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

Guide to Strokes

Introduction

This page will provide a quick reference guide to common mirror making strokes. We aim to provide text, diagrams and a short video of each stroke, explaining when it is used and showing how to do it.

The videos on this page are of file type .WMV for use with Windows Media Player. The are generally 0.75 to 1.0 MB in size, so they may take a long time to download on a slow link. They are approximately 30 seconds long, and are intended to illustrate the proper execution of each stroke.

Index to Strokes

The first three strokes are the basic ones used to produce a mirror, while the last four are used in figuring to correct errors:

Stroke Stage Use
Chordal Rough Grinding "Hogging Out"
(Initial generation of curve)
Normal or
1/3 Center-Over-Center
Rough, Fine, Polishing Generating a Sphere
W Stroke Figuring Parabolizing
Stroke Stage Use
1/4 Center-over-Center Figuring Correcting Turned Down Edge (TDE)
1/2 Center-over-Center Figuring Correcting Turned Up Edge (TUE)
W Center Deepening Stroke Figuring Parabolizing
W Edge Deepening Stroke Figuring Parabolizing

There are many other strokes that can be used to correct other figuring problems or that are alternatives to the strokes we present here. Please refer to books in our bibliography for additional information, particularly [Berry94], [Brown96] and [Texereau84].


The Chordal Stroke

USE: Used at the beginning of rough grinding to initially "hog out" a lot of glass and generate the initial curve in the glass.

MIRROR POSITION: Mirror on Top, Tool on Bottom

ABRASIVE: Your coarsest grit (usually #60 or #80)

PRESSURE: Firm downward pressure will result in short wets and faster glass removal.

DESCRIPTION:

With mirror on top, overhang mirror about 40% of its diameter over the tool, so that most of the grinding action is concentrated on the center of the mirror and the edge of the tool. Take straight strokes such that the center of the mirror travels along an imaginary chord1 on the tool (The red arrow in the diagram). This stroke will rapidly remove glass from the center of the mirror. After 5 to 10 chordal strokes, rotate the mirror and the tool in opposite directions a random amount, and repeat.

As a depression begins to develop in the center of your mirror (check with a straight edge after 15 to 20 minutes of work), reduce the overhang amount. Now you will be taking longer strokes, as the chord has moved inward towards the center of the tool. The effect of this reduction in overhang is to grind more of the mirror surface and tool surface, and spread the depression over more of the mirror surface. You may find your progress seems to slow as you do this, but this is because you are grinding a large surface, and therefore removing more glass which takes longer.

Continue to examine and measure your mirror, and continue to reduce the amount of overhang so that the depression in the mirror finally spreads to its edge. When this happens, you need to change to a 0% overhang stroke, the Normal Stroke.

1Chord: A line segment that joins two points on a curve.

DIAGRAM:

Click for Chordal Stroke DiagramClick
Picture at right to View Diagram
Illustrates initial chordal stroke with maximum overhang of 40%.
Gradually reduce overhang as curve develops to spread curve to edge.
In the diagram, the red arrow would move to the right (towards the center of the tool) and grow longer to keep in contact with the tool edge.

VIDEO:

Click to Play VideoClick
Picture at right to Play Video
Format: Windows Media Player Format (WMV)
Size: 859 kB     Length: 30 Seconds
Please ignore the background chatter; this was recorded in the  busy and crowded Stellafane Mirror Workshop - but please listen to and appreciate the grinding noise.

The Normal Stroke or 1/3 Center-Over-Center Stroke

USE:

  • Rough Grinding after the chordal stroke.
  • All of Fine Grinding.
  • All of Polishing.
  • May be used in Figuring to return to a Sphere.

Proper use of this stroke creates and maintains (a section of) a spherical surface on your mirror.

MIRROR POSITION:

Mirror my be on Top or Bottom. Alternate regularly to maintain Radius of Curvature (RoC).

  • Put Mirror on Top to Deepen Curve and Shorten RoC.
  • Put Tool on Top to Flatten Curve and Increase RoC.

