Allen Wrench Set for Bench Mills and Lathes: Sizes and Uses
By Products for bench top machinists | LittleMachineShop.com | Published: 2026-10-30
Category: Buying Guides
Learn which metric hex wrench sizes you need for bench mill and lathe setups, why a 3-6 mm set covers most tasks, and when to look for larger or smaller keys.
Quick answer
A compact 3, 4, 5, 6 mm metric allen wrench set covers the most common hex fasteners on bench mills and lathes, but if your machine uses imperial bolts or larger metric sizes, you will need an extended set.
Why most bench top machines use metric hex fasteners
Bench mills and lathes from manufacturers like Sieg and others rely on metric cap screws and set screws for adjustments, gibs, and tool post clamps. The four most frequent sizes are 3, 4, 5, and 6 mm, which is why a set covering exactly those sizes is practical for routine maintenance and setup.
If you own only a single-piece wrench or a loose key, the risk of rounding a fastener increases when the fit is not snug. A dedicated set with a black oxide finish resists corrosion and provides a better grip.
Trade-off: what the compact set does not cover
A 3-6 mm set leaves out 2.5 mm and 8 mm hex sizes, which appear on some tailstock locking bolts, drawbars, or spindle lock nuts. It also omits imperial (SAE) sizes such as 1/8" or 5/32", which older machines or certain accessories still use.
For a dedicated home workshop that services only one or two newer bench top machines, the missing sizes may never be needed. For a shop that works on mixed hardware, an additional larger or imperial set becomes necessary.
Decision rules for choosing an allen wrench set
- Check your machine's fastener sizes first: Look at the cap screws on the saddle, cross-slide, compound rest, and tailstock. Most bench lathes and mills use 3, 4, 5, and 6 mm hex sockets. If all four sizes appear, a 3-6 mm set is sufficient. Confirm each size by trying a known wrench before buying.
- Decide whether you need 2.5 mm or 8 mm: Some drawbars use an 8 mm hex, and certain small set screws use 2.5 mm. If your machine has either, you will need an additional single key or a larger set that includes those sizes. If unsure, buy the 3-6 mm set first and add singles only if needed later.
- Choose black oxide for grip and rust resistance: A black oxide finish reduces glare and provides a mild corrosion barrier. It also helps the wrench seat fully in the socket, reducing the chance of stripping. Avoid plain steel keys if you work in a humid garage.
Product fact
- Allen Wrench Set 3, 4, 5, 6 mm: The Allen Wrench Set from Sieg Industrial Group includes four metric Allen wrenches in 3, 4, 5, and 6 mm sizes with a black oxide finish. This compact set matches the most common metric hex fasteners on bench mills and lathes, so you can adjust gibs, tool posts, and tailstocks without hunting for loose keys.
Allen Wrench Set 3, 4, 5, 6 mm
Is this set right for you?
Best for
- Owners of newer bench mills and lathes with metric fasteners in 3-6 mm range
- Hobbyists who want a compact, low-cost set for routine adjustments and setup
Not ideal for
- Shops that frequently work on imperial hardware or need 2.5 mm or 8 mm sizes
- Users who require a full set from 1.5 mm to 10 mm for varied machinery
