BIZOMAT MAKEOS · Manufacturing
Drilling Fixture Clamp Check (Drill Torque, Clamp Force to Stop Rotation, Strap Clamp Screw)
When drilling a part held by strap clamps, what torque does the drill put on it, how much clamping force stops it spinning, and are the clamp screws strong enough?
Check of the clamping for a drilling operation: spindle speed, cutting and motor power, drill torque and thrust (as the drilling power tool), the tangential force the torque applies at the effective friction radius of the clamped faces, then the total clamping force by friction at the clamps and locators with a safety factor, the force on each strap clamp screw, its tightening torque and tensile stress (as the fixture clamp force tool), against the machine power and the screw proof strength.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Drill diameter | mm | Drill diameter. |
| Cutting speed | m/min | Cutting speed. |
| Feed | mm/rev | Feed per revolution. |
| Specific cutting force kc | N/mm² | Specific cutting force for the material. |
| Point angle | ° | Drill point angle. |
| Machine efficiency | % | Machine efficiency. |
| Available spindle power | kW | Spindle motor power. |
| Clamping | ||
| Effective friction radius of the clamped faces | mm | Average radius from the drill axis at which the clamp and locator friction acts. |
| Safety factor | Safety factor on the clamping force. | |
| Friction, clamp to work | Friction between the clamps and the work. | |
| Friction, work to locators | Friction between the work and the locators. | |
| Number of clamps | Number of strap clamps. | |
| Strap clamp and screw | ||
| Screw to workpiece contact distance | mm | Distance from the clamp screw to where the strap presses the work. |
| Screw to heel distance | mm | Distance from the screw to the heel of the strap. |
| Screw diameter | mm | Clamp screw diameter. |
| Screw pitch | mm | Clamp screw pitch. |
| Nut factor | Nut factor for the screw torque. | |
| Screw proof strength | MPa | Proof strength of the clamp screw. |
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: Drill diameter
- What it is
- The diameter of the drill that cuts the hole, which sets the cutting torque the fixture must resist.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Drill torque, and so clamping force, rises with the square of the diameter at a fixed feed.
- Get it right
- Use the largest drill run in this fixture, as it produces the most torque.
What this means: Cutting speed
- What it is
- The surface speed at the drill’s outer corner, from the tool maker’s data for the work material. It sets the spindle speed and the power.
- Units and format
- Enter a number in m/min: metres per minute. The form accepts 0.000001 m/min to 2000 m/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 m/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter metres per minute, not rpm; the tool works out the spindle speed itself.
What this means: Feed
- What it is
- How far the drill advances into the work in one spindle revolution.
- Units and format
- Enter a number in mm/rev: millimetres per revolution. The form accepts 0.000001 mm/rev to 5 mm/rev. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.25 mm/rev as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque and power rise in direct proportion to the feed.
- Get it right
- Enter feed per revolution, not per cutting edge: a two-flute drill’s per-edge feed is half of this.
What this means: Specific cutting force kc
- What it is
- The cutting force per square millimetre of chip cross-section for the work material, used to find drill torque and 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 10000 N/mm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2000 N/mm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque, power and clamping force all rise in direct proportion.
- Get it right
- Take it from the cutting tool maker’s material data; it varies with material and chip thickness.
What this means: Point angle
- What it is
- The included angle of the drill tip between the two cutting lips. It is used only for the drill thrust the tool reports.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 60° to 180°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 118° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Read it from the drill maker’s data; it does not change torque or clamping force here.
What this means: Machine efficiency
- What it is
- The share of motor power that reaches the drill after losses in the spindle drive.
- 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
- A lower efficiency raises the motor power needed.
- Get it right
- Type the percentage itself; take it from the machine maker or your own measurements.
What this means: Available spindle power
- What it is
- The power the machine’s spindle motor can deliver, compared with the motor power the drill needs.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 10000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7.5 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the continuous rating, and check the power available at the spindle speed being used.
Clamping
What this means: Effective friction radius of the clamped faces
- What it is
- The average distance from the drill axis to where the clamp and locator friction acts. The drill torque divided by this gives the friction force that must hold the part.
- 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 60 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 friction radius reduces the clamping force needed.