ABRASIVE: Used with all grit sizes, and in polishing and figuring with a Pitch Lap and Polishing Compound (usually Cerium Oxide. Well thinned CeO seems to work better and faster than thick CeO.)

PRESSURE: With coarse grits, firm downward pressure will speed glass removal. Less pressure is called for with finer grits. Polishing calls for slow, steady strokes and moderate downward pressure will speed results.

DESCRIPTION:

(Written from the perspective of Mirror on Top. Just reverse Mirror and Tool in the description below if you are working with Tool on Top) Carefully place the mirror on the tool so they are concentric. Pull the mirror straight back towards you until it is overhanging 1/6 of the mirror diameter. Then push straight forward, passing the center of the mirror directly over the center of the tool, until the mirror is overhanging 1/6 diameter on the far side. Continue pushing to and fro, with a 1/6 overhang at each extreme for 5 to 10 strokes. Then rotate the mirror and tool in opposite directions a random amount, and repeat.

A common error for beginners is to overhang 1/3 at each extreme, resulting in a 2/3 diameter stroke. This will deepen the center of the mirror, and will not maintain the Radius of Curvature.

In order to maintain your desired Radius of Curvature, you should use this stroke with equal amounts of work with Mirror on Top and Tool on Top, reversing every few wets. If you are drifting away from your desired Radius of Curvature, you can correct small deviations by doing more work in one position over the other:

  • Use Mirror on Top to deepen the sagitta and shorten the RoC and Focal Length.
  • Use Tool on Top to flatten the mirror and lengthen the RoC and Focal Length.

DIAGRAM

Click to view Normal Stroke Diagram

Click Picture at right to View Diagram
Illustrates the normal stroke.
The red arrow indicates the path of the mirror center, which passes directly over the tool center. The total stroke length is 1/3 of the mirror diameter, which indicates a 1/6 diameter mirror overhang on either side at the beginning and end of the stroke.

VIDEO:

Click to Play Video

Click Picture at right to Play Video
Format: Windows Media Player Format (AVI)
Size: 42.8 mb     Length: 30 Seconds
Recorded at the Stellafane Mirror Workshop - No Audio - Using very fine grit (Aluminum Oxide) but the same stroke, albiet with more effort and therefgore a slower pace, is used for rough grinding and polishing.

This is an 8" mirror, so the stroke overhang is about 1.3" on each side for a 2.6" stroke length. The end of the video moves to an overhead view and you can see this stroke length quite well through the narly transparent mirror.


The W Stroke

USE: Used to parabolize a mirror when figuring. Note that some people use a slight W movement when grinding and polishing, augmenting the normal stroke which we do not recommend or describe; here we are describing a wide W stroke used for parabolization.

MIRROR POSITION: Mirror on Top, Tool on Bottom

ABRASIVE: Use with polishing compound (usually Cerium Oxide) and a Pitch Lap Tool

PRESSURE: Light - with good contact between lap and mirror, no additional downward force is necessary.

DESCRIPTION:

The W Stroke is just what it sounds like, the center of the mirror traces a broad, wide W pattern over the face of the tool. The normal stroke amplitude is 3/4 mirror diameter - that means the mirror center comes to within 1/8 diameter of the tool edge on all sides. In the diagram, you can see the imaginary dotted-line ring drawn at 3/4 diameter - the spacing from the tool edge is uniformly 1/8 diameter.

For a 6" or 8" mirror, the 7 segment W as shown is just about right. Use proportionally more segments for bigger mirrors. Work to keep the spread (right-to-left motion in the diagram) even, so that the edge and center deepen the same amount.

On a small mirror, this stroke can work fast. Berry in [Berry94] claims a mere 72 strokes with CeO will fully parabolize a 6" f/8 mirror. So take it easy, and test often. Better to under-correct, and do a few more W strokes, than to over-correct and have to flatten back out form a hyperbola.

DIAGRAM:

Click to view W-Stroke Diagram

Click Picture at right to View Diagram
Illustrates the normal W-Stroke, which evenly deepens the center and edge.

