Making a riving knife out of 2mm mild steel plate

[baldpate]

Hi,

I need some advice on how best to cut a riving knife out of 2mm mild steel sheet with the tools I have available; if it helped to do so, I could use as a template a prototype I have made out of 2mm aluminium. I cut the prototype (very slowly!) on a cheap scrollsaw with a ordinary 15tpi pin-ended blade (but I suspect this won't be up to cutting the steel), finished by hand filing; other likely power tools available to me include a jigsaw, a small dremel-type tool, a cheap bench grinder, a pillar drill. My metalworking skills/tools are limited.

The background is that I was persuaded by this recent thread
https://www.ukworkshop.co.uk/forums/home-made-table-saw-t61771.html
that I needed to improve the safety features on my home small made table saw (inverted 5" circular saw). The first step on this road will be to make a riving knife - the blade has a 1.6mm plate and a 2.4mm kerf, so 2mm sheet seemed reasonable. I made the prototype out of aluminium, and it fits well just a few mm behind the blade and co-planar with it. However, the 2mm aluminium is just too flexible - even though the prototype it is solidly bolted to the mounting stud designed to take the (missing) original riving knife, it doesn't take much pressure to deflect it laterally enough (I think) to let the cut material get caught by the rising back teeth. Hence the need to copy the shape in steel. The most challenging part will, I think, be to accurately cut the curve which sits behind the blade. On the prototype, it is an arc of a circle radius 68mm (so it sits just outside the 64 mm radius of the blade), the arc subtending about 120 degrees.

How can I cut this shape in 2mm mild steel - specifically, the circular arc? The remaining cuts are mostly straight & could be done with a hand hacksaw, but I don't think a hacksaw will negotiate the curve I need.

Any ideas gratefully received.

Thanks

Chris

 

[Phil Pascoe]

A metal cutting blade in a jigsaw will cut 2mm easily - a little lubrication helps.

 

[Steve Maskery]

FIrst of all I think your choice of material could be improved. 2mm mild steel is not very rigid and in the course of making it, it will very easily get dmaged. Once bent, it will be very difficult to flatten again.
A much better choice of material is tool steel, also known as gauge plate. 2mm gauge plate will be much more resistant to being bent whilst working.

As for how to cut it, I made mine using a jigsaw. Clamp the steel down firmly to a piece of scrap and jig away, finishing with a file.

Remember that gauge plate hardens if it gets hot, so the secret is not to dwell. This is especially true when drilling the fixing holes. If you dwell and the steel gets hot, you won't be able to continue drilling, the drill with just get very blunt. You'll have to anneal the plate again.

I have successfully made just such a riving knife exactly this way.

HTH
Steve

 

[deserter]

Have you measured your blade to check 2mm is the correct size? It seams a little neat to my mind.
For a riving knife to work it must be thicker than the plate of your saw whilst being thinner than the kerf. So of you have a 2mm blade plate and 3mm kerf around 2.5mm would be good. You are using a 2mm riving knife which would imply you have a blade thinner than 2mm which would seem thin to me.


~Nil carborundum illegitemi~

 

[Steve Maskery]

My TK freud has a 2.4mm (IIRC) kerf on a 1.8mm plate.
S

 

[RogerP]

If you don't have either a metal cutting bandsaw or a jigsaw drill a series of small holes (almost touching), snap off (it helps to cut between the holes with a cold-chisel) and finish with a file.

 

[Stormer1940]

Sorry not trying to hijack but I'm interested in making a riving knife myself.... I have read before that it should be made from the suggested steel. Does anyone know of a online supplier of this stuff or is it best to go to a sheet metal fabricator?

 

[Steve Maskery]

Drop Bob9Fingers a PM. I got mine from him.
S

 

[baldpate]

Thanks for the suggestions.

I'll try the jigsaw first. If I can't follow the curve well enough, I'll try drilling holes. I expected to have to clean up with a file, whatever the method.

A much better choice of material is tool steel, also known as gauge plate. 2mm gauge plate will be much more resistant to being bent whilst working.

