12DP 25-tooth pinion (module 2.117, OD 57.15 mm) versus m2 rack: pitch 6.652 mm vs 6.283 mm, a 0.37 mm (5.6%) mismatch found in drawing review before quoting.
Target language: English Article title: A 12DP pinion paired with an m2 rack: converting one set of imperial drawings to metric, and how to lock the basis before quoting
Markdown block 1/1 — translate it:
Imperial diametral pitch (DP) and metric module (m) are two parallel gear bases, and only one basis can be used within a single drive train. This time the pinion that came in was 12DP with 25 teeth, while the rack was m2 — the circular pitches of the two differ by 5.6%, and once installed the teeth would interfere after only a few meshes. The problem is not in the machining, it is in the drawings themselves. Geyon Transmission’s approach is: parse item by item, cross-verify against the drawing’s coordinates, and lay out the basis conflict, the missing material and heat treatment entries, and the accuracy grade all at once, asking the end-user factory to fix the basis first before price is discussed.
In late September 2026, a trading company in Guangzhou that exports transmission parts forwarded a set of rack and pinion drawings. The end user is an overseas equipment manufacturer, and both sheets are the end user’s formal production drawings. One pinion, matched to one set of racks; the pinion drawing shows 12DP, 25 teeth, and the rack drawing shows m2, single piece 900 mm. What the customer first sent was a one-page imperial stock-item catalog (pinion 12DP, 1 inch face width); by the time the end user’s formal production drawings arrived, the rack had been redesigned as a whole piece, no longer the off-the-shelf item in the catalog.
Diametral pitch DP is “how many teeth per inch of pitch diameter,” and the conversion to module is m = 25.4 ÷ DP, so 12DP converts to 2.117, which is itself not in the standard module series. Put the two drawings together and measure the circular pitch: the 12DP pinion has a circular pitch of 6.652 mm, the m2 rack has 6.283 mm, a difference of 0.37 mm per tooth, that is 5.6% — accumulate that over a dozen or so teeth and the tