CNC Workholding & Vise Selection: Holding Small Parts Right
When machining a small metal part on a CNC, workholding is one of the main things that decides the result: if the part doesn't move, the cut is clean, the dimensions are consistent and the job is safe. This guide explains which part to hold with which method, which vise type to choose when you need one, and how to work within your desktop CNC's Z travel and table design.
📑 Contents
1. Quick Summary — Workholding Table by Part
The right workholding method depends on the part's size, shape and material. A rough decision table first:
| Part | Recommended method |
|---|---|
| Thin sheet / PCB / large flat surface | Double-sided tape, spray adhesive, PCB spoilboard |
| Small/thick metal block (aluminum, brass) | Vise (low-profile) — the most solid choice |
| Medium metal part, single-sided work | Vise or clamps |
| Thin/small parts (risk of breaking loose) | Vise + soft jaws, or leave tabs in CAM |
| Plastic / EVA / wood | Double-sided tape is enough — cutting forces are low |
General rule
When cutting forces are light and the surface is wide, tape/adhesive is enough; when forces are concentrated and the part is small, mechanical clamping (vise/clamps) is required. For metal blocks, a vise gives the most repeatable results.
2. When Is a Vise Needed, When Not?
A vise is not "the answer to every job". Using it where it's not needed wastes time; not using it where it is needed creates a safety risk.
| Situation | Tape / adhesive | Vise / clamps |
|---|---|---|
| Thin sheet, plate, PCB | Best | Sheet gets crushed / bent |
| Large flat surface (wood, EVA, plastic) | Enough and fast | Not needed, takes up space |
| Small thick metal block | Won't handle cutting force | Right choice |
| Machining every face of a part (flipping) | Peeling and re-sticking isn't practical | Re-mount in the same alignment |
| Series / repeat parts | Hard and slow | Fast, repeatable |
Practical range
On a desktop CNC (Mini and Pro), a vise is usually ideal for metal blocks roughly between 50×50 mm and 150×100 mm. Very small parts need soft jaws; very large parts are better held with tape/clamps.
3. Vise Types
The main vise families you will meet in CNC milling are:
| Type | Construction | Advantage | Disadvantage | Best for |
|---|---|---|---|---|
| Machinist vise | Flanged base, lead-screw clamping | Solid, cheap, easy to bolt to the table | Wide and tall — eats Z travel on a desktop machine | Manual mills; first vise |
| CNC vise | Flangeless, ground body, replaceable jaws | Compact; can be mounted on side/end; multiple setups | Tall; flangeless mounting is harder | VMCs / production |
| Toolmaker vise (screwless) | Clamps with a cam/latch, not a screw | Low profile, precise, ideal for small parts | Clamping/releasing is more fiddly | Small jobs, low-Z machines |
| Angle vise | Body adjustable to a set angle | Machining angled surfaces | Single-purpose | Occasional angled jobs |
| Hydraulic / pneumatic | Clamps with pressurized fluid/air | Constant clamping force, automation | Expensive, needs infrastructure | Production, robotic lines |
| Dual-station | Clamps two parts from one screw | Two parts per setup | Takes more space | VMC production work |
What really matters on a desktop CNC: height (Z)
The Mini series has roughly Z ≈ 80 mm of travel. A classic CNC vise body alone can reach 60–90 mm — add the workpiece and most of your Z is consumed by the vise, sharply reducing usable depth of cut. That's why low-profile options (toolmaker/screwless or a compact machinist vise) stand out on desktop CNCs.
4. Choosing for a Desktop CNC
When choosing a vise for the Raptorex Mini and Pro, look at three numbers: height, jaw opening and base width.
- Height (most critical): Body + workpiece height must leave the top face to be machined under the bit. On the Mini, keep the body height around 40–50 mm; on the Pro (Z 120 mm), about 60–70 mm is tolerable.
- Jaw opening: It should grip your largest workpiece. A 50–75 mm opening covers most desktop jobs; hold larger parts with clamps/tape.
- Base width and mounting: The base must sit on your table and be secured with T-nuts (if you have T-slots) or with clamps (if not). Table mounting is covered in Section 5.
First-vise recommendation
When starting on a desktop CNC, a low-profile (compact) machinist or toolmaker vise is a good start: ~50 mm body height, 50–75 mm jaw opening and parallel jaws. Instead of buying an expensive CNC vise, start with this class and customize it with soft jaws as needed (Section 6).
5. Mounting to the Table: T-Slots, Flat Plates and Clamps
Raptorex table design can vary by model: some tables are slotted (T-slot-like), others are flat aluminum plates. Before mounting a vise, check your own table's design from the photo/specs and pick the appropriate method below:
On a T-slot / slotted table
- T-nut + bolt: The most solid route. Bolts through the vise base holes thread into T-nuts; the vise cannot move.