I take your point, Steve. I think I'll see what I can do first with the material at hand (I've already bought a small piece of mild steel in the expectation that the aluminium would be no good). If the MS is no good either, I'll follow your suggestions.

Have you measured your blade to check 2mm is the correct size?

I think it's OK - this is only a dinky little saw. The plate thickness is 1.6mm as measured by callipers, and the kerf is 2.4mm, as printed on the blade plate.

 

[deserter]

That's alright then, just didn't want you spending time making something which didn't work.


~Nil carborundum illegitemi~

 

[tool613]

I like stainless steel plate for RKs because its does not rust and stays slick over its life. It is Also stronger than mild steel and easier to work than tool plate(mild is a poor choose as is aluminum i have seen as shop made and i have read that mild is not regulation) i just cut mine with a angle grinder and a zep cut off blade in the shark fin pattern. I then shaped it on the disk sander and or hand file angle grinder. Leading the front edge is very important.
Riving/spliter knives are supposed to be slightly thicker than the body of the blade, but slightly thinner than the tips, so for example a typical 10/12" blade has a 2.8mm thick body with 3.2mm tips(set /kerf), the riving knife would therefore need to be around 3.0mm thick. The front edges of the knife should both be slightly tapered and it needs to be aligned with the blade - 1mm out is way too much. I'd suggest trying to shim the riving knife if at all possible (with paper, card, etc) if this is possible and size the knife to the blade.

The thicknesses of riving/splitter knifes offered on north ameriacan saws are actually just standard steel gauges. 13, 12 and 11. The smaller the number, the thicker the stock. Here is a link to standard steel sheet gauges. powder coat if it has it adds 3 or 4 mils to each side
http://www.coasteltools.com/tech_steel_gauge_chart.htm
Most factory supplied spliters use 14 gauge and even less in some cases. Very few use thicker, but there are some. It has been my experience that getting a splitter very close to the kerf thickness is bad. It can often result in sticking and pinching on nearly every cut. This is also not safe. It will cause you to want to apply more pressure to feed and that is undesirable.



jack
English machines

 

[sickasapike]

Sorry if I should be opening a new thread, I just joined up to ask a follow-up question, seems to make sense to rejuvenate this thread since I'm looking to do exactly this too (albeit several years later) - to the same thickness even.

I have a new Sheppach table saw (TS82), which has a 2.6mm riving knife for the supplied 2.2mm / 3.2mm blade, all good.

I just treated mysef to a pair of Freud blades, which are slimmer; 1.6mm blade plate width and 2.3mm kerf so the standard RK is too thick. I'll be using the standard blade for a while as I'm doing bigger/rougher stuff with pine and pallet wood for a while but at some point I want to start playing with making smaller things fron nicer wood so I'd like to make a thinner RK from steel so I can use the nicer blades safely.

The Sheppach tech support guy I've been talking to seems very knowledgeable but appears to have a mental block regarding blade sizes other than the supplied one - i.e. won't give me any suggestion as to the correct RK thickness for the thinner blades; or for that matter, even acknowledge the possibility that any other blade might find it's way into the saw.

From my reading, of the above thread, the HSE document mentioned and lots elsewhere I reckon I also want a 2mm RK, (1.6 < 2.0 < 2.3) or slightly thinner if anything (all specific examples I've found have the RK slightly closer to the blade, not kerf width, but I'm not clear on this yet) but I want it as stiff as possible so think I'm looking for a 2.0mm (or 1.8mm... 1.9 ?? - more a matter of what's avaiable i guess) riving knife from some kind of steel that's rigid enough at that thickness.

May I ask those more knowledgeable than I on here (err, probably the entire membership !) if that all makes sense, and if 2.0mm would be a good size for a 1.6/2.3mm blade, since that's a lt easier to get hold of than slightly thinner.

I've asked that question of Sheppach twice and both times was told that the correct blade thickness for the supplied (2.6mm) RK is 2.2 / 3.2mm, like the supplied blade. They won't tell me if there is a reason why you can't, or shouldn't use a thinner RK on that saw (like complications with shims or religious dogma or something...), I'm assuming it's just not allowed for liability reasons, but is a bit annoying; if it's not safe to do, I'd like to be told why (or at least told so) - I'm inexperienced sure, that's why I'm seeking advice, but I'm not dim

The other question, again, a follow up to the thread above is what's the best steel ? - and how to cut it ? - hopefully in the intervening years, the budget end of kit has got better at cutting this stuff.

