Make a Mini-Router For Installing Swivel Bases

’m I sure you’ve seen those twoscrew swivel bases in the Brownells catalog—the ones that are reminiscent of those used on pre-’64 Winchester lever and bolt-action rifles. If they are installed properly, these swivel bases add a nice touch to the appearance of a custom stock. But it’s no easy task to get the socket for each base on top dead center and then keep everything in line with the bore while trying not to chip the sides of the mortise away.

Although I’ve installed dozens of these two-screw base sets by laying out a position for the front and rear base and then cutting in a mortise that would accept each, I felt there had to be a more accurate, less time-consuming method.

An Adapted Concept The method I used to use for installing these swivel bases involved scribing a line around the base, once the base was positioned on top-dead-center of either the forearm or bottom of the stock, while striving to keep both bases in line with the bore and one another. Once the position looked correct, I used radiused and straight-end chisels to cut a mortise that would accept the bases.

The swivel base was then painted with inletting black and the final fit was then scraped until the swivel base fit properly. This tedious process took an hour or more per swivel base—not an efficient process if the intention is to make a couple of bucks on the job. While watching a woodworker on TV make a whole kitchen full of cabinets during his half-hour show, I noticed this guy had several routers at his disposal, each on, it seemed, set up for one specific purpose.

As I was watching him speedily route out a mortise for door hinges with one of his routers, the thought occurred to me that these swivel bases could be installed in a more expedient manner than the process I’d been using. Obviously, a full-size router would be quite cumbersome and would only get very limited use in my shop. Nevertheless, this is how I began the thought process, and then I began sketching various concepts on paper in an effort to attack this time-consuming task.

The Concept Takes Shape My rotary tool is a hand-piece on a three-foot flexible shaft that ends at the motor, which is plugged into a variable speed-control. It has always served me well on the many projects where I need its help. This tool would become the heart and soul of my "mini-router," once a few

components were made that would bring the concept to fruition. The mini-router body was made from a short length of one-inch hexagonal aluminum stock that was left over from another project. The handpiece on my rotary tool measures .718 inch on the outside diameter, so that made it easy to simply drill a 23/32-inch hole into one end of the hex-stock, 1-3/4 inches deep, to accept the hand-piece.

Ahead of this hole, another hole, 1/2 inch in diameter was drilled to a depth of 5/8 inch beyond the 23/32 diameter hole, so that the nut used to tighten the collet that holds the cutting tool would freely spin when the tool is under power. At the working end of the aluminum hex-stock, a throughhole was drilled with a #30 (.1285) diameter drill so that the cutting tool, which is 1/8 (.125) inch in diameter will clear the sides.

Apiece of 2-inch-diameter aluminum bar stock was then set up in the lathe where a center hole was drilled at 15/16-inch diameter and then tapped with a 1"-12 tap. This piece will act as the up/down adjustable base for the mini-router. The aluminum round stock was left at 2 inches at the front end for a width of 1/4 inch and turned to 1.300 inches in diameter. Behind the front flange, a #10-32 set-screw hole was drilled and tapped into this diameter to lock the collar in place until further adjustment is necessary.

The outside diameter of the 1/4-inch flange was knurled to provide a non-slip surface when adjusting this collar either up or down. The hex-stock was then chucked in the lathe and the working end was turned to a diameter of .998 inch, 3/4inch long, with an undercut diameter of .900 inch at the hex shoulder. The next step involved threading this end at 1"-12 TPI to accept the adjusting collar. The thread cutting was not done in the conventional manner, however.

It’s very difficult to cut threads from right to left when a shoulder is close to where the threads should end. The method I use when threading barrel shanks involves using the thread-cutting tool upside down in the tool-post holder, and then running the lathe in reverse. This way I can start at the shoulder, in the undercut, and cut the threads from left to right without any danger of banging the threading tool into the shoulder because I didn’t react quickly enough.

The threading tool cuts until it reaches open space on the right end of the work, and I don’t need to hold my breath until the job is done. As the threads get close to final form, try the collar that was tapped at 1"-12 TPI to see if it starts to fit on the threads being cut into the .998 diameter. We want these threads to fit rather snugly together, with slight turning resistance, but not so much the collar will not turn.

