Fastener & Fabrication Calculators
The numbers on a machinist’s cheat sheet now live in Fastener & Fabrication Calculators. Set bolt torque and preload, size threads and clearance holes, and find spindle speed. Check welds, sheet metal bends, punch force, and true position.
About Fastener & Fabrication Calculators
Shop work runs on a short list of formulas. Torque, thread pitch, cutting speed, and bend allowance come up every week. Fastener & Fabrication Calculators keep that math in one place.
This page works like a reference sheet. Each section covers one kind of job. It names the tools that handle it and shows the core formula behind them.
Bolted Joints
Six tools cover bolts. The Bolt Torque Calculator and Bolt Preload Calculator link wrench torque to clamp force. The Bolt Friction Coefficient Calculator works backward from measured torque and preload. The Bolt Area Calculator finds tensile stress area. The Bolt Length Calculator sizes a bolt for the grip. The Bolt Pull Out Force Calculator estimates how much load threads can hold before stripping.
Torque and Preload
The short-form torque formula is the one most shops use.
$$T = K \times D \times F$$
T is torque. D is the nominal bolt diameter. F is the target preload. K is the nut factor, which lumps all friction into one number.
| Condition | Typical nut factor K |
|---|---|
| Dry steel | About 0.20 |
| Lightly oiled | About 0.15 |
| Rough or corroded | About 0.25 |
Most of the applied torque is lost to friction under the head and in the threads. Only a small share stretches the bolt. That is why lubrication changes torque so much. A lubricated bolt torqued to a dry value can be badly over-tightened.
A common preload target is about 75% of the bolt’s proof load. That leaves margin below yield.
Worked Example: 1/2-13 Grade 5
For inch threads, tensile stress area follows the ASME formula.
$$A_s = 0.7854 \left(D – \frac{0.9743}{n}\right)^2$$
Here n is threads per inch. For a 1/2-13 bolt, the stress area is about 0.1419 sq in.
Grade 5 has a proof strength of 85,000 psi in this size. Proof load is about 12,060 lb. At 75%, the target preload is about 9,050 lb.
With a dry nut factor of 0.20, torque is 0.20 × 0.5 × 9,050. That is about 905 in-lb, or about 75 ft-lb.
Torque control is not precise. Real preload can vary widely from one bolt to the next. Critical joints should follow the maker’s torque spec or tension testing.
Thread Engagement
Pull-out strength depends on how many threads carry load and how strong the tapped material is. Soft metals like aluminum need longer engagement than steel. The Bolt Pull Out Force Calculator shows how engagement length changes the result.
Threads and Holes
The Thread Calculator gives thread dimensions for common screw threads. The Thread Pitch Calculator converts between pitch and threads per inch. The Pitch Diameter Calculator finds the diameter used to gauge a thread.
Pitch is the inverse of threads per inch. A 13 TPI thread has a pitch of about 0.0769 in. For standard 60° inch threads, basic pitch diameter is the major diameter minus 0.6495 times the pitch. For a 1/2-13 thread, that is about 0.4500 in.
The Clearance Hole Calculator sizes the hole a bolt passes through. Standard tables list close, normal, and loose fits. Close fits help alignment. Loose fits make assembly easier.
The Countersink Depth Calculator finds how deep to cut for a flat-head screw. Depth depends on the head diameter and the countersink angle. Inch flat-head screws commonly use 82°. Metric screws commonly use 90°.
The Bolt Circle Calculator gives X and Y positions for holes spaced around a circle. Each hole sits at radius times cosine and radius times sine of its angle. Six holes on a 4 in circle sit 60° apart, with the first at X = 2.000, Y = 0.
The Screw Length Calculator finds screw length from material thickness and embedment. The Solid Rivet Length Calculator sizes rivets from grip and diameter. A common aircraft rule adds about 1.5 rivet diameters beyond the grip to form the shop head.
Machining
The Spindle Speed Calculator turns a cutting speed into RPM using the exact circumference-based formula, not the rounded shop shortcut some references use.
$$\text{RPM} = \frac{12 \times \text{SFM}}{\pi \times D}$$
SFM is surface feet per minute. D is the tool or part diameter in inches. A 1/2 in drill at 100 SFM runs about 764 RPM — the same result the 3.82 approximation gives here, though the two formulas diverge by roughly 5% at other diameters and speeds.
Cutting speed depends on the material and the tool. Use the tool maker’s chart for your starting SFM.
The Material Removal Rate Calculator estimates how fast metal comes off. In milling, it is roughly feed rate times width of cut times depth of cut. The Taper Calculator finds taper per inch and angle from two diameters and a length.
The True Position Calculator checks a hole’s location against its tolerance. True position is twice the distance from the measured center to the true center.
$$TP = 2\sqrt{\Delta x^2 + \Delta y^2}$$
A hole off by 0.002 in X and 0.003 in Y has a true position of about 0.0072 in. Compare that with the tolerance on the drawing.
Sheet Metal and Punching
The K-Factor Calculator finds bend allowance for press brake work. K is where the neutral axis sits, as a fraction of the thickness.
$$BA = \frac{\pi}{180} \times A \times (R + K \times T)$$
A is the bend angle in degrees. R is the inside radius. T is the material thickness. K often falls between about 0.3 and 0.5. Test bends on your own brake give the most reliable value.
The Punch Force Calculator finds the tonnage needed to punch a hole. Force equals the cut perimeter times thickness times the material’s shear strength.
Take a 1 in round hole in 1/8 in plate. With a shear strength of 50,000 psi, the force is about 19,600 lb. That is roughly 9.8 tons. Use the shear strength of your actual material.
Welding
The Welding Calculator estimates weld metal, filler, and weld size for a joint. For an equal-leg fillet weld, the effective throat is about 0.707 times the leg size.
The Weld Shrinkage Calculator estimates how much a joint pulls in as the weld cools. Knowing that ahead of time lets you pre-set parts or add length.
The Carbon Equivalent Calculator rates steel weldability from its chemistry. The IIW formula is the most widely used.
$$CE = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15}$$
Values below about 0.40 generally weld well without preheat. Higher values call for more care, and preheat becomes more likely as CE rises. AWS D1.1 uses a version that also adds silicon. Take the chemistry from the mill test report, not a generic grade sheet.
Surface Prep and Lifting
The Sandblasting Calculator estimates abrasive use and blast time for a surface area. The Screw Jack Force Calculator finds lifting force from input torque and screw lead. Friction lowers the real output, so a no-friction result will read high.
FAQs
How do I calculate bolt torque?
Multiply the nut factor by the bolt diameter and the target preload. A dry steel bolt often uses K = 0.20.
Why does lubrication lower the torque spec?
Less friction means more of the torque becomes clamp force. The same wrench setting on a lubricated bolt can stretch it much further than on a dry one.
How do I convert SFM to RPM?
Multiply SFM by 12, divide by π times the diameter in inches. The 3.82 shortcut some shops use (12 ÷ π rounded up) is close but overstates RPM by about 5%.
What is a good K-factor for sheet metal?
Many shops start between about 0.3 and 0.5. The best value comes from test bends in your material on your brake.
What carbon equivalent is weldable?
Under the IIW formula, steel below about 0.40 is generally easy to weld. Higher values often need preheat and low-hydrogen practice.
How is true position calculated?
Take the X and Y deviations from true center. Find the straight-line distance and double it.