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Surface Gage

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Posted 09 January 2009 - 02:43 AM

Although now retired, I spent 30 some years in manufacturing. The rodbuilding machinists will know what Iím up to immediately. I can work both, fast and accurately because of knowledge Iíve gained in manufacturing. Hereís a little tidbit that will help take guesswork out of planning, but more specifically, hitting the numbers right on the nose, every time!

Iím starting with an Interapid test indicator, which is graduated at .0005 per division. Perhaps a little pricey, but Iíve gained a lot of tools through the years. I suppose one could be had from ebay for perhaps 50 bucks give or take. Donít cringe guys, but Iíve epoxyíd it to a hardened and ground parallel. In doing so, made a usable surface gage for rodbuilding. Just place the surface gage on a true surface and adjust the dial to zero. Any variation under the indicator point will be reflected on the dial as being above or below the reference zero.

Hereís a picture of my small surface gage resting on a surface plate. It is un-adjusted inasmuch as it is not yet set to zero. Setting it to zero is easily accomplished by moving the indicator point up or down to touch the surface. It has a friction bearing setup which allows for itís adjustment independent of the dial itself. How to use it? I use it to set my my planes, all of which have a rodmakers groove. I can take an iron from the strop, install it in my plane and adjust the plane and verify where the edge is really at in less than a minute.

Here you see the surface gage resting across the rails of a block plane with a rodmakers groove. The surface gage is adjusted to zero on the rail and becomes the reference point. The camera angle makes it look slightly off but it is zero nonetheless.

Below, youíll see the surface gage shifted slightly so the indicator point is resting on the bottom of the rodmakers groove. You can see from the picture that the groove is .005 deep. Itís important to understand that the surface gage is determining that the surface is below the reference zero by a magnitude of .005 in. Bear in mind, that all readings counter-clockwise of the ( 0 ) on the indicator dial, would be an indication of the surface above the zero reference point (the rails). In this case, the reading is clockwise of ( 0 ) which indicates that the surface is below.

Below youíll see the surface gage set so the indicator point is resting on the very edge of the planeís iron. From the picture, youíll easily see that the cutting edge is below the plane rails (zero reference) by a magnitude of .001. Now, to keep this in perspective: A human hair is typically .004 in. thick. This plane is adjusted to within ľ the thickness of a human hair. Hereís why a rodmakers groove is so valuable. If the iron is below the rails, it is impossible to gouge the planning form, and you can be accurate on your planning to a tolerance to well under .001 in. with a plane alone. Typically, with this setup, I finish plane to .0005. I donít even need the scraper anymore.

Iím sure some of people are thinking Iím crazy by now but wait, thereís a method to my madness. Below, youíll see the surface gage again zeroíd on a flat surface (planning form)

Next, The surface gage is moved so the indicator point rests on the strip itself. You can see that the .001 the iron was adjusted to (below the rails), after planning, is now transferred to the strip, and shows the strip to be .001 above the planning form. I used .001 for illustration purposes, but normally set my plane a breath closer to zero. Itís easy to see that this fast and easy method of adjusting your plane will be helpful in two unique ways. Firstly, Youíll never gouge your planning form, which saves forms and irons, sharpening notwithstanding. Secondly, you can plane with ultra precision, and predictability.

If anyone decides to make one, keep the thickness of the parallel a little on the thin side so none of the machinists out there start crying about indicator cosine error which is beyond the realm of whatís important in this application.

Perhaps all of this seems like way too much for nothing, but if you want accuracy, and predictability, youíll give this some thought. This is way more difficult to explain than it is to do.


Sometimes while sleeping I set the hooks so hard I wake myself up! It's really going to get ugly if I ever get one on!

#2 OSD

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Posted 09 January 2009 - 09:00 AM

Great information Todd thumbsup.gif

#3 graydog

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Posted 02 August 2010 - 10:46 PM

Great info Todd!

Is that a socket head screw in the side of the plane? I was thinking of using centering screws on each side of my plane to keep the blade from skewing.

Work and love are equivalent passions. Do good work!

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