This article is part of the CNC Machining Processes Guide: Milling, Turning, EDM & Multi-Axis on CNX Precision.
Soft jaws for CNC are machinable vise jaws that shops cut to match a specific workpiece profile. They solve three common problems at once: they hold irregular shapes without crushing them, grip finished surfaces without marking them, and protect delicate parts that hard jaws would deform. For any job shop that machines aluminum, plastic, or thin-wall components, soft jaws for CNC are one of the fastest ways to improve repeatability and reduce scrap.
What Are Soft Jaws for CNC?
Soft jaws are blanks made from aluminum, plastic, or mild steel that you mount on a standard vise and then machine in place. Instead of relying on the serrated teeth of a fixed jaw, a soft jaw is cut to the exact shape of the part you plan to make. The result is a custom nest that spreads clamping force across a broad surface.
Machining the jaws while they are clamped on the vise transfers the vise position directly to the jaw face. Once the jaws are machined, every workpiece sits in the same location. That is the core idea behind soft jaws for CNC: cut the fixture once, then load identical parts with confidence.
Choose blanks that cover the full part height plus a little extra. That extra material lets the jaws be refaced or re-bored for the next run. Blank height should also clear the part flange so the jaw grips the strongest section of the workpiece.
Why Use Soft Jaws Instead of Hard Jaws
Hard jaws offer fast setup for simple square stock. They also leave marks, crack thin walls, and struggle with odd shapes. Soft jaws fix those issues because you can cut them to match the part exactly.
Finished surfaces are the biggest reason shops switch. A machined soft jaw grips on a clean face that matches the part contour, so there is no scoring and no rocking. Delicate parts gain the same benefit. When wall thickness is small, a form-fit jaw spreads the load and prevents deflection during second-operation work.
Repeatability is another benefit. Every blank loads against a jaw that was machined in the same vise it now sits in. This makes soft jaws for CNC a natural choice for medium runs where you want consistent location without expensive fixtures.
The material cost is low. One bar of aluminum makes many jaw sets, and the machining time is short. Hard jaws still have their place, but soft jaws remove the guesswork from irregular and finished work.
Common applications follow the same logic. Near-net-shape parts are the most common use. Investment castings, forgings, and laser-cut blanks sit in a machined nest that matches the raw contour. This shortens cycle time because the operator does not probe each blank.
Second-operation work also depends on soft jaws. After the first side is machined, the finished feature must not be touched by hard metal. A form-fit jaw protects the machined surface while the second side is completed.
Thin-wall and delicate components benefit from full-surface support. Instead of three point contacts from hard jaws, the soft jaw contact patches spread the load. This reduces distortion and lets shops hold tolerances that hard jaws cannot.
How to Machine Soft Jaws In Place
Machining jaws in place starts with clean vise surfaces and parallel blanks. Squeeze the soft jaw blanks lightly, indicate the top face, and skim it flat. Then create the part profile with an end mill, measuring carefully so the finished contour matches your part model.
For round or near-round workpieces, bore a pocket in the jaw with the correct diameter. A bored pocket gives true concentricity because the bore and the jaw mounting share the same setup. After boring, open the vise lightly and place the workpiece in the pocket.
Match clamping pressure during jaw machining. If you plan to clamp parts firmly, cut the jaws at that same pressure. Jaws machined while barely tightened can bow when production clamping begins, and the pocket will not match the part.
Plan the order of operations before you start. The block must be cut, drilled, or shaped before the soft jaws that will hold it are machined. Shops that follow this sequence find that making soft jaws for CNC takes minutes, not hours.
Soft Jaw Materials and Their Trade-Offs
Aluminum is the default choice for most shops. It machines quickly, costs little, and is soft enough to avoid marking steel or finished features. Use aluminum soft jaws for light to medium clamping loads and for parts with delicate edges.
Plastic jaws work well for polished or plated workpieces. Nylon, Delrin, and similar materials grip without marring and can be re-machined several times. Their main weakness is limited strength, so keep clamping load modest and support the part from below.
Mild steel jaws are for heavy cuts and repeated use. Steel holds its shape under strong clamping force and survives hundreds of cycles. The trade-off is longer machining time and the risk of marking softer workpieces.
Material choice depends on the part, not the habit of the shop. A thin-wall aluminum housing does not need steel jaws. A large steel block with deep roughing cuts does. Confirm your material choice with a quick test before committing a production run.
Best Practices for Soft Jaw Workholding
Start with quality blanks and square them before use. A soft jaw that rides on chips or a burr will hold every part slightly wrong. Keep the vise clean and deburr the jaw base each time you remount it.
Machine an accurate bottom reference. Use a soft jaw height that clears the part flange so full contact occurs on the clamping face. This reduces the chance of tilting and keeps the part level.
Cut the part profile slightly undersized, usually by a few thousandths, so the jaw grips with light interference. Do not machine the pocket too loose. Clearance causes chatter and lets the part shift between each clamping.
Inspect every pair of jaws before production. Check that both jaws close evenly and that the profile matches the model. A quick feeler-gauge check between jaw and part prevents expensive surprises.
Store matched jaw pairs with the job data. Label the jaws with the part number and the machine they were cut on. When the order repeats, the setup is already done. This habit makes soft jaws for CNC a true time saver across reorders.
Frequently Asked Questions
Can you reuse soft jaws for CNC?
Yes. Soft jaws can be re-machined and reused many times, as long as enough material remains. Most shops reface or re-bore the jaws between runs and store them with the part program for quick reorders.
What is the best material for soft jaws?
Aluminum is the best all-around choice for soft jaws because it machines fast and protects finished parts. Use plastic for polished surfaces and mild steel only when heavy clamping loads demand extra strength.
How much clamping force do soft jaws need?
Use the minimum force that keeps the part from moving. Soft jaws spread load across a large contact area, so they typically need less torque than hard jaws. Test a few clamping pressures at the start of the run to find the reliable setting.