- Strap clamps: Clamps sitting on the vise base lugs are secured into the T-slot.
On a flat plate table
- Drill holes: Drill controlled holes at the corners/suitable spots of the table and sink T-nuts or nuts underneath — a permanent, solid solution. (Since the table is aluminum, the drilling can be done with a Raptorex machine itself.)
- Clamps: Squeeze the vise base with C-clamps/edge clamps sitting on the table edge — a practical no-drill route, though repositioning is harder.
- Sacrificial plate: Screw a thick aluminum/MDF plate onto the table and put your vise-mounting holes in that plate; it adds flexibility without drilling the table.
Safety
Never machine with a vise that is not secured to the table. "It feels snug" is not enough; if cutting force moves the vise, both the part and the bit are damaged and the part can fly off.
6. Soft Jaws: Machining Jaws to Fit the Part
A vise's hard steel can crush or scratch many parts. The answer is soft jaws: soft aluminum jaws bolted over the vise's standard jaws and machined to fit the part on the CNC.
- Material: 6061-T6 aluminum is the standard — easy to machine and won't scratch the part.
- How it's done: Two aluminum blocks are bolted onto the vise jaws; then on the CNC you machine the negative form of the part into the jaws (e.g., if the part is a 12 mm square, cut a 12.1 mm pocket into the jaws). The part now sits in the pocket — located and gripped at the same time.
- Benefit: In series production of the same part, every part lands in the same position — repeatability improves and setup time drops.
- Machining them on a Raptorex is a great example: Cutting your own soft jaws trains both workholding and aluminum machining skills; having the machine make its own fixtures is very useful in a shop.
Pocket allowance for locating
Machine the soft-jaw pocket 0.05–0.15 mm larger than the part: the part should sit in freely but not move when clamped. If clamping lifts the part slightly, add a shallow support or parallel under the pocket.
7. Using a Vise: Parallels, Squareness and Clamping Force
- Parallels: Hold the part raised between the jaws so there is clearance under the machined surface and the bit won't hit the table. Place parallels at equal height along the part.
- Squareness (tram): The vise must be mounted square (90°) to the table. Verify against the surface with a dial indicator or square; a skewed vise makes the whole job crooked.
- Clamping force: Clamp with the minimum force that holds the part. Too much force bends/deforms the part; too little lets it shift during cutting. On thin or hollow parts, keep the force especially balanced.
- Seat the part against the jaws: The part must sit fully on the jaw base (or parallels); a part hanging in mid-air bends when clamped.
- Repeated setups: If you'll run the same job again, fix the vise position (e.g., a mark or stop pin on the table); don't re-zero every time.
Watch for lifting
During clamping, the part can lift slightly out of the jaws (especially round or plastic parts). After clamping, check that the top face is parallel to the parallels; if needed, seat it gently with a soft mallet.
8. FAQ
- Which vise for a desktop CNC? A low-profile (body ~40–60 mm) machinist or toolmaker/screwless vise. On the Mini with Z 80 mm, height is the most critical selection criterion.
- Can I hold a small aluminum part with tape? For short, light cuts, yes; but deep/heavy cuts may exceed what tape can hold. For a metal block, a vise is safer.
- How do I mount a vise on a flat table? If you have no T-slots: squeeze it with clamps, use a sacrificial plate, or drill controlled holes in the table and sink nuts. See Section 5.
- What are soft jaws, and are they necessary? They are aluminum jaws machined to the part on the CNC. Very useful for position repeatability and scratch protection in series work; not required for one-offs.
- How much Z does a vise eat? Machinist vises have ~60–90 mm bodies, compact/toolmaker ~40–55 mm. On the Mini this directly reduces usable depth of cut — that's why you should choose low-profile.
- Do I need an angle vise for angled jobs? No — for occasional angled work you can also use an angled fixture or a rotated coordinate system in CAM.
References
References and further reading
- Tormach — What Kind of Vise Is Best for CNC Work?
- Gimbel — The Vise Buyer's Guide
- LittleMachineShop — Vise Selection Guide (machinist & toolmaker vise range)
- Linquip — Types of Vises
- CNCCookbook — Soft Jaws for CNC Machining
Note: Vise height and jaw-opening values vary by manufacturer; compare against your machine's Z travel and table design before buying.
Related topics
🔗 Which Machine Cuts Which Material, and How? — stepdown/feed tables to consider alongside workholding
🔗 End Mill Selection for Metal Cutting — bit and parameter selection
🔗 Mechanical Accessories — table and mounting solutions
What part will you be machining?
Tell us your part size, material and machine; let's work out the right workholding method and vise recommendation together.