polsim - using a smaller tool, with offset

mirror size: 406 mm
tool size: 330 mm

simulation with Tool on top, moving along x direction, changing chordal stroke parameters: 

 

Effect of varying stroke,  y offset kept same (center of Tool above edge of Mirror)

1a) y offset of 185mm, stroke 102 mm

 

1b) y offset of 185mm, stroke 135 mm ( I use this to grind down the untouched mirror edge)

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Effect of varying y offset, keeping same stroke.

for rough and fine grinding, Mel Bartels suggests to use 1/3 W to 1/2 W, ( ie varies y offset from 1/3 D to 1/2 D travel) continuously, with stroke ( x direction) kept at 1/3D. D is diameter of mirror(NB this W stroke is different from w stroke used in polishing and parabolizing, eg in stellafane) :

 

In rough grinding:

When you reach 2/3 of the desired depth( by hogging with a metal tool), flip the mirror face up. You will see that the edge of the mirror has hardly been touched. 

Put the tool on top(TOT), and use 1/2 'W' pattern strokes. If the mirror is a 20", then the stroke length is 10". 

'W' strokes start a little to one side of the mirror center( positive y offset), and end a little to the other side of the center(negative y offset). About a half dozen push and pulls constitute a 'W'. 

At the end of the 'W' pattern, spin the upper piece (tool) clockwise and take a step to your right (counter clockwise). These strokes will extend the curve out to the edge and continue to deepen the mirror center. 

2a)

tool diameter is 80% of mirror

x stroke @ 1/3D, y offset 0 :

 

x stroke @ 1/3D, y offset 1/6D

x stroke @ 1/3D, y offset 1/4D

For comparison with full size tool, same 1/3D x stroke

 

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If desired depth is reached too soon, use very wide ( > 1/2 D) long strokes to extend the curve without deepening the mirror center. 

2b) y offset of 60%D, x stroke 135 mm

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Effect of varying x offset, pls note this is not chordal stroke, but more like a off-centered normal stroke, but very different effect to normal stroke. 

For 25% dia tool, the shape is generally a s-curve back to back, lower in center, higher on edge

3a) tool 115mm dia, x offset of 115mm, stroke 115 mm

3b)tool 115mm dia, x offset of 95mm, stroke 135 mm

3c)tool 115mm dia, x offset of 85mm, stroke 155 mm

 

chordal stroke from stellafane 

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