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Poor Riving Knife Design (large)

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CHJ

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One of the limitations with my "economy" table saw was the design of the Riving Knife and top guard/dust collector hood.


The Knife protruded above the blade to facilitate the support of the guard/hood.

This of course prevented the cutting of part thickness channels for plywood cabinet backs etc. (a case of quick meeting of EU safety rules overriding original design essential of adjustable blade height maybe.) And meant the temptation to operate without the knife and risk kickbacks.

Also the supplied Riving Knife was some 0.6 of a mm thinner than the blade kerf, and without constant re-shimming of knife to the required side of the blade for job in hand, resulted in chipping of edge especially of laminate surfaces on anything passing the rear teeth.


Although I always plane the edges of any boards cut on my Router Table (don’t have a Jointer) the mere fact that the saw was not producing a better job niggled.


Solution: Manufacture new Knife of increased thickness to spread cut and prevent chipping.

Checked Dimensions:
Existing Blade kerf: 3 to 3.1 mm dependant on material, feed rate etc.
Existing Riving Knife: 2.4 mm thick
New Knife material: 3.2 mm thick (selected from scrap bin, been there 5 yrs+)

Having marked out new material using original as template, cut it out with judicious use of drilled holes, hacksaw and file. A trial fit determined the max height by lowering the blade alongside a scrap of wood on the saw table and marking off the new knife.


A trial run with some cheap paper veneered chipboard proved that the rear teeth no longer caused as much surface damage and a run through with a piece of hardwood proved that the spread of the cut did not produce binding.

On trialling a partial thicknesses channel, the binding of the new knife in the cut proved to much for comfort and unusable.


Solution: Reduce the thickness of the new knife over the top 20 mm or so to be an easy fit in a channel but still provide the spread in through cut boards.


The final result, which relies totally on careful use for finger retention and IMOHO no more dangerous than the view impairing, jig blocking guard supplied.
 

Midnight

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yeaaa.. I like that. Nice solution Chas..

just one thought; given that you need to put another mounting hole into the Mk2 splitter, would the blade guard still work??
 

Chris Knight

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Good job Chas. It is indeed very frustrating to have a riving knife that doesn't perform properly - especially when it is of such a dumb design as the original.
 

jasonB

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Worth making a few different thicknesses, once you get your blade sharpened the kerf will be reduced and it may start to bind.

Assume you have some form of overhead guard when doing part depth cuts :?:
 

johnelliott

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Just one petite comment-ette, isn't one of the points of having a guard that bolts to the riving knife to prevent the back part of a solid wood board starting to rise as it contacts the rear of the blade thus initiating the kick-back sequence? Obviously I refer to a situation where the stress-release caused by cutting a particular board is enough to cause one side of it to touch the back of the blade
John
 

jasonB

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I find it also stops a long length of timber droping off the table when I'm not using my outfeed roller for support. :wink:

Jason
 
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Nice job Chas

Although John's comment is valid, you should be OK as long as the knife thickess is at least the same as the kerf - rear teeth can't lift the wood if this is so.

I had the opposite problem with my knife as the new Freud blade was about .5mm thinner that the original kity
 

johnelliott

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Tony":3rkge1ep said:
Nice job Chas

Although John's comment is valid, you should be OK as long as the knife thickess is at least the same as the kerf - rear teeth can't lift the wood if this is so.
Not if the knife bends, remember it is only 3.2mm thick and is secured some distance below the table.

John
 

CHJ

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Midnight":29ihvez2 said:
yeaaa.. I like that. Nice solution Chas..

just one thought; given that you need to put another mounting hole into the Mk2 splitter, would the blade guard still work??
Job for tomorrow if the weather is as bad as predicted.

Some thing on the lines of two holes and a vertical extension, although that is going to limit the max thickness that will pass under fixings.
 