The reason I chose 12 threads per inch is that one full revolution of the collar will achieve movement, up or down, of .083 inch or around .013 inch per each side of the hex body. Two full turns of the collar will move the cutter, once installed in the rotary-tool collet, up or down .166 inch which is just about the height of these swivel base bottoms. I marked the hex faces with #1 through #6 to act as points of reference in the progress of reaching the necessary depth of cut for these bases.

Every other one of the hex faces were drilled and tapped to receive a socket set-screw that keeps the hand-piece in place, preventing it from moving up or down in the holder during use. Determining Pilot Diameter Like all routers that follow a template, this one will need to have a pilot. The pilot is critical because it’s the only dimension that’s easily modified. As we go through the mathematical gymnastics a bit later, it will become obvious as to why.

Common cutter diameters are used throughout the making and the use of this mini-router, so most of the compensation necessary for the cutter to

create a receptacle for these swivel bases is applied to the pilot diameter. The two-screw swivel bases I use most are made by Dakota Arms Inc. These are very nice looking bases and are very well made. Dave Talley also makes this style of base and they work equally well for the purpose. The bases shown here are from some of my older stock and have a width and length dimension that varies slightly from those that are currently available.

Most all of these bases have a 2-degree draft angle along the sides, so the bottom of the base is around .006 inch smaller than the top, exposed surface, once they are installed. Depending on which base you use, measure the width and length with a caliper as close to the bottom as you can and record these dimensions. The front end of the 1"-12 threaded adjusting collar was set up in the lathe and a recess was cut 1-1/8 inch in diameter, .100 inch deep from where the threads are, out toward the outside diameter.

Because each manufacturer of these two-screw bases has their own idea on the sizes involved and because I wanted to have a replaceable pilot in case this one became damaged, the ability to replace this pilot would be a handy option. To create the pilot, the 1-1/8-inch-diameter round stock was chucked in the lathe and a #30 hole was drilled completely through the center.

A pilot diameter of .263 inch was calculated to be optimal, using the nominal dimension of a .500-inch slot width, cut with an end mill of that diameter. End mills rarely cut to exact size so a rough diameter was determined for the pilot diameter to temporarily be turned to .275 inch diameter for a length of .375 inch. The pilot diameter was then chucked and the back side of the 1-1/8-inch stock faced until it reached a thickness of .100 inch.

A small 45-degree angle was cut into the back face inner edge to clear the radius from the turning tool used to cut the recess in the 2-inch diameter adjusting collar. The flanged pilot needs to slip fit into the collar recess. To keep the pilot from falling out of place, two #6-32 flat-head set screws were installed 180 degrees apart and set slightly below the surface in 82-degree chamfers. We’ll need one more dimension before arriving at the final pilot diameter—the actual width of our template slot.

Making the Template To be able to route an accurate receptacle in place for these two-screw swivel bases, we’ll need a template to guide the cutter in the mini-router. For this base, I used 1-1/2- by 1-1/2inch aluminum angle stock with a 1/8-inch wall thickness purchased from our local farm-supply store. This angle stock came in an 18-inch section for under $6. I cut a 7-inchlong piece and then squared up each end.

This section was then set up in the mill vise and a 1/8-inch flat was milled along the top edge where the two top ends intersect. A 1/2-inchthick aluminum block, 1-1/2 inches wide by 2-1/4 inches long was then set up in the vise. A 90-degree angle was then milled into the bottom surface of this block so that it will sit on top of the aluminum angle stock.

Once this block was fit to the top of the aluminum angle stock, the angle plate was drilled, tapped, and chamfered from the underside for two #5-40 flat-head screws on each side to anchor it in place. All four screws go through the angle plate and up into the aluminum block. The angle plate was then tightened up in the mill vise and the top of the aluminum block was milled so that the top of the block blends with the 1/8-inch wide cut previously made on the top of the angle plate.

