Screws and Oak

[Freetochat]

I know it is always advised to use brass screws in oak and not ordinary steel, but does that include stainless steel?

 

[OPJ]

I think I'm right in saying you're okay to use stainless steel screws.

I think it has something to do with the name, you know, being stainless... Therefore, it shouldn't react with the tannin and what-not to stain the timber, unlike aluminium, mild steel or some other metal.

Brass is very good in its appearence, but I wouldn't like to compare it to others for its strength. :S

Yes, I'm sure stainless steel is okay...

Okay, so perhaps someone else will come along now to put this to bed!

 

[Newbie_Neil]

Hi FTC

I know it is always advised to use brass screws in oak and not ordinary steel, but does that include stainless steel?

Neither stainless nor brass will react with the tannin in oak.

Cheers
Neil

 

[LyNx]

To add a twist. I have had this happen to oak and red cedar, both require stainless screws. One brand didn't effect the timber but another brand did.

Lucky the brand of screws that did blacken the cedar was out of sight.

Andy

 

[Freetochat]

Thanks for the feedback.

Regards

John

 

[Chris Knight]

Anticipating Tim replying to this, you can get failures with SS in oak. Phosphor bronze screws for a boat builders is another solution but they are expensive.

 

[Newbie_Neil]

Hi Chris

I'd always believed that stainless was ok in oak and when I did a search I found Scrit agreed here.

The problem that Andy mentions, was in the Cedar and not the oak.

Cheers
Neil

 

[CHJ]

It is worth remembering that there is no such thing as "Stainless Steel" despite the common designation. A more accurate name is "Stain Resistant" and there are several common (generic?) grades normally available, If I remember correctly A2 should be treated as general indoor use and at least A4 needed for corrosion prevention in such areas as marine use.

Normal household bleach can pierce a standard gauge 18/80 stainless sink in 24hrs if left to form reactive crystals. (anyone who has done colour photography developing in the kitchen sink may have fallen foul of this also)

 

[Chris Knight]

Neil,

Tim had some failures of SS bolts in oak. I will leave him to fill in the gory detail.

I think Chas has hit the nail on the head, there is stainless steel, then there is stainless steel...

 

[tim]

Talk about being stitched up - thanks Chris :shock: :roll: :roll: .

The problem that occurred for me was not so much a reaction between the oak and SS but the fact that SS can be quite brittle (which I now know).

I made an oak table and benches last summer for a client.

Some of the leg components were held together using SS screws so that they could be plugged and hidden (client's request!). Having used this type of construction before (although not SS) I was confident that it would do the trick.

Anyway, even though clearance holes and pilot holes were correctly made it appears that the torque used to tighten them was too high (although never experienced any probs with any other fixings at this level) and started an invisible shearing process.

Got a call late one evening from the client (about a week after delivery!) saying that part of one of the legs was a bit wobbly, went over the next day perplexed to find that all of the screws bar one in one leg panel had sheared and only required a small pressure to break the final screw.

Given that the table was designed to be collapsible and semi portable, the strength of the structure is reliant on the integrity of all components working together, but obviously critical is the strength of the legs parts. As I was checking the other legs I noticed that all of them had failed in some way so I realised that it was a combination of possible design failing and screw quality.

In the workshop I tried to recreate the problem to see what had gone wrong and used ordinary steel screws and SS ones. The SS ones sheared MUCH more easily than the steel ones. Anyway the problem is now resolved and I have used SS coach bolts instead.

Was it the oak or the torque or a faulty batch? Who knows but I don't think I'll use the same method again.

If you are interested - these were the screws:

http://www.screwfix.com/app/sfd/cat/pro.jsp?ts=53724&id=50896

Cheers

Tim

 

[Taffy Turner]

Funnily enough, I had the exact same problem as Tim driving the same screws into Oak (pilot holes etc all correct).

I found that reducing the torque on my drill / driver didn't work, as it wouldn't then drive the screw fully home.

I solved the problem in the end by dipping the screws in paste wax before driving them - worked a treat!

Regards

Gary

 

[ike]

Screwfix screws eh. I've found their 'Gold' screws (zinc+yellow passivate) are much prone to breakage - more so than other brand same-size pozi-screws I have used in the past.

