Specification
Where to buy| Item | Value |
|---|---|
| Type | Microstop countersink cage |
| Maker | Martin Aircraft Tool, Zephyr Tool Group |
| Depth adjustment | Positive locking in increments of .001, by hand, with a locking ring |
| Metric spindles | 6 mm on MM-303 and MM-303-HS, 8 mm on MM-303-M and MM-303-J, 10 mm on MM-303-SJ, on request |
| Status | Maker's catalogue, read 30 Oct 2026. Check the current catalogue before you order. |
Part numbers
| Part no. | Size | Cutter thread | Max. cutter dia. | Max. RPM | B | C | D | L |
|---|---|---|---|---|---|---|---|---|
| MM-303 | MM-303 | 1/4"-28 | 5/8" | 3,000 | — | — | — | — |
| MM-303-HS | MM-303-HS | 1/4"-28 | 5/8" | 5,000 | — | — | — | — |
| MM-303-M | MM-303-M | 3/8"-24 | 7/8" | 3,000 | — | — | — | — |
| MM-303-J | MM-303-J | 3/8"-24 | 1-1/4" | 3,000 | — | — | — | — |
| MM-303-SJ | MM-303-SJ | 7/16"-20 | 1-1/2" | 3,000 | — | — | — | — |
| MM-303-EXFT-1 | EXFT-1 | 10-32 | .250 | — | .275 | .375 | — | — |
| MM-303-EXFT-2 | EXFT-2 | 1/4-28 | .300 | — | .325 | .425 | — | — |
| MM-303-EXFT-3 | EXFT-3 | 1/4-28 | .375 | — | .400 | .500 | — | — |
| MM-303-EXFT-4 | EXFT-4 | 1/4-28 | .562 | — | .656 | .750 | — | — |
| MM-303S-1 | MM-303S-1 | 1/4-28 | 7/16" | — | — | — | 11/16" | 3-3/16" |
| MM-303S-5 | MM-303S-5 | 1/4-28 | 3/8" | — | — | — | 11/16" | 3-3/16" |
Every model in the catalogue. A dash means the catalogue does not state it. B, C, D and L are the letters of its drawings: B and C at the extended footpiece, D and L on the midget model.
What each code means
| Part | Meaning |
|---|---|
| MM-303-1 | Skirt: Closed, max cutter diameter 5/8" |
| MM-303-2 | Skirt: Closed, max cutter diameter 3/4" |
| MM-303-3 | Skirt: 2 open sides |
| MM-303-4 | Skirt: 1 open side |
| MM-303-5A | Skirt: Nylon foot |
| MM-303-5B | Skirt: Hardened steel foot |
| MM-303-6 | Skirt: 2 open sides, cutter diameter 3/8" |
| MM-303-9 | Skirt: Olympic-type bulb fastener |
Notes
- MM-303: full ball bearing for thrust, oilite radial bearing. MM-303-HS: full ball bearing for thrust, roller bearing for spindle support.
- MM-303-EXFT: basic design and adjustments of the MM-303, with an extended foot piece and reduced diameters for hard to reach areas. The drawing shows 2.75" of reach. Its table is printed without the decimal point (250, 275); it is shown here as .250, .275.
- MM-303-M takes 3/8"-24 shank cutters; MM-303-J is the same with a 3/8"-24 spindle; MM-303-SJ is an extra heavy duty unit. MM-303S-5 has a nylon insert on the footpiece.
Where to buy
AmericaDistributor coming soonOur partner for this region is on its way
EuropeDistributor coming soonOur partner for this region is on its way
OceaniaDistributor coming soonOur partner for this region is on its way
Distributors coming soon.
Sources
- Martin Aircraft Tool Catalog — Zephyr Tool Group, pages 1 to 4 — models, cutter threads and capacities, speeds, bearings and footpieces, from the maker's catalogue.
Educational reference only. not approved data. Always consult the applicable SRM, AMM, or AC 43.13-1B and your certifying authority.