From my previous calculations, a 1/2-inch-wide slot would work best for this template as 1/2-inch end mills are common cutting tools and a reasonably strong wall for our cutter guiding pilot will result. The aluminum angle was then held in the mill vise upside down, clamping on the sides of the block screwed into the top, side faces of the angles. This makes it much easier to locate the exact center where the two inside faces of the angle stock intersect.

The length of these two-screw bases measure around 1.094 inch. For calculation purposes, we need to subtract our theoretical pilot diameter of .263 inch and then the cutter diameter to be used in the mini-router of .125 inch diameter. The result, .706 inch, is the length we need to make the slot in the top plate. A dial indicator with at least one-inch of travel will be a big help in recording when this length is reached.

Once the end mill has cut the slot completely through the top plate, remove the template from the vise and deburr all the edges. We now need to measure the actual inside width of the slot with the inside recording jaws of a caliper. Most likely, it will not be the same diameter as the end mill (.500 inch). Run-out and size discrepancy of the cutter will cause the cutter to cut oversize somewhat.

Whatever the dimensional difference above .500 inch we end up with, half of that amount needs to be added to the diameter of our theoretical pilot diameter (.263). For example, if the

template slot width ends up at .508 inch wide, we would need to add half the amount over .500 inch, or .004 to the diameter of the pilot. With the pilot mounted in the hex-stock holder, chuck the hex end in the lathe and then turn the pilot diameter to a diameter of .267 inch. Mounting Hole Drill Guide Try as I may, it’s difficult for me to drill a hole perfectly perpendicular to a surface using a hand drill.

If the hole is crooked, the screw will surely go in crooked, and then the screw head will not fit the counter-bored holes correctly. I made up a spring-loaded 1/8" Drill .312" Diameter Spring Housing Conclusions If you invest the time up front to make this mini-router, it will save you time later when installing these two-screw swivel bases.

Without altering the pilot diameter, I have used the router and template to install the latest version of the Dakota Arms bases, though a bit of final scraping is usually Figure 1. required to achieve a perfect fit. ■ .218" Diameter Drill Guide guiding tool that has the front end of the tool fitting into the counterbore of the screw holes in these bases while a hand drill powers the drill bit (see Figure 1).

The counterbore holes in these bases that accept the screw heads are .219 to .220 inch as measured with gauge pins. Starting with 1/4-inch round stock 7/8 inch long, I turned one end down to .218-inch diameter for a length of 5/8 inch. A #30 hole was then drilled through this piece. The back end of this part was turned and then polished to .248 inch.

A 1-inch length of .312-diameter stock was then chucked up in the lathe and a .250-inch-diameter hole was drilled to a depth of 7/8 inch, and then a .218 hole was drilled through. There should now be a shoulder around 1/8 inch long in one end of this piece. The front end was then faced off until the shoulder reached 1/16 inch in length. If all goes well, the first piece should slide inside this last piece until the .248 diameter hits the shoulder in the front end with the .218 diameter protruding out the front end.

Another short piece of .312-inch stock was then chucked in the lathe, drilled through the center with a 1/8-inch drill, and the outside diameter turned until the rear section of the drill guide, with the .250 hole, would just slip on. Once this was accomplished, the short piece of .312 stock was sawed off just behind the rear section of the drill guide. A hole was then drilled and tapped for #3-56 through the outside wall of the rear section and through the plug into the 1/8-inch hole.

The rear face was then faced flush in the lathe, and the 1/8inch through-hole edge chamfered along with the outside edge of the rear section of the drill guide. Remove the screw holding the plug in place and then remove the plug. We now need to insert a coil spring that will slip into the rear end of the drill guide with the .250-diameter hole. A spring of the appropriate diameter came out of one of Wolff’s service packs.

Once the spring is installed, the plug can be slipped back in place and kept there with the #3-56 screw. A 1/8-inch drill should now be installed through the hole in the plug until you can just see the tip of the drill protruding out the front end of the drill guide. Turn the #3- 56 screw until it hits the drill shank and tighten it up. Test the action by holding the rear of the guide while pushing the front end of the guide backward. The front end should move back with some resistance and spring freely forward.

AGARAD4 Armorer’s Course, amg version pick up from Sept 05 issue

AGARAD4 Armorer’s Course, amg version pick up from Sept 05 issue