VIDEO:

Click to view video of the normal W Stroke

Click Picture at right to Play Video
Format: Windows Media Player Format (WMV)
Size: 378 kB     Length: 30 Seconds
Please ignore the background chatter; this was recorded in the  busy and crowded Stellafane Mirror Workshop.


The W Center Deepening Stroke

USE: Used to parabolize a mirror when figuring, after testing shows the center needs deepening more than the edge.

MIRROR POSITION: Mirror on Top, Tool on Bottom

ABRASIVE: Use with polishing compound (usually Cerium Oxide) and a Pitch Lap Tool

PRESSURE: Light - with good contact between lap and mirror, no additional downward force is necessary.

DESCRIPTION:

Very much like the W Stroke except there are additional strokes when the mirror center is near the tool edge at both extremes. This causes the mirror center to wear more than the mirror edge, deepening it.

DIAGRAM: Click to view W Center Deepening Stroke Diagram

Click Picture at right to View Diagram
Illustrates the W Center Deepening Stroke, which deepens the center more than the edge.

VIDEO: Click to view video of the center deepening W Stroke

Click Picture at right to Play Video
Format: Windows Media Player Format (WMV)
Size: 343 kB     Length: 27 Seconds
Please ignore the background chatter; this was recorded in the busy and crowded Stellafane Mirror Workshop.


The W Edge Deepening Stroke

USE: Used to parabolize a mirror when figuring, after testing shows the edge needs deepening more than the center.

MIRROR POSITION: Mirror on Top, Tool on Bottom

ABRASIVE: Use with polishing compound (usually Cerium Oxide) and a Pitch Lap Tool

PRESSURE: Light - with good contact between lap and mirror, no additional downward force is necessary.

DESCRIPTION:

Very much like the W Stroke except there are additional strokes when the mirror center is near the tool center. This causes the mirror edge to wear more than the mirror center, deepening it.

DIAGRAM: Click to view W Edge Deepening Stroke Diagram

Click Picture at right to View Diagram
Illustrates the W Edge Deepening Stroke, which deepens the edge more than the center.

VIDEO: Click to view video of the edge deepening W Stroke

Click Picture at right to Play Video
Format: Windows Media Player Format (WMV)
Size: 364 kB     Length: 29 Seconds
Please ignore the background chatter; this was recorded in the  busy and crowded Stellafane Mirror Workshop.


1/4 Center-Over-Center Stroke

USE: Correct Turned Down Edge (TDE)

MIRROR POSITION: Mirror on Top

ABRASIVE: Use with polishing compound (usually Cerium Oxide) and a Pitch Lap Tool

PRESSURE: Light - with good contact between lap and mirror, no additional downward force is necessary.

DESCRIPTION:

This is just a short-stroke version of the Normal Stroke. The short, 1/4-diameter strokes tend to work more on the mirror center. Deepening the center, and spreading this deeper curve to the edge while avoiding edge wear should bring the whole mirror to a nice sphere without TDE.

DIAGRAM:

Not provided. See diagram for the Normal Stroke, but make total stroke 1/4-diameter, with overhang on each side 1/8-diameter.


1/2 Center-Over-Center Stroke

USE: Correct Turned Up Edge (TUE)

MIRROR POSITION: Mirror on Top

ABRASIVE: Use with polishing compound (usually Cerium Oxide) and a Pitch Lap Tool

PRESSURE: Light - with good contact between lap and mirror, no additional downward force is necessary.

DESCRIPTION:

This is just a long-stroke version of the Normal Stroke. The long, 1/2-diameter strokes tend to work more on the mirror edge. This stroke works very quickly, test often! It is far more work to correct the Turned Down Edge you may get if your overshoot with this stroke. You can moderate the effect of this stroke by not going the full 1/2-Diameter, but you need to stroke more than the 1/3-diameter used by the normal stroke to get the desired effect.

DIAGRAM:

Not provided. See diagram for the Normal Stroke, but make total stroke 1/2-diameter, with overhang on each side 1/4-diameter.

Back to the ATM Index Page

Page last revised on Tuesday, 2013-Jun-04 16:34:52 -0400