and that's all you need.

Any T- handle will do. It might be worth buying a reasonable tap and die kit (Aldi recently had them) as this won't be the last thread hole you need on a boat.

 

[anoccasionalyachtsman]

With Alu alloy any sharp drill will be fine.

A drill chuck can actually be a better than a T handle for a tap because 'square' is often more obvious.

EDit: Chuck turned by hand unless you're in a real hurry to mess it up.

 

[Chalker]

I usually use a cordless drill in place of a T handle in aluminium. 4mm drill, No1, then No2 tap. Slow speed.
Taught by a NZ rigger that I trust.
My gooseneck and gas strut are held this way with 8mm macine screws rather than rivets

Yes, I was taught the proper way 55 years ago!

 

[Boo2]

1. For a clean hole in aluminium would you use an HSS, Tn or Cobalt bit?

Any of those will do for ally but HSS drills are likely to be good quality and to make a closer to the right size hole so I would use that.

2. If i used an undersized drill, i.e. 4.0mm, would i reduce my risk of oversizing the hole for the tap?

Taps snatch like a b*****d in ally and I personally go for a god quality 4.5mm drill rather than the full-engagement 4.2mm. I would under no circumstances go below 4.2mm or you stand a good chance of breaking the tap

3. Is M5 normally 0.8 thread pitch or is it possible Selden use something coarser/finer?

According to Wikipedia there are two common thread pitches for M5, you will have to ask Selden which they use but I'd be prepared to bet it is 0.8mm.

4. I understand taps come in three types, taper, plug and bottoming; I know I don't need the last one but can I start a clean thread with the intermediate one alone?

Yes, depending on the exact alloy, taper taps are generally useless on aluminium as the entry is so gentle that it is hard to stop smearing out the threads when using one. I'd only buy a "second" tap for ally. Note also that "plug" tap means "bottoming" tap to some people and "second" tap to others...

5. How critical is the quality of the tap - does £5 or £6 sound about right?
6. What is the best type of T-handle to use, bearing in mind I probably won't use it again.

I'd buy this tap and this wrench. Both from Chronos who are good guys IME (no relation). Use kerosene or paraffin or wd40 to lubricate the tap while using it.

Hth,

Boo2

 

[Yngmar]

Meh, threads in masts. If there's no way to rivet it instead, make very sure you apply some silicone grease every single time you put those bolts back in, else the corrosion will gnaw off the alu threads (the screws are presumably stainless steel).

I have tapped a mast on a friends boat, but it wasn't my preference (guy sent to fetch the rivets had the wrong length and they were heading off soon). It's okayish for light loads (lazyjack cleats) that don't get undone regularly. Always use a bedding compound - Duralac if permanent, silicone grease if not or no Duralac to hand.

Having decided to go for a screw in sail entry gate I'm going to have make a couple of threaded holes in the aluminium mast. I have zero experience of this and no general metalworking skills as evidenced by my attempts at drilling out rivets.

The Selden instructions say use a 4.2mm drill and an M5 tap. I have a few beginner questions;

1. For a clean hole in aluminium would you use an HSS, Tn or Cobalt bit?
2. If i used an undersized drill, i.e. 4.0mm, would i reduce my risk of oversizing the hole for the tap?
3. Is M5 normally 0.8 thread pitch or is it possible Selden use something coarser/finer?
4. I understand taps come in three types, taper, plug and bottoming; I know I don't need the last one but can I start a clean thread with the intermediate one alone?
5. How critical is the quality of the tap - does £5 or £6 sound about right?
6. What is the best type of T-handle to use, bearing in mind I probably won't use it again.

Sorry if this is a bit OTT but if I make a hash of it, I imagine I would need a pretty expensive mast repair (if that is possible).

1. Doesn't matter what the bit is made of, but it must be sharp. Aluminium is soft, so easy to drill, but also easy to make an oval out of with a dull bit. Use cutting oil/spray and a rag to collect the filings before they're tread permanently into the deck.
2. No, just increases the chance of snapping off the tap. Sharp drill bit!
3. http://www.metrication.com/engineering/threads.html
4. Cheaper kits only have one type, and work fine in this situation (soft metal, through-hole). Differences illustrated: http://www.kmstools.com/blog/wp-content/uploads/2012/01/Taper-Plug-Bottom.jpg
5. As long as its not total crap and will snap right off, pretty much any will do. This is the kit I've got on board and used. Works sufficiently fine with some care (i.e. back up often).
6. One that you actually have room to turn! On a mast there is usually lots of kit interfering with turning a wide handle, so you often end up using something smaller, or the already mentioned drill chuck.

Good luck

 

[eddystone]

Wow, brilliant information thanks; this thing only needs to be removed twice a year i.e. when I take the main off so I was going to use Duralac as I didn't think silicone grease had any anti-corroion properties. Only reason I asked about type of drill bit was only previous experience of drilling into aluminium was through the traveller to insert a steel retention pin (didn't know about duralac then so my traveller is probably dissolving as we speak!) for end stops - I broke a lot of drill bits but it was a very awkward angle.

