Modulus Arms AR-15 User manual

Universal AR-15 80% Lower
Receiver Jig Instructions

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Modulus Arms Jig Contents
The Modulus Arms Universal AR-15 80% Lower Receiver Jig contains the following parts:
1. 1x Template
2. 2x Side Plate
3. 1x Drill Guide
4. 1x Front Support
5. 1x Rear Support
6. 1x Buffer Support
7. 1x Depth Gauge
8. 14x Screws [8-32 x ½”]
9. 2x Side Plate Screw [8-32 x 1.5”]
10. 1x Rear Take Down Bolt [1/4-20 x 2.25”]
11. 1x Flat Washer [1/4”}
12. 1x Rear Take Down Nut [1/4”}
Figure 1: Jig Contents
2
9
10, 11, 12
8
3
6
7
1
5
4

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Required Tools
The following items are required to finish the 80% AR-15 Lower Receiver:
(Available for sale on Modulus Arms Website)
1. ¼” Dia, ¼” Shank, 4” Long, 1” Cut, Square End Mill
2. 3/8” Drill Bit
3. 19/64” Drill Bit
4. 5/32” Drill Bit
5. 3/8” Drill Stop
Figure 2: Jig Fabrication Kit
The additional tools needed to finish the 80% AR-15 Lower Receiver:
1. Router (Laminate/Trim Recommended)
2. Front Takedown kit or ¼” bolt
3. Drill (Hand or Drill Press)
4. 1/8” Allen Wrench
5. Vise or device to hold Jig
6. 9/64” Allen Wrench
7. Phillips Screwdriver
8. 3/16” Allen Wrench
Figure 3: Example Tools
Routers that we recommend to finish the 80% AR-15 Lower Receiver:
1. Ridgid R24012
2. Dewalt D26670
3. Makita RT0701C
4. Porter-Cable PCE6430
5. Porter-Cable 7310

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Modulus Arms Jig Assembly
Front Support: The front takedown pin secures the front support as shown in Figure 4 (Left). For best
results, the spring and detent from the Modulus Arms AR Lower Parts Kit may be used. Optionally,
Modulus Arms offers an AR-15 Front Takedown Kit for sale on the website. To install the Front Support
you must first install the front takedown pin. Next, you place the front support in the proper location
and secure it with the front takedown pin as shown in Figure 4 (Right). If a front takedown pin is not
available, you may use a ¼” diameter bolt (not supplied).
Figure 4: Front Takedown Pin (Left), Front Support (Right)
Buffer Support and Rear Support: Thread in the buffer support (clockwise) until the buffer support is
below the rear surface of the lower, then further turn clockwise until the three screw holes are aligned
vertically shown in Figure 5 (Left). On some lower receivers, especially polymer, the buffer support can
go in very snug. Threading in two of the screws to use as a handle can help to install the buffer support.
If it is too tight, do not thread it in as it can get stuck. Using two of the ½” long screws, attach the rear
support. Do no tighten, yet. Make sure the rear support is in contact with the lower receiver rear
surface. If it is not in contact, you must thread the buffer support in further.
Figure 5: Buffer Support (Left), Rear Support (Right)

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Drill Guide: Align the drill guide under the rear pocket and trigger slot cutouts in the template. Use four
½” screws to attach the drill guide to the template as shown in Figure 6. Tighten the screws to 1 ft-lb.
Figure 6: Drill Guide Attachment
Template: Align the template over the rear and front support as shown in Figure 7. Use four ½” screws
to attach the template. Tighten all four screws to 1 ft-lb. Tighten rear support screws to 1 ft-lb.
Figure 7: Template Attachment

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Side Plates: Place the side plate into position and thread two ½” screws into the top through the
template as shown in Figure 8. Leave loose. Repeat the same steps on the other side plate. Each side
plate is identical and can be placed on either side. When both side plates are attached loosely, squeeze
the side plates together against the sides of the lower receiver. When they are tight to the lower
receiver, tighten the four ½” screws to 1 ft-lb as shown in Figure 9. Screw the two 1-1/2” screws through
the side plates to connect them together as shown in Figure 10. Tighten these two screws to 1 ft-lb. Do
not install the 1/4-20 bolt.
Note: Many questions have been asked about these two 1-1/2” screws. They are used to
prevent the side plates from flexing in when clamped in a vise. That is why both sides are
threaded.
Note: The side plates are identical and can be used on either side. If you wear out any hole, you
have three more sets of holes to use.
Note: It is recommended to put masking tape or some other protection over the sides of the
lower before installing the side plates. Doing so will help prevent scratches or wear.
Figure 8: Side Plate Alignment
Figure 9: Side Plate Tightening
Leave Loose
Squeeze then
Tighten