I will need to cut it to the existing one as a template so need to be able to do that (and want to gain experience so prefer not to pay someone to do it for me), I have a new dewalt jigsaw and a new-but-budget grinder, both with metal cutting blades/disks (and don't mind buying more), I've a press for drilling the round corners and oil etc so think I have all I need apart from the steel itself.

I noted the comment below about possibly bending the piece while hacking away at it, would sandwiching it in something help, or just make the cutting operation more strenuous and maybe make things slower/hotter/worse ?

I do have some 1mm steel plate, it's a brand new kick plate and I thik it'd be ideal if it wre twice as thick. I did get to thinking why can't I cut the template twice (or do a few and pick the 2 best) and bond them together with some magical substance. Excuse my ignorance but would this be stronger/stiffer somehow - probably not, just a thought, it's not wood, glue won;t make it stronger.... the vibration might crack the join / god knows what else, I gave up on the idea as dodgy but am curious as to why it might be a disastrous approach if anyone cares to explain

Any advice appreciated :^)

 

[Steve Maskery]

If you do use gauge plate (and I would) you must ensure that it does not get hot. If you cut slowly it gets hot and hardens and then you can't do anything with it. So drill holes hard and fast, for example.

As for cutting it out, I've not actually tried this but I think it would make sense to mark the shape out on the steel, then freeze it, outline face down, in a shallow tray of water. You could then just saw away through the ice and it would not overheat.

That's the idea, anyway!
S

 

[MikeG.]

Ooh, water and lecktrickery! That could be fun!

 

[sickasapike]

Hi Steve, thanks for replying.

gauge plate

I only heard about this steel first in the thread above, if it's needed fo the stiffness then that's the stuff, I see a few places that sell it, makes sense to get a 500mm square, I might need a few goes ! - any suggestions for a supplier ?

I think I'll try to make a template with the 1mm steel I already have as practice (and so I don't have to keep taking the current one out to cut the new one !); by the time I've done that, I'll hopefully have got a 2mm piece in the post to do it for real.

freeze it

Nice idea, though as the following chap says, without the ice perhaps, just the chilled metal itself - no bother to chuck it in the freezer before major cuts, am in no hurry; I guess the surrounding area will heat up quickly but will cool again when the bulk of cold metal re-imposes itself.

I just watched a video of somebody cutting 3mm steel with a saw like mine but previous model, went very smoothly, so pretty happy after seeing that. Cutting metal is still a bit of a big deal for me, when using a grinder to cut something I keep stopping to check I've got all my limbs and haven't set fire to the building every 10 seconds and have a 'phew, no disasters yet then'-congratulatory-roll-up-break, which is probably making the whole process more dangerous overall but I'm starting to get there confidence-wise !

 

[julianf]

If i had to do the job, id print out the plan of what i wanted, and then stick it to the sheet (either cut around the paper, and then use brown parcel tape, or, possibly, some sort of photomount type stuff, if i had it)

Id then just go at it with a grinder, and a thin disc (they call them "plasma" discs). Id then finish up the curves with a flap wheel.

You dont need a fancy expensive grinder. Those cheap aldi ones are ok, and you will probably get a few years out of one.

I have a plasma cutter here, and id still just use a simple angle grinder. Any concave curves too tight to get the flap disc too could be filed down by hand, or, if you have a power file (they have b&d ones cheap at FFX at the moment - id never be without mine now!), that would do a great job there. A grinding disc may also give you some purchase on concave edges.

Funny you mention grinders giving you the fear - im fine with grinders, plasma cutters, cnc mills etc. but really dont like table saws, or chainsaws!