 

[AndrewL]

It is very common on bikes to use aluminium bolts for fittings that attach to the frame, such as bottle holders. They are normally M5 and you can buy them cheaply in any bike shop. That would remove any corrosion worries.

For example http://www.cyclomonster.com/token-aluminium-bottle-cage-bolts-4-pcs

 

[PaulRainbow]

Meh, threads in masts. If there's no way to rivet it instead, make very sure you apply some silicone grease every single time you put those bolts back in, else the corrosion will gnaw off the alu threads (the screws are presumably stainless steel).

I have tapped a mast on a friends boat, but it wasn't my preference (guy sent to fetch the rivets had the wrong length and they were heading off soon). It's okayish for light loads (lazyjack cleats) that don't get undone regularly. Always use a bedding compound - Duralac if permanent, silicone grease if not or no Duralac to hand.



1. Doesn't matter what the bit is made of, but it must be sharp. Aluminium is soft, so easy to drill, but also easy to make an oval out of with a dull bit. Use cutting oil/spray and a rag to collect the filings before they're tread permanently into the deck.
2. No, just increases the chance of snapping off the tap. Sharp drill bit!
3. http://www.metrication.com/engineering/threads.html
4. Cheaper kits only have one type, and work fine in this situation (soft metal, through-hole). Differences illustrated: http://www.kmstools.com/blog/wp-content/uploads/2012/01/Taper-Plug-Bottom.jpg
5. As long as its not total crap and will snap right off, pretty much any will do. This is the kit I've got on board and used. Works sufficiently fine with some care (i.e. back up often).
6. One that you actually have room to turn! On a mast there is usually lots of kit interfering with turning a wide handle, so you often end up using something smaller, or the already mentioned drill chuck.

Good luck

+1 for all of the points here, especially the lubricant, most important.

 

[BabaYaga]

The thread on the M5 bolts is normal, coarse, 0.8mm.

The alloy is very soft, therefore the threads are easily damaged if you wobble while turning the tap. Personally I find it easier to use a screw driver type handle than a T-handle.

One thing to keep in mind: The holes in the fitting where the two bolts sit are also threaded. So when you put the fitting in place, you do not want a distance between the fitting and the surface it is attaching to. Therefore be careful to unscrew the bolts as much possible before mounting and go lightly, especially the first time.

 

[PetiteFleur]

Rather than tap the aluminium, and as you want to remove it every year, I would use a Rivnut/Nutsert as they have a longer thread engagement. They have a splined shaft which is knocked into a drilled hole in the mast. You then 'collapse' the end of the rivet inside the mast with a lubricated M5 hex head screw which clamps the nut to the mast wall, the screw is then removed. You can get these in stainless, used with Duralac should last for years. That is what I used for my mast gate. Only possible disadvantage is that the insert sits a millimetre or two.

 

[lw395]

Rather than tap the aluminium, and as you want to remove it every year, I would use a Rivnut/Nutsert as they have a longer thread engagement. They have a splined shaft which is knocked into a drilled hole in the mast. You then 'collapse' the end of the rivet inside the mast with a lubricated M5 hex head screw which clamps the nut to the mast wall, the screw is then removed. You can get these in stainless, used with Duralac should last for years. That is what I used for my mast gate. Only possible disadvantage is that the insert sits a millimetre or two.

I was going to suggest that if the tapping went off-piste.
In practice, tapping a thread in a mast section is not hard.
Use a decent HSS tap. Dormer, Warrior, other good brand. There are some absolutely fine Chinese taps on ebay.
There are a lot of crappy sets on market stalls, ebay and glorified pound shops.
Practise on a bit of scrap.
Use a lubricant, any is very much better than none.
On thin(ish) ali sections, tapping drip diameter is not hyper critical, the taper tap will ream the hole within reason. If it won't go in, ease with a file or something!
M5 is small. Not much torque is needed to tap. Back off if it goes tight, half a turn back to clear swarf.
And when you put the screw in, use either duralac, loctite or copper grease. Bare stainless and ali is DOOM.

Breaking a tap in the work is always embarrassing awkward and annoying.
Often expensive!
When the work is your mast it is going to be worse!
Best learn on scrap and take care IMHO.

