Stellafane 'Little Man' - click for Stellafane Home Page ATM Header

Rough Grinding

This page describes the rough grinding process.

Before You Start

Use this checklist to make sure you are ready to start rough grinding:

You may click on the hyperlinks in the above checklist to visit the pages where we discuss each of these items. When all the check list items are done, you are ready to start grinding!


With any glass grinding operation, you must work wet, to prevent glass dust from entering your lungs. Always keep the glass and tool wet to prevent glass dust from becoming airborne. Never grind glass dry!

Inhaling glass dust can result in Silicosis, a serious lung disease.

Keeping your work wet is a proven technique to prevent inhalation of glass dust. Keep your work and wet and you should not have to worry!

Rough Grinding or "Hogging Out"

Peter grinding
Peter uses the Chordal Stroke
on a new 8" Mirror

Rough grinding is often called "Hogging Out", as your goal is to remove the majority of the glass needed to form the curve of your mirror. It is not particularly elegant work, being the noisiest and messiest part of the whole mirror making process.

GOAL: You have completed rough grinding when you have:

  • Removed enough glass to reach you target sagitta (0.001")
  • The curve you have ground extends evenly and smoothly to the edge of the mirror.

ABRASIVE: Use your coarsest grit. This is usually for #60 Silicon Carbide, but may be #80 for 6" or smaller mirrors, and might be #40 for very large mirrors.

TOOL: Use your hard tool (Glass or Tile). Start with the tool on the bottom.

STROKES: Begin with the Chordal Stroke and as the curve spreads towards the edge end with the Normal Stroke (See text below for transition information).

PRESSURE: The harder you press, the faster the grinding will go. But pressing is hard is tiring, and you need to grind over a period of time, so don't press so hard that you wear out easily. If you don't press at all, grinding will go much more slowly, but it will still get done. Some people use barbell weights on top of the tool or mirror so that gravity provides the pressure instead of them.

TIME: A 6" f/8 or 8" f/6 mirror will typically be rough ground in 3 to 6 hours or work, depending on mirror size, grit size, pressure applied (more pressure means short wets but more glass removed) and personal work style.


Start with the Chordal Stroke at 40% overhang. Do about 15 Wets, which is about 15 to 20 minutes of work. Be sure to rotate you tool and mirror (or rotate you tool and walk around your grinding stand) about every 5 to 10 strokes. This is critical in all stages of grinding, so get in the habit now.

The Spreading Curve
Curve Depth
Greatly Exaggerated

Then stop and measure. Clean your mirror by dunking it in a bucket of water. Just use a straight edge to see the initial, central depression in the mirror, like the second picture from the top at right. Then measure your sagitta, to see how deep you have dug the hole. Your goal is to get to your target sagitta just about the time the curve spreads to the mirror's edge.

You would like to be at about 1/4 of your target sagitta and have the curve about 1/4 of the mirrors diameter at this first stage. These are very rough goals, don't try to hit them exactly, just get close. If your are short of these goals, do a few more wets until you get close. If you feel you are going too deep too fast, reduce the chordal stroke overhang to slow deepening and put the work of grinding towards spreading the curve. Similarly, if you have curve that is wide and shallow, increase the chordal stroke overhang to concentrate the grinding work on deepening the curve. Do more wets, and clean the mirror and tool about every 10 wets.

Once you have achieved the initial 1/4 sagitta, 1/4 curve diameter goal, continue on with anther set of wets with less overhang on the chordal stroke (perhaps 30% to 25% overhang) to get to a curve that is about 1/2 of target sagitta and spread to about 1/2 of the mirrors diameter. Stop and measure with a straightedge for curve spread and check your sagitta about every 15 minutes, and adjust your stroke overhang as necessary.

In order to spread the curve outward to the mirror's edge, you keep reducing the overhang of the chordal stroke. In the last stage, you will want to reduce the overhand to nothing. At this point, convert to using the Normal Stroke for the remainder of your work.

With the curve out to the edge of the mirror, check you sagitta. If it is not deep enough, do more wets with the Mirror on Top. If it is too deep, flip the mirror and tool positions, and do some wets with the Tool on Top, which will wear the edges down and reduce the depth. Using these techniques, you should be able to get within 0.001" of you target sagitta


Cleaning a Turntable

Check: that the curve is really to the edge of the glass everywhere, and that you are at or very near your target sagitta.

Bevel: Have you reduced or eliminated your bevel? If so, now is a good time to put it back to where it needs to be. Instructions for How to Bevel.

Clean: Now is the time to clean everything, and make sure none of your coarse grit is lurking about to cause a scratch in the next stages. Start by cleaning the mirror and tool, and put them safely aside in a clean place. Then clean you work area, including your grinding stand or turntable, water buckets, spray bottle, and anything else that may be hiding that odd piece of grit. Finally, clean yourself. Change your clothes and take a shower. Clean under your fingernails, also, a favorite grit hiding place. You must clean thoroughly between each grit - this is the first of many cleanings!

CONGRATULATIONS! You have finished rough grinding. On to Fine Grinding

What is a "Wet"?

A wet is one unit of work on your mirror, begun with a spray of water, a charge of grit, and then grinding until the grit breaks down and you are no longer doing useful work, or on finer grits until the wet dries out. It doesn't matter if you spray or sprinkle first - I like to spray first, but the young man I was photographing (below) preferred to sprinkle the grit first.

For rough grinding a 6" mirror, about 1/2 teaspoon of grit is used per wet. With finer grits, proportionately less grit is used per wet. Grits in the25 micron range are often premixed with water to form a slurry, and water and grit are squirted onto the work together from a plastic squeeze bottle (not a spray bottle).

Sprinkle on Grit
Sprinkle on Grit
Spray on Water
Spray on Water
Always Work Wet!
Grind Until Grit Breaks Down
or Work Dries Out

Cleaning the Mirror & Tool

Cleaning a Mirror
in a Dunk Bucket

After several wets, your mirror and tool will be covered in a gray mud of ground glass and spent abrasive. You will need to clean them off periodically, because the mud will inhibit efficient grinding or you need to take a measurement. You need not do this after every wet; every 5 to 10 wets is typical practice.

The best way to clean is to use a plastic dunk bucket. Just dunk the mirror and tool into the plastic bucket filled with water a swish a bit; all the mud will wash right off and fall to the bottom. DO NOT RINSE MUD DOWN YOUR DRAIN - it will build up into a hard cement and you will be calling the plumber very shortly. To clean out your dunk bucket, you can pour off the water down a sink drain, then scrape out the gray mud into a plastic bag, or hose it out in your yard (it is very harmless and inert stuff, not much different than beach sand).

If you need to dry off the mirror for a measurement, a soft cloth towel or paper towels will do the job.

Back to the ATM Index Page

Page last revised on Sunday, 2011-Jan-02 20:27:12 -0500