A guide to machine screws

Engineer using machine screw

Exactly what is a machine screw? The American Society of Mechanical Engineers (ASME) defines machine screws as featuring a diameter of up to 0.75 inches. While machine-screw diameters can be smaller than this, they can’t be any larger, which means machine screws are typically smaller than most other screws.

Another feature that characterizes these screws is uniform threading, as opposed to tapered threading. This simply refers to the exterior threading. On a machine screw, it remains the same size from top to bottom. A machine screw also fits into a nut or a tapped hole.

Machine screw vs. bolt

If it can be used with a nut, what’s the difference between machine screws and bolts? Remember, small machine screws are the norm. However, the main difference is in how they’re tightened in a tapped hole. Bolts are driven in and then tightened by turning the nut. Machine screws aren’t driven in but screwed in and require torque applied to the heads.

Another key point of difference: all machine screws can be bolts but not all bolts can be machine screws. Machine screws are used with a threaded hole to join two components together, sometimes requiring a nut. Bolts rely on nuts and are fitted through a clearance hole to secure parts together.

Types of machine screws

Machine-screw head types are varied. So are their applications, which include securing components within engines, electronic products and general machinery. Another interesting aspect is how machine-screw head dimensions can differ from screw to screw.

Machine screw type and characteristics

Pan machine screw

Pan head screw

  • Wide, circular head with flat, bearing surface for a secure hold. The head is either flat or slightly domed
  • Typically slotted or Phillips drives, but also available in other drive types
  • Smooth with no sharp edges for safety
  • Aesthetic appeal

Applications: Machinery and automotive parts and accessories, such as license plates and headlights

Binder machine screw

Binder head screw

  • Thicker head than pan with a deeper slot
  • Head has a chamfered outer edge

Applications: Binding material swatches and large manuals – also, electrical applications, with the undercut area under the head providing space for wire connections

Cheese head screw

Cheese head screw

  • Deep head with deep recess – ideal for slot drives
  • Head is around half of its diameter
  • Flat disc top, and a flat bearing surface
  • Accuracy and outstanding driving compacity

Applications: Appliances, automotive and fixing electrical components; Phillips-drive version used in automotive lights

Fillister head screw

Fillister head screw

  • Available with slotted or Phillips drive
  • Convex top and flat, bearing surface
  • Over-sized head is similar to pan and cheese heads, but fillister has greater height
  • Used for counterbored holes
  • Protects surface by raising the level which the screw driver is used

Applications: Precision instruments and machine assemblies

Flat head screw

Flat head screw

  • Also called countersunk screws
  • When installed, the head sits flush with the surface
  • Pointed machine screw
  • No protrusions when fully countersunk– avoids snagging skin and clothes

Applications: Handrails, furniture and lighting fixtures

Oval head screw

Oval head screw

  • Also called raised countersunk head screw
  • Rounded top surface and conical bearing surface
  • Used when finished surfaces need a flush fastening unit
  • Aesthetic appeal

Applications: Switch covers and construction, especially when a decorative finish is needed

Round head screw

Round head screw

  • Large dome
  • Extra grip when adjusting
  • Attractive appearance

Applications: Machinery

Machine-screw materials

These screws are mostly used in industrial machines, vehicles, construction equipment, engines, electronic devices and other applications that vibrate. Machine screws must be able to withstand vibration. This is where material plays a critical role. Machine-screw strength depends not just on the screw’s design, but also the material it’s made of. Get it wrong, and the screw could crack while the application is vibrating. It’s also important to use a corrosion-resistant material if the screw will be exposed to moisture.

So, what are machine screws made of? Common materials used include plastics and metals. The chart here is an overview only. Polypropylene has good tensile strength, but not high, so we’ve not given that a check mark. Talk to your screw manufacturer to learn more about the material you’re considering. Where materials provide the same attributes you’re looking for, cost might help you make the final decision. You may find our fastener materials guide helpful.

Overview:

Priority

Stainless steel

Zinc-plated mild steel

Nylon

PEEK®

Polypropylene (PP)

Polyvinylidene fluoride (PVDF)

Durability

x

x

x

x

x

x

High-tensile strength

x

 

x

x

  x

Corrosion resistance

x

x

x

x

x

x

Chemical resistance

x

 

x

x

x

x

Vibration resistance

   

x

x

x

 

Lightweight

   

x

x

x

x

How to measure a machine screw

Machine screw sizes are wide-ranging. The nominal machine screw length is measured from below the head to the tip of the screw. That’s the case for most machine screws. Machine countersunk screws – or any countersunk screw for that matter – are measured differently. The length of any screw typically parallels with the length of the hole. Countersunk machine screw dimensions are designed to sit flush with the surface of its housing. Thus, these screws are measured from the top of the head to the bottom of the screw.

Both imperial and metric machine screw sizes are available. You can learn more about sizing, along with standards, by checking out What is the difference between metric and standard fasteners?

What about machine screw thread sizes? See the machine screw size chart below.

US Machine Screw Diameters

Machine- screw number sizes

Nominal thread diameter

Decimal (mm)

Nearest fractional (in)

#0

0.060

1/16

#1

0.073

5/64

#2

0.086

3/32

#3

0.099

7/64

#4

0.112

7/64

#5

0.125

1/8

#6

0.138

9/64

#8

0.164

5/32

#10

0.190

3/16

#12

0.216

7/32

 

Nominal thread diameter is measured on the outside of the threads per ASME B18.6.3. For information on how to measure diameter see our What are screw heads, drives and threads?

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