Wednesday, August 31, 2011

Riving Knife 1st proto type



I got a prototype done. There were a few iterations, but the rough one made from wood works well enough to prove the theory.


Here's a video of it in action:

video


A few things I learned, I originally planed to make this out of 3/8" steel, but that is just a little to thick, so it will be from 1/4". My sketchup model appears to be off a little bit (measure don't make assumptions). There is more room around the arbor that I thought. The tops off in this video but there is about 1/2" between the top of the table and the top of the arbor so I can make a larger ring and trap the outside. originally I was going to have some wide head bolts trap on the ring (the arc slots in the sketchup model were the clearance slots for the bolts). But the trap design is better. This is hard to explain so I will be posting pictures once I make it.


I'll have to have a few custom shoulder bolts made, other than that nothing to complicated. The prototype also showed me that I need have some way to adjust the pivot arm if I am off a little. I'll have to put some thought into that.


I need to make a few adjustments to the mounting plate. Nothing radical, trim a few places for clearance, remove some excess, but the basic shape appears to be a good one.


Next step is to make 1 or 2 more prototypes. Once I have the design finished then I'll need to find a machine shop to make one. Hopefully I will find a hobbyist machinist who wants one for their saw and will make one for free!

Monday, August 29, 2011

homemade riving knife part 4

So I didn't get a lot done this weekend, Other responsibilities. But the bearings came in. That was really fast! so I give Accurate bearings 2 thumbs up for having the parts in stock and fast shipping.

I did get a chance to make a quick model in sketchup. After moving things around a bit here's what I came up with.

in the up position:


and in the down position:



Next I plan to make a prototype out of wood. then I'll work out the kinks in my design.



Friday, August 26, 2011

Making a riving knife part 3

So now that the table saw's stripped down I can focus on the riving knife. I decided to focus on this first, and the dust collection second, since the riving knife will determine the room for the dust collection. Here is what I have to work with:

I guess I never really explained what a riving knife is. I'm not going to go into to much detail, but basically it's a table saw splitter that rides close to the blade. Wikipedia has a great description. If you google homemade riving knife what you mostly see are adjustable splitters, and while these are better than the factory splitters they are not riving knifes.


The problem with adding a riving knife is that most saws use a trunnion that rotates from a fixed point. If you just slap a splitter on the yolk, the top of the knife will not stay even with the top of the blade. It will just rotate down into the cabinet.

All of the pictures I found of riving knifes on newly manufactured saw change the trunnion design to lift the blade up and down in a vertical manor. Looking at all this additional complication makes me wonder how these tools will last in the long run. A pivoting arbor will wear and just not raise as high but for the most part will not go out of left to right alignment. But these systems have a lot more parts and allow a lot more room for errors. And besides, I don't have the capabilities of making an new trunnion.

I've been doing a lot of looking around on the Internet, the first homemade riving knife I found was on a saw just like mine! I studied this design inside and out and it works great. But there are a few drawbacks. I would need to make a new pulley for the arbor and motor. I would need to machine the arm, and I would need to buy preloaded linear bearings. The last part was the killer, the preloaded bearings are ~$250 each! Plus I'm not sure how well this will hold up under long term use.

The second design I found was right in line with what I was thinking. The yolk on the saw rotates about a fixed point. If I were to add a parallel arm of equal length a set distance from the from the pivot point as the yolk rotational center, and the arbor center, these 2 things would remain parallel. Here's a picture to illustrate my point:



If the arms are the same length and parallel then the part will remain level. So all I need to do is make a plate to hold the riving knife. In the picture above the top arm would be the Yolk in the saw and I would add the second arm.


The space is pretty tight I have about 1/2" between the blade and the yolk, and even less around the arbor because of the flange that holds the blade on the arbor. I will have to make some sort of ring to go around the arbor that mounts to the yolk. If you look in the first picture you'll see that there is about 1/4" of space to drill a hole and tap into to mount the ring. This might be a problem.


Fortunately this space appears to have been machined, and seems to be parallel with the blade. If not I will break out the file and level it off.


The parts that stick out on the yolk around the arbor hole, pivot hole, and gear rack are about 1/4" So this will provide some clearance for pivot arms etc.


