Manual machining
How do you cut soft jaws for a milling vise?
Clamp a spacer between the jaws, then cut the pocket to the part size with the jaws held closed on it. The spacer must be thicker than the difference between the pocket and the smallest part (the full tolerance band when the pocket is cut to the largest part), or the jaws close on each other before they grip a small part.
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
Spacer thickness: g > pocket size - smallest part size. Pocket depth: no more than part height - deepest cut on top - clearance. A 2.000 in part with a -0.005 in tolerance needs a spacer thicker than 0.005 in, and a 1.250 in tall part with a 0.500 in cut on top and 0.100 in clearance takes a pocket no deeper than 0.650 in.
Source: Plane geometry of the jaw gap and pocket depth as shown; the steps are common shop practice, not a standard.
Soft jaw spacer and pocket depth
Minimum spacer thickness (more than)
in
d = S - P = 2 - 1.995 = 0.0050 in
The spacer g must be thicker than d. A 0.1250 in spacer leaves 0.1200 in between the jaws on the smallest part. gap left = g - (S - P) = 0.125 - (2 - 1.995) = 0.1200 in.
Maximum pocket depth
in
max depth = H - C - c = 1.25 - 0.5 - 0.1 = 0.6500 in
Cut the pocket no deeper than this, so the part stands above the jaws by the cut plus clearance.
g > d, with d = S - P; max depth = H - C - c
| Symbol | Meaning | Unit |
|---|---|---|
| g | Spacer thickness: the jaw gap while cutting | in or mm |
| S | Pocket size as cut | in or mm |
| P | Smallest part size | in or mm |
| d | How much smaller the smallest part is than the pocket | in or mm |
| H | Part height | in or mm |
| C | Deepest cut on top of the part | in or mm |
| c | Clearance between the tool or holder and the jaw top | in or mm |
A part that is smaller than the pocket by d closes the jaw gap by d. The jaws still grip while g > d. For a pocket cut to the part's nominal size, the spacer has to be thicker than the minus tolerance.
Step by step (shop practice)
Soft jaws on a milling vise, pocket cut to the part size.
- Mount the jaws on a clean vise and tighten the jaw screws.
- Clamp a spacer (a parallel, a ground pin or a bar) between the jaws at the clamping pressure you will use on the part, away from where the pocket goes.
- Indicate the fixed jaw if the pocket must locate the part to the machine, and set work zero on the jaws.
- Cut the pocket or bore to the part size, no deeper than the maximum pocket depth from the formula.
- Remove the spacer, load a part and check that it seats on the pocket floor or on parallels and that the jaws do not touch each other.
- Mark the jaws and their positions so they go back on the same vise the same way.
Result: jaws that grip every part in the tolerance band, with the part standing clear of the jaws for the cut.
Worked example 1: jaw gap (inch)
Part 2.000 in wide, +0.000/-0.005 in. Pocket cut at 2.000 in.
- Smallest part: 2.000 - 0.005 = 1.995 in.
- g > 2.000 - 1.995 = 0.005 in.
- A 0.125 in spacer leaves 0.125 - 0.005 = 0.120 in between the jaws on the smallest part.
Result: any spacer thicker than 0.005 in works; 0.125 in leaves 0.120 in of gap.
Worked example 2: pocket depth (inch)
Part 1.250 in tall, deepest cut on top 0.500 in, clearance 0.100 in.
- Maximum pocket depth = 1.250 - 0.500 - 0.100 = 0.650 in.
Result: cut the pocket no deeper than 0.650 in (16.51 mm).
Worked example 3: round part (metric)
Part 40.00 mm diameter, -0.10 mm tolerance, bored at 40.00 mm.
- g > 40.00 - 39.90 = 0.10 mm.
- A 3 mm spacer leaves 3.00 - 0.10 = 2.90 mm on the smallest part.
Result: any spacer thicker than 0.10 mm works; 3 mm leaves 2.90 mm.
Common mistakes
- Cutting the pocket with the jaws touching each other. Small parts then go loose.
- Cutting at a lighter clamp pressure than the job uses.
- A pocket so deep the cutter or holder reaches the jaws.
- Moving jaws to another vise and expecting the same location.
FAQ
Why clamp on a spacer when cutting soft jaws?
So the jaws grip the part before they touch each other. A part 0.005 in under the pocket size closes the gap by 0.005 in, so the spacer has to be thicker than that.
How deep should soft jaw pockets be?
No deeper than part height minus the deepest cut minus tool clearance: 1.250 - 0.500 - 0.100 = 0.650 in in the example.
Should you cut soft jaws at the same clamping pressure you run?
Yes, as shop practice: the jaws and vise deflect under clamping, so cut them the way they will hold the part.
Aluminum or steel soft jaws?
Shop practice: aluminum cuts fast and is the common choice; steel lasts longer on long runs.
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