Every mold making, precision machining, aerospace and tool-and-die shop uses a granite surface plate as the foundation for using precision measurement tools; read on…
Keep your granite surface plate clean
Granite is extremely hard, but not so hard that it cannot wear out and become uneven and no longer truly flat. Because the variations are so small it is easy to become deceived into thinking that your surface plate is still extremely flat.
Many shops neglect to properly use and care for their surface plate and suffer as a result.
Care should be taken to keep the surface clean and free from the inevitable dust and grit that settles throughout the day. It is truly amazing to watch somebody slide a heavy steel plate onto a dirty surface plate and expect to obtain an accurate reading.
Yet this happens all the time. Just imagine all those little dust particles as sandpaper grit, and the heavy plate as a lapping plate. That is exactly what is happening. Over time the plate obviously wears from this type of abrasion and becomes inaccurate.
The Metrology Handbook, Second Edition
An authoritative book, The Metrology Handbook, Second Edition provides a foundation for understanding basic metrology and calibration principles and practices.
Keep a lid on it
Companies such as Starrett, Mitutoyo, Standridge or Rock Of Ages sell covers to protect surface plates. These should be used regularly to help maintain accuracy.
These same companies, and others, sell surface plate cleaners that are specially formulated for granite. Simply follow the directions, or just use common sense when cleaning. The idea is to keep your plate clean and free from the damaging grit that wears it out prematurely.
Keep your granite surface plate calibrated
Setting up a resurfacing schedule is essential. This will ensure that the measurements are actually what they appear to be. Because the wear is so gradual, it is easy to overlook little inaccuracies, until they become a big problem.
Basic Metrology for ISO 9000 Certification
An authoritative book, Basic Metrology for ISO 9000 Certification
gives a nice introduction to ISO 9001 requirements for calibration of test and measuring instruments.
Keep your surface plate stable
If the granite is heavy or not mounted on a benchtop, make sure to use a sturdy steel stand. You could fabricate your own, or simply buy one from the same manufacturer as the plate. Buying one from the manufacturer has the advantage of having been properly designed and built for this application.
If your plate is placed on a toolbox or benchtop, be sure to support it evenly. This will contribute to consistent accuracy and help to prolong the life of the plate.
Keep accessories in a safe place
If you have granite parallels, angle plates, vee-blocks, squares, toolmakers flats or tri-squares, make sure they are kept out of harm’s way. A great place to keep them is in a wooden cabinet, or in their own case.
Some bad habits of toolmakers and machinists
We all have some bad habits, and some of us have more than a few. When it concerns granite surface plate use and care, here are a few of the habits to avoid:
- Do not use your surface plate as a lapping plate. You see it all the time, a sheet of sandpaper lying on the plate, to be used for removing burrs, evening out surfaces and so on. The plate works great for this, but over time the grit from the sandpaper will damage your plate. If you do use sandpaper, clean it up afterward and don’t leave it lying around.
- Never allow oils, such as cutting oils, tapping oils, or kerosene to gather on the plate. This mixes with the dust and debris to make a nice slurry to lap your plate out of flatness.
- Sliding dirty plates onto the surface plate. You would think this is so obvious that it would not need mentioning, but it happens all the time. Chips, oil, and burrs all slide along the granite, leaving a trail of white dust.
- Hammering on the plate. Duh? Hammering dowel pins, fitting inserts, assembling components or any other set-up that requires pounding should obviously be done on a bench or anywhere else but your granite.
- Using your granite surface plate for a storage shelf is certainly not what it was intended for! Sliding books, boards and such just cannot be a good idea.