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Figure 10: Vise Screws
Drilling the Trigger and Rear Shelf Holes
Trigger Slot Drilling: With the two exposed holes in the trigger slot, use the 19/64” drill bit to drill all the
way through the lower. View Figure 11 for locations.
Note: If a hand drill is being used be sure to hold the drill upright. If the drill is not held upright
blemishes will be visible in the trigger slot after assembly. This is true for all drilling of the lower.
Note: Use a vise, C-clamp or other mechanism to hold the Jig.
Note: Be sure not to start spinning the drill until after the drill bit is placed into the drill guide or
premature wear will occur. This is true for all drilling of the lower.
Note: Use plenty of WD-40 or cutting fluid and remove the chips regularly. This is true for all
drilling of the lower.
Figure 11: Trigger Slot Holes

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Rear Shelf Drilling: Slide the 3/8” drill stop onto the drill bit. Insert the drill bit into the depth gauge in
slot labeled “B” as shown in Figure 12. Make sure the drill bit is inserted all the way into the slot and
tighten the drill stop onto the drill bit. Next, drill the three holes visible in the rear shelf template
opening as shown in Figure 13. Be sure to drill to the depth specified with the drill stop and do not go
any further.
Note: Modulus Arms forged lower receivers have some of the rear pocket already machined.
You must only drill the hole closest to the trigger slot template.
Note: Be sure the drill stop is tight. Check the placement of the drill stop with the depth gauge
after each hole.
Figure 12: Slot B
Figure 13: Rear Shelf Drill Location

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Milling the Trigger Slot
Configure Jig for Milling Trigger Slot: Remove the four screws holding the template to the front and rear
Support. Remove the four Screws holding the template to the side plates. Remove the template from
the Jig. Remove the drill guide from the template. Reinstall the template by inserting and tightening the
eight screws to the side plates, front and rear supports. The Jig should be configured like Figure 14.
Figure 14: Configuration for Trigger Milling
Trigger Slot Milling: Loosen the router base so that it is as far down as it can go. Insert the end mill into
the router. Adjust the end mill in the router until it hits the bottom of the depth gauge slot “A”. When
the end mill is at the correct height, tighten the collet of the router locking the end mill in place. Move
the base of the router until the tip of the end mill is at the first line in slot “A”, shown in Figure 15. Place
the router into the trigger template and into one of the holes previously drilled as shown in Figure 16.
Be sure not to be touching the sides of the holes when you start the router. Start the router and allow it
to come completely up to speed. Start to move the router removing material. Start by moving in semi-
circular motion in small clockwise steps. Press down firmly to prevent the router from lifting. Do not try
to move the router in one straight direction but rather in smooth half circles removing small amounts of
material as you proceed. Join the two drill holes together and then start to widen the opening until you
reach the template. When you reach the template, trace the inside diameter of the template in a
clockwise motion. Be sure to remove as much material as possible before touching the template or
tracing the template. Reducing the amount of time in contact with the template will increase its
longevity. Do not push hard against the template. A gentle touch is enough. Mill the rounded corners in
the template last, as they are most likely to chatter. When the opening is the same size as the template
turn off the router, wait for it to completely stop and remove it. Adjust the router base so the end mill is
at the next mark on the depth gauge as shown in Figure 17. Continue in this pattern until you reach the
bottom of the depth gauge. At this point, you should be out of the bottom of the lower receiver.
Note: DO NOT INSERT OR REMOVE THE END MILL FROM THE TEMPLATE WHILE THE END MILL IS
SPINNING. THIS IS DANGEROUS AND THE MOST COMMON CAUSE OF BROKEN END MILLS OR
DAMAGED JIGS/LOWERS.
Note: Be sure the end mill is very tight in the router. Tighten it as much as possible.

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Note: If the router begins to whine or vibrate during milling, turn the router off and remove it
from the template. Inspect the end mill and be sure there are no chips jammed into the teeth of
the end mill. If there is, use a tool to remove the stuck chips. A small screwdriver or pencil works
well. Be sure not to damage the teeth.
Note: The top 90% of the milling is removed in future steps so this is your practice session. Use
it to adjust the router speed settings for optimal results. We recommend a mid-speed setting on
most routers. Adjust as necessary for your exact router.
Note: If you have a shop vacuum, use it. We recommend sticking it between the lower and the
template over the magazine opening. It typically fits and stays. Run the vacuum whenever you
are milling. If you do not have a shop vacuum, stop regularly are remove the chips. If they build
up, they can become stuck between the end mill and the template and melt. This build up can
be removed from the end mill with sand paper. Use of a cutting fluid or WD-40 is
recommended.
Figure 15: Trigger Slot Depth Setting