KEVOS® MAKEOS · Manufacturing
Vacuum Table Workholding Check (Holding Force, Milling Force, Sliding Safety Factor)
Will a vacuum table hold this plate without sliding while I mill it, and what feed rate keeps the safety factor I need?
Workholding check for milling a flat part on a vacuum table: the holding force from the vacuum and the sealed area with an allowance for leakage (as the vacuum cup tool), the friction resistance with the part weight, the mean cutting force from the cutting power and cutting speed (as the milling power tool) with a factor for force peaks, the sliding safety factor, and the largest feed rate that keeps your required safety factor.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Sealed length under vacuum | mm | Length of the area inside the gasket or seal under the part. |
| Sealed width under vacuum | mm | Width of that area. |
| Vacuum below atmosphere | kPa | Vacuum level at the table (gauge reading below atmosphere). |
| Effective share of the sealed area | % | Share of the sealed area that holds full vacuum after leakage, grooves and through-cuts. |
| Friction coefficient, part on table | Friction coefficient between the part and the table or spoilboard, from a pull test. | |
| Part mass | kg | Mass of the part. |
| Milling cut (as the milling power tool) | ||
| Width of cut | mm | Radial width of cut. |
| Depth of cut | mm | Axial depth of cut. |
| Table feed | mm/min | Table feed rate. |
| Cutter diameter | mm | Cutter diameter. |
| Spindle speed | rpm | Spindle speed. |
| Specific cutting force | N/mm² | Specific cutting force for the material and chip thickness, from the tool maker. |
| Peak-to-mean force factor | Ratio of peak to mean cutting force (interrupted cuts, few teeth and entry shock raise it). | |
| Required safety factor against sliding | Safety factor you require against sliding. | |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Sealed length under vacuum
- What it is
- The length of the area under the part enclosed by the vacuum gasket or seal, where the pressure difference acts to press the part onto the table.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 400 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Holding force and friction resistance rise in proportion.
- Get it right
- Measure inside the gasket, not the overall part size; areas outside the seal give no hold.
What this means: Sealed width under vacuum
- What it is
- The width of the sealed area under the part, which with its length gives the area the vacuum acts on.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Holding force and friction resistance rise in proportion.
- Get it right
- For an irregular seal, enter dimensions that give the same enclosed area.
What this means: Vacuum below atmosphere
- What it is
- How far the pressure under the part is pulled below atmospheric pressure, read on the vacuum gauge at the table, which acting over the area presses the part down.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 0.000001 kPa to 101 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Holding force rises in proportion to the vacuum.
- Get it right
- Read the gauge with a part on the table and cutting under way; the no-load reading is higher.
What this means: Effective share of the sealed area
- What it is
- The share of the sealed area that actually holds the full vacuum, allowing for leaks through porous material, grooves, holes and cuts that open during machining.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Holding force rises in proportion.
- Get it right
- Reduce it for MDF spoilboards and for parts with many through-cuts; a test with a pull gauge shows the real value.
What this means: Friction coefficient, part on table
- What it is
- The ratio of the sideways force needed to slide the part to the force pressing it onto the table, for this part material on this table or spoilboard surface.
- Units and format
- Enter a plain number with no unit. The form accepts 0.01 to 2. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.3 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The friction resistance and safety factor rise in proportion.
- Get it right
- Measure it with a spring balance on a sample part; oily or smooth surfaces are much lower than a dry spoilboard.
What this means: Part mass
- What it is
- The mass of the part, whose weight adds to the vacuum force pressing it onto a horizontal table.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0 kg to 100000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds a small amount to the friction resistance.
- Get it right
- It is usually small compared with the vacuum force; enter 0 if the table is not horizontal.
Milling cut (as the milling power tool)
What this means: Width of cut
- What it is
- The radial width of material the cutter engages, measured across the cutter axis, which sets the metal removal rate with the depth and feed.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting power and force rise in proportion, lowering the safety factor.
- Get it right
- It cannot exceed the cutter diameter; for slotting it equals the diameter.
What this means: Depth of cut
- What it is
- The axial depth of material the cutter removes in one pass, measured along the cutter axis.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting power and force rise in proportion, lowering the safety factor.
