AR-15 Jig Instructions - Modulus Arms Jig Instructions … · · 2014-07-29 June 16, 2014 | AR-15...

AR-15 Jig Instructions

www.ModulusArms.com June 16, 2014

www.ModulusArms.com | AR-15 Jig Instructions 2

Introduction

If your lower can accommodate an AR-15 upper, it is compatible with the Modulus Arms 80% AR-15

lower jig. The Modulus Arms jig utilizes standardized features within the lower receiver designed to

accommodate an AR-15 upper to properly locate the templates, resulting in a universal fit. A forged

lower completed with the Modulus Arms 80% jig using basic hand-held power tools is shown below.

For the first time user, the Modulus Arms jig offers increased safety (no milling with a drill press), a simple

and easy to use process, compatibility with commonly available hand power tools, and reduced risk of

failure.

For the experienced builder, the Modulus Arms jig offers universal fit, increased precision, and reduced

fabrication time. Modulus Arms also offers individual replacement parts.

The lower is easily completed with four simple steps to fabricate the : 1) the trigger slot, 2) the rear shelf

pocket, 3) the fire control pocket, and 4) the fire control pin holes (trigger pin, hammer pin, and safety

selector). To reduce the fabrication time and to improve the finish, a router with an end-mill is used to

finish the cavity that is first hollowed out with a drill bit in the first three steps.

Once any of the required finishing features are started, the lower is considered a firearm and all local,

state, and federal firearms laws will apply. It is your responsibility to check the legality of finishing and

owning an 80% lower in your area. Please make sure you are familiar with the safe operation of power

tools before using the Modulus Arms jig to complete your lower.

Modulus Arms Jig Part Description & Key Features

The Modulus Arms 80% Jig kit contains the following parts as shown in figure 1 and listed below: 1. Template

2. Side Plate (2x)

3. Drill Guide

4. Front Support

5. Rear Support

6. Buffer Support

7. Depth Gauge

8. Long Cap Screws [8-32 x ¾”] (12x)

9. Side Plate Support Screw [8-32 x 1.5”] (2x)

10. Rear Take Down Screw/Nut [1/4-20 x 2.25”]



Figure 1: Modulus Arms Jig Kit Contents (in production kits, short and long screws are the same size)

Unlike conventional 80% jigs, all of the features that require milling with the ¼” end-mill are contained on

a single template that is designed to be used in two orientations (figure 2). The fire control pocket is

fabricated in one orientation while the rear shelf pocket and trigger slot are fabricated in the other

orientation. The template is attached to the front and rear supports. The side plates are attached with the

oval holes allowing for a universal fit regardless of variations in the lower receiver widths.



Figure 2: Template Features

To speed up the milling process, excess material is removed with a drill. The drill holes are located with a

drill guide installed under the template as shown in figure 3. Since the template only exposes a limited

number of holes in the drill template, there is no chance to drill a hole in the wrong location.

Figure 3: Drill Guild installed under template in both orientations

The same side plate is used on the left and right side as shown in figure 4. The rear takedown hole is

provided to assure proper placement for the drilling of the safety selector and the trigger group pins. The

side plate support holes are designed to reduce flex and marring with anodized lowers when the side

plates are placed in a vice to secure the lower during the milling process.

Figure 4: Side Plate Features

The depth gauge is used for both drilling and milling as shown in figure 5. The gauge is used 1) to set the

drill depth using a drill stop on the 3/8” drill bit and 2) use to set the router depth for the final cut. Depth

gauge “A” is used to set the end mill so it exits the bottom of the lower for the trigger hole. Depth gauge



“B” is used for the rear shelf (depth = 0.630”). Depth gauge “C” is used for the fire-control pocket (depth=

1.249”). Production depth gauges will have engraved hash marks, the first hash mark represents the top

of the lower. Each hash mark is ¼” deep and represents the maximum depth of cut for a pass. The user

is encouraged to start at 1/16” - 1/8” of an inch depth of cut and work up to a comfortable depth that

should not exceed more than ¼” depth of cut. Typically, a deeper depth of cut requires a router with

variable speed so the end-mill speed can be slowed down for more control.