- Get it right
- Measure from the drill axis to the middle of the clamped contact areas; it must exceed the drill radius.
What this means: Safety factor
- What it is
- The multiplier you apply to the force that just stops the part turning, to cover dull drills, breakthrough grab and friction uncertainty.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 10. 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
- Clamping force rises in direct proportion.
- Get it right
- Set it from your fixture design rules; it multiplies the total clamping force directly.
What this means: Friction, clamp to work
- What it is
- The friction coefficient between the clamp pads and the top of the workpiece.
- Units and format
- Enter a plain number with no unit. The form accepts 0.02 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.15 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The two friction coefficients are added, so higher friction reduces the clamping force needed.
- Get it right
- Use a value for the actual surfaces and their condition; oily surfaces grip much less.
What this means: Friction, work to locators
- What it is
- The friction coefficient between the workpiece and the locators or rest pads it sits on.
- Units and format
- Enter a plain number with no unit. The form accepts 0.02 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.15 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Added to the clamp friction, so higher friction reduces the clamping force needed.
- Get it right
- Use a value for the actual surfaces; hardened ground locators and coolant lower it.
What this means: Number of clamps
- What it is
- How many strap clamps share the total clamping force equally.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 100. Use a whole number. 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
- More clamps lower the force, screw torque and stress per clamp.
- Get it right
- Count only clamps that bear on the part during drilling.
Strap clamp and screw
What this means: Screw to workpiece contact distance
- What it is
- On the strap clamp, the distance from the clamp screw to the point where the strap presses on the workpiece.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 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
- A longer distance raises the screw force needed for the same force on the work.
- Get it right
- Measure from the screw centre to the contact point, along the strap.
What this means: Screw to heel distance
- What it is
- On the strap clamp, the distance from the clamp screw to the heel, the end of the strap resting on the heel block.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.1 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
- A longer heel distance lowers the screw force needed.
- Get it right
- Measure from the screw centre to the heel contact; putting the screw nearer the work raises the clamp force.
What this means: Screw diameter
- What it is
- The nominal (major) diameter of the clamp screw or stud that pulls the strap down.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 200 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the thread size, for example 12 for M12, not the tapping drill size.
What this means: Screw pitch
- What it is
- The thread pitch of the clamp screw: the distance between neighbouring thread crests. With the diameter it sets the stress area.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.75 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use pitch in millimetres; for inch threads, pitch = 25.4 ÷ threads per inch.
What this means: Nut factor
- What it is
- The nut factor that relates tightening torque to screw tension, lumping thread and face friction together.
- Units and format
- Enter a plain number with no unit. The form accepts 0.05 to 0.5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The tightening torque the tool reports rises in direct proportion.
- Get it right
- Take it from the fastener supplier or a bolting reference for the actual finish and lubrication.
What this means: Screw proof strength
- What it is
- The proof strength of the clamp screw material, the stress it can carry without permanent stretch.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 2000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 600 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Take it from the fastener standard for the screw’s property class or grade.
Method and formulas
- n = 1000 v ÷ π D; cutting power = f v D k_c ÷ 240 000; torque = P × 30 000 ÷ π n; thrust from the point angle (as the drilling power tool).
- Tangential force = torque ÷ friction radius; clamping force = SF × force ÷ (μ₁ + μ₂); the thrust presses the work onto the locators and is not counted.
- Strap clamp screw force = force per clamp × (A + B) ÷ B; torque = K F d; stress on the tensile stress area (as the fixture clamp force tool).
- Example: the drill turns the work with 25 N·m; 2,777.8 N of clamping holds it; each screw sees 32.964 MPa.
Limitations
- Friction holding only; a stop or pin against rotation takes the torque directly and needs far less clamping.
- Steady drilling; breakthrough and a grabbing drill can multiply the torque.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check clamps on a drilling fixture
- Find whether a part will spin under a large drill
- Size the strap clamp screws
Who it is for
- Fixture designers
- Machinists
- Manufacturing engineers
What you get
- Drill speed, power, torque and thrust
- Clamping force and force per clamp
- Screw force, torque and stress
Interpreting the result
- A pin or stop against rotation is far more reliable than friction for large drills.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1089–1093, 3131–3134, 1522–1532. Topic reference; no source tables or recommended settings are reproduced.
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