Five ways to steer a router
| Guide | Follows | Good for | From the manuals |
|---|---|---|---|
| Bearing on the bit | The edge of the work or a template | Edge profiles, flush trimming, pattern copies | Bearing and cutter sizes are the bit maker's |
| Edge guide | The edge of the work, on rods from the base | Grooves and rabbets parallel to an edge; circles and arcs | Bosch's deluxe guide does straight cuts, circles and arcs; Milwaukee's straight edge guide fits the fixed base |
| Clamped straightedge | A board or rule clamped to the work | Dadoes and grooves away from an edge | Bosch: securely clamp a straightedge and guide the sub-base along it |
| Template guide (bushing) | A template fixed to the work | Repeated shapes, hinge recesses, inlays | Bosch, Colt and Milwaukee each fit guides in their own sub-bases |
| Table fence | The fence of a router table | Narrow and small parts, profiles on edges | See router tables |
A clamped straightedge is the simplest jig there is, and the dado guide in router projects turns it into a T-square that marks its own cut line. Clamps and bench work are covered in clamps and the workbench guide.
Template guides and the offset
A template guide is a short collar fixed in the sub-base; the bit passes through it and the collar's outside rides the template. Because the bit is smaller than the collar, the cut lands outside the template line. Bosch's manual says the template must be made to compensate for that distance — the offset — because the finished work will differ in size from the template by that amount; the Colt manual says the same.
Template-guide offset
offset = (guide outside diameter − bit cutting diameter) ÷ 2
template edge = finished edge ± offset
- guide outside diameter
- The collar's outside, as its maker lists it (in)
- bit cutting diameter
- The bit's cutting diameter, as its maker lists it (in)
Assumes the guide is centred on the bit and both are at their stated sizes. Bosch sells centering devices (the RA1051, and the RA1151 for the Colt); Milwaukee calls for a centering cone. Routing around the outside of a template, the part comes out larger by the offset on every edge, so make the template that much smaller; routing inside a template's opening, the recess comes out smaller by the offset on every edge, so make the opening that much larger. A test cut in scrap confirms it.
| Bosch guide | Outside diameter | Inside diameter | Largest bit (Bosch) | Offset with a 1/2 in bit |
|---|---|---|---|---|
| RA1113 | 5/8 in | 17/32 in | 15/32 in | Bit too large for this guide |
| RA1115 | 3/4 in | 21/32 in | 19/32 in | 1/8 in |
| RA1117 | 13/16 in | 5/8 in | 9/16 in | 5/32 in |
| RA1119 | 1 in | 25/32 in | 21/32 in | 1/4 in |
| RA1121 | 1-3/8 in | 1-19/64 in | 1-15/64 in | 7/16 in |
Diameters and the largest bit are from Bosch's 1617 manual, which asks for cutters 1/16 in less than the guide's inside diameter. Offsets are calculated with the formula above. Other makers' guides have other sizes.
Two limits sit around these numbers. The guide's collar has a depth below the sub-base — Bosch lists it for each guide — and the template must be thicker than that, or the collar rides on the work instead of the template. And not every tool takes a guide: Milwaukee allows only guides up to 1-3/16 in on its 2723, with its 5-3/4 in template sub-base, and Bosch's RA1181 table accepts none.
Bearings instead of bushings
A bit with a bearing the same diameter as its cutter needs no offset: the cut lands on the template line. Freud describes its 50-101 top-bearing bit, with the bearing above the cutter, for template work; a bearing at the tip follows a template fixed underneath. The trade-off is that the template must be as thick as the bearing is tall, and that every bit you use must carry its own bearing. Bit types are drawn in the bit profile chart.
Circles, joints and inlays
- Circles and arcs. Bosch's deluxe guide pivots for circles and arcs; a shop-made trammel does the same job with the router on a pivoting arm.
- Mortises. A plunge router with an edge guide or a mortising jig — see plunge routers and the joint itself in mortise and tenon.
- Dovetails. Commercial jigs pair a template with a template guide and a dovetail bit; Freud says its dovetail bits suit popular jigs. The jig's own manual sets the guide, bit and depth; the joint is covered in dovetails and choosing a joint.
- Box joints and sliding dovetails are usually cut on a table — see box joints and using a router table.
- Inlays and bow ties. Bosch lists inlays among template-guide work; matching an inlay to its recess depends on the offsets of the guides used, so test both in scrap.
Accuracy without a universal tolerance
No maker cited here gives a universal fit, clearance or tolerance for a router jig, and none is given here. Bosch suggests test cuts in scrap to confirm depth settings; Milwaukee asks for a sample cut on scrap before the work. Make the template from stable sheet stock — Bosch lists plywood, hardboard, metal or plastic — and lay it out from one reference edge as in measuring and marking.
Before routing with a template
- Guide and bit sizes known; offset worked out and drawn on the template
- Bit no larger than the guide allows (Bosch: 1/16 in under its inside diameter)
- Guide centred on the bit with a centering device
- Template thicker than the guide's depth, fixed so it cannot shift
- Work clamped; feed direction set as in your router's manual
- Test cut in scrap measured before the real part
More on the machine in the router guide; feed and setup in using a router.
Sources
The offset formula is geometry, not a maker's tolerance; it assumes a guide centred on the bit and a bit and guide at their stated sizes. No universal fit or clearance is given: test cuts in scrap settle them, as Bosch and Milwaukee both advise. Joint jigs are sold with their own instructions, which govern.
- 1.
1617, 1617EVS, 1617EVSPK, 1618EVS Operating/Safety Instructions. Bosch, 2018.
Used for: Clamping a straightedge and guiding the sub-base along it for grooves and dadoes; the deluxe edge guide for straight cuts, circles and arcs; templet guides, the offset between bit and guide, and the template made to compensate for it; the guide table of outside and inside diameters and the largest bit for each, cutters 1/16 in less than the inside diameter; centering devices; test cuts in scrap.
- 2.
GKF125CE Colt Palm Router Operating/Safety Instructions. Bosch, 2018.
Used for: The PR110 round sub-base for threaded template guides on the Colt; centering the guide with the RA1151; the finished work differing from the template by the offset.
- 3.
M18 FUEL Compact Router, Cat. No. 2723-20, Operator's Manual. Milwaukee Tool.
Used for: Template guides on the 2723 only up to 1-3/16 in, with the 5-3/4 in template sub-base and a centering cone; the straight edge guide or a clamped board as a guide; a sample cut on scrap before the work.
- 4.
1617EVS 2.25 HP Electronic Fixed-Base Router. Bosch.
Used for: Accessories listed for the 1617EVS: the RA1054 router guide, template guide system and sets, and the RA1051 centering device.
- 5.
RA1181 Benchtop Router Table. Bosch.
Used for: The RA1181 table accepting no template guides.
- 6.
50-101 Top Bearing Flush Trim bit. Freud.
Used for: A top-bearing flush-trim bit for template work.
- 7.
22-102 Dovetail bit. Freud.
Used for: Dovetail bits made to suit popular dovetail jigs.
About this guide
Written by BenchRoot Editorial. How BenchRoot sources, checks and corrects its pages is set out in the Editorial Policy. If something here is wrong or unclear, the contact page explains how to tell us.