Figure 5: Depth gauge used to set the drill depth and end-mill depth

The following parts are required to complete the jig and can be purchased as an optional kit: 1. ¼” x 4” 3-flute End-Mill with 1” Depth of Cut

2. 3/8” Drill Bit (fire-control, rear-shelf, and safety

selector)

3. 19/64” Drill Bit (trigger slot)

4. 5/32” Drill Bit (hammer/trigger pin)

5. 3/8” Drill Stop

Figure 6: Optional kit required to finish the lower

The following tools are required / recommended (examples shown in the figure below): 1. Router (full size or laminate)

2. Drill (hand drill or drill press)

3. Vice (drill press shown)

4. 3/64” Allen Wrench (for drill stop)

5. 3/32” Allen Wrench (8-32 cap screws)

6. 3/16” Allen Wrench (Rear Take Down Screw)

7. Philips Screwdriver (Side Plate support)

8. Front Take Down (pin, detent, & spring)

9. C-clamp (optional if vice is not mounted)



Figure 7: Example of common tools that can be used to complete the lower

Modulus Arms Jig: General Assembly

Front Support: The front takedown pin is used to secure front support as shown in figure 8. For best

results, the spring and detent from the parts kit should be used install the pin.

Figure 8: Install front take down pin and use it to secure the front support like an upper

Rear Support: Thread in the buffer support (clockwise) until the buffer support is below the rear surface

of the lower (figure 9), then further turn clockwise until the two screw holes are at 12 o’clock. Cap screws

can be partially threaded into the buffer support for additional leverage. Using the long cap screws, attach

the rear support. Make sure the buffer support is recessed so that the rear support touches the back of

the lower for proper alignment.



Figure 9: Install buffer support and rear support

Template and Drill Guide: The template attaches to the front and rear support using four small cap

screws. For milling, the template is used alone (figure 10, left). For roughing, the drill guide is used (figure

10, right). The first three of four steps involves installing the template with the drill guide to rough out the

pocket, then removing the drill guide to mill the pocket. Removing only the front support makes it easy

to remove the drill guide.

Figure 10: Template only (left) or Template & Drill Guide (right) installed

Side Plates: The side plates attach to the jig through the template with four small cap screws (figure 11,

right). The jig is designed to accommodate different lower widths and should be pressed against the lower

before tightening the screws. For the first three steps, the main purpose of the two side plates is to allow

a vice to secure the jig, so the screw through the rear take down hole is not needed. For the fourth step,

the side plates serve as the template for the fire control group pins holes. For increased accuracy, the rear

take down screw/nut can be installed through the rear-take down hole.



Figure 11: Side Plate Installation

Step 1: Trigger Slot

Configure jig for drilling: The template is installed with the rear pocket template closest to the buffer as

shown in figure 1.1 with the drill guide installed.

Figure 1.1: Jig configured to drill he rear pocket and trigger slot

Trigger Slot Drilling: With the two exposed holes in the trigger slot, use a 19/64” drill bit to drill all the

way through the lower (figure 1.2, left). Figure 1.2, right shows the lower after completion. For best

results, start the drill after the drill bit is partially or completely through the drill guide to prevent

accidental damage to the template.



Figure 1.2: Drilling the trigger slot holes (left), lower after drilling (right)

Configure jig for milling: The configured jig is shown in figure 1.3 after the drill guide is removed.

Figure 1.3: Jig configured to mill the rear pocket and trigger slot

Trigger Slot Milling: A router with an end-mill is used to form the cavity (figure 1.4, left) and the completed

lower with the trigger slot is shown in figure 1.5, right).

1. Adjust the end-mill to touch the top of the lower (using the rib between the two drill holes) – use

the depth gauge to set the depth and verify.

