A compact mag drill is the quickest way to put accurate holes in steelwork on site: carry it up, clamp it on, broach. Most are bought for annular cutters from about 12 to 36 mm, but with the right adapter they will also twist drill, step, countersink and run TCT HoleCutters.
They have limits, though, and most problems with them come from running into those limits: a magnet with poor hold, a fixed spindle speed that doesn't suit the cutter, or tooling the machine has no stroke for. This covers what works, what doesn't, and why, whichever make of compact mag drill you use.
Know your machine
Check four figures before choosing a cutter: cutter capacity, stroke, spindle speed, and whether it reverses. Using the Steelbor S36 as an example:
| Annular cutters | TCT up to 36 mm, HSS up to 32 mm, cutter length up to 110 mm |
| Twist drilling | Up to 13 mm |
| Countersinking | Up to 25 mm |
| Tapping | Not suitable |
| Speed | 800 rpm no-load, single speed |
| Stroke | 147 mm |
| Magnet | 160 × 80 mm, 1,015 kg adhesion |
| Spindle | Integral 3/4" Weldon |
| Weight | 10 kg |
For work away from mains power, the Steelbor V36 cordless covers similar compact work on 18V batteries.
Magnet hold comes first
Every other point in this guide depends on the magnet staying put.
- Thickness. Mag drills are rated for steel at least 10 mm thick. On thinner material, a flat steel plate clamped tight underneath improves hold, but it is not a full substitute. Check your machine's manual and rely on the safety strap.
- Surface. Paint, mill scale, rust, swarf and curvature all create an air gap, and an air gap kills holding force. Clean the area and make sure the whole magnet face sits flat.
- Material. Magnets won't hold on stainless or non-ferrous metals. These need a mechanical clamping arrangement.
- Power supply. Don't run other tools off the same supply, and take extra care on generators. A voltage dip weakens the magnet.
- Welding. Don't drill a workpiece that is being welded at the same time.
- Safety strap. Fit it every time, whatever the position. An illuminated magnet switch only tells you there is power, not that the magnet is holding. In an emergency, hit the motor stop, not the magnet switch. The motor stop leaves the magnet holding.
Speed: why cutter choice matters on a single-speed drill
Most compact mag drills run at one fixed speed, so you can't slow the spindle down for a bigger cutter. You have to choose a cutter that suits the speed. Recommended speeds for annular cutters in structural steel:
| Cutter diameter | TCT (rpm) | HSS (rpm) |
|---|---|---|
| 12–19 mm | 505–800 | 335–530 |
| 20–25 mm | 380–480 | 255–320 |
| 26–32 mm | 300–370 | 200–245 |
| 33–39 mm | 245–300 | 160–190 |
TCT cutters are designed for higher cutting speeds than HSS, so on a fast single-speed compact they are generally the closer match, particularly as diameters increase. Whichever you use, give the cutter steady feed and plenty of lubricant, and watch the swarf. If it comes off blue, the cutter is running too hot. See the Annular Cutter guide for the full tables.
Annular cutting technique
- Centre punch, don't pilot drill. The pilot pin locates in the punch mark and ejects the slug at the end of the cut.
- Feed gently for the first millimetre while the cutter establishes its groove, then apply firm, steady feed.
- Choose a cutter deeper than the material you are drilling.
- On thick plate, back the rotating cutter out periodically to clear swarf.
- Before breakthrough, ease off the feed. The slug will be ejected hot and sharp, so make sure nobody is below or in line with it, especially when working at height.
- Guard and entanglement. Keep the guard closed while cutting, and wear eye and hearing protection. Keep loose gloves, sleeves and jewellery away from the arbor, and only clear swarf ribbons with a hook or brush once the spindle has stopped.
- Check the slide and handles regularly. Vibration loosens slides, handles and arbors, and slack in the slide shows up as chatter, oversize holes and chipped cutters.
The Steelbor annular cutter range covers both HSS and TCT.
Beyond broaching: VersaDrive tooling
With a VersaDrive Rapid-Lock Weldon adapter (supplied with the S36 Pro kit), a compact mag drill takes the VersaDrive hex-shank range. That gives quick-change drilling and countersinking without needing a chuck.
- Twist drilling: any VersaDrive drill bit will fit via the 111130 Weldon adapter. 201070 drill bits have a Weldon shank and fit directly. Stay within the machine's capacity, which is 13 mm on the S36.
- Step drills and step cutters: suitable via the 111130 Weldon adapter.
- TCT HoleCutters: suitable with the 111035 adapter. Swap the supplied pilot drill for a 101030P-0003 pilot pin, and check there is enough stroke for the cutter. See the HoleCutter guide.
- Countersinking: because the speed is fixed, a compact is best with countersinks below about 20 mm. Clear the swarf so the countersink can't chatter over it. A piloted MultiSink gives the cleanest seat. See the Countersink guide.
What a compact mag drill shouldn't do
- Tapping. A tap needs low speed and reverse, which most compacts, including the S36, don't have. That rules out ImpactaTaps, DrillTaps and clutched tapping adapters, even where they physically fit. Tap with an impact wrench, or with a larger mag drill that has low gearing and reverse.
- Reaming. A compact doesn't usually have the stroke or working length for a reamer. Use an impact wrench, a pistol drill or a large mag drill.
- Large cutters. Above the machine's rating, or where the speed chart says the cutter needs much lower RPM, move to a larger variable-speed machine.
Lubricant
SpeedLube for general broaching and drilling. BioCut Blue for harder steels and stainless. In vertical and overhead positions, don't use liquid coolant or the coolant bottle. Use BioCut Paste on the cutter instead, and keep fluid out of the motor. Overhead drilling carries the highest risk if the magnet lets go. Only drill overhead where your machine's manual allows it, with the strap fitted and nobody below. See the full range.
Pre-start checklist
- Is the steel at least 10 mm thick? If not, is a backing plate clamped flat against it, with the strap fitted?
- Is the contact area clean, flat and free of paint and swarf?
- Is the machine on its own supply, with no welding on the same workpiece?
- Is the safety strap fitted and tight?
- Is the cutter within the machine's rating, and suited to its speed?
- Is the cutter fully seated in the arbor, with the correct pilot pin and the grub screws tight?
- Are the slide, handles and arbor free of play?
- Is the guard closed and lubricant ready (paste if working vertical or overhead)?
- Is the area below and in line with the slug clear?
If it goes wrong
| Symptom | Usual cause |
|---|---|
| Machine moves or lifts during the cut | Thin steel, paint or scale under the magnet, curved surface, or a shared or weak power supply |
| Holes oversize, cutter chipped | Play in the slide, a loose arbor, or excessive feed |
| Blue swarf, cutter dulls quickly | Speed too high for the cutter, or not enough lubricant |
| Cutter stalls in deep holes | Swarf packing: back the cutter out to clear it |
| Slug won't eject | Wrong or worn pilot pin, or a cutter too short for the material |
| Countersink chatters | Countersink too large for a fixed-speed machine, or swarf in the hole |
Watch: S36 Compact Magnet Drill
The Magnet Drill Training Series also covers the S50 and V36 cordless.
Technical guides and support
Annular Cutters · Drill Bits · HoleCutters · Countersinks · Mag Drill Registration & Warranty
Before you start: Always refer to the operating manual for the exact model of drilling machine you are using, as well as the instructions for your cutting tools. Portable drilling conditions vary widely from job to job, so an individual risk assessment and method statement (RAMS) is recommended for every application, and remains the responsibility of the user.