Making Bit Opening Inserts

Cutting List

Having the inserts does you no good unless you also have a mounting plate set up to accommodate them. So make the plate at the same time you make the inserts; you do use the same template Here's how to make a plate and an insert.

1. Make the mounting-plate template. I do hate-to do this to you, but. . . . Keep your finger at this page, and turn to the step-by-step text on making the Universal Router Mounting Plate on page 163. Follow the procedure outlined there for making the template.

When you bore the bit opening in the template, make the opening 4l/z inches in diameter, not 1 Vi inchcs.To cut a hole this large and have it centered perfectly, use a router and trammel (the Trammel Baseplate shown on page 39 is great for this operation) or do it at the drill press with a fly-cutter or hole saw.

2. Make the mounting plate. Follow the directions in the "Universal Router Mounting Plate" chapter on page 163 for using the template to make the mounting plate. Drill and countersink the mounting-screw holes for the router, and flush-trim the plate to duplicate the template.

But skip the directions in that chapter on routing the bit opening.

Instead, study the drawing Making Hit Opening inserts. As shown there in part A. use a 1 ii-inch-O.D. template guide bushing and a ! i-inch straight bit to rout the opening through

Part Qty. Dimensions Material

Mounting plate blank 1 H" X 8" X II Ji" Clear acrylic*

Insert blank* I fc" X 12" X 12" Clear acrylict


2 brass flathead machine screws, 8-32 x W

*You should get c minimum of four inserts from a blank this size.

t Phenolic is also suitable.

the mounting plate. Note the feed direction specified.

Then switch to a 34-inch straight bit in the same template guide to rout the vWmch-dcep rabbet, as shown in part B of the drawing.

At this point, you can pry apart the template and mounting plate.

3. Cut the insert blank. The insert must be made from the same material as the mounting plate. Obviously, you can't attach the template to a small square workpiece; so cut as many inserts as you want from one large work-piece. Bond the template to the workpiece with carpet tape, as you did in making the mounting plate. Use the tape to bond the workpiece to some scrap.The scrap will protect the work surface.Taping the workpiece and scrap


Retaining-screw hole, countersunk and tapped for 8-32 flathead machine screw

H"-thick acrylic, polycarbonate, or phenolic plastic

Mounting-screw holes.countersunk

Retaining-screw hole, countersunk and tapped for 8-32 flathead machine screw

H"-thick acrylic, polycarbonate, or phenolic plastic

together keeps the insert blank from shifting as it is cut free; if it would shift, the router bit would gouge it, and thus ruin it.

Set up the router with a /io-inch-O.D. template guide and a toft-inch straight bit.

Keeping the template guide tight against the inner edge of the template, rout out the insert blank.The plastics typically used for mounting plates arc all very hard. A /16-inch straight bit is thin and consequently frail. So take very shallow cuts, or you risk breaking the bit.

If you are making more lhan one insert—and surely you are—pry up the template and reposition It 10 cut the next one. The carpet tape always seems to stick to the wrong part, and when it does, you can't successfully peel it up and reapply ii to the right part. So you should figure on using new strips of tape for each new insert.

4. Rabbet the insert blank. Each insert must be rabbeted so it will nest into the mounting plate. The depth of the rabbet is critical, since you want the insert neither standing proud of the plate nor setting recessed. It must be perfectly flush.

To hold the insert blank for rabbeting, attach it to a 2-inch-square post with carpet tape. Clamp the post in a vise, and you have unrestricted access to the blank's edge. Because of its compact size, a laminate trimmer is a dandy router to use for this job. Use a rabbeting bit, of course.

making bit opening inserts


l'/S"-dia. guide bushing


Waste ^

, Mounting plate Va" straight bit Resu,t js a

Scrap to protect work surface 3!/2"-dia. hole

Template with a 4'/2"-dia. port


5/i6"-O.D. guide bushing

Insert blank


Insert blank


Waste Vi6H straight bit

Scrap to protect work surface

Template with 4'/2"-dia. port

4"-dia. disk

Template with 4'/2"-dia. port

4"-dia. disk


Va" straight bit

Va" straight bit

Scrap to protect Mounting / Template with a \ work surface plate 4l^" dia. port

!4"-wide x Vi6M-deep rabbet 4" dla. circle, which yields a Va" rabbet between 4"-dia. cut and 3l6"-dia. rut


Insert blank t


Shop-made edge guide is bonded to baseplate with carpet tape.