2. Important: Set the first cut depth to be ¼” (maximum) below the surface using the drill guide,

verify that the cutting surfaces of the end-mill are below the edge of the template. Modulus

Arms suggests that the user first start with 1/16”-1/8” cutting depth before working up to a

comfortable depth not to exceed ¼”.

3. Insert the end-mill into a drill hole and without the end-mill touching the lower, carefully turn on

the router allow the router to come up to full speed.

4. With firm downward pressure applied to the base, mill using shallow clockwise passes with light

pressure. The goal is to first adjoin the two holes, then widen until the end mill touches the side

of the template. Turn off the router, wait until the end-mill stops spinning, and remove from the

jig. Figure 1.5, left shows the lower after the first pass.

5. Increase the cutting depth ¼” using the depth gauge and repeat steps 3 and 4 until the end-mill

breaks out of the bottom of the lower. The “A” depth gauge can be used to set the final depth as



shown in figure 1.4, right. Modulus Arms suggests that the user work up to a comfortable depth

not to exceed ¼” depth of cut based on adjustment of router speed and user experience.

Figure 1.4: Router to mill out trigger-slot ¼” at a time (left) and final cut depth set with “A” depth gauge

Figure 1.5: Example of partially milled trigger slot (left) and complete trigger slot (right)

Step 2: Rear Shelf

Configure jig for drilling: The template is installed with the rear pocket template closest to the buffer as

shown in figure 2.1 with the drill guide installed. DO NOT install the rear takedown screw/nut for this step.



Figure 2.1: Jig configured to drill out the rear pocket and trigger slot

Rear-Shelf Pocket Drilling: The three holes exposed through the rear-pocket template are drilled out with

a 3/8” drill bit (figure 2.2, right) to a depth set with the “B” depth gauge (figure 2.2, right). Finished lower

is shown in figure 2.3. Make sure to start the drill with the bit surrounded by the drill template.

Figure 2.2: Drilling the trigger slot holes

Figure 2.3: Drilled lower –most 80% lowers will have three holes. The lower in this example has additional reference

slot, which does not require the two right most holes.




Figure 2.4: Jig configured to mill out the rear pocket and trigger slot

Rear-shelf Pocket Milling: A router with an end-mill is used to form the cavity (figure 2.5, left) and the

completed lower with the trigger slot is shown in figure 2.6.

1. Adjust the end-mill to touch the top of the lower (using the rib between drill holes) – use depth

gauge to set the depth and verify.

2. Set the first cut depth to be ¼” below the surface using the depth gauge, insert router into a drill

hole, and without the end-mill touching the lower, carefully turn on the router allow the router

to come up to full speed. Modulus Arms suggests that the user first start with 1/16”-1/8” cutting

depth before working up to a comfortable depth not to exceed ¼”.


pressure. First adjoin the three holes by removing the rib between the holes, then widen until the

end mill touches the side of the template. Turn off the router, wait until the end-mill stops

spinning, and remove from the jig.

4. Check the depth using the “B” depth gauge after each pass. The final depth is shown in figure 2.5,

right. Adjust the cutting depth by ¼” at a time and repeat steps 2 and 3 until the final depth of

0.630” is reached as indicated by the depth gauge. Modulus Arms suggests that the user work up

to a comfortable depth not to exceed ¼” depth of cut based on adjustment of router speed and

user experience.



Figure 2.5: Milling the rear-shelf pocket (left) “B” depth gauge at the maximum depth for a 0.630” rear pocket

Figure 2.6: Finished rear-shelf pocket

Step 3: Fire-Control Pocket

Configure jig for drilling: The template is installed with the fire-control pocket template closest to the

buffer as shown in figure 3.1 with the drill guide installed.



Figure 3.1: Jig configured to drill the fire-control pocket

Fire-Control Pocket Drilling: The five holes exposed through the rear-pocket template are drilled out with

a 3/8” drill bit (figure 3.2, right) to a depth set with the “C” depth gauge (figure 2.2, right). Finished lower

is shown in figure 3.3.

