Miter slots NOT parallel - problem?

[Chip Lindley]

It is understood that both miter slots are milled in a CI tabletop at the same time on the same machine! Thus by all reason, slots will be parallel! I will qualify that statement by adding: *If the table has not otherwise distorted, or the milling operation was flawed!*

Another issue to consider: Look into your miter slots closely and you will see some very coarse mill marks. Your indicator point could be *in* or *out* of these rough spots. File the sides of your slots ever-so-slightly with a flat bastard file! Same amount of strokes on each side! See if your measurements do not improve!

In the event both slots will not accept absolutely parallel bars (such as sled runners) the right runner can be made narrower, drilled, and those spring-loaded buttons inserted to the right, to take up the inconsistency. Lots of ways to *skin the cat*!

[Kevin Downs]

Wow! So many thoughts. Using some of the suggestions above I went about retesting my measurement. I ended up using an Incra Miter Slider (what I may use for the sled) and locked it down in the slot.

Then I used a different tip on the A-Line-It indicator (great tool) to remove some more 'varables' and guess what!?

-.0015

That's .0005 different than the saw blade measurement. Check out the pics.

Now to prove that there isn't any issue with using double runners I'm going to build a temp sled and run it through! I'll post the results.

Thanks to everyone for ALL the ideas/thoughts. I'm going to go out on a limb and say that having a supper accurate gauge made me think the worst.

Now that I think about it I don't remember seeing David Marks using a digital indicator. And Charles Ingalls didn't have a supper flat, super accurate table saw - and he built a house!

I guess I can live with .0015.

Kevin

[Tom Veatch]

... I'm going to go out on a limb and say that having a supper accurate gauge made me think the worst...

Sometimes having too much information is as bad as not having enough. At least it can certainly be unnecessarily unsettling.

If your miter slots are parallel within .0015 TIR, I'd hazard a guess that's about as close to perfect as you can expect from a mass-produced consumer product.

[Bob Wingard]

"are your miter slots parallel?"And the angled indicator *will* read more than the actual runout in this case. So if it was 'straight' when he measured the blade, and angled in the slot that will make up some of the difference. Not a lot though. At 30°, you should subtract about .001 from a .007 reading.

NO .. NO .. NO !!

The angled indicator will read LESS than the actual amount.

Look at it this way .. using your method, if the indicator reads more than the actual amount at, say 30 deg .. it will read even more at 45 deg .. more still at 60 deg .. even more at 90deg !!!!

At 90 deg. if you put pressure on the stylus, you will destroy it.

Now, my way .. at 0 deg = accurate & actual .. 30 deg. .. less resolution and less accurate .. all the way up to 90 deg .. zero accuracy, 'cause you're pushing sideways on the stem, which won't move the needle at all.

Now, does that make sense ??


<<<__ Bøb __>>>

[keith ouellette]

Sometimes having too much information is as bad as not having enough. At least it can certainly be unnecessarily unsettling.

Man, You said it there.

[Kevin Downs]

Now, does that make sense ??

Almost totally! Please add pictures as I am only 99.999999 sure I understand.

[Wes Grass]

Nope, doesn't make sense at all.

Set it 10° from vertical, and the tip an 1/8" past the slot. How far will you have to pull the indicator stem up so the tip will drop into the slot?

Answer: ~.72"

[Phil Thien]

C'mon .. we're talking about a thou. or so .. how much can it possibly affect the reading ??

Maybe your table saw is flat to within a thousandth, most aren't. The specs from Rockwell and Grizzly call for tables to be flat to within .01" (one-hundreth of an inch), IIRC. Many will be less. Some will be .0099 and will be within spec.

The angled plunger will actually make the indicator read a lesser number .. not greater. The shortest distance between 2 points is still a straight line, and the best resolution for a dial indicator is a path along that line.

No, sorry, that isn't correct.

When the plunger is angled, you are no longer measuring a straight line from one miter slot to another.