Va" rabbeting bit

Scrap clamped in vise (bond insert to scrap with carpet tape)

Va" rabbeting bit

Scrap clamped in vise (bond insert to scrap with carpet tape)

Router base

Insert Va" rabbeting bit

Router base

Insert Va" rabbeting bit

Chuck a rabbeting bit in the router. Make sure you have the appropriate pilot bearing lor making a 1 »-inch-wide rabbet. Set the cutting depth, using the mounting plate's rabbet. Set the mounting plate on the baseplate, with the flange formed by the rabbet beside the bit. as shown. Set the height of the bit to exactly equal the thickness of the flange. If you rabbet just this thickness from the insert, it will nest Hush into the mounting plate.

The drawing shows a shop-made edge guide stuck to the router's baseplate. This isn't essential, but you may find it helpful in keeping the router steady on the workpiece.The insert blank is small, and you have to work its entire edge. To secure the blank yet minimize the handling, stick it to a block of scrap wood with carpet tape. You can improve die bond if you squeeze the blank to the block momentarily, using a clamp. (The improv ed bond, of course, means the insert will l>e harder to pry off the wood when the roudng is done.) Clamp the wood in a vise, and rout the rabbet around the insert.

5. Drill, countersink, and tap the retaining-serew holes. Retaining screws are needed to keep the insert in place. Since there's no weight on it, as there is on the mounting plate, the router's vibrations will prompt it to pop out of place—a real disaster.

The screws are located in the seam between the plate and die insert, as shown in the drawing Hit Opening Insert Layouts. As you can see. the mounting plate has through holes, while the inserts have only two crescent-shaped notches on the edge. All arc slightly countersunk.

Because the holes in the mounting plate are tapped (so the screws thread into them), their diameter has to be coordinated with the tap to be used. Cheek the tap: the size of the bit to use will be embossed on it. If you are using 8-32 JJJ screws, use what's called a «29 bit. (You can buy the tap and Jia the bit at a good hardware store.)

JQ The easiest way to bore the holes is on the drill press. Fit AS an insert into the mounting plate, and position this work-r piece on the drill press table. Drill the two holes, then 1L, countersink them. Remove the insert. Tap the holes in the Q) mounting plate. Details on using a uip are found in the

Create a countersink arc in the insert using a round file. Mark the locations of the needed countersinks—two in each insert. A couple of swipes with the file will form the profile needed.

appendix, in the feature "CuttingThreads" on page 304.

L'sc the insert that's been drilled as a pattern for filing similar notches in any other inserts you are making. Here's the situation: The half-hole or notch in the insert merely provides clearance for the screw shank. The screw head needs a skosh of contact to hold the insert down. But a precise fit isn't dial important here. Now it's dam hard to bore half a hole. A Forstncr bit will do it, but you won't find a «29 Fortsner. If you fit each insert into the mounting plate and drill all of them, you'll distort the mounting-plate holes, making them impossible to tap. So that's out. A little work with the comer of a file should do die trick.

6. Bore the bit opening in the insert. The best way to bore the bit openings is with the router. This will provide true zero-clearance holes in the inserts. Mount the plate on die router. Chuck the bit you want to use in the router's collet. Fit an insert into place and drive in the retaining screws. Tlirn on the router and plunge-bore through the insert. Because you are boring plastic, you need to bore in increments, plunging and retracting the bit again and again until the insert is penetrated.

Repeat this process with other bits and inserts, as you desire.

For very large openings, bore the holes on the drill press or with a a router and trammel To mark the insert cen-terpoint, which you need to do for either of these approaches, install it in the mounting plate. Then use a V-gnxjving bit, chucked in the router but turned by hand, to dimple the insert.

If you use the drill press, use a Forstncr bit or fiy-cuttcr to bore the holes. To begin, chuck a centering pin or small-diameter bit in the chuck, and lock the quill just above the insert surfacc. line up the insert's ccntcrpoint directly under the bit axis, then clamp the insert. Unlock die quill, and switch to the bit to be used to bore the bit opening.

If you want to make an insert for template guides, this is the approach to use. After the insert is aligned on the drill press table, bore a 752-inch-deep counterbore for the template guide flange using a 1 Vinch Forstncr bit. 'I*hcn switch to a 1 l-i-inch Fortsner bit to bore completely through the insert.

If you use a router and trammel, you still should use a drill press to bore a hole in the insert for the pivot pin. For the trammel to guide a cut dial's concentric with the center-point, the pivot hole needs to be perpendicular to the insert's face. A drill press will bore such a hole, lacking a drill press, use a drill equipped with a right-angle guide.

With the pivot hole drilled, lx>nd the insert to a fairly large scrap, which can be clamped to the work surface. Since the pivot Is in the waste, you need to ensure that it won't shift when the waste is cut completely free of die insert. And, of course, you don't warn the insert to move, cither.

Adjust the trammel to the desired radius, and rout the opening.

If you want to try routing a rabbeted opening for template guides, you should rout the larger-diameter circle first, cutting inch deep. 'Ilien readjust the trammel to rout the smaller-diameter opening.

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