Figure 3.2: Drilling the Fire-Control Pocket Holes



Figure 3.3: Lower with Fire-Control pocket holes drilled


Figure 3.4: Jig configured to mill the fire-control pocket

Fire-Control Pocket Milling: A router with an end-mill is used to form the cavity (figure 3.5, left) and the

completed lower with the fire-control pocket is shown in figure 3.7.

1. Adjust the end-mill to touch the top of the lower (using the rib between drill holes) – use depth

gauge to set the depth and verify.

2. Set the first cut depth to be ¼” below the surface, insert router into a trigger slot and without end-

mill touching the lower, carefully turn on the router, and allow the router to come up to full speed.

Modulus Arms suggests that the user first start with 1/16”-1/8” cutting depth before working up

to a comfortable depth not to exceed ¼”.


pressure. First adjoin the all holes to the trigger slot by removing the rib between the holes, then

widen until the end mill touches the side of the template. Turn off the router, wait until the end-

mill stops spinning, and remove from the jig. Figure 3.6 shows the lower after the first pass.

4. Check the depth using the “C” depth gauge after each pass. The final depth is shown in figure 3.5,

right. Adjust the cutting depth by ¼” at a time and repeat steps 2 and 3 until the final depth of

1.249” is reached as indicated by the depth gauge. Modulus Arms suggests that the user work up



to a comfortable depth not to exceed ¼” depth of cut based on adjustment of router speed and

user experience.

Figure 3.5: Milling the fire-control pocket (left) “C” depth gauge at the maximum depth for a 1.249” rear pocket

Figure 3.6: Fire-control Pocket after the first pass



Figure 3.7: Completed fire-control pocket

Step 4: Fire-Control Pin Holes

The last step is to drill the holes which accommodate the fire-control trigger and hammer pin as well as

the safety selector as shown in figure 4.1. A finished lower is shown in figure 4.2.

1. Insert the rear take down screw through the side plates (and through the lower rear take down

pin hole), and hand tighten the nut.

2. Using a wooden block or metal parallels, support the side so the side plate is parallel to the work

surface.

3. Optional: Clamp the bottom side piece in a vice, parallel to the work surface using wooden blocks

or metal parallel for spacing.

4. Using a 3/8” drill bit, drill through only one side of the lower (figure 4.1, left) for the safety

selector. (Try to make sure the drill is perpendicular to the jig before drilling)

5. Using a 5/32 drill bit, drill through only one side of the lower to form the hammer (figure 4.1,

right) and trigger pin holes (figure 4.1, center). (Try to make sure the drill is perpendicular to the

jig before drilling)

6. Turn over the lower and repeat the process.



Figure 4.1: Drilling the fire-control pin holes using the side plate as templates to locate the holes

Figure 4.2: Trigger/hammer pin holes and safety selector holes

Helpful Hints

The completed lower is shown in the figures below. This section will focus on additional hints and

suggestions to improve the quality of the lower. The hints and suggestions may be useful to both the first

time novice as well as experienced 80% lower fabricators.



Finishing Power Tools: The Modulus Arms jig is compatible with a number of different tools that can be

used to complete the drilling and milling process. Please make sure that you can safely operate the power

tools, and understand the manufacturer’s product manual.

A hand-drill or a drill press can be used to complete the 80% lower using the Modulus Arms jig. The hand-

drill is the lowest cost option, and its portability allows it to be easily borrowed from a friend or rented. A

drill press is a rarer and more expensive option, but results in reduced build time and increased precision.

Drill Press

o Set the drill-press speed to approximately 2500 rpm.

o Make sure the drill bit is square to the table in both the x and y directions before using.

Hand-Drill

o Using the low (2-speed) or medium (3-speed) gear option if available.

o Best results are obtained with a more powerful corded drill.

o Cordless drills will work; however, more than one battery and additional cool down times may

be required.

o For 7075 alloy lowers, the alloy is significantly more difficult to drill – a drill press is highly

recommended.

