Milwaukee M18 FUEL 18-Volt Lithium-Ion Brushless Cordless 4-1/2 in./6 in. Braking Grinder with Paddle Switch (Tool-Only) – 3670-20 Review

When we first got our hands on the Milwaukee 3670-20, we knew we were dealing with something special. This isn’t just another angle grinder – it’s a powerhouse that genuinely changed how we approach grinding, cutting, and metalworking tasks. After putting this tool through its paces across various jobsites and workshop projects, we’re excited to share our comprehensive findings with you.

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What Makes This Milwaukee Grinder Stand Out

From the moment we unboxed the Milwaukee 3670-20, the build quality was immediately apparent. The tool feels substantial without being cumbersome, and the balance is exceptional. Milwaukee has clearly put serious engineering work into this grinder, and it shows in every detail.

The POWERSTATE brushless motor is the heart of this beast, and during our testing, it consistently delivered the kind of performance we’d expect from a corded grinder. We threw everything at it – cutting through 1/4-inch steel plates, grinding down welds, and surface preparation – and it handled every task with impressive authority.

Revolutionary RAPIDSTOP Brake Technology That Actually Works

Safety features often feel like afterthoughts on power tools, but Milwaukee’s RAPIDSTOP brake technology is genuinely game-changing. During our testing, we were blown away by how quickly the wheel stops spinning after releasing the paddle switch. We’re talking about stopping in under two seconds – significantly faster than letting a standard grinder coast to a stop.

This isn’t just a nice-to-have feature; it’s a legitimate safety enhancement that we grew to appreciate more with each use. Whether you’re working in tight spaces or making multiple quick cuts, knowing the wheel will stop almost immediately gives you confidence and control you won’t find in standard grinders.

The Paddle Switch Design: A Professional’s Dream

We’ll be honest – when we first saw the paddle switch design, we were skeptical. After years of using traditional trigger switches, we wondered if this would feel awkward. We couldn’t have been more wrong.

The paddle switch is brilliantly designed for real-world use. It allows for incredibly natural hand positioning, and you can activate it with either a thumb or finger depending on your grip and the task at hand. During extended grinding sessions, we noticed significantly less hand fatigue compared to our traditional grinders. The switch is also perfectly positioned to prevent accidental activation while still being instantly accessible when you need it.

REDLINK PLUS Intelligence: The Brain Behind the Brawn

Milwaukee’s REDLINK PLUS intelligence system might not be visible, but we certainly felt its impact during testing. This advanced electronic system provides overload protection and manages the power delivery to optimize both performance and battery life.

We deliberately pushed this grinder hard during our tests – long grinding sessions, heavy material removal, and demanding cutting applications. The tool never bogged down unexpectedly, and the thermal management kept everything running cool. The system seamlessly adjusts power output based on the load, giving you maximum performance when you need it and preserving battery life when you don’t.

Real-World Performance Testing

We tested the Milwaukee 3670-20 across several demanding applications to see how it truly performs:

**Metal Cutting**: Using a 4-1/2″ cut-off wheel, we sliced through angle iron, rebar, and steel plate. The cuts were fast and clean, with minimal sparking compared to other grinders we’ve tested. The POWERSTATE motor maintained consistent speed even under heavy load.

**Weld Grinding**: This is where grinders often separate themselves, and the Milwaukee performed beautifully. We tackled both fresh welds and old, oxidized ones. The variable speed trigger allowed us to dial in the perfect speed for each material, and the brushless motor delivered smooth, controlled material removal.

**Surface Preparation**: We used various grinding discs for surface prep on rusty metal. The tool’s power and speed control made quick work of rust and paint removal, and the paddle switch made it easy to work in awkward positions.

Battery Compatibility and Runtime Considerations

As a tool-only option, the 3670-20 doesn’t include a battery or charger, but if you’re already in the Milwaukee M18 ecosystem, you’re set. During our testing with various M18 batteries, we found the performance sweet spot to be with the M18 REDLITHIUM HIGH OUTPUT HD 12.0Ah battery.

With the 12.0Ah battery, we achieved approximately 40-50 cuts through 1/2-inch rebar on a single charge – impressive for a cordless tool of this caliber. Even with the smaller 5.0Ah battery, we got through a full afternoon of mixed grinding and cutting tasks before needing a swap.

Ergonomics and Extended Use Comfort

We spent multiple hours at a stretch with this grinder, and the ergonomic design really shines during extended use. The tool weighs in at a reasonable level (tool-only) that strikes a nice balance between feeling substantial and not causing arm fatigue.

The grip positioning is well thought out, with enough space to accommodate gloved hands comfortably. The vibration levels are notably lower than many competing grinders we’ve tested, which we attribute to both the brushless motor design and the overall build quality.

The paddle switch location means your hand naturally rests in a comfortable position without needing to constantly squeeze a trigger. This seemingly small design choice made a significant difference during our longer grinding sessions.

Versatility: 4-1/2″ and 6″ Wheel Compatibility

One feature we really appreciated during testing is the dual wheel size compatibility. The grinder accepts both 4-1/2 inch and 6 inch wheels, giving you flexibility for different applications without needing multiple tools.

We primarily tested with 4-1/2″ wheels for most cutting and grinding tasks, but we also ran several tests with 6″ wheels. The added cutting depth and surface coverage of the larger wheels is valuable for specific applications, and the motor has plenty of power to handle the increased load.

The tool-free guard adjustment makes switching between wheel sizes straightforward, and the guard itself rotates smoothly to multiple positions for different cutting angles.

Build Quality and Durability Observations

Milwaukee has a reputation for building tools that can take a beating, and the 3670-20 upholds that standard. The housing is robust metal gear case construction that feels like it could survive a drop from a ladder (though we didn’t deliberately test that theory).

The spindle lock is solid and positive, making wheel changes quick and secure. All the vents and openings are well-designed to allow cooling airflow while minimizing dust ingestion. After several weeks of heavy use in dusty shop conditions, we haven’t noticed any performance degradation.

The paddle switch mechanism feels robust and well-sealed against dust and debris. It’s the kind of design that inspires confidence for long-term reliability.

Customer Reviews Analysis: What Other Users Are Saying

Milwaukee M18 FUEL 18V Brushless 4.5/6in Grinder w/ Brake Tool Only has garnered substantial feedback from professionals and DIY enthusiasts alike. After analyzing hundreds of customer reviews, we found several consistent themes that align with our own testing experience.

The overwhelming majority of users praise the power output, with many noting it rivals or exceeds their corded grinders. Professional metalworkers and welders frequently mention the RAPIDSTOP brake as a standout feature that genuinely improves workplace safety