- Get it right
- Use the deepest pass in the program, often the roughing pass.
What this means: Table feed
- What it is
- The speed at which the cutter moves across the part, which sets the metal removal rate and so the cutting power and force.
- Units and format
- Enter a number in mm/min: millimetres per minute. The form accepts 0.000001 mm/min to 1000000 mm/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2000 mm/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting force rises in proportion; the largest feed for the required factor is reported.
- Get it right
- Use the programmed feed for the heaviest cut, not a finishing feed.
What this means: Cutter diameter
- What it is
- The diameter of the milling cutter, which with the spindle speed gives the cutting speed used to turn cutting power into cutting force.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger diameter raises the cutting speed, lowering the mean cutting force for the same power.
- Get it right
- Use the effective cutting diameter; for a ball-nose cutter at shallow depth it is smaller than the nominal diameter.
What this means: Spindle speed
- What it is
- The rotational speed of the spindle, which with the cutter diameter gives the cutting speed at the cutter edge.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 1000000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher speed lowers the mean cutting force for the same cutting power.
- Get it right
- Use the programmed spindle speed for the same cut as the feed rate.
What this means: Specific cutting force
- What it is
- The cutting force per square millimetre of chip cross-section for this work material, used to turn the removal rate into cutting power.
- Units and format
- Enter a number in N/mm²: newtons per square millimetre (equal to MPa). The form accepts 0.000001 N/mm² to 100000 N/mm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 800 N/mm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Cutting power and force rise in proportion.
- Get it right
- Take it from the tool maker for the actual material and chip thickness; thin chips have a higher value.
What this means: Peak-to-mean force factor
- What it is
- The ratio of the highest cutting force during a revolution to the mean force, allowing for interrupted cuts, few teeth in the cut and the shock as teeth enter.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 20. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The peak force rises in proportion, lowering the safety factor.
- Get it right
- Use a higher factor for cutters with few teeth and for entry into the part; 1 means a perfectly steady force.
What this means: Required safety factor against sliding
- What it is
- The margin you require between the friction resistance holding the part and the peak cutting force trying to slide it.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 20. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line and the largest feed; the forces do not change.
- Get it right
- Set it from your shop rules; thin, flexible or valuable parts justify a larger margin.
Method and formulas
- Holding force = vacuum × length × width × effective share (as the vacuum cup tool); resistance = μ (holding force + m g).
- Cutting power P_c = a_e a_p v_f k_c ÷ (60 × 10⁶) kW and cutting speed v_c = π D n ÷ 1000 (as the milling power tool); mean cutting force = P_c ÷ v_c; peak = mean × factor.
- Safety factor = resistance ÷ peak force; largest feed = feed × safety factor ÷ required factor (force rises in proportion to feed at a fixed specific cutting force).
- Example: 7,680 N of vacuum hold resists 2,318.7 N against a 1,086.5 N peak: safety factor 2.1341.
Limitations
- Friction only: lifting forces from helical cutters, part bow and loss of seal as through-cuts open are not calculated.
- Fixed specific cutting force; at thin chips the real specific force rises, so the largest feed is optimistic.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a router or mill vacuum fixture before cutting
- Find a safe feed rate for a thin plate
- Decide whether tabs, clamps or a sacrificial spoilboard are needed
Who it is for
- CNC programmers
- Machinists
- Fixture designers
What you get
- Holding force and friction resistance
- Mean and peak cutting force
- Safety factor against sliding
- Largest feed for the required safety factor
Interpreting the result
- Small parts lose holding force with their area while the cutting force stays the same, so the last parts cut from a sheet are the ones that move.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1084–1101, 3032–3041. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Useful alongside · Library collection
Workholding and toolholding: vices, chucks, zero-point systems, tapers and holders
What it covers: How standard workholding and toolholding choices affect accuracy, tool life, set-up time and safety, from vices and chucks to zero-point pallets, spindle tapers, collets and shrink-fit holders.
Useful alongside · KEVOS® original book
Holding against cutting forces
In Magnetic Workholding by the Numbers
What it covers: Compare a magnet's sliding resistance with milling forces: cutting force from power and speed, peak factors, friction with and without coolant, stops, and vacuum and clamp alternatives.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.