Friday, January 11, 2019

T19 Build Guide - Part 5 - Grip Assembly

Previous: Part 4 - Stock Assembly

  • Hand Drill Motor
  • 9/64" Drill
  • 7/64" Drill and/or 6-32 tap drill size to suit material
  • 6-32 Tap
  • Tap Handle
  • File
  • Mini Grinder/Zip wheel
  • Soldering Iron
  • Heat Gun/Heat source for shrinking HST
  • T19 Grip Frame
  • T19 Grip Base
  • T19 Grip Frame Insert Plate
  • T19 Trigger
  • T19 Left Grip Panel
  • T19 Right Grip Panel
  • 6-32 1/4" SHCS x 4 (Grip Panels to Frame)
  • 6-32 1/2" SHCS x 2 (Grip Panel to Microswitch)
  • 6-32 1.5" SHCS x 1 (Trigger Pivot)
  • 6-32 3/4" SHCS x 6 (Grip Frame to Grip Base; Grip Base to Drive Housing)
  • 6-32 5/8" SHCS x 2 (Grip Base to Drive Housing, front 2 bolts)
  • "Signal gauge" stranded hookup wire (~24AWG) x 3 colors x 12" length
  • Leadfree flux-core (electrical) solder
  • Flux
  • 1/8" Heatshrink Tubing
  • 0.1" male breakaway header (3 pins required)
  • Full-size microswitch, no actuator (exposed plunger)
  • 2-part methacrylate adhesive

Clean out all 6-32 through holes in the grip base and panels with 9/64" drill. Also, clean out the pivot hole in the trigger and the trigger bolt hole in the grip frame.

Drill with appropriate tap drill size and tap 6-32 threads in the top 2 bolt holes on the grip frame (frame to base) and the grip panel mounting holes.

Drill and tap 6-32 threads in the insert plate hole.

Clean any burrs out of the trigger pocket if necessary.

Deburr/Hand fit the insert plate into the grip frame with a file. Ensure the plate doesn't try to bend the frame when inserted and that it seats flush and even with the top surface.

Bond it in with Devcon. Clean out ALL excess adhesive quickly from the inside of the trigger pocket.

Tap the microswitch case holes with 6-32 threads. No drilling required. (Yes, this is a bit dirty a methodology to cut threads in the switch case, but I can't be bothered to use nuts or put the bolt heads on the other side and threads in the grip panel. It works and it's clean.)

Solder 3 wires to the microswitch in this orientation. Watch out for wire/solder joint clearance to the grip frame on the back of the common terminal - best to put the wire on the INSIDE of the terminal against the switch case. Make note of color coding of the common terminal for later. Do not terminate the other end with a connector yet!

A microswitch selection note: This switch supplies ALL the return force for the trigger. There is no external spring. I use high-current switches typical of old mechanical DC blaster controls (10+ A rated), even though there is only a few mA in this signal application. Many dedicated low current switches have lower operating forces, and won't feel right or could even fail to reliably return the trigger and cause a runaway gun!

Twist that wiring into a cable as shown, as typical of T19 signal harnesses.

Hand fit the grip frame to the grip base: Filing WILL BE REQUIRED. A tight fit is desired. The interference fit into the grip base channel significantly strengthens this assembly over simply bolting the frame onto a flat surface, but superhuman force should not be required to mate the parts, and the grip base should not distort when the frame is inserted.

Bolt the right grip panel on with 2, 1/4" length 6-32 SHCSes

Feed the switch cable through the grip frame wiring hole.

Use 2 5/8" length 6-32 SHCSes to bolt the switch onto the grip panel. Don't overtorque, there is limited thread engagement in the tapped switch case.

Load the trigger into the frame.

Install the trigger pivot bolt (1.5" 6-32 SHCS or equivalent unthreaded length).

You should know the drill by now with cutting bolts down: mark this and zip off the excess on the threaded end so that it doesn't protrude from the threaded boss on the insert plate.

Don't overtorque it, why should be obvious.

(Builder's note: It may be necessary to add washer(s) under the head of this bolt to keep the "slightly too long" unthreaded shank from bottoming on the threads in the insert plate. If you do that, account for the washers being there when cutting the bolt to length. If you don't, the clean alternative method is to drill a small clearance pocket into the inside end of the plate to allow the shank to be there.)

Now is the time to check/set trigger travel. Ensure the trigger moves far enough forward to allow the switch to reset. There should be a slight, but not rattling, clearance between the trigger and switch plunger when at rest (think valve clearance on an engine). If too tight, file the BOTTOM flat of the trigger, which is where it stops on the grip frame. DON'T file the back of the trigger or the switch plunger. If too loose... That's perfectly fine. Switches vary slightly. You can shim it and work it back to correct fit if you are super OCD about having no play in the trigger.

Don't worry too much about trigger axial play/binding at this point. That will be done after the frame-to-base pressing and bolting to allow for part stresses.

Mount the left grip panel.

Things are starting to come together and look like a blaster now eh? Looking shooty.

Feed the trigger cable through the grip base wire hole.

Mate the frame to the base (Careful of bolt hole alignment) and bolt it on. 2, 3/4" length 6-32 SHCS.

Now is the time to set trigger axial clearance - If the trigger binds, which is LIKELY, remove the trigger pivot bolt and unload the trigger out the top of the assembly through the trigger access slot. It's a bit tight in the plate, just jam it through. File the sides of the trigger evenly. Don't do just the pivot area, the clearance in the frame cavity is uniform, so make it uniform thickness.

I would thin the trigger by a layer or two to give always-working assemblies with minimal work, but a hand fitted trigger without any play is nice if you want that, so I leave you that option in the prints. To get there, keep filing a bit, and checking. If you don't, just attack that thing pretty good with the file and make sure it has clearance and moves freely. You don't want your trigger to stick down. That's bad.

Reinstall the trigger bolt.

Terminate the trigger cable with a male 0.1" 3 pin header/connector. Center pin is switch common.

You're done, stick the base to drive housing bolts in that for safekeeping. Note that 4 rear bolts are 3/4" length and front 2 are 5/8" length. Put this assembly aside.

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