@4nthony You really need a CALIBRATED reference surface plate and an indicator holder with no flex to take meaningful measurements of the plate.

Woodpeckers has a 30 day return policy for a full refund without a restocking fee. You would normally be required to pay shipping. Given that you seem to have gotten an example that slipped through quality control, I'd be sure to request a return shipping label on their dime.

Up to you on whether you want to try again with another one or toss in the towel and just get the refund.

Ron

A guy like you should have a granite test plate. More generally anyone who owns a dial indicator should have a truly flat reference surface and a couple of ways to hold the indicator.

To properly test that WP plate you need to clamp the opposite side arbor area on top of a 1-2-3 block which is on top of a granite surface plate/stone. (You need to test whether the part that engages the arbor is parallel to the surface you are try to make parallel to the miter slot. To that end, both sides of the plate should be surface ground.)

Then mount your indicator to a stable enough fixture that you can run it across the same surface plate. An 8x10 stone might be big enough but bigger is better.

But your test is good enough to show that your WP plate should be replaced, or if you’re generous, just don’t use that really bad end.

Are you measuring the plate or your table?

I considered that, hence my previous comment:

Then I started thinking my tables are probably not perfectly flat and maybe the 1-2-3 block is riding across undulations in the table. Do I want to check every inch of cast iron to look for a perfect 12" section that I can use to check if the plate is flat? How far down this rabbit hole do I want to go?

I did a few more tests, passing the plate under the dial indicator. My thinking was the 12" of plate would pass over any changes in table flatness. I thought I had a clip of the surface ground side but I guess I only hit record on the blanchard ground side.



Is the glass I rested the block on flat? None of my cast iron is perfectly flat. I'm chasing perfection in an imperfect environment.

Woodpeckers has a 30 day return policy for a full refund without a restocking fee. You would normally be required to pay shipping. Given that you seem to have gotten an example that slipped through quality control, I'd be sure to request a return shipping label on their dime.

I'll see what they say on Monday. Unfortunately, I tossed the packaging it came in on the day it arrived.

My local Woodcraft has the granite plate in stock. Maybe I'll dive just a tiny bit deeper down this rabbit hole...

Thanks all!

Ah, one of my pet peeves of indicator measuring, and extremely common. With this type of indicator, the arm needs to be parallel (or as close as possible to parallel) to the surface being measured or you get the dreaded "cosine error". In the screen capture above, the arm is about 45 degrees to the surface being measured, and hence a change in the surface of say 0.010" will read about 0.007" on the dial, i.e. 0.010" times cosine 45 degrees = 0.010" times 0.7071 = ~0.007".

With the "other type" of dial indicator (plunge type), a similar error can occur if the plunger isn't perpendicular to the surface being measured, but this is less commonly abused to a great extent.

You can confirm my analysis by sticking a 0.010" feeler gauge under the arm or plunger and see what reading you get.

Quote from: dicktill on March 18, 2023, 01:08 PM

Ah, one of my pet peeves of indicator measuring, and extremely common. With this type of indicator, the arm needs to be parallel (or as close as possible to parallel) to the surface being measured or you get the dreaded "cosine error".

Ya, that's always been an issue and most people are not aware of it. Brown & Sharpe and Starrett claim the difference is negligible if the arm angle is 15º or less, while Interapid indicators are designed to give a true reading when the arm angle is 12º.

I find it easier to adjust the arm angle to 15º or so rather than trying to place the indicator at 12º, although I'm sure Interapid's method is more accurate. Interapid even refers to its vertical & horizontal indicators as the INTERAPID® 312 Vertical & Horizontal Models.

Makes sense: the cosine of 12 degrees is 0.978, and of 15 degrees is 0.966, so only "a-couple-three" percent error anyway, even if they didn't calibrate at that/those spots. It's when you get into larger angles that the error soars.

Very informative and interesting thread.


I have a bunch of woodpeckers stuff and the couple of times I've had a problem I picked up the phone and they've promptly fixed it.

I got my replacement plate a couple weeks ago and had some time to mount it on my saw today.

In lieu of taking a bunch of photos showing blade alignment and measuring how flat the plate may or may not be, I'll just say that the new plate shows a nice linear progression from 0.0 at the start, to .003 at the arbor, to .006 at the far end.

Exactly the type of results I was expecting to see.