tormek T7 - Page 6

4 Grinding Angle Adjustmentl
Consider the triangle determined by the knife jig, via R
1
to the centre of the stone
and via the long side D (=R
2
+S) back to the jig on the universal support.
The angle opposite side D (=S+R) is equal to 90°+! because the grinding angle !
is equal to the angle between the knife blade and the tangent to the stone while the
radius R
1
is perpendicular to the tangent.
With the cosine-law we get
D
2
= K
2
+R
1
2
! 2!K!R
1
!cos (90 °+ ! )
which can be simplified to:
D
2
=K
2
+R
2
+2!K!R!sin(!)
[F0]
So the grinding angle ! is determined by:
!=arcsin
(
D
2
"K
2
"R
2
2!K!R
)
[F1]
and D should be adjusted to
D=
!
(K
2
+R
2
+2!K!R!sin(! ))
[F2]
If D is fixed, then the grinding angle can be adjusted by changing the distance K
with the adjustable stop.
The required length K can be determined by solving K from the following equation
derived from [F0]:
K
2
+ 2!R!sin (! )!K + R
2
!D
2
=0
That gives:
K="R!sin(! )+
!
R
2
!sin
2
(! )+D
2
"R
2
[F3]
We can also express the formulas as a function of the distance S between support
and stone. Then D in formula F1 and F2 should be replaced by S+R:
Formula F2 can then be rewritten as:
S="R+
!
R
2
+K
2
+2!K!R!sin(!)
[F4]
Distance S will normally be adjusted in coarse steps of e.g 5-10mm whereas fine
adjustment of the grinding angle will be done via the adjustable stop (A).
By substitution of S+R for D in formula F3 we get:
K="R!sin( !)+
!
R
2
!sin
2
(!)+S
2
+2!S!R
[F5]
As the diameter of the support bar is 12 mm, it follows that A=K+6
Hence
A=6"R!sin(!)+
!
R
2
!sin
2
(!)+S
2
+2!S!R
[F6]
Fig. 4.1: Positioning of the grinding jig
A
grinding jig
R
1
!
D=S+R
tangent
universal
support
adjustable
stop
adjustable
clamp
grinding
stone
S
R
2
K
90°
© 2013 Ton Nillesen 13
Adjustment of the length A between adjustable stop and knife-edge
for a grinding edge !
at a preset of the distance S between support-top and stone
Table 7. Stone diameter=190 mm
S= 65 70 75 80 85 90 95 100 105 110 115 120 125
!(°) Adjustable length A (mm)
10 119 125 131 137 143 149 155 161 166 172 178 183 189
11 118 124 130 136 142 148 153 159 165 170 176 182 187
12 116 123 129 135 140 146 152 158 163 169 175 180 186
13 115 121 127 133 139 145 151 156 162 168 173 179 184
14 114 120 126 132 138 143 149 155 161 166 172 177 183
15 112 119 125 130 136 142 148 153 159 165 170 176 181
16 111 117 123 129 135 141 146 152 158 163 169 174 180
17 110 116 122 128 134 139 145 151 156 162 168 173 179
18 109 115 121 127 132 138 144 149 155 161 166 172 177
19 107 113 119 125 131 137 142 148 154 159 165 170 176
20 106 112 118 124 130 136 141 147 152 158 164 169 175
21 105 111 117 123 129 134 140 146 151 157 162 168 173
22 104 110 116 122 127 133 139 144 150 156 161 167 172
23 103 109 115 120 126 132 138 143 149 154 160 165 171
24 102 108 114 119 125 131 136 142 148 153 159 164 170
25 101 107 112 118 124 130 135 141 146 152 157 163 168
26 100 106 111 117 123 128 134 140 145 151 156 162 167
27 99 105 110 116 122 127 133 139 144 150 155 161 166
28 98 103 109 115 121 126 132 137 143 148 154 159 165
29 97 102 108 114 120 125 131 136 142 147 153 158 164
30 96 102 107 113 119 124 130 135 141 146 152 157 163