A4 stainless

[David AmbroseParticipant @davidambrose86182]

I’ve recently bought some Timco A4 grade stainless steel wood screws, 4mm by 60mm, for an exposed maritime application.  I was using an impact driver, and in quick succession I broke the head off one screw, then chewed up the head of another.  I had naively assumed stainless steel would be strong and hard, but this was not the case.  Is A4 stainless good for corrosion resistance, but poor for strength?

[JasonBModerator @jasonb]

Stainless woodscrews are nowhere near as resistant to overtightening as their modern steel versions. So set the torque of your driver low, use a good bit and keep the pressure on the driver and make sure you keep it lined up with the screw.

clearance drill the top piece of wood if joining two and even countersink particularly if a hardwood or you are going through a knot in softweed as they may not pull themselves in without the head coming off.

[noel shelleyParticipant @noelshelley55608]

I would use a new tip and be wary of using an impact driver, the more so on hard woods. Doing them by screw driver may be a pain but you know when to stop and open the hole some more. Stainless is good for marine use and its corrosion resistance takes precedence over it’s mechanical strength. Plan B would be bronze – at a price. See also Jasons comment. Noel.

[Richard SimpsonParticipant @richardsimpson88330]

I think mainly as a result of modern tools and things such as self drilling and self tapping wood screws we seem to have got out of the habit of drilling the wood appropriately.  I would still always advocate a clearance drill to clear the shank diameter and a pre-drilled hole of the same diameter of the root to ensure only the thread is cutting into the wood.

I think we sometimes fall into the trap of thinking that a screw is tight because it requires a lot of effort to screw it in.  In fact a wood screw should require little effort to fit it.  The quality of some wood screws nowadays is pretty appalling with a tendency for the head to snap off  if excessive torque is applied, so pre drilling is the best way to minimise the torque required.

[roy entwistleParticipant @royentwistle24699]

My dad used to say Use a hammer to put a screw in, use a screwdriver to take it out

[Mark EasingwoodParticipant @markeasingwood33578]

Yes stainless steel wood screws are not as strong as steel, some brands are better than others, and the ones supplied with fittings can be very poor. I find the Hafele ones are pretty good.

As said pilot holes and countersinking will help, and use of hand screwdriver for final tightening after using the impact driver/drill for the first 80/90%.

Running a similar size steel scew in first will cut a “thread” for the SS screw.

Dipping the SS screws in Vaseline before insertion also helps, but the Vaseline will bleed into the surrounding wood, paste wax is another option. Tallow is the traditional method for dunking screws in.

[JasonBModerator @jasonb]

Bar of soap or candle wax also work.

It may be worth driving with a cordless drill/driver rather than an impact driver as they tend to have more torque settings and will apply a continuous load.

[FulmenParticipant @fulmen]

A4 is equivalent to 316 and should be well suited for marine applications. It usually comes in 3 classes, -50, 70 and 80 with a corresponding tensile strength (500, 700 or 800MPa). 500MPa is stronger than a wet noodle although it doesn’t really feel like it, while 800MPa is decent.

[David AmbroseParticipant @davidambrose86182]

Thanks for that – lesson learned.  I had drilled clearance holes, as I usually do. I can’t see how a fully-threaded wood screw can join two bits of wood tightly unless you do, but many people don’t seem to bother.

[VicParticipant @vic]

Another point is to choose your fasteners carefully in terms of size. Some stainless screws I’ve bought had comparatively large threads on a slimmer core, others not so much. Probably the best I’ve bought were Robertson screws but not always easy to source over here. I avoid Pozidriv whenever possible so I normally end up with Torx. Spax stainless Torx are generally pretty good.

I use Spax Wirox screws for many outdoor jobs but they’re not stainless, just coated.

[Bo’sunParticipant @bosun58570]

Plenty of good advice above.  But do check you’re driving with the right bit.  Phillips and Pozidriv are the likely culprits, but they’re not the same.  If the screw head has a small groove between each driving face, then it’s Pozidrive.  It’s also important to select the right size bit.  You’ll probably need a size No.2.

Vic has eluded to Torx drive and a good quality screw.  A bit more expensive, but that would be my choice in this case.  Torx drive will also negate the tendency for “cam-out” which can reduce the tightening force and make screws difficult to remove in the future.

[bernard towersParticipant @bernardtowers37738]

Why not bronze screws? we always used them with straight slots and dipped in a tin of grease beforehand. The thing with older style screws is that with no thread at the neck it had greater strength.

