I Bought a New Granite Surface Plate - ToolGuyd

I bought another new granite surface plate, with this one being 12″ x 18″ x 3″ and with 2 ledges.

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It’s a Grade A plate with 0.″ surface accuracy and 0.″ repeat gage reading.

Generally, B is toolroom grade, A is inspection grade, and AA is laboratory grade. The higher the grade, the higher the accuracy – and price.

Granite surface plates provide a flat reference surface.

In total, I have a Starrett 12″ x 8″ x 2″ “toolmaker flat” for portable or office use (also a recent purchase), a new Precision Granite-branded 12″ x 18″ x 3″ surface plate (shown above), and an older B-grade surface plate that I bought 15 years ago from Enco for $15.

While I was hoping to buy a larger Starrett surface plate, they’ve been backordered at every supplier for many months. If I ordered one last spring, I likely still wouldn’t have one. I luckily managed to find another USA supplier.

I upgraded more of my metrology equipment as well, such as splurging on a digital height gauge to replace a much cheaper one with a dial readout. I bought the dial height gauge more than 10 years ago, and never liked using it.

I also bought a cylindrical square for use with the plates, and the cost was painful. I went with a USA-brand there too, and regret it a bit, as it’s hard to move around. Import squares come with handles.

There are measuring tasks where all of this comes in handy, but there are also some checks that can only be done with a flat reference surface.

Imported toolroom B-grade surface plates can be found for less than $100, and they’re still decent. My older one will continue to be used in other ways, such as a sandpaper lapping plate for polishing or sharpening tasks.

If you’re interested in more detailed posts about precision measurement tools and the like, please let me know!

For more Inspection Surface Plateinformation, please contact us. We will provide professional answers.

In the meantime, I’m going to have to find a cover for the surface plate to protect it from dust and debris. Any ideas?

Symbol:

Definition:

Datums are theoretically exact points, axes, lines, and planes or a combination thereof that are derived from datum features. A datum feature is the tangible surface or feature of size (comprised of multiple surfaces or revolved surfaces) that is indicated by the datum feature symbol. You can think of them as anchors for the entire part. They are the surface or feature where the other features are referenced from. It is usually an important functional feature that needs to be controlled during measurement as well. 

All GD&T symbols except for the form tolerances (straightness, flatness, circularity and cylindricity ) can use datums to help specify what geometrical control is needed on the part.

Datum Features vs Datums:

Its important to know the difference between Datum Features and Datums. While they are related to one another they are indeed different entities. It is especially important to know this when studying the theories of Geometric Dimensioning and Tolerancing and while setting up a Datum Reference Frame.

Datums are theoretical and only simulated by Measurement Equipment (Gauge pins, Granite slabs, angle plates, computer-generated planes, etc) 

Datum Features are real, tangible features on a part where the measurement equipment would physically touch or measure.  They are usually important functional surfaces. 

How Datum Features are Shown on a Drawing

The datum features on a drawing are denoted with a series of capital letters. These letters are in boxes and tied to the datum features with a black triangle. This letter will also show up in any feature control frame that uses this datum feature as a reference. A feature control frame will reference as many datums features as necessary to lock in all the necessary degrees of freedom, this creates what is known as a Datum Reference Frame.

Notation is Important on Drawings

As stated before, datums can be points, axes, lines, and planes or a combination thereof. It is important though that datum features are indicated correctly on the drawing to ensure that the right type of feature is being controlled. Datum features can be either a surface or a feature of size, depending on how the symbol is applied. 

Datum Feature Symbol on a Surface

1. Point with a solid or dashed leader line with the datum feature symbol placed on it. A dashed leader line indicates the far side surface.
2. Datum feature symbol placed directly on a planar surface.
3. Symbol placed on an extension line off the datum feature surface.
4. Leader arrow with the datum feature symbol pointing to a planar surface.
5. Symbol placed on a size dimension but placed offset of the dimension arrows.
6. Placed off a feature control frame that is applied to a surface. (Datum feature is geometrically controlled.)

When the symbol is indicated in the methods stated above this means that the datum feature is the surface of the associated with that symbol. 

Datum Feature Symbol on a Feature of Size

1. Symbol is placed on a dimension line directly in line with the dimension arrows.
2. Symbol placed on the leader arrow of a size dimension.
3. Symbol placed on a non-planar feature of size.
4. Placed off a feature control frame that is applied to a feature of size.
5. Symbol placed on the arrows of a size dimension.
6. Placed as one half in an open-ended size dimension (in line with the arrow).

A few common Feature of Size Datum Features are bores, cylinders, slots, or tabs. These features will generally result in a datum that is an axis or plane (midplane). For example, the datum of the bore labeled as Datum Feature C above is going to be the axis of that bore, NOT the surface of the bore itself. In this example note the tangible Datum Feature is the surface of the bore itself while the Datum is the theoretically exact axis of that cylindrical surface. 

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