CHJ

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Tony":1on0556j said:
Nice job Chas
I had the opposite problem with my knife as the new Freud blade was about .5mm thinner that the original kity
Now reviewing the scrap bin for different gages of material to put in the "Known Use" bin just in case some kind fairy drops a better quality blade my way.

PS: Tony: the table saw I think is the same pattern as an old one of yours that forced you to 'engineer' a replacement fence, just a different badge and colour finish.
 

CHJ

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jasonB":6042flcg said:
Assume you have some form of overhead guard when doing part depth cuts :?:
The usual basic flat piece of MDF or ply fitted to top of fence a la simple router guard when chopping off small stuff, but for channeling, the workpiece acts as a guard used with an appropriate push block.

I use a modified version of a bench saw hook for 'pushing' panels to give more control and keep hands away from emerging blade.

EDIT:

Sorry for the delay with the pics.


Opted for a day of Sun, Sea & Sand down the southwest before the Snow got to us.



The Crosscut Cutting board and simple guard. (polycarbonate guard used when more accurate cutting to line needs more eyeball input)


Push Board used when channelling (trenching) panels, It allows more controlled pressure of advancement and semi-automatic hold against fence.


Small brads act as grippers on chipboard, If working with semi finished wood I increase depth of rear locators and apply a strip of glass paper to mating face.

I also call in the TLOML to assist with supporting panel as it leaves the saw with all sheet or panel cuts if I see the need.
 

johnelliott

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Tony":1mc22owz said:
only 3.2mm thick? It is steel John
As an engineer, Tony, I am sure you will be able to tell us how much force will be needed to bend it the .5mm or so that will be necessary to allow a twisty piece of hardwood to climb up the rear of the blade.
That's always presuming that the mounting for the riving knife is completely rigid?

John
 
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John

I take your point on board and am not totally in confilict with the notion that the wood may release large stresses and move.
Whether this will deflect a piece of 32.x40mm steel that is clamped firmly in place with two M8 bolts is subject to debate - if the knife does move, then the results will be catastrophic :shock:

Chas will, of course, stand to the side of the wood when feeding it through :wink:

Anyone on here use a similar design to Chas's? Be inbterested to hear if you have/haven't experienced kickback
 

CHJ

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Tony":1zh3rxrc said:
John
Chas will, of course, stand to the side of the wood when feeding it through :wink:
Having been raised on a farm since the mid 40's where all our wood was logged with a 5ft diameter OPEN (unguarded) blade driven by a JAP Vee twin Engine, I am very much aware of what a wood jamb can do.

If however I am lucky enough to come into possession of a select piece of hardwood that may cause a 'pinch' problem I will do what we always did then, drive a wedge in the split behind the blade as soon as enough material passed to alow it. (stopping blade of coarse)

Tony":1zh3rxrc said:
Anyone on here use a similar design to Chas's? Be inbterested to hear if you have/haven't experienced kickback
Fortunately I have never had a kickback and can never remember us having one reported or talked about in the nine years I had a wood shop under my control.
 

johnelliott

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CHJ- It seems to me that your new riving knife has two distinct features. Feature A) that it is slightly wider than the saw kerf, and
Feature B), that it no longer extends above the blade and this has two results first that it no longer interferes with partial depth cuts and secondly that it is no longer capable of being fitted with a guard.

I have no argument with feature A.

My argument with Feature B is that the blade cannot be fitted with a guard. The only advantage that this has is that you can carry out partial depth cuts, from your first post in this thread-

"This of course prevented the cutting of part thickness channels for plywood cabinet backs etc. (a case of quick meeting of EU safety rules overriding original design essential of adjustable blade height maybe.) And meant the temptation to operate without the knife and risk kickbacks. "

I frequently (several times a week) cut slots to accommodate plywood backs. I use a Trend 6.5mm cutter in an old Elu 96 router. This set up can in one pass cut a 5mm deep slot with perfect accuracy, quickly and easily. The 6.5mm is ideal for 1/4" birch ply which is usually slightly oversize. When making drawer stock I usually cut a full 8 foot length with this set up.