 

[MusicMan]

There is a misunderstanding in this thread concerning strength and stiffness. Tool steel is no stiffer than mild steel and might even be less. Stiffness goes mainly with the density of the metal. There isn't a substantial difference between any of the metals called something-steel. If you don't believe me, look it up. You want the Young's Modulus figure.
https://www.engineeringtoolbox.com/youn ... d_773.html. And yes, steel is about three times denser, and stiffer, than aluminium.

What there is a big difference in is the strength, as this depends on the detailed microstructure. That is where tool steel has a big advantage. Since riving knives primarily need stiffness, this is not an issue and mild steel is fine from this point of view.

I probably would use gauge plate for a riving knife but not because of its stiffness. Gauge plate is flatter, so wouldn't have any bends or kinks in it (unless you put them in during the making). And it would also resist bending out of shape better, for example if you twisted the wood in the riving knife. So yes it's a bit better, but mild steel isn't as bad as has been implied, and would perform just as well until it got bent.

 

[sickasapike]

Thanks chaps, I've ordered some Bosch 1mm metal disks for the grinder, I have a 3mm generic one for metal that I'll use to shape the leading edge but at 13 quid for a stack 10 of the thin slicers from screwfix it seems rude not to.

Ah yes, thanks for the reminder, I do have a flap wheel, thought it was a cool idea so picked one up in a late night online 'what else could i possibly need ?' session !

I bought the grinder almost as a throwaway when I needed to cut the top off the riving knife on a previous saw (t-piece guard holder thing stopped it being used with sleds), that was before my coming down with a bad case of yellow fever (which is becoming expensive !) so it's the cheapest available from Toolstation... 25 quid maybe ? - but it seemed study enough for that job.

I have a great jigsaw with some bosch metal blades too so will try that on the long straight bits and little cutouts (I promised myself I'd put the little notch in for the guard... that I've never used..)

I'm going to make a template from the 1mm steel kick plate I have (enough for a few attempts there and order some 2mm to make a real one with.

I'll go for mild steel initially then, if it's stiff enough and easier to work, if I bend it when cutting I'll make another, then look at the guage stuff if I bend that one too

 

[sickasapike]

I started on the 1mm template today (a treat after a day of grout, scraping and wrestling old screws buried in brick, you could have probably hung the building from the old cistern surround I removed), realised I didn't have any proper cutting oil so tried 3-in-1, it started to get smoky so found some proper stuff on the toolstation site, will go get that tomorrow before continuing.

As an aside, I took the standard blade out just to clean it and just because I'd not done so before, and got to comparing it with the slimmer one and the riving knife with a magnifying glass, to the assisted eye the RK looks to be same width as the kerf on the thin blade, tired eyes perhaps, optimistic eyes quite possibly too, will look again tomorrow, am starting to suspect my budget digital caliper is a bit variable, or maybe I'm just not holding it straight enough.

I was thinking, if I do a test rip with the thinner blade and the RK is too wide, as suspected (and the blade properly centred in the RK, that might be the problem that stops me trying it); it'll pinch or even stop the fence side piece, which I can be prepared for, full battle gear, standing to one side etc. - and can then stop the saw and know empirically that it's too wide.

I'd not want to try that with a RK that I suspect is too narrow but am I right in thinking that it's a safe experiment ? - also I can then compare the kerf cut with the RK - I suppose I could do that anyway by running it without the RK for one cut, with the piece clamped on a sled.

 

[sickasapike]

Over the weekend I dug out my old titan table saw, the riving knife on that is bang on 2mm, perfect.

Ah well, I was just waiting to flog/donate it anyway now my new one is settled in so it'll have to go without a riving knife now, I'll be sure to point that out to potential new owners !

A bit of filing and the long slot at the bottom fits over the lower bolt nicely with the second hole lined up with the hole it's supposed to for the upper bolt (well, close to it; I can't see the hole except from above, steel casing round the blade assembly would be in the way even if I opened it up but a 6mm bolt goes through, needs to be an 8mm), so that needs drilling out a little. The mount for the riving knife has a steel plate on either side, it's not hanging on the washers.

The blade for the titan saw is 255mm, 65mm more than the blade this is for so it's more than tall enough, I'll get it nicely in place then mark off a slice to remove along the top and file down from there.