Kinetic Precision
Kinetic Precision

Precision Flat-Ground (PFG) Stones

Precision Flat-Ground (PFG) Stones

Order PFG Stones here.

Precision Flat-Ground (PFG) Stones are like magic. These are no ordinary abrasive stones. The four largest surfaces are diamond ground very, very flat. They are intended to deburr normally flat surfaces without harming the original precision grind.

Use PFG Stones on:

  1. Magnetic chuck surfaces prior to putting parts on

  2. Part surfaces prior to putting on magnetic chuck

  3. Machine ways

  4. Deburring during hand scraping

  5. Flat-ground surfaces prior to putting on a surface plate or another ground surface

  6. Toolholder tapers (CAT-40, etc.)

  7. Collet tapers (ER-32, etc.)

When you first get your PFG Stones, and every time prior to using them, rub the “working” stone on the “conditioning stone”. Don’t be afraid to use pressure, five pounds will not hurt. You will feel and hear surface condition, and you’ll notice the stones get smoother in a few seconds. This is the foreign particles being worked out, and the surface getting smoother. Once this happens, you can transfer to the work.

Notes on using your PFG Stones:

  1. Don’t throw out the dark grey packing material!!!  This is Absorbent Mat, and we use it in our shop for soaking up oil.  Please give it a job. (McMaster 7193T84)

  2. Please report any damage to your PFG Stones.  We do our best to pack them, but the USPS is pretty clever.  We want your Stones to arrive PFG.

  3. Please watch Robin Renzetti’s video on the use and care of precision stones.  He was the inspiration for PFG Stones, though he will remind us that he did not invent them.

  4. Keep your stones in the tray when not in use.  The “cleaning” stone can stay in the tray during use.

  5. DO rotate and randomize your PFG Stones.  Use either one for “working” or “cleaning”, and switch them up.

  6. BOTH sides are diamond flat-ground and PFG!  The fine side is 320 grit, the coarse side 100 grit.  The long sides are also flat-ground square, with the same recipe as the faces.  The edges are all deburred. The small ends are not flat-ground. Grinding of a lip on one or both sides is available as an option at additional cost.

  7. Clean your stones with a toothbrush (or similar) and isopropyl alcohol (McMaster 3190K812). They work just fine when they are amazingly dirty, but what if guests show up?

  8. In the supplied tray, keep one PFG Stone fine side up, and the other coarse side up. If, for example, you want to use the fine side, you only need one hand to pick up the stone which is fine side down and clean it on the other stone. And you can put it back in the tray and store it with one hand. You won’t believe how useful this is.

Instructions for use

  1. Clean the work.

  2. Rub two stones against each other to remove foreign material on the stones.  You’ll feel and hear them get clean.

  3. Carefully apply one stone onto the work and slide with light pressure.

  4. If loose debris can be felt, stop and go to Step 1.

  5. Burrs and upsets on work can be easily felt if the work is clean.

  6. Continue stoning with moderate pressure until the work feels smooth.

  7. Look for bright spots on the work which indicates the bearing surface. If there are no identifiable bright spots, your work is very flat!

  8. Ideally, periodically clean the stones in an ultrasonic cleaner with an anionic detergent solution.  We use LPS Precision Clean (1:10 dilution). Rinse in clear water and allow the stones to air dry completely before use. Isopropyl alcohol (99%) and a toothbrush works well, too, for routine cleaning.

Thank you for your business.  PFG Stones are made to order. Lip grinds are available upon request, at additional cost.

These are precision instruments, please take good care of them and they will give you a lifetime of faithful service.  And they'll listen to your bad jokes without complaints.

Useful Links:

Here is an excellent article by Tom Lipton about precision flat-ground stones.

Here are instructions by Starrett about how to stone gage blocks. You may use the fine PFG stone for this purpose.

An excellent paper: Marsh, Eric. Repairing a Damaged Part with Flat Stones, Penn State University. Originally from: <>, recovered from the Internet Archive for your convenience. Date of publication unknown.

In Robin Renzetti’s video on Precision Ground Flat Stones, he mentions Professional Instruments Company. Their website has some limited information about flat stones.

Order PFG Stones here.