I was thinking making a note for myself for future reference, and I decided to put it online so that someone may get use for it. It is not the Fastest nor Easiest way to make it.
There are many ways for adjusting the fence of sliding table saw so that it can cut square (90 degree), I just came out my own way. The easiest one is just finding a trust Framing Square and use it to calibrate the fence. I can't found one in my area and still having a lot of acrylic sheets on hand. So I decided to make one.
My way just involve a large sheet of solid material and the sliding table saw itself. Having a measuring tool like caliper can help but you don't need one for this.
I archived a squareness of <0.1 mm for 45 cm in length, or less than 0.013 degree. It's overkill for a sliding table saw, since a small chip stuck on the fence will create a larger error than this. But it feel good :)
Sliding Table Saw, Solid Sheet Material
Step 1: Finding Material
You need a solid sheet of material to done this, I just use a 5 mm Acrylic Sheet.
Use the biggest one that still can fit nicely on your sliding table.
Step 2: Cleaning the Rails
Clean the rails and roller bearing for smoother cut and reduce error when we making square.
Step 3: Mark a Reference Corner
Choose a reference corner and mark it, the corner should has one trusted straight edge and putting it at the fence side. Mark the lower left corner.
If you don't have a trusted straight edge on your material, cut one. The squareness is not important now.
Step 4: Start Cutting 1st Edge
Blow out any debris within the sheet and the fence. Any small chip will affect the result.
Make sure the material touch perfectly to the fence, if any gap found and nothing between it, you may have a curve on material or the fence. See the Troubleshooting at below.
Cut the 1st edge if you don't have a gap problem.
1. Try re-cut the material on the saw again, use enough support to push down the material to the table. If still spotting a gap, it may because your machine's rails are not straight. Try the next step to find out.
2. Find the other same material (or 2 new but same material, like 2 pcs of lumber), cut again on the saw. Placing them together and see any gap found. If there's a gap, you have a curve on the machine rails. Unfortunately I don't have a easy way to solve this :(
3. If both material fit nicely together, you have a not straight fence, just like my case, the original steel fence is curvy and not square to the table too (tilted upward). So I just cut a piece of lumber and then use it as the fence.
Step 5: Cutting the 2nd Edge
After cutting the 1st edge, flip the sheet horizontally. The edge that facing you, still facing you. but the 1st edge just goes to the right.
Cut the 2nd edge. Same precaution as the 1st one
Step 6: Cutting the 3rd
Use a file to clean off the burr if needed, my acrylic has some paper burr that need to clean off before the 3rd cut.
After that, placing the mark on the lower right corner just like in the picture. NOTE that the acrylic was flip back to the state that same as 1st cut and then turn 90 degree to the left.
Start cutting if you don't have problem.
Step 7: Checking the Square
After the 3rd cut, flip the acrylic vertically. The mark now goes to the upper right and then facing the table.
Placing the acrylic tight to the fence, and slowly push it near to the saw. Sliding the table to check if the top meet the saw 1st or the bottom one.
Case 1: Bottom meet the saw 1st, leaving a gap at the top
Problem: This final uncut corner has a lower angle than other 3 cut corners.
Reason: Other 3 corners have the angle larger than 90 degree.
Case 2: Top meet the saw 1st, leaving a gap at bottom
Problem: The final uncut corner has a larger angle than other 3 cut corners.
Reason: Other 3 corners have the angle smaller than 90 degree
Case 3: Small or no gap between the saw and the sheet
Problem: Might still have a problem with angle but not so visible, proceed to final checking stage for confirming
Reason: Other 3 corners have the angle that very closed to 90 degree, you can stop here if you want
Measure the gap if you need, or just eyeballing and remember it.
Step 8: Explaining the Situation
In the 1st case, you have the last corner smaller than 90 degree, means that other 3 corners must have more than 90 degree. Since for a Quadrilateral (Basically means a shape with 4 corners but not limited to rectangle and square, all corner sum up should equal to 360 degree.
And also all 3 cut corners have the same angle due to same setup on the saw, so they have the same error, and that error sum up in the final corner. For example, you have 91 degree on all of them, the final corner should have 87 degree.
For case 2 is all the same, just all the cut corner have angle that less than 90 degree and making the last corner has 3 times more error than the rest.
Step 9: Problem Solving
Now is the simple and final step.
If you have an angle that More than 90 degree on the last corner, you need to Increase the fence angle relative to the saw blade. And vice versa.
But adjust by how much?
Since for the last corner, there is 3 times the angle error. So using the last corner as a guide, adjust the angle by 1/3 of the last angle related to 90 degree.
But still has one problem.
Step 10: One More Problem
Since if you proceed to loosen the knobs just to adjust the angle, you will quickly find out you will lost the original angle and have no idea how much you need to adjust. You need to redo again all the process if you are in this situation.
If you want to increase the fence angle, the solution is, using the sheet itself as a guide, stick back the sheet firmly to the fence, just using some tape or weight to secure the sheet to the table, don't give it any chance of moving during adjustment.
Now loosen the knobs and then increase the fence angle. If you unable to increase the angle due to no room to adjust. Tighten it back and make sure it still remain contact with the sheet.
You might have the problem of the holes on the fence not large enough to give you room to adjust. In my case, I made the fence out of wood, later found out the hole is not big enough for adjustment, so I just drill bigger holes on it.
If you want to decrease the fence angle, secure the sheet as above, but this time just set the final angle on the sheet and secure it. Then just loosen the fence and then move it to the sheet.
Step 11: Checking the Result
To check the result, repeat all the process, and looking for the final angle. If it looks 90 degree on the last corner, congratulation you are done! The fence are now square to the saw, and you can Keep This Sheet as your reference square for your other project.
But you are picky as me, just checking with eyes may not be so accurate, so I push the table back and forth, slowing bringing the sheet near to the saw until it barely touch. Making some scribing sound with the tooth of the saw. If the sound hear to be constant, the saw are now very very square to use.
If the scribing sound disappear means the gap become larger, and if the saw tooth start moving or turning, the gap become too tight.
Keep repeating the process and you will get a very square sheet and a sliding table saw that also cut very square.
I manage to make the error below 0.1 mm in the 1st 45 cm cut, more than that the material will overhang and the square cannot be guarantee. (My table are short and I add a plywood to extend it)
I marked the sheet as reference square and keep it on a safe place, in case one day the fence is out of square, I can take out the sheet and then just use it as a reference to quickly set the square. And also the sheet is quite useful for other project although it's not quite easy to use.
The process can be very painful if you make some tiny mistake. My recommendation is still get a proper, trusted framing square, and then straight away you can use it to set the fence angle.
In my case, I am too lazy and stingy to buy a framing square (I live in a small town in Malaysia, I never seen a proper framing square in any store I visited in my area, all the "square" I can get is not square either).
And then I challenge myself to make my saw square without any fancy tool, and I did it. :D
If you have any smarter idea, please comment in below.