Accurate cursors, elinating parallax errors

[Randy]

I just read another magazine article complaining about the error induced if you don't look straight down on a cursor that isn't close to the scale. They recommended shimming the cursor down to bring it closer to the scale, (they were discussing a Powermatic Accu-fence).

The better solution is to scribe the opposite surface and fill in the scribe line with a magic marker, and then remove the excess marker with a white vinyl eraser. Then you just line up the two marks, and you can't induce a parallax error. (FYI, the white vinyl eraser will take all sorts of marks off of most impervious surfaces, i.e. magic marker and stamp pad ink off formica).

[BobS]

Another old trick to eliminate parallax error is real simple. Close one eye then align.

Of course, for those that have tested their laser embellished chop saws and now only have one good eye - they already knew this....

Bob S.

[Michael Daly]

??? Parallax isn't caused by binocular vision. How does closing one eye solve the problem?

Mike

[BobS]

Short answer - Just try it before you doubt that it works or not.

Longer Answer - Close left eye, place head over cursor and reasonably perpendicular to it. Move the fence to the alignment mark that you want while keeping head above the cursor and perpendicular. If you consistently do this the same way each time you make an adjustment you will minimize the parallax error and your measurements will be within a few thousandths of the measurement. If you align your fence to your blade and zero the cursor using the same method, you will just about eliminate any noticeable parallax error.

One other item is the width of the cursor mark. From what I've seen on the market, the lines on the cursor have become wider and introducing even greater error. My Jet cabinet saw came with a fine line cursor but the newer ones have a cursor line that must be at least 1/16" wide. I align the cursor using the above method and split the line on the attached ruler mounted on the saw.

If you need more of a scientific answer than that, someone else will have to provide it. This has worked for me for years and I learned it from someone else long ago. If it doesn't work for you - find another way.

Bob S.

[Michael Daly]

Your description of how to align things will work fine, but that requires that you keep your eye perpendicular to the scale. If you don't do that, even with only one eye open, you still get parallax. Closing one eye doesn't eliminate the problem. However, your description involves a lot more than just closing one eye.

Mike

[Steve DeMars]

Would appreciate original info. Read reply. How do you read and eliminate parallax?

[Leon]

The absolute way to eliminate parallax is to scribe a second line on the opposite side of the current cursor. Then when you are correctly aligned you will only see 1 line. If you see to lines you are not reading correctly.

[David]

That's what I did to my Twin Linear fence cursor. When doing dovetails if you miss a mark by 1/32 there goes your nicely cut dovetails...

Dave

Le>

[Prometheus]

Perhaps it's just me, but I don't use any cursor to set up cuts. Generally, I use a cabinetmaker's rule with the zero end held tight to the fence, and line it up with the tip of the blade on the corner that is nearest the cut. Look at the blade with both eyes open, and make sure that you can only see the kerf. If you can see any part of the side of the blade, you're not aligned correctly. Move your head, and then check the measurement again. Never have to worry about the silly gauges on the fence system that way, and wonder if they've gotten bumped or bent or otherwise distorted. Once the fence is locked down, it should be good to go. I run a saw for a living and it works for me, generally with a tolerance of 1/64" or less.

The same process applies when cutting miters on the edge of stock, but you must bring your eyes to a level that allows you to look straight at the blade from angle that is close to parallel to the table- if you look from above, it will also introduce error.

[Leon]

I did it that way before getting a good fence. Good advise if you have a less than desirable fence, A good fence is going to give you repeatable set ups using its silly gauge. ;~) Using my Jet fence I get repeatable setups using the cursor way better than you are getting. Personally I cannot see how parallax would throw a set up off much more than 1/128" given a worst case scenario. But if you dont have a great fence, you do with what you have.

[Guess who]

I use exactly the same procedure, and I don't do it fo a living. it jus makes sense ot measure directly. Besides, I have a feeling it was a troll [one of those that comes close enough to reality, but not wuite.] The reason I say that is that is that although it can not be denied that some parallax exists, it means about as much as those arguments that measure to the nearest ten thou or thereabouts, as if they were building jet engines instead of crunching wood.

[David]

[Edwin Pawlowski]

That won't work with a Biesemeyer fence. You line up the cursor, lock the handle, and the fence may move slightly as it lock in place exactly where set, as it should. Your method is good for my old Craftsman, or to double check on the Bies cursor, but one in place, the Bies is exactly where set.

[Prometheus]

There is that. I do have a less-than perfect fence with my little TS200. But I got in the habit of hand measuring at work, where we cut bar ends on a vertical bandsaw by lining up the final cut manually (the rest of the cuts are power fed), and parallax problems really come into play. It's tough to see how it could be a signifigant problem for someone who does have a good fence, so I figured I'd toss out the method for us bums. :)

That, and FWIW, I was was checking out a $600 Delta contractor's saw this weekend (didn't note the model no.), and the one problem I could find with it during my impromptu inspection was that the measurement gauge was off by at least 1/4". Not a problem if you know it, but it can drive a guy who likes to trust the scale nuts!

[Prometheus]

Well, parallax can become a big problem quickly, but not too terribly often on the table saw. Where I've run into problems with it is when you take 5 or 6 2.5" round tubes, and try to line up a 45 degree angle on them using a vertical bandsaw (where the blade tilts, not the table) If your eyes aren't in the right spot, it can throw off the measurement by a quarter inch or better. Of course, that's not using the OPs concern about the minimal gap between the indicator and the scale, it's using a rule and the blade as references. You're right though, in the case of a 1/16" or 1/32" gap between the cursor and the fence rail, it's a fairly trivial thing to worry about.

[Prometheus]

It's certainly no Biesmeyer! It's just the little tube fence that comes on the nicest version of the Delta benchtops. It's not a bad saw overall, but it's still a benchtop- and the table is too small to upgrade the fence with any of the aftermarket fences I've seen.

But, the fence is straight and it is parallel to the blade, and that's really all you need!