General Guidelines and Recommendations

o Make sure the drill bits are sharp – sharpen if necessary. Dull drill bits will result in

considerably higher effort and excessive temperatures.

o To prevent damage to the template edges, start the drill when the drill tip is partially in the

drill guide. After drilling, stop the drill bit before moving to the next hole.

o Frequently clear the chips by pulling the drill bit up but not out of the drill guide. The space

between the drill guide and lower is designed to accommodate the chips.

o Clear the chips between the drill guide and lower after each hole to prevent chips from

accumulating and compacting.

o Cutting oil or WD-40 applied with a toothbrush can reduce the drilling effort.

o After each hole, use the drill guide to check the depth to make sure the drill stop did not slip.

For the milling process, the Modulus Arms jig is compatible with all common methods such as using a drill

press, router, or mini-mill. Using a drill press as a milling tool is NOT recommended and can be dangerous.

A router is common tool that is affordable and commonly used, making them easy to borrow or rent. A

mini-mill is a significantly more expensive option but results a significant reduction in build time, and an

increase in quality for the experienced user.

Drill-Press as a mill (NOT RECOMMENDED)

Router

o A router commonly used for woodworking with a standard ¼” collet.



o The end-mill must be a 4” long, ¼” diameter end mill. Three flutes with 1” cutting depth is

recommended for best results. Longer cutting surfaces can damage the jig. Always check with

the first cutting pass that the cutting flutes are at least 50% below the jig template surface.

o Routers with variable speed give the bests results, although fixed speed routers can be used.

o Beware of inexpensive routers with quite a bit of flex – they can result in non-parallel walls

and damaged lowers.

o The results in these instructions were achieved with a Bosch PR20EVSK Colt Palm Grip Router

with the speed set at 4.75.

o End-mill cut in a clockwise fashion. Best results are achieved with light clockwise semi-circles,

trying to keep only ¼ of the cutting surface on the metal to be removed.

o This jig has been tested with a number of different routers.

Mini-Mill

o A mini-mill with a Modulus Arms jig in a machine vice can be used. The x-y table offers

additional control, improved surface finish, increased speed, and reduce fatigue.

o Start with the end-mill in a drill hole and start the mill. Slowly move table front and back

without touching the jig wall to cut a slot. This must be done at a slow feed speed since the

end mill is cutting on both sides.

o Cut either toward the front or back without touching the jig wall, then move front to back

removing more material (should be done in about two passes on each side of the starting

channel).

o Carefully control both the x and y to generate a light pressure against the template for the

final pass.

General Guidelines and Recommendations

o The user should start with a shallow 1/16” to 1/8” depth of cut to gain experience before

increasing the depth of cut to a comfortable level that should never exceed ¼”.



Variable speed router is high recommended when approach the maximum depth.

The lower end mill speed allows for greater control

o If the end-mill has larger than a 1” cutting surface, the first few passes need to be made

without the end-mill touching the end of the jig. Only once the cutting surfaces have cleared

the jig, should the end-mill touch the template edges to obtain the proper finished

dimensions.

o The end-mill when working properly will feel like its cutting through butter. When the effort

increases, it is possible the flutes are clogged with metal. This can be removed with a small

pair of needle nose pliers.

o The clogging effect can be reduced by

Frequently coating the end-mill with cutting oil or WD-40 applied with a toothbrush.

If possible, reduce the speed of the end-mill.

Misc. Tools: The following are suggestions based on our experience.

Vice: The vice and work surface must not move or flex when routing. The vice needs to be either

clamped or bolted to the work surface. A drill vice shown in the figure was selected due to its low-

cost, common availability, and low profile that reduces flexing.

Shop-Vac: Great at removing chips and cleaning up the work area to prevent spreading of metal chips

Toothbrush: Can be used to apply a small amount of oil on the end-mill.

AR-15 Jig Instructions - Modulus Arms Jig Instructions … · · 2014-07-29 June 16, 2014 | AR-15...

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Transcript of AR-15 Jig Instructions - Modulus Arms Jig Instructions … · · 2014-07-29 June 16, 2014 | AR-15...