Screws for Aluminium?

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STATUE

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I am to screw an aluminium enclosure box to an aluminium bracket on top the mast.

What type of screws/machine screws should I use ?
 
Obviously, it depends on load and location.

Self-tapping (sheet metal screws) are generally a big no-no because...
  • They seize. Generally they are very hard to get out, even with anti-seize.
  • They snag, chafe, or cut anything inside the mast they touch, such as wires and halyards. Too sharp. So never self-tapping screws on a spar.
So your choices are machine screws, pop rivets, and Rivnuts.
  • Machine screws (bolts) are most common. Strong and removable. You will have to tap the holes, so that means buying a tap and the correct drill (they often sell the set). Standard metric threads are good, do NOT get fine threads. Try to get just the right length screw, so there is little extra sticking inside. Perhaps the easiest up the mast, but it's close. Do practice tapping a few holes on scrap before you try it on the mast, up in the air. There is a slight learning curve. Be careful not to snap a tap if the boa sways with wakes--this can be a good reason to use rivets. Also, a lubricant makes the threads better and the process easier. For this, oil is fine.
  • Pop rivets are just as strong, or stronger if the mast is less than 4 mm thick. You can remove them by drilling out the heads, which is really quite easy, and then reuse the hole. Stainless is good, monel is better, 5 mm minimum. Perhaps the easiest if you are not very handy and have never tapped a hole. Many small boats are put together with these at the factory, not because they are cheap but because they are better in many cases.
  • Rivnuts are also good for thin masts, allowing removal. But you have to buy the kit and they do not pull the part in tight (they leave a small gap). I've used them but don't favor them for most things.
 
If you do go for SS machine screws, which I would advocate, then when tapping the hole fit the tap into a battery drill which has a 2 speed gear box. Use low speed high totque. It makes it easier to get the tap straight and very quick. Use reverse to remove. Assuming your drill hole is big enough to make tapping easy. Use duralac or grease on screw to avert corrosion. ol'will
 
William_H said:
If you do go for SS machine screws, which I would advocate, then when tapping the hole fit the tap into a battery drill which has a 2 speed gear box. Use low speed high totque. It makes it easier to get the tap straight and very quick. Use reverse to remove. Assuming your drill hole is big enough to make tapping easy. Use duralac or grease on screw to avert corrosion. ol'will
Click to expand...
Interesting, William. I would not have considered using an electric drill.
.
 
Helped a friend a while back who wanted to fit rubbing strake to a little GRP fin keeler. Got a team together with four battery drills. 2 guys to hold wood in place one drill for hole in wood another for hole in GRP another for running the tap in and out again. Last one to drive screw in. Worked a treat we fitted about 40 screws in about 2 hours. I always tap holes in GRP for mounting stuff. It means you don't have to hold the screw when fitting a nut. Now GRP is softer than ali but mostly ali can be tapped using a drill. If it gets too hard you leave the tap in but release the chuck. The use a spanner on tap. As said don't break the tap. ol'will
 
William_H said:
If you do go for SS machine screws, which I would advocate, then when tapping the hole fit the tap into a battery drill which has a 2 speed gear box. Use low speed high totque. It makes it easier to get the tap straight and very quick. Use reverse to remove. Assuming your drill hole is big enough to make tapping easy. Use duralac or grease on screw to avert corrosion. ol'will
Click to expand...
I like to 'feel' when the tap is locking up and drills are a bit insensitive. Maybe I need more practice and/or maybe it depends on what you are tapping.

Jonathan
 
thinwater said:
Machine screws (bolts) are most common. Strong and removable. You will have to tap the holes, so that means buying a tap and the correct drill (they often sell the set). Standard metric threads are good, do NOT get fine threads
Click to expand...
I wouldn't tap aluminium for a screw unless very light load and you are never going to take it out. Aluminium threads strip very, very easily. if you do do it, Loctite them in. The proper way to use machine screws into aluminium is used a helicoil or Tappex insert but then you need the tap for them and the insert tool.
For thin walls Rivnuts or jacknuts work well but, as you say, you get the little bit of standoff
 
ingenious said:
I wouldn't tap aluminium for a screw unless very light load and you are never going to take it out. Aluminium threads strip very, very easily. if you do do it, Loctite them in. The proper way to use machine screws into aluminium is used a helicoil or Tappex insert but then you need the tap for them and the insert tool.
For thin walls Rivnuts or jacknuts work well but, as you say, you get the little bit of standoff
Click to expand...