 

[William_H]

If you have a battery drill with a gear box for low speed and high torque I would go for driving the tap in with the drill powered. This I have found fine for ali and grp. (I seem to be stuck in 3/16 ww time warp but suits me fine.) The battery drill means easy to reverse and much easier to get the tap straight.
As a matter of interest but not applicable here. I had some success recently tapping a hole in a great lump of stainless steel used as a filler adapter on a friends diesel tank. The tap was for a brass fitting for vent. The size was 5/16 inch 24 tpi and I had a tap. The hole was drilled with a drill press (pillar drill) and was about 8mm thick. I fitted the ss lump fitting into a horizontal vice for the hole drilling and this turned out suitable for the tapping as well. I put the tap deep in the drill chuck I was then able to use the drill press action to get lots of pressure square on to the tap. I turned the tap with a spanner on the flutes of the tap. (high up on the flutes). Back and forth. Lubrication helped but very satisfying to get the thread cut. The tap did cut marks in the jaws of the old crescent adjustable spanner. I used the battery drill with tap in chuck to run it in and out a few times to clean out the thread. (easy with reverse). Anyway just for a bit of fred drift. olewill

 

[TiggerToo]

Rather than tap the aluminium, and as you want to remove it every year, I would use a Rivnut/Nutsert as they have a longer thread engagement. They have a splined shaft which is knocked into a drilled hole in the mast. You then 'collapse' the end of the rivet inside the mast with a lubricated M5 hex head screw which clamps the nut to the mast wall, the screw is then removed. You can get these in stainless, used with Duralac should last for years. That is what I used for my mast gate. Only possible disadvantage is that the insert sits a millimetre or two.

PF is absolutely right. Go for RIVNUTS. I recommend a stainless steel one. You can then add Duralac to the SS-aluminium interface which will not be subjected to further screwing /unscrewing. Then you have a nice "machine" thread (which cloud be lubricated) for the screw.

https://en.wikipedia.org/wiki/Rivet_nut

 

[Daydream believer]

Do you actually need to tap the hole?
If the mast gate is already tapped then one would assume that the screw is being held at 90 degrees to the mast & if the gate is inside the groove of the mast rather than in the open part it cannot fall out, only slide up and down. So the screw would act in shear & stop the gate sliding up & down. In that case a 5 mm hole would be sufficient for the end of the screw to locate into.
However, if the gate can fall out then it would need to be tapped

 

[BabaYaga]

Do you actually need to tap the hole?
If the mast gate is already tapped then one would assume that the screw is being held at 90 degrees to the mast & if the gate is inside the groove of the mast rather than in the open part it cannot fall out, only slide up and down. So the screw would act in shear & stop the gate sliding up & down. In that case a 5 mm hole would be sufficient for the end of the screw to locate into.
However, if the gate can fall out then it would need to be tapped

The fitting is in the open part, so could fall out if not secured.

The rivnut idea might work, but as already mentioned the mast gate will sit 1 or 2mm proud. Perhaps not ideal.

Another idea that might work, but which I have not tried, is to tap through the already threaded holes with the fitting in place. I imagine that could ensure a continuous thread through the two pieces.

 

[savageseadog]

The fitting is in the open part, so could fall out if not secured.

The rivnut idea might work, but as already mentioned the mast gate will sit 1 or 2mm proud. Perhaps not ideal.

Another idea that might work, but which I have not tried, is to tap through the already threaded holes with the fitting in place. I imagine that could ensure a continuous thread through the two pieces.

There's various types of Rivnuts, some have a collar which will sit proud but they can be slightly conical, countersunk and will sit flush. I would recommend Rivnuts as a mast wall is quite thin and soft aluminium there will only be two or three full turns of thread to hold onto.

 

[eddystone]

Do you actually need to tap the hole?
If the mast gate is already tapped then one would assume that the screw is being held at 90 degrees to the mast & if the gate is inside the groove of the mast rather than in the open part it cannot fall out, only slide up and down. So the screw would act in shear & stop the gate sliding up & down. In that case a 5 mm hole would be sufficient for the end of the screw to locate into.
However, if the gate can fall out then it would need to be tapped

The existing "sprung tongue" type gate (505-501) is held in with rivets, not screws but I have just found this in a different Selden document!

Sail feeder Gate Insert for Full length Batten Cars
For sails with full batten sliders, a sail groove insert is available (see fig. 3.8.1.c).
This insert was developed for full batten sliders from other brands than Seldén. Seldéns own full batten
slider (OWS slider)
does not need this extra insert as the OWS slider can passes through the
original Seldén sail feeder gate
505-501-01/505-503-01.



When they say original, they mean the Mk2 nibless version. So I don't need to drill into the mast after all - however I am extremely grateful for the advice which I'm sure will be put to good use in the future.

 

[davidej]

deleted

 

[Boo2]

Wow, brilliant information thanks; this thing only needs to be removed twice a year i.e. when I take the main off so I was going to use Duralac as I didn't think silicone grease had any anti-corroion properties.

I use tef-gel and SS self-tappers into the ally mast for my mast gate. Tef-gel is a pfte loaded grease and I have found it works great and is non-setting - I've had zero problems with corrosion on the screws / mast "threads". Advantage of self-tppers is the thread is fairly coarse for any given screw diameter so there is less chance of damaging the threads cut into the mast.

Hth,

Boo2