making a riving knife question

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gog64

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I'd like to make a narrow riving knife for my table saw for a ripping blade. As it needs to be about 2mm thick, I think that mild steel plate might deform too easily, so wondered about using gauge steel. I've not worked hardened steel before, so had a couple of questions if people wouldn't mind sharing their expertise? Firstly, am I going to be able to shape the steel with normal tools, i.e. angle grinder, files, drill press, etc? Secondly, would grinding the steel heat it up sufficiently to affect the heat treatment?
 

chaoticbob

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Gauge plate is normally sold in the annealed (soft) state. It's still tougher than mild steel, but it is easy enough to file, grind, drill etc. If you want to harden it you need to heat to red, quench in oil then temper back .
Robin.
 

MikeG.

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Help me out here. This is a genuine question based on my ignorance..........but why would a riving knife ever deform, no matter what it was made of?
 

SammyQ

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Mike, they do get 'whacked' by tired or clumsy woodworkers. Also the moment of force applied by a substantial section, long, piece of timber as you manoeuvre it, can bend them. (No, I haven't with the AGS, but I did on the piddly TS200). So, being stiff is a priority requisite. More so as the dust hood is hung off them. Would not like to see THAT deflected into the spinning blade.
Sam
 

gog64

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Mike, I'm far from being an expert, but my understanding is that you want hardened steel because (a) when the riving knife is doing its job, i.e. when the wood wants to close up over the blade, the force either side is not always equal and this can bend mild steel (b) when cutting it to shape, thin mild steel can easily deform and (c) because sh*t happens and hence I like to over engineer things when it's sensible to do so!

Thanks Bob for the point about gauge steel being supplied annealed. I'd got a bit confused as some of the stuff on ebay says "fully hardened". I've got a little bit of metal working experience, but haven't done anything like this before. I think that what I need to do is:

(1) Anneal the plate when I get it, i.e. heat evenly until non magnetic and let cool (skip this step if supplied annealed).
(2) Once finished shaping the knife, heat to non magnetic and quench in an oil bath.
(3) Anneal to manufacturer's specification (the wife will love me using her oven for that I'm sure).

My concern with this is whether the blade will warp. Is that likely and can I do anything to prevent it?

Thanks for all the help.
 

Steve Maskery

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I have a home-made RK on my saw. I used gauge plate and have not hardened it.
The issue of deforming is not so much in use, it is in the making. You have to cut out, drill, file and generally muck about with it. It is the easiest thing in the world for it to get a bit bent during those processes, and, once it is bent, you will never ever get it flat again. It really must be flat, otherwise the workpiece will catch on it one side or the other, the temptation is to force it and then things go wrong.
Also, it is not hardness that matters here, it is stiffness. A 2mm RK is going to be, if my mental arithmetic is even vestigially present, about half as stiff as a full-kerf one.
It's a good idea to keep the metal as cool as possible, because if you allow it to get hot, it will harden and you will only be able to grind it rather than file and drill it.
 

MusicMan

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Scott and Sergeant sell riving knives of a variety of diameters and thicknesses if you want to buy one.

Unhardened gauge plate will do fine as long as it is not bent, as mentioned.

The stiffness of mild steel and gauge plate is very similar. The strength of gauge plate is much higher and it is more precisely flat.

If you think you can anneal steel in the domestic oven, you will not be popular with your wife. It needs a temperature of about 1000 C (red heat). Might just about do for tempering after quenching but you don't really need to harden this way.
 

chaoticbob

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MusicMan":h4601s28 said:
...
Unhardened gauge plate will do fine as long as it is not bent, as mentioned.

The stiffness of mild steel and gauge plate is very similar. The strength of gauge plate is much higher and it is more precisely flat.

If you think you can anneal steel in the domestic oven, you will not be popular with your wife. It needs a temperature of about 1000 C (red heat). Might just about do for tempering after quenching but you don't really need to harden this way.
I think the OP was referring to tempering in the oven, not annealing MM!
I agree that there's nothing significant between mild steel and gauge plate (hardened or not) regarding stiffness - by which I mean how much it deflects under a given stress. Also agree that strength (in the sense of yield stress, the point at which you bend the RK or whatever and it doesn't spring back) is more important in this context and gauge plate is stronger than MS.
Having now retrieved now a piece of 2mm gauge plate from the dungeon and subjecting it to highly scientific tests :wink: , I wouldn't hesitate to make a RK from it without heat treating - more than adequate I'd say.

At risk of hijacking slightly: Steve, have you ever personally experienced gauge plate hardening when being machined? I've read of this elsewhere, but it's never happened to me when making cutting tools. Maybe I've been lucky! I'd be interested to know under what conditions it happens.
Bob.
 

gog64

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Yes, sorry fat fingers, I did mean temper in an oven.

Unless I'm using a cross cut sled, I keep a "full size" riving knife on the machine as the crown guard attaches to that on my machine (I don't mean that there's anything wrong with the other styles, just that the crown guard won't attach to them). So that rules out the S&S etc. knives. It's no big deal, I'll enjoy making the riving knife. Like most people I just like to do my research first.
 

Steve Maskery

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chaoticbob":3o051s21 said:
Steve, have you ever personally experienced gauge plate hardening when being machined? I've read of this elsewhere, but it's never happened to me when making cutting tools. Maybe I've been lucky! I'd be interested to know under what conditions it happens.
Bob.
Hi Bob,
Yes I have, when drilling. I'm no metal-man, so my metal experience is not extensive, but yes, when drilling the stuff you have to be bold and decisive. Get through the darn stuff before it starts to fight back. If you let it get hot it will simply destroy your drills.
Well, mine, anyway.
 

chaoticbob

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Steve Maskery":hhxkqkq4 said:
chaoticbob":hhxkqkq4 said:
Steve, have you ever personally experienced gauge plate hardening when being machined? I've read of this elsewhere, but it's never happened to me when making cutting tools. Maybe I've been lucky! I'd be interested to know under what conditions it happens.
Bob.
Hi Bob,
Yes I have, when drilling. I'm no metal-man, so my metal experience is not extensive, but yes, when drilling the stuff you have to be bold and decisive. Get through the darn stuff before it starts to fight back. If you let it get hot it will simply destroy your drills.
Well, mine, anyway.
Thanks, that makes sense. Sounds like drilling stainless - you have to show the metal who's boss. It might be work rather than heat hardening then. I guess I've escaped the problem because I tend to lean heavily enough to keep the bit cutting rather than rubbing.
Bob.
 

wallace

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I needed a one for a machine so got that youtube guy 'doubleboost' to make some out of 4mm steel plate. He used his cnc plasma and they came out really nice. I sold a couple so it has paid for itself
 

wallace

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It was for an 18" blade with big chunky lumps of tungsten on its tips. I copied an original wadkin one
 
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