Retrofit Riving Knife for the Hafco SB12

**Preso** · 18th February 2016, 11:27 PM

I recently purchased a used Hafco SB12 TS sight unseen due to missing out on two other saws which were advertised online. I paid a deposit based purely on the photos in the ad and also because the price seemed good given the saws age. I had to drive two hours to get the thing but it checked out OK although I noted at the time of pickup that there was no riving knife, splitter or guard attached to the saw. Now the guard I was not too bothered about. I have recently retired as an Industrial Technology teacher and during my 35 years of service I mostly used Woodfast combination saws in school workshops and not one of then ever had a guard fitted. And before any WH&S experts pass judgement, these saws were not for students to use! I learned early on to respect the cutting capacity of the blade however the missing riving knife on my Hafco saw was an issue for me. I have had some bad kickbacks using a DeWalt radial arm saw for ripping when I had neglected to fit the riving knife after changing over from crosscutting to ripping. At the time I paid for the Hafco saw I was fairly confident that I could make a suitable riving knife given that I have a fairly well equipped workshop with plenty of metalworking machinery. When I got the saw home I removed the throat plate and saw straight away that there was nothing to bolt the riving knife to!
After a few days of pondering I realised that all that was required was to bolt an extension arm to the main spindle and then the correct thickness of riving knife could just be bolted to the arm. Of course I should have realised that such a solution was just a bit too easy and the reality was that the geometry of the rise and fall mechanism, which pivots around a fixed centre rather than being lateral meant that the tip of the riving knife would not stay at the same point relative to the top tangent of the blade.
I modelled the blade, bearing casting, extension arm and riving knife on my 3D CAD software and with a bit of tweaking I was able to design a riving knife with a curved slot that would always stay at the correct radial distance from the circumference of the blade as it was raised and lowered (manually) relative to the blade. In practice, for most through cutting the knife can stay fixed in one position. It is only if the blade is lowered for rebating that the knife needs to be manually raised to bring it back close to the top tangent point of the blade.
DSC_9538.JPG
The image above shows the completed assembly attached to the spindle bearing casting.
I was able to mount the riving knife support arm to the blade side of the bearing housing which was already (fortunately) machined flat. I used four 6mm button head socket screws and flat washers which were just narrow enough to miss the side of the blade by a few millimetres. I transferred all the holes to the cast iron bearing housing, the riving knife spacer and nut plate and these were tapped M6 where necessary. I made my riving knife from 3mm aluminium plate simply because I had some. Steel would probably be better and although I have specified 3.2mm material on the drawings, I only had 3mm sheet. But in practice I prefer to have the knife slightly thinner than the saw kerf so there is less chance of the knife binding on every cut.
I am fortunate enough to have a laser cutter/engraver and a CNC milling machine so I cut a few versions of the riving knife from 3.2mm acrylic first before machining a finished one from hard aluminium sheet on the CNC mill.
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Here is the riving knife being cut out on the CNC mill.
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Here are all the parts prior to powder coating and assembly.
I chose to powder coat the arm, nut plate and spacer because I have a home powder coating unit (Eastwood's unit, imported from USA) and it is more durable than paint.
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This pic shows the parts assembled prior to fitting the blade.
I did search for a solution to my riving knife problem on this forum and it quickly became apparent that plenty of other saw owners have the same difficulty as I was having.
So, in the spirit of sharing, I have included as attachments the assembly and detail drawings of my riving knife solution. I have also included both DXF files of the support arm, riving knife, nut plate and spacer plate. For users who don’t have access to metal working tools, it should be possible to have these parts processed by firms who do laser cutting of metal stock. They will require the DXF files to do this. The other possibility is to print the drawings out on paper and stick this to the metal stock and then get busy with saws and files. I have also included full size PDF files of all the parts if you want to do this.
Oh, and by the way. The standard throat plate on the SB12 saw does not really allow for the full extension of the riving knife. I am in the process of making a pattern so that I can cast a new alloy one but I am guessing that many users will probably make a zero clearance timber insert instead. And just for the record, my saw came with a blade of around 290mm diameter but I have ordered two Freud blades of 305mm so the files in the attachments are for a blade of this diameter (305).
Hope this is of some help.

