RepairMachineryEasy

Replacing a Stripped Set Screw

A shaft collar or pulley has stopped gripping because the socket in its set screw is rounded out. Identifying the thread, choosing the point form, and knowing why the length is as critical as the diameter.

Designthe dimensions everything else is derived from

Typical size
M6 x 8 socket set screw
Thread
M6 x 1.0 coarse
Point form
cup point for a shaft, flat point for a flat or a key
Property class
45H — the class set screws are supplied in
Seat
the point must bear on the shaft, not on a key or a shoulder

Bolt lengths are given as the length under the head, which is how the standards in this database list them. Check the grip before ordering: a bolt has to clear the nut by at least two full threads.

Materialswhat each part is made from, and why

All materials
Alloy Steel · the replacement screw
Set screws are supplied in alloy steel at property class 45H. A mild steel screw of the same size will round its own socket out under the same torque, which is how the first one failed.
Carbon Steel · the shaft it clamps
Worth knowing before torque is applied: a cup point bites into a plain carbon steel shaft, and on a hardened shaft it does not, which changes the point form you want.

Fastenersquantities and lengths that follow from the joints

All standards
ItemQtyGripStandard
M6 x 8 socket set screw, cup point2—DIN 916
M6 x 8 socket set screw, cup point — product gradealternative—ISO 4029

Grip is the sum of the clamped thicknesses. A bolt has to clear the nut by two full threads, which is what sets the length that appears in each row — not a preference.

M6 x 8 socket set screw, cup point
Cup point is the default against a plain cylindrical shaft: the rim bites and resists rotation. Two at 90 degrees is the usual arrangement, because one screw on its own lets the collar rock. DIN 916 →
M6 x 8 socket set screw, cup point — product grade
The ISO twin of the DIN screw, with the same 1.0 mm pitch and the same cup point. Either number may be on the packet; they are interchangeable at this size. ISO 4029 →

Toolsand what each one is for on this job

All tools
Thread Pitch Gauge
Confirming the pitch is 1.0 mm and not 0.75 mm. Both exist at M6 and only one of them fits.
Vernier & Dial Caliper
Measuring the shaft diameter, which is what decides the collar bore and therefore the screw size.
Hex Key (Allen Key)
The socket is what strips. A worn key of the right size is itself a cause of rounded sockets.
Twist Drill Bits
A left-hand bit is the reliable way to extract a screw whose socket has gone, since the drilling torque tends to unscrew it.

Order of workthe sequence, and where it is not free to change

  1. Check the point form before ordering. A cup point against a hardened shaft will simply skate.
  2. Confirm the thread with a gauge. M6 comes in 1.0 and 0.75 mm pitch and they are not interchangeable.
  3. Extract the old screw. If the socket is rounded, a left-hand bit is usually faster than an extractor.
  4. Inspect the hole. A set screw that loosened repeatedly has usually raised a burr on the shaft, and the new screw will sit on the burr rather than in the hole.
  5. Fit the new screw and bring it to the torque the screw size allows, not to the limit of the key.

Where it goes wrongthe three failures worth designing out

  • Buying a longer screw because it was in the box. A set screw that bottoms out in the collar or reaches the key stands the collar off its seat, and the joint is loose no matter how hard it is tightened.
  • Reusing a rounded key. It rounds the socket of the replacement screw in exactly the proportion that it is worn.
  • Substituting a flat point for a cup point on a plain shaft. It will hold until the first reversing load and then slip.

Frequently asked questions

How do I know whether the thread is M6 or 1/4 inch?

Measure the outside diameter first: 6 mm is M6 and 6.35 mm is 1/4 inch. Then measure the pitch. The two are close enough that a screw of one will start in the other and then damage the thread, which is why the gauge matters rather than a visual check.

Why does the length matter more than the diameter?

Because the diameter only has to match the hole that already exists, while the length decides whether the point reaches the shaft. A screw that is too short bears on nothing and a screw that is too long lifts the collar off its seat.

Should the new screw be retained with anything?

On a vibrating machine, a medium-strength threadlocker is the usual answer. This database does not carry threadlocker as a product; what it does carry is the dimensional standard for the screw itself.