[noel shelleyParticipant @noelshelley55608]

There are many ways of driving a screw, slot, hexagon, phillips, pozidrive, square, torx and many others. If one looks at a phillips tip it is machined on 4 driving points to a radius, getting thicker at the top and marked PH 1,2,3 No 2 being the most common. Pozidrive is machined so the 4 driving points are parallel, marked PZ 1,2,3, again PZ 2 is the common one. Hex, H and torx, T go by size number   Noel.

[David AmbroseParticipant @davidambrose86182]

I nearly bought the screws from a regular eBay supplier until I noticed that although they were advertised as Pozi the photo clearly showed them as Phillips.
Comparing the price of Spax with Timco, I guess you get what you pay for.

[Martin KyteParticipant @martinkyte99762]

In days gone by, if assembling a piece of joinery using brass screws identical steel screws would be fitted first and then the brass screws sequentially exchanged into the preformed and tapped holes. This was all done by hand  tools. It was also important to get the brass screw to ‘pick up’ the thread by turning backwards until it dropped into register. This avoided any recutting of the thread. Using candle wax is excellent advice (and can be used to great advantage on bolts and nuts)

The technique is pretty much a must on oak with brass screws.

I suspect a similar technique could be employed with the Stainless screws but using power drivers. You do need to find identical screws though.

[noel shelleyParticipant @noelshelley55608]

Martin trick is also applicable when refitting self tappers into plastic, ie wind back till the thread drops into the original thread, then screw in. Noel.

[DalboyParticipant @dalboy]

Drill a pilot hole full depth of the screw and a clearance hole through the top piece only as mentioned earlier and also as Jason candle wax will aid insertion of screw

[VicParticipant @vic]

Candles aren’t something I used to have around so when putting screws into plastic plugs by hand I’ve used a bar of soap before to make it easier to put the screws in.

[Martin KyteParticipant @martinkyte99762]

Night lights are the most convenient way of having a bit of wax in your toolbox

[NealebParticipant @nealeb]

I always had an odd stub of candle in the box with the spare Yankee bits. A cordless impact driver and Torx and Posidriv screws saw the end of the Yankee but there is still a stub of candle in the box of driver bits.

It’s interesting that there is an immediate assumption that stainless means stronger, but sadly not necessarily true!

[Mark EasingwoodParticipant @markeasingwood33578]

I still have a “Prices” type candle or too in my tool bag/box, used to use ’em a lot when we made wardrobes/kitchens with traditionally fitted drawers, makes a world of difference to a sticking drawer.

The Paste wax, such as “Liberon” is easier to dip screws into ‘tho.

[old martParticipant @oldmart]

The museum was given a huge quantity of stainless aircraft bolts in UNF and metric fine by Rolls Royce, they are likely a type of A4 which is much higher tensile than commercal stuff, I know what they feel like to cut with a hacksaw and turn on a lathe. Woodscrews in stainless would benefit from a small pilot hole first.

[Nigel Graham 2Participant @nigelgraham2]

Unless knocking up something quick and dirty from a few bits of old pallets and those “self-drilling” mild-steel screws sold to shuttering-carpenters on piece-rates, I always clearance-hole the upper and usually pilot-hole the lower, wood.

The clue to identifying ‘Pozidrive’ from ‘Philips’ is that the ‘Pozidriv’ head is marked with a tiny radial “hyphen” between each slot. The ‘Philips head’ is not marked. Using the wrong screwdriver is likely only to damage the screw or driver, or both.

 

What though is all this about “impact drivers”? Surely you want a steady, smooth torque, not a series of shocks? Almost by definition, an impact driver is for loosening nuts and bolts by combining the torque with a circular hammer action, not for inserting wood-screws on decent-quality work!

 

[Bill PhinnParticipant @billphinn90025]

I’m not averse to using an impact driver for the initial driving of wood screws; no hammer action need be involved if you stop before things get tight and finish by hand.

Torx or Torx security heads are my preferred choice for stainless self-tappers.

I’ve often wondered why Torx screws are not used more than they are in the building trade and Pozidriv or Phillips continue to be so popular. Is it something to do with the fact that if PZ and PH screws are inclined to have the wrong bit used on them, Torx screws, with their very close sizing in some parts of their range (e.g. T25 and T27, and all the sub-T10 ones) are even more likely to be driven with the wrong bit?

[howardbParticipant @howardb]

“I broke the head off one screw, then chewed up the head of another.  I had naively assumed stainless steel would be strong and hard, but this was not the case”

If in hardwood you need to pilot drill for s/s woodscrews, softwood probably not.

 

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