Regards,
Preso

**Preso** · 3rd April 2016, 12:17 PM

After I machined up and fitted all the bits and bobs necessary to allow for the fitting of a riving knife I purchased two new Freud blades for my saw. It was at this point I realised that the blade that was supplied with my (used) Hafco saw was around 295mm diameter and the Freud blades were 12 inch or close to 305mm diameter. The lovely CNC machined riving knife I had made for my original blade would not now fit. It also turned out that the Freud blades were of the thin kerf variety and were less than 3mm thick. I modified the original CAD model of the riving knife to allow it to clear the larger diameter and machined a new one from some 2.5mm thick hard aluminium sheet.
Great! Now all I had to do was to modify the original Hafco throat plate so that the riving knife and blade could be set to their maximum cut height without the blade sawing into the rear of the throat plate.

I figured that I could cut out the slot in the plate and have an expert welder friend weld in a new piece of aluminium and then I could tidy it all up on the milling machine. It turns out that the throat plate was some sort of die cast zinc alloy which proved to be all but impossible for my expert welder friend to join.
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The photo above shows where we tried and failed to weld in a new edge for the throat plate so that the slot for the blade and riving knife could be joined into one long slot.

Somewhat dispirited I decided to make up a pattern for a new throat plate and then cast it myself in aluminium. I modelled the new throat plate in my CAD software using the exact sizes I would need for the finished machined part and then modelled the pattern around the finished plate with allowances for machining and draft angles of 2.5 degrees.
insert casting CAD.JPG
This image shows the CAD model of the original throat plate (in red) and the slightly larger model of the pattern for the casting.

I then used my 3D printer to print off the pattern in three parts, (my 3D printer has a build platform of 200 X 200mm and the throat plate was around 500mm long)
The finished pattern was glued to a plywood moulding board then filled and painted to make a smooth surface for the sand mould.
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Here are the original throat plate, the painted pattern and the freshly cast aluminium blank in the background. The riser and runner are still attached to the casting. If you look carefully you can see the two joins in the 3D printed pattern. (The purple bit!)

As luck would have it the first casting was a good’un so it was on to the milling machine where I cleaned up the top and bottom surfaces and then machined out the slot for the blade the perimeter and the holes for the levelling screws. I also machined two small pockets in the edge of the plate to take two neodymium magnets which would help to keep the throat plate in place.
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Machining the slot for the blade. The first attempt at this was a slot 8mm wide but I found that the blade would foul the right hand edge when tilted over to a full 45 degrees so the slot was later widened to 12mm.
The photo on right shows the rare earth magnet fitted into a CNC machined pocket to keep the throat plate firmly in place when in use.

As a final touch I powder coated the riving knife and the throat plate in red to match the very pretty Freud blades. In use, the new throat plate allows for the full rise and fall of the blade plus a full 45 degree tilt and the riving knife can be raised and lowered independently of the blade to allow for partial through cuts to be made. There is a tendency for the riving knife to vibrate side to side but once the kerf in the timber which is being cut slides onto the knife, everything settles down. I really would be better to have it laser cut from stainless steel but I am happy to see how this one goes and I am figuring that if it does come into contact with the blade it is less likely to do some serious damage.
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Here is the finished riving knife and throat plate in place and the second photo shows the throat plate removed. I had to enlarge the finger hole slightly for my finger to fit. No. I don't have fat fingers,,, the hole was just too small.

All in all I am quite happy with the upgrade. I certainly feel safer knowing the there is a riving knife to stop material from pinching in on the blade and ejecting said material violently rearwards on a collision course with my tender bits!

I have also upgraded the mobile base which came with my saw. It was manufactured from timber and was of the type that allowed four wheels to roll up a ramp which raises the saw base off the floor. The wheels were ball bearings and the 19mm axles were just made from dowel. I felt a bit uneasy about the dowels taking all the weight of the saw when it clunked up and down so I rebuilt the whole thing out of SHS tube. I also fitted a lever to pull the saw up the ramp and to lower it again smoothly. I am still to clean it up and paint it but when it is done I will post some pics. (I figure a coat of paint will hide my rubbish welds).

Regards
Preso

**Preso** · 19th August 2018, 01:57 PM

Hi Coolamon,
I had forgotten about this thread. Did you want photos of the mobile base that I made for my saw?
Regards,
Preso

**coollamon** · 26th August 2018, 12:42 PM

Gday Preso,
Mate do you sell the riving knife assembly for the sb12 that you have made?
Any other pics would be awesome
Cheers
John

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