It depends entirely on ...
  • Tapping clean threads.
  • How thick it is. Preferably, the aluminum is > than the diameter of the screw, preferably >150%. This is more than is required in steel. In thin materials pop rivets are highest strength, which is why they are used on even high quality dinghies so much. They pull the fitting tight and they last. Drilling them out is not hard.
  • The alloy and state of temper. There is a big range. Spars are somewhere in the middle, but not the highest because of corrosion compromises.
  • Coarse threads for soft metals. In the US we would use USS threads, which are coarser than metric standard. Fine threads strip easily in soft metals. Some folks think fine is better in thin material, because there are more threads, but that is not true (Google engineering guides).
 
thinwater said:
Obviously, it depends on load and location.

Self-tapping (sheet metal screws) are generally a big no-no because...
  • They seize. Generally they are very hard to get out, even with anti-seize.
  • They snag, chafe, or cut anything inside the mast they touch, such as wires and halyards. Too sharp. So never self-tapping screws on a spar.
So your choices are machine screws, pop rivets, and Rivnuts.
  • Machine screws (bolts) are most common. Strong and removable. You will have to tap the holes, so that means buying a tap and the correct drill (they often sell the set). Standard metric threads are good, do NOT get fine threads. Try to get just the right length screw, so there is little extra sticking inside. Perhaps the easiest up the mast, but it's close. Do practice tapping a few holes on scrap before you try it on the mast, up in the air. There is a slight learning curve. Be careful not to snap a tap if the boa sways with wakes--this can be a good reason to use rivets. Also, a lubricant makes the threads better and the process easier. For this, oil is fine.
  • Pop rivets are just as strong, or stronger if the mast is less than 4 mm thick. You can remove them by drilling out the heads, which is really quite easy, and then reuse the hole. Stainless is good, monel is better, 5 mm minimum. Perhaps the easiest if you are not very handy and have never tapped a hole. Many small boats are put together with these at the factory, not because they are cheap but because they are better in many cases.
  • Rivnuts are also good for thin masts, allowing removal. But you have to buy the kit and they do not pull the part in tight (they leave a small gap). I've used them but don't favor them for most things.
Click to expand...
"Pop rivets.....stainless is good.."

How do you prevent corrosion between the ss and the Al?
 
STATUE said:
I am to screw an aluminium enclosure box to an aluminium bracket on top the mast.

What type of screws/machine screws should I use ?
Click to expand...

In fitting an aluminiun box to an alumiiun bracket I would use alumiium pop rivets the size would depend on the load involved but 5 mm should be OK in most applications.

If tapping into aluminium use a coarse thread so the thread has a higher shear load

I have fitted nylon folding steps to my mast using M6 ss set screws tapping into my 6 mm thick mast
 
STATUE said:
I am to screw an aluminium enclosure box to an aluminium bracket on top the mast.

What type of screws/machine screws should I use ?
Click to expand...
Is the aluminium bracket on top of the mast the mast head itself, so quite thick. If so I would tap a thread and use t-gel on the thread and use stainless steel bolts again covered with t-gel.

If the material is thin I would use pop / blind rivets, either monel or aluminium depending on the load the box has to carry.
 
IanCC said:
How to use goo with a pop rivet?
Click to expand...
You cannot be serious :)

The same way you protect a bolt or stud in a windlass. You dip the rivet, or stud, in the goo and then apply as normal. I think you will find your SS pop rivets use 'goo' but Seldom wipe the surface clean after attachment - its more professional. If you use Duralac as your goo you don't need to worry about not coating the rivet correctly - the stuff gets everywhere :)

Jonathan
 
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