Obviously you do not wish to be talked out of your guard-less set up, but I am sure you won't mind me making it clear to other readers that working a table saw without a guard is an entirely unnecessary risk. Maybe a small risk, but definitely unnecessary
John
 

Keith Smith

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I'm 100% with John on this, using a table saw without a guard is so risky. Unfortunately the solution (a SUVA guard) is expensive, but what price losing a hand.

On the subject of riving knives, mine (on my TS2500) was out of line and I was surprised just how easy it was to bend it back.
 

CHJ

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johnelliott":26gekuj8 said:
Obviously you do not wish to be talked out of your guard-less set up, but I am sure you won't mind me making it clear to other readers that working a table saw without a guard is an entirely unnecessary risk. Maybe a small risk, but definitely unnecessary
John
John
I fully appreciate the increased risk the lack of a fixed guard implies, I do however come from a lifelong engineering background that worked on one-off's and experimental machine processes that made the fitting of 'production' standard guards on lathes, milling machines, grinders and presses (up to 300ton capability) etc. totally unworkable.

I have even passed the odd comment to 'professional' woodwork demonstrators at shows like Stoneliegh who walk away from their lathes leaving a chuck key in the chuck.

The kickback risk I believe is negligible with my care and attention to riving knife condition, and the supplied dust cover/guard supplied being of lightweight plastic may prevent a hand accidentally touching the blade whilst NOT processing wood, but would in my opinion certainly shatter on a serious kickback.

I do endeavour to make all jigs in such a way as to keep hands clear of cutters on saws and router tables etc.

I have yet to see any guard that can stop me from removing my fingers in a home saw or lathe (wood or metal) if I do not exercise common sense.

A senior craftsman in my village modified his fingers on a table saw just because he did not have a push stick within easy reach. It was fitted with a guard, but of coarse it was two inches off the bed when the wood passed under it.

I am currently working on a homemade dust collector guard assembly that will be attached to the side of the table, admittedly its prime aim is to reduce the airborne dust whilst cutting MDF but I hope to make it rigid enough to satisfy the guard conscious amongst us. But try as I might I have yet to come up with a design that prevents a finger following the wood under it.

I have a simple idea re: enhanced kickback containment that I will try out tomorrow on my knife and pass it on if it works.

Sorry for this grossly overlong reply, I just want all who may read this thread to know I do not remove guards lightly and in no way wish to imply that guard-less use of machine tools is to be encouraged.
 

Midnight

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Feature B), that it no longer extends above the blade and this has two results first that it no longer interferes with partial depth cuts and secondly that it is no longer capable of being fitted with a guard.
John..

given that the OME splitter is alive, well and unmodified in any way, it can't be refitted in order to carry the guard when ripping or cross-cutting... because...??

just curious...
 

CHJ

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johnelliott":36sznytw said:
I frequently (several times a week) cut slots to accommodate plywood backs. I use a Trend 6.5mm cutter in an old Elu 96 router. This set up can in one pass cut a 5mm deep slot with perfect accuracy, quickly and easily. The 6.5mm is ideal for 1/4" birch ply which is usually slightly oversize. When making drawer stock I usually cut a full 8 foot length with this set up.
John
John

I to use a router quite a lot but find that the kerf width produced by the saw blade is just right for the ply or faced hardboard that is used in most of my DIY jobs for self and friends. A 3 mm router bit would need to be almost as expensive as my routers I think(solid cobalt perhaps?) to be capable of doing the job with any measure of speed.

How would you take out 25 mm X 25 mm corner rebate out of a piece of 100 mm SQ X 2.4 m stock for instance with a riving knife that is above the blade? I don't fancy using the router (hand or table) to generate all those shavings and I don't have a hand held circular saw or spindle molder to hand.

Not "having a go" just trying to explain that as someone who 'plays with wood' for a hobby I am working within what I have got and can afford.
 

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