Helilap Bore Finishing Tools

For extreme accuracy in production bore finishing, there is only one way to guarantee precision. This involves a Helilaps and an abrasive lapping and polishing medium or Kemet Diamond Compound.

Manufactured from a uniform grade of annealed, close grained, grey cast iron, the lap incorporates a unique helical slot and external circumferential grooves. When the lap is slipped over the arbor and tapped gently with the lap expander, the internal taper in the lap and the taper on the arbor make contact, therefore allowing a precise degree of diametrical expansion to be achieved uniformly over the entire lap. This means that a Helilap gives a fast lapping action, perfect for bore roundness, exceptional straightness and superb surface finish. They are produced in a large range of sizes from 3/64" dia. to 3" dia as well as metric sizes.

Helilap Design

The Helilap, made of a good quality, close-grained, lapping iron has a tapered bore to enable it to slide on a high-tensile, hardened and ground steel Arbor. When the lap is driven up the Arbor, expansion occurs, and by utilisation of uniform points of contact around the circle, the roundness of bores and diameters can be held to extremely fine limits. Also, because the points of contact extend on the circle through the length of the lap, parallelism can be guaranteed. The lap will contract on being pulled down the slope of the Arbor but this contraction is limited and care should be taken in selecting the correct lap size. After use the lap should be removed from the Arbor so that it "relaxes" back to its original state.

internal lapping tools for barrel finishing

Helilap Use

  1. If a bore in steel has a finished size of 0.252" (6.4mm) for example, consult the range chart (Appendix A). From this, choose the Ø 1/4" Helilap with No.3 Arbor and No.3 Expander. It can be seen that this lap covers a range of Ø 0.249" to Ø 0.264".
  2. If the component can be held by hand, rotate the lap in a lathe chuck at approx 500-600 rpm for Ø 1/4"
  3. Put some Kemet O.S Lubricating Fluid onto the Lap. Slide the component over the Lap. Keep on gently expanding the lap until slight resistance against the component can be felt
  4. At this point, stop the lap and apply some Kemet KD Diamond Compound. Restart and resistance should be felt.
  5. Transverse the component backwards and forwards along the rotating lap until resistance fades
  6. Re-expand the lap and continue.
  7. Regularly check the size and finish of the bore.
  8. During use reapply Kemet Diamond Compound. If the lap dries out, apply small quantities of Kemet OS lubricating fluid.

Kemet Diamond Compound

Kemet 14 Micron Diamond Compound used on steel will give a visual silky matt finish with Ra = 0.08 μm. This grade will also remove stock.
Kemet 3 Micron Diamond Compound used on a clean lap after Kemet 14 Micron Diamond Compound will give a polished finish with Ra = 0.03 μm.

Blind bores

When lapping blind bores, it is necessary to machine the lap so that only 50% of the lap contacts the bore. The arbor must also be reduced in length so that it does not foul the end of the blind bore. It is recognised that as the lap is expanded by driving the arbor into the lap, the end of the arbor could contact the end of the bore, so it is necessary to have different lengths of arbor for one lap. The accuracies achieved rely entirely on the supreme accuracy of that portion of the lap which is working within the bore.

Blind bores


Expanding Internal Helical Slot Laps

Laps Corresponding Arbors Expanders Pullers
Nominal Lap dia ‘E’ Length ‘D’ Code Arbor No. Shank Dia ‘A’ Shank Length ‘B’ Taper Length ‘C’ Code Expander No. Code Size Code
3/64in 1/2in 303101 4/0 1/8in 3/4in 1 1/2in 303401 4/0 303201 Extra Small 303301
1/16in 5/8in 303102 3/0 1/8in 3/4in 1 1/2in 303402 3/0 303216
5/64in 303103
3/32in 3/4in 303104 2/0 1/8in 3/4in 1 5/8in 303403 2/0 303202
7/64in 303105
1/8in 7/8in 303106 0 1/8in 3/4in 1 3/4in 303404 0 303203
9/64in 303107
5/32in 303108
11/64in 303109
3/16in 1 3/8in 303110 1 3/16in 1in 2 3/4in 303405 1 303204 Small 303302
13/64in 303111
7/32in 1 3/4in 303112 2 3/16in 1in 3in 303406 2 303205
15/64in 303113
1/4in 2in 303114 3 3/16in 1in 4in 303407 3 303206
17/64in 303115
9/32in 303116
19/64in 303152
5/16in 2in 303117 4 7/32in 1in 4in 303408 4 303207
21/64in 303153
11/32in 303154
23/64in 303118
3/8in 2 1/2in 303119 5 9/32in 1 1/2in 5in 303409 5 303208
25/64in 303120
13/32in 303121
27/64in 303122
7/16in 2 1/2in 303123 6 5/16in 1 1/2in 5in 303410 6 303209
29/64in 303124
15/32in 303125
31/64in 303166
1/2in 3 1/4in 303126 7 3/8in 1 1/2in 6in 303411 7 303210
33/64in 303127
17/32in 303155
35/64in 303156
9/16in 303157
37/64in 303174
19/32in 303128
39/64in 303168
5/8in 3 1/4in 303129 8 13/32in 1 1/2in 6in 303412 8 303211 large 303303
41/64in 303170
21/32in 303130
43/64in 303175
11/16in 303131
45/64in 303132
23/32in 303133
47/64in 303171
3/4in 3 1/2in 303134 9 1/2in 2in 7in 303413 9 303212
49/64in 303158
25/32in 303135
51/64in 303167
13/16in 303136
53/64in 303159
27/32in 303160
55/64in 303161
7/8in 3 1/2in 303172 10 9/16in 2in 7in 303414 10 303213
57/64in 303176
29/32in 303144
59/64in 303162
15/16in 303137
61/64in 303177
31/32in 303138
63/64in 303165
kemet ISO 9001

BSI number Q05919

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