My theory is it's a combination of an agressive thread profile with a small root diameter, and the material work-hardening when the thread is formed, and the material not getting or not suitable for subsequent remedial heat treatment. I suspect it's the same issue with the SS screws.

Trouble is, these screws are much better in end-grain/don't need a piklot hole etc, but the downside is they are weaker than some other types/brands of screw.

Ike

 

[Freetochat]

Thanks Tim, they are the screws I have in stock and was going to use them. I'd better have another look.

 

[CHJ]

...snip...My theory is it's a combination of an agressive thread profile with a small root diameter, and the material work-hardening when the thread is formed, and the material not getting or not suitable for subsequent remedial heat treatment. I suspect it's the same issue with the SS screws.....snip...Ike

I guess "made to price point" has a lot to do with it, I have a stock of most sizes from Screwfix as a means of economically avoiding the 'Hunt the Screw' bind in the middle of a quick job. I would be nervous of relying on their integrity where safety was a factor though. A recent 'top-up' of some sizes with a bargain purchase from Lidl resulted in better shear strength items.

 

[Keith Smith]

I've had exactly the same problem with the stainless "Ultra" screws the neck is far too thin to be used in anything but softwood.

If you use slotted stainless screws rather than pozi you should have no problems.

Keith

 

[Newbie_Neil]

Hi Andy

To add a twist. I have had this happen to oak and red cedar, both require stainless screws. One brand didn't effect the timber but another brand did.

Lucky the brand of screws that did blacken the cedar was out of sight. Andy

I'm sorry, but I'd just like to clarify whether you had a problem with stainless in oak?

Thanks
Neil

 

[Scrit]

As I've been quoted from the past I'll tread carefully! My experience is has been that you can use s/steel with oak, however s/steel and brass screws both require careful installation. You need to pilot drill everything, drive a standard steel screw (check that the thread is similar pitch to the brass/stainless screw, though) to between 3/4 and 7/8 depth into the hole, back it out then insert the s/steel or brass scews and tighten using a torque-adjustable screwdriver. This is the traditional way to avoid shearing the heads off brass screws and so far as I am aware. Tim, I reckon you may have torqued them up too much. BTW, why did you opt for screws? A more traditional approach might have been to dowel/glue together, possibly using squared oak dowels (into round holes).

As to screw quality I normally only buy German Spax s/steel screws these days as they are recommended for external use in corrosive environments. I reckon that you get what you pay for with screws - there's no such thing as a free meal.

Edit: Tim, having pondered the construction I have to ask if a shallow cross halving joint (still with screws) with an appropriate exterior grade glue (say UF) would have worked better in that much of the potential racking in the joint would then have been countered by the wood joint itself and the screws would not have had so much shear force fed into them. Another suggestion (for the future) would be to consider using a single coach bolt beneath the plugs instead of screws and again use the shallow halving type joint construction. I've done something similar with recycled plastic (polyethylene) picnic tables where screws just don't work and glue doesn't either (the bolt heads were visible and the hex nuts replaced by domed nuts) and they stand up to abuse in public use quite well (when the scallies don't torch 'em). Yet another approach might be to make the joint more rigid with 2 or more hidden hardwood dowels on the inside. I am bearing in mind that a bench such as this would be expected to seat two 80 to 100kg adults and be subjected to racking forces as they moved around whilst seated. Bent beechwood chairs are much more flimsy than your bench but the glue surface area of the joints is huge in comparison to the cross section of the timber and they seem to survive heavy use for many years without the use of any/many screws. Sorry to have written one of those "if I were you I wouldn't have started here" type postscripts.

Scrit

 

[Losos]

Many years ago I was heavily into restoration of a classic car. One of the materials people use as replacement parts is so called 'stainless steel' but it's forbidden to use this material for any 'stress' related parts, e.g. wishbones, anti-roll bars, etc. In the classic car world the only parts which should be replaced with stainless steel are things like exhaust pipes, body panels (If the car has a chassis) and small trim parts etc.

 

[LyNx]

I had a little problem with oak and a big problem with cedar. The offending screws came from screwfix, both self tappers. I think it's a cheap grade of Stainless steel?? :?

Andy

 

[Newbie_Neil]

Hi Andy

Thanks for clearing that up.

Cheers
Neil