A router is probably the most versatile power tool in any woodshop, but with the bit spinning at up to 35,000 rpm, it's also capable of causing instant mayhem if not properly controlled. Few are the times when a router is used freehand - it's just too likely to end with disaster.
Routers are, therefore, always used with some kind of bearing, that is, something that bears on the work to control or limit the cut. Many router bits have sealed-bearing rollers at the top and/or bottom to guide them, but to open the potential of the hand-held router it is necessary to learn how to use a template guide. And to use a template guide creatively, you have to learn to make your own templates.
This instructable shows how to make a free-form template to allow copying of a pattern repeatedly.
Step 1: How to Make the Template
This past summer, I had the privileged task of building the head of the Burning Man effigy for the event. The head was about 30 times the volume of the usual head, given that the man was 3x its usual scale and the head was about 3.1x. At that size, the cloth sides of the traditional head wouldn't have lasted long what with the high winds 100' in the air above the Black Rock Desert, so the designer drew a plan for the sides with 44 cutouts in plywood backed by cloth.
Making the head entirely in the desert involved cutting the pattern 88 times 22 cutouts each side and two layers of plywood. No CNC machine on hand, so it's into the tool-bag of tricks and techniques developed over 25 years to do CNC by hand. Let's call it "Cerebral Nimbleness Control".
First, after figuring out the dimensions needed to make the designer's plan work out sensibly with the actual dimensions of the enormous structure standing nearby, I laid out the lines on a piece of plywood sized to make the module easily repeated, staggered and nested. I added 1/8" to the final dimensions I wanted, so that when the template was completed, it would give the correct size cutout in the material when using a 5/8" template guide bushing with a 1/2" diameter straight bit. A 5/8" bushing, correctly centered, will have the bit cutting 1/16" inside the template. You use a template guide and a plunge router so that you don't damage your template with the spinning bit and you don't damage your router by trying to cut away too much material at once.
I nailed the first piece of wood in place to just hit my layout lines and then added the rest of the pieces to complete the template to guide the router to make the template (to guide the router to make the cutouts in the actual piece).
Step 2: Building Up the Template
You can see that it's not critical to get the acute corners perfect. The 5/8" diameter of the template guide means that the curve will prevent the bit from getting all the way into the corners and a gap smaller than the arc of the circle won't make any difference. This means you can stay on your lines and not fuss with the corners - the guides are to make the router stay on the inside and what the outside is like doesn't matter. What does matter is that the router base be adequately supported at all times since tipping the router will spoil the cut and the router will then follow the divot faithfully every time thereafter if you don't repair the mistake.
Step 3: Cutting Out the Waste
Next, you set the plunge depth of the router. This is so the router isn't straining to remove too much material at once. A router is a high-speed motor with a cutter outboard of the end bearing for the armature, and with a long straight bit, the cutting end has a lot of leverage on the bearing. Putting too much strain on the bit can cause vibration, excess heat build-up and bearing wear. Plunge routers make it easy to remove a sensible amount of material at once, like 1/8" deep x 1/2" cutting diameter, but do it incrementally to go through your material in multiple passes. A router cares about the volume of material you're removing, so removing 1/8" deep x 1/2" wide is the same (approximately) as 1/2" deep x 1/8" wide with the same bit.
To set the depth, place the router with the template guide and appropriate bit on the template (in this case, the pieces of wood nailed to the eventual template). Release the lock so the motor can be lowered and lower the motor until the tip of the bit just touches the work. Set the lock to hold the motor down in this position. Then, release the vertical plunge depth stop and bring it down to touch the topmost of the stepped levels on the little turret that limits the depth of plunge. This is zero cut. Turning the turret allows the router to plunge past zero cut and start removing material.
The router I used for this was a Bosch 1617EVS in a plunge base which has 3mm, or 1/8", steps on the depth stop turret, so cutting 4 steps was enough to go through the 1/2" material into the sacrificial piece below. I made 12 passes, 4 per cutout, keeping the template guide running along the edges of the 1x material nailed to the plywood. The last pass is the one to make sure to faithfully follow the template with as it will clean up any little bumps left by any previous passes, usually at outside corners or anywhere dust has built up and impeded the guide bushing.
Step 4: Finishing Up
After removing the bulk of the material with the template guide and straight bit, I put a different bit into play to remove the remaining excess. This bit is a top-indexing or top-bearing flush-trimming straight bit. The bearing rides along the template edge and since it is the same diameter as the cutter, it trims flush. It would be possible to simply start with this bit, but you would have to plunge it into the material enough to get the bearing to engage the guide surface, so you're removing a LOT of material at once and the chance of wrecking your desired product is unacceptably high. Remember, NO TIPPING.
Removing the pieces tacked in place reveals the template, which you then use only with the plunge router and template guide bushing.