The plan is to take a rubbing of the yolk, get measurements off of it, make a model in sketchup and play around until I get something that fits. Then make a mock up in wood, work out any kinks, and take the mock up and accurate drawings to a friend who is a machinist, make the parts out of metal and start sawing safely!


Thursday, August 25, 2011

Making a riving knife part 2

So I tore down the table saw. When I got the top off (which must weigh more than 300 lbs!) this is what I saw.


So I decided that I will need to improve the dust collection as well. Not visible in this picture is a little deflector that prevents the dust from coming out the slot where the lift lower crank travels when tilting the blade. That's about it for dust collection. So I will try to improve the dust collection as well. I'm thinking some shroud around the blade with some flexable DC host to handle tilting. Hooked up through the factory dc port. I will still need to clean it out once in awhile, but I hope I will be able to collect most of the dust.








After getting the top off, I vacumed out the saw and cleaned it up. Then I removed the motor shroud and using a brass punch a knocked out the arbor. It came out much harder than I expected, which is probably why the arbor didn't move once the retaining nut came loose.



I disassembled the arbor, and ordered new bearings from accurate bearing. They were helpful and with the part numbers from the old bearings it was easy to get the right parts. But when I called the first person I got started to take my order and then said "Wait, are you a woodworker?" I said "Yes"; then she transfered me to someone else who took my order. I thought that was odd. But they told me they would have them out today so that's good. The bad was that the front bearing is a double bearing and was $38.00! thats alot for a bearing even the rear bearing I thought was pricey at $9.00. but both of these are special bearings the front being a double and the rear being a raised flange. And Accurate was the only place that had them in stock, and the other places I called wanted around $75.00 for the pair and I would have to wait 4 to 6 weeks.


As for the riving knife I plan to put some sort of pivot arount the arbor hole on the yolk. I only have 1/4" of room to work with, so it's going to be tight. I will probably lose a 1/2" or so of height, If I lose that I will still have about 4 1/4" of cutting, so I can still get through my 16/4 stock. So I'm okay with that. Forgot to take a picture of that! maybe tommorrow I will get one.

Wednesday, August 24, 2011

A homemade riving knife

Recently I heard my table saw making a funny noise when it was winding down. That's never a good sign. I have a delta-rockwell 34-350 with a 3 hp motor in it, if your not familiar with the saw this saw takes a 12" or 14" blade, and the motor is a real 3 horse, it weighs in at about 150 lbs, and needs a 220V 20 amp circuit.





So when this saw makes funny noises I respect it and try to find out why.








After a little searching this is what I found. This picture is looking down through the insert hole at the arbor. In the middle you'll see the threaded thing. That piece is a retaining ring and threads in to the yolk to hold the arbor bearing in place. Now I'm not a mechanical engineer, but I am certain that loose bearings holding a 12" blade spinning at 3400rpm is a bad thing.


So although I had other projects going on at the time (don't we all) I decided I needed to put the ring back. So after a few checks I wasn't convinced that I could just grab a spanner and tighten up the retaining ring, I was afraid sawdust had gotten around and in the threads and in any space left from the bearing moving. So I decided to pull the arbor, clean it up, and re-assemble.


And of course in order to pull the arbor I have to remove the table, remove the motor, and pop the whole thing out. And when growing up on a farm you learn that any time you pull something from a bearing, you replace the bearings. And since I was tearing it down, and this tablesaw is at least 50 years old, I would have the motor rebuilt at the same time. So this will include a bearing job, and motor rebuild as well. And if you're wondering about rebuilding a motor, it's a really easy job. Just take it to a motor rebuilder and pick it up a week or two later.


At this point your probably wondering why this post is titled a homemade riving knife, since I haven't mentioned a riving knife yet. It's because I decided that since I will have the entire saw stripped down I will add a riving knife. I had a close call a few weeks ago with a piece of wood pinching the blade. Fortunatly this piece was only about 18" long so I was able to finish the cut, but it made me pause. A pause that made me consider going out and buying a sawstop, and if it was in the budget I would have. But anyway over the next few weeks I will post my trials, and try to detail the design process I go through to make the riving knife.


I won't claim that what I'm doing will be successful, or that it's even smart, but I'm hoping that some of you will help in this effort and offer some suggestions on improving my design, and in the end I'll end up with something that's safe and works reliably and well.