Here, let me give you a for instance. Assume a perfectly flat table saw. Measure the distance between slots at the front of the table using the setup as indicated in the photo (with angled plunger). Now move the gear to the rear of the saw, but add .005" of shim under the indicator to simulate a table that isn't flat. If you just solve for the triangle (figure a right triangle with side a of 1" and side b of maybe 2 or 2.5") you can see that a table that is out of flat by .005" can cause an error on the indicator of as much as .002".

.002" is 1/4 to 1/2 of his error.

[Bob Wingard]

If it makes you feel better, go on believing that ..

[John Lucas]

A couple of simple statements: 1) when you make the sled, use only one runner...I would use the left miter because I am right handed and I generally work on the left side of the blade. With only one runner, it should track fine and no need for 2nd anyway. 2) don't measure; cut wood

[Tom Veatch]

...Look at it this way .. using your method, if the indicator reads more than the actual amount at, say 30 deg .. it will read even more at 45 deg .. more still at 60 deg .. even more at 90deg !!!!...

And that is correct except for the singularity at 90° which involves a division by zero and is undefined.

The tip of the indicator moves along the hypotenuse of a right triangle while the carrier of the indicator moves along the base of that triangle. When the carrier moves a distance X along the base of the triangle, the indicator tip moves along the hypotenuse a distance X/cosA (or X*secA) where A is the included angle between the base and hypotenuse of the triangle. The larger the angle between 0° and 90°, the smaller the cosine and the greater the multiplier effect.

Get a piece of paper and sketch out the geometry involved.

[John Grossi]

Kevin, As I was reading your initial post, my first reaction was the way you were measuring the slots to the blade were the reason for those numbers. When I got my 1023 I set it up blade to the left slot, then blade to the fence. I was in such a hurry to see dust, I never checked the right slot. My first project was to make a sled, and I used 2 runners. It is not pretty but it works great. I use it every chance I can. John

[John Keeton]

I have no idea how parallel the slots on my 1023 are - nor do I care. They are close enough for me to work safely - and that is the important part for me. Without a high tech instrument, I took a reasonably accurate measurement of the blade-to-left slot alignment. My double runner sled works fine - using some tips from other creekers. I don't get any burning on my cuts. No binding or hint of kickback. Using my crude methods of measurement (same ones they used 200 years ago) I get good cuts.

Well, here is the amazing thing! When I cut a bunch of wood, after it gets through moving and doing its thing, I get through planing and sanding, I'll be danged if it all doesn't go together just fine! But, I will admit, it may be off by .001 or more

You guys are all apparently machinists or engineers. I am just a lowly weekend woodworker. I agree with John Lucas - just cut wood. Odds are, if you don't have black saw marks on the wood, or hear funny noises or feel some odd forces on wood going thru the saw - everything will be OK.

Think of the wood you could have cut while worrying about this stuff. Lighten up and enjoy this hobby - unless your hobby is finite adjustments on your woodworking tools. If that is the case, have a good 'ol time. I'll be cuttin' wood

[Kevin Downs]

I think I need to make my problem clear. I'm VERY happy with the left slot, the blade and the fence - heck the saw itself! My 'problem' was with the slots to each other and a double runner table saw sled. I wanted to know if having my slots spread away from each other (going front to back) would BIND the sled. This problem had nothing to do with cutting and everything to do with a double runner sled.

Turns out my measurements were off and due to the MANY GREAT posts in this thread I was able to remeasure and get a truer reading.

[Duncan Horner]

C'mon .. we're talking about a thou. or so .. how much can it possibly affect the reading ??

The angled plunger will actually make the indicator read a lesser number .. not greater. The shortest distance between 2 points is still a straight line, and the best resolution for a dial indicator is a path along that line.


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That's the same mistake in reasoning you made in another thread on checking runouts Bob. It still doesn't work. Best resolution is not along the shortest distance, it's the opposite. Once again, draw it on paper.