Blade Alignment
What
The saw arbor needs to be perpendicular to the miter slots. This ensures that the blade will be parallel to the miter slots.
Before launching into blade alignment, we need to bust a couple of woodworking myths. The first involves the notion that it’s better to use a flat blade replacement plate than using the blade for alignment. The fact is, using a flat blade replacement plate is one of those comfortable intuitive ideas which seems right but is actually really bad. Unless you are certain that it’s truly flat then it could actually lead you to misalign your saw. This isn’t a topic of preference or opinion, it’s a matter of fact.
It takes a good deal of skill and knowledge to produce a plate which is flat to less than 0.001″ that will stay flat over time. I haven’t seen any product on the market that could possibly qualify. No 1/4″ or 3/8″ thick piece of aluminum plate could every qualify no matter how much skill and expertise is applied to it’s manufacture. Both the material and thickness are completely inadequate. Buying such a product is a waste of good money no matter what the price.
Not too long ago I purchased a popular blade replacement plate to test it’s accuracy. It was warped so much that it rocked noticeably on my surface plate. A customer, who had purchased the same product, sent me his to test. It was also severely warped. If I attempted to use this plate as it’s maker insists, there would be no way to determine if a change in reading on the dial indicator was caused by alignment error or warp in the plate. If I adjusted the saw to minimize the change in reading, then I would be guaranteed to misalign the saw by several thousandths to compensate for warp in the plate.
There is a better method and understanding the geometry is pretty simple. If you mark a spot on your blade with a felt tip pen, and then rotate the arbor on your saw, that spot will travel in a circular path. The circle traced out by the spot will lie in a plane. That plane will be perfectly perpendicular to the axis of rotation of the arbor. It doesn’t matter if the blade is flat or not. It could be bent like a potato chip and the spot will still trace a circle which lies in a plane perpendicular to the arbor’s axis of rotation. Arbor runout (from being bent) will not affect the path of the spot. Flange run-out won’t affect the path of the spot either. If you always place the stylus of your dial indicator on the spot when you make a measurement, rotating the blade as necessary, then your results will be more reliable than any flat plate can provide. All measurements (alignment and blade tilt) can and should be done in this manner. It guarantees accurate results, saves you time (not needing to swap out the blade), and saves you money (not needing to buy the plate).
The second myth crops up less often. It involves the choice of the alignment reference. The goal of fence and blade alignment is to ensure that they are parallel to each other. So, the intuitive comfortable idea would be to make your measurements directly between the blade and the fence. Advocates of this idea insist that using an intermediate reference (like the miter slot) compounds measurement errors. So, they will guide their dial indicator jig with the fence and place the stylus tip directly on the surface of the blade. Unfortunately, this practice often results in a poorly aligned saw. Why? Because blades and fences are rarely very flat. When you see a change in reading on the dial indicator, you won’t have any way to know if it’s due to a fence flatness problem, or alignment error. The correct procedure is to take all your readings with the dial indicator jig riding in the miter slot and the stylus against the blade (for blade alignment) or the fence (for fence alignment). The miter slot is machined and much more reliably straight. You are far better off trusting it as the blade alignment reference because when you see variation, you will know that it’s due to alignment error. And, when aligning the fence, minimizing the change in reading will yield the best possible results. As an added bonus, cross cuts using the miter gauge will also be accurate - which is completely ignored when aligning the fence directly to the blade.
Why
Few calssic symptoms of poor blade alignment:
- Burning of the cut edge
- Board wandering away from the fence during a rip cut
- Sawdust being thrown up at the operator
- Kickback
How
Watch the video…
Or read the text… (and get more info)
Start by marking a spot on the surface of the blade body. Don’t mark a tooth. The teeth are deliberately ground at an angle so getting consistent and reliable readings on a dial indicator will be impossible. The recommendation to mark a tooth comes from an alignment method which involves judging the scraping of a tooth against the end of a stick. It has no place in the process when using a dial indicator (or in this blog).
Place the stylus of the dial indicator against the blade body on the spot that you marked. You’ll notice how handy the Stylus Offset Bar is for this procedure. It allows measurements to be made at the widest part of the exposed blade. Some tilt the indicator to make measurements at this level but this is a bad practice that produces inaccurate and unreliable results. Others argue that this is yet another good reason to use a flat blade replacement plate. I think the Offset Bar is far more convenient, less expensive, and doesn’t introduce error into the alignment process. It doesn’t matter how wide your measurement is if the object being measured has error. Rotate the scale so that the large hand points to zero.
Now rotate the blade so that the spot is located at the trailing edge and move the dial indicator so that the stylus is on the spot again. Any change in reading indicates misalignment. Correcting the error on a cabinet saw involves loosening the bolts which hold down the table, and shifting the table to minimize the change in reading. On a contractor’s saw, the trunnion bolts are loosened and the blade carriage is shifted to minimize the change in reading.
A future update will include photos of the procedure on a cabinet saw and a contractor’s saw. In the meantime, please refer to your owner’s manual for details.
Here’s a few tips that can help make the process easier.
- Loosen three bolts all the way and leave one snug. When you shift the table (or blade carriage), pivot around the snug bolt. This will help you to keep the blade centered in the throat plate and make the process a lot easier.
- Leave the dial indicator in contact with the blade while you make the adjustment. Watch the reading as the large hand goes back to zero. Since the pivot point is not located at the first measurement position, it will take a few iterations to get the blade properly aligned. So, go back to the leading edge of the blade, rotate the scale so that the large hand points to zero, go to the trailing edge and shift the table (or blade carriage) to eliminate the error.
- You can make fine adjustments by tapping the table (or blade carriage) with a block of wood (or a wooden mallet). Don’t use a steel hammer as this can cause the cast iron to crack. For a contractor’s saw, the PALS product can make fine adjustments easier.
- Tighten each bolt a little at a time. Placing two or three flat washers between the bolt heads and the casting can help minimize squirming as you tighten - especially if you put grease between the washers. Again, leave the dial indicator stylus against the blade and watch for any change in reading as you tighten. You can make minor corrections as you go.
Ed Bennett
ejb@tablesawalignment.com
July 7th, 2008 at 6:48 am
I have a Craftsman table saw circa 1975 model number 113.298031
I got it from my dad and have done some cool stuff with it. I was tightening up some bolts a few weeks ago and the adjustment bolt for the motor snapped off. I drilled it out and re-tapped and thought I was good to go. Unfortunately now the pulley’s must be out of alignment because it will not get up to full speed and it bind’s up very easy. it also shakes quite loudly. Do you have any information on how to resolve this?
Also what is the penny / nickel test?
Thanks,
Jeremy Howard
July 7th, 2008 at 12:09 pm
Hi Jeremy,
Without knowing which bolt you snapped off it’s hard to tell exactly what is going on. However, it sounds to me like it was a bolt used to tension the belt by holding the motor in position. On most saws the motor mount is hinged. Tension on the belt is applied by spring force or a clamping bolt. If there is not enough force to maintain tension on the belt, the motor will bounce up and down causing the whole saw to shake and vibrate. And, of course, the belt will slip and you will lose power during the cut.
I would check to make sure that there is proper belt tension and that the motor has sufficient force (spring or bolt) to maintain that tension while running. It could be that your new clamping bolt isn’t actually doing its job (clamping).
Generally, if the pulleys are out of alignment, the belt will just jump off. You can check pulley alignment by laying a straight edge across the faces of both pulleys. They are in alignment if there is full contact between the straight edge and the faces of both pulleys.
The nickel test is an old method for checking table saw vibration. Supposedly, the saw is running smoothly if a nickel will remain on its edge when placed on the table of a running saw. Of course, there are a lot of uncontrolled variables in this “test” which can influence the outcome. For example, do you put the nickel on the table before starting the saw or after it’s up to speed? Many saws will shudder a bit while starting up but run very smoothly when at full speed. And, if the saw isn’t level then the nickel will be more likely to fall over when oriented in one direction than another. Cleanliness and smoothness of the table surface will also influence the outcome. I’ve never heard it done with a penny but I suppose it’s possible. I would say that a saw that can’t pass the nickel test under any circumstances could use some TLC.
Many people go to the trouble of installing new machined pulleys and a link belt. I’ve never done this but I’m not surprised to hear that it helps. If nothing else, the process would involve aligning the new pulleys and adjusting the belt tension (something you should do anyway). And, the balanced pulleys will definitely run smoother.
Let us know what you come up with!
Thanks,
Ed Bennett
ejb@tablesawalignment.com
August 4th, 2009 at 2:05 pm
Hi Ed,
Good information. Nice scientific/engineering level approach to things. I was getting tired of all the low level stuff I find so this is excellent.
I just acquired an older Craftsman contractors saw. While the saw appears in excellent condition and the internals show no abnormal or excessive wear marks (they really look like new) after aligning the blade, and my method is nearly identical to yours, it will not remain in alignment. It moves from alignment simply by operating the blade adjustment cranks or cutting almost anything. When it misaligns, it is visibly misaligned–or by about a sixteenth of an inch.
Any suggestions? I am unfamiliar with these older Craftsman saws but I bought it on the recommendation of people who feel they are both reliable and accurate. I have found the inverse to be true. I replaced the arbor bearings and carefully shimmed everything to eliminate what I considered to be excessive free play in both directions. Then I had to replace the blade tilt bushing with an actual bearing as well as heavily reinforce the side of the case in which it was originally seated. The way it was, when tilting the blade back to vertical, I would have to go around back and tug on the motor to get that thin walled side back to flat. It moves too easily as factory produced and is difficult to keep reliably adjusted. I placed some small sealed aircraft control bearings in the inboard end to remove what was about a sixteenth of play there.
August 4th, 2009 at 10:41 pm
Hi John,
Welcome to the group and thanks for the encouraging words!
My first thought is that the trunnion bolts are bottoming out before they firmly clamp the trunnions to the table. Check to see that they really do clamp the trunnions down good and tight. A couple of flat washers underneath each bolt would solve such a problem (and a common squirming problem).
It’s pretty common for derogatory words that sound similar to “Craftsman” to be used in describing these machines. Having owned one (a bit more than 25 years ago) I would have to say that the reputation is not completely undeserved. It seems you are likely to arrive at the same conclusion. Sometimes well intentioned free advice isn’t worth what you paid for it!
Let us know if this doesn’t solve the problem.
Thanks,
Ed