Aluminium riving knife - bad idea?

[julianf]

Im in the market for a table saw, and a commonly misplaced part is the riving knife.

I cut cnc cut metal all the time, but only non-ferrous. Im wondering if i made a riving knife out of a bit of anodised plate, if that would be ok? I mean, if i needed to replace it every year it would hardly be the end of the world.

Any reason why not to?

 

[MattRoberts]

The purpose of a riving knife is to prevent the wood from pressing on the back of the blade. Assuming the aluminium is strong enough not to flex at that thickness (very slightly less thick than the blade), I can't think of a reason not to use it...

One consideration is if you're looking to attach a crown guard, and whether it could put too much strain on the fixing points...

 

[TFrench]

They're normally made from hardened steel I think so I wouldn't use ally.

 

[Brandlin]

That means an ali sheet just 2-3mm thick. Annodising it will not effect its rigidity.

It needs to be rigid enough not to bend if you put lateral pressure on it, ie a force perpendicular to the plane of the saw blade. If it deflects because your feed is somewhat skewed then its not doing its job and you'll get kick back.

Not sure what you mean about replacing it every year? if it gets bent, you're unlikely to straighten it enough to guarantee it doesn't catch on the material you're pushing past the blade.

Ali is much cheaper than steel. Theres a reason manufacturers use the more expensive material for this purpose.

 

[sunnybob]

It would depend on what you cut. If youre only ever going to cut thin flat wood (1" or less high) I cant see any problem with alloy, especially if it was duralamin or the like.
But if youre thinking of cutting stuff 2" or more, then there is a risk of a warped pieced of wood moving around the blade and jamming everything, which would be very dangerous and most likely ruin your blade.

IF, and its only IF your new to you saw doesnt have one, its less than an hours work to hand cut a new one from a piece of steel. not worth the risk.

 

[novocaine]

Youngs modulus of steel ~200 Gpa
Youngs modulus of aluminium ~70 Gpa
also know as less than half as resistant to bending and staying bent.

different alloys of aluminium are going to have different properties, the most common is 6061 which is 69 Gpa, 7075 is 71Gpa, 2024 is 73 Gpa.

in short, no, aluminium is not going to be good enough for a riving knife, you'll bend it way to easily.

 

[sunnybob]

point of order Mr chairman :lol:
Take a piece of steel, put it in a vice, and hit it at the top with a club hammer.
Do the same again with a piece of alloy and you are a 100% correct.

BUT.... We are talking about a riving knife, which is positioned behind the blade.
The force exerted on the knife isnt a bending force at the top of it.
Its a shear force at the level of the table.

Under those circumstances, the alloy becomes a lot stronger.
Still not as strong as steel obviously, but strong enough for thin pieces of wood as I described, which could not possibly exert enough shear force to cut that alloy like a guillotine.
Once the wood becomes tall enough to exert a bending force as opposed to a shearing force..... its gotta be steel.

Of course the poster could try a truly radical approach to solve the problem, he could always buy a new one (hammer) =D> :roll:

 

[novocaine]

in retort your honour.
It isn't supported at the table, it's supported at some point (typically 50mm or more and dependent on the blade height) below the table . it passes through a slot in the table, that by design has to be wider that the knife and is normally the same slot as the blade, so has to be wider still to accommodate the blade. so it is still a bending force and it could still bend the knife out of place and allow the timber to bind on the rear of the blade.

I stand by my ascertain that aluminium is not a suitable material me'lud.

 

[sunnybob]

point conceded, but what about a zero clearance insert?

An honourable draw i believe. (hammer) (hammer)

 

[julianf]

IF, and its only IF your new to you saw doesnt have one, its less than an hours work to hand cut a new one from a piece of steel. not worth the risk.

That's probably the most valid comment in this thread - thank you.

Ali is much cheaper than steel.

In a previous thread, you suggested i was doubting the advice i was given, when really i was just trying to understand it. I do, however, know that your statement above is inaccurate.

 

[julianf]

point conceded, but what about a zero clearance insert?

An honourable draw i believe. (hammer) (hammer)

The reason i mentioned anodising before was not in relation to rigidity at all, but resistance to abrasion. Rigidity can, as mentioned, be controlled to some extent by the alloy, but, like you say, ill just make one from steel if that time comes.

For an insert, an anodised surface would, im sure, be preferable, if it were to be made of alloy.

 

[Brandlin]

Ali is much cheaper than steel.

In a previous thread, you suggested i was doubting the advice i was given, when really i was just trying to understand it. I do, however, know that your statement above is inaccurate.

Raw material price added to processing and finishing costs, means aluminum is almost always cheaper than a comparable steel component. Thats why manufacturers use cast or extruded aluminium rather than steel where they can.

 

[RobinBHM]

That surprises me, I wouldve imagined aluminium is generally more expensive than steel.

Certainly sheet steel is much cheaper than aluminium. Laser cutting a sheet into a riving knife wouldnt cost much.

I can see cast iron would be more expensive than cast aluminium though.

I suppose extruded aluminium section for machines fences would be cheaper than a cast fence

 

[julianf]

That surprises me, I wouldve imagined aluminium is generally more expensive than steel.

Just because it's said with authority, does not mean it's true! : )

If you want an outside opinion, maybe run a handful of Google searches?

 

[novocaine]

the cost of a part is heavily dependent on the amount of labour and what the part needs to withstand. building a bridge from aluminium would cost the world, simply because you'd need so much of it and it's difficult to work (welding aluminium is still considered a special service) in comparison to steel. take a bike frame as a good example, to make a frame of comparable strength and resistance to stresses from aluminium costs about twice what it does to make it from steel, you'll use about 4 times more material too (steel tubing profile can be sub 1mm thick in places, aluminium will be lucky to get down to 2mm).

if you are replacing like for like (identical part with no alterations to provide the same level of resistance), then yes, aluminium will most likely be cheaper, but if you are replacing based on good design and suitable engineering, thus able to withstand to the same level, typically aluminium is more expensive (pressed steel saw table vs cast aluminium).

so you are both right, sort of.
aluminium is still not suitable for a riving knife.

 

[Geoff_S]

You need to do what I did.

Take the riving knife out and merrily cut down lengths of timber, walnut in my case but I don't think that makes a big difference.

Then as a length of timber kicks back, hits the end of your finger opening it up like the petals of a bleeding red tulip and ripping off the finger nail at the same time, then think to yourself "Did I actually need to remove that riving knife?".

Some may say that it was nothing to do with removing the riving knife, but I am never going to mislay that riving knife ever again when ripping timber.

 

[siggy_7]

What novocaine says about the modulus of the two metals tells you what you need to know - aluminium will not be stiff enough. Aluminium can be used in many situations to make stiffer structures than steel, but this is achieved by increasing section depths and wall thicknesses to compensate for the lower stiffness. For a riving knife, the thickness is by necessity determined by the saw kerf, so you will end up with something not stiff enough to do its job. Steel is the right choice.

Stiffness (modulus) doesn't change much between different alloys - aluminium is about 70GPa and steel about 210GPa pretty much whatever alloy you choose (although some alloys retain stiffness at high temperature better than others). Don't confuse stiffness - the deflection when a load is applied - with strength - the load that can be taken before the deflection becomes permanent and the metal deforms plastically. Strength varies greatly between alloys. For a riving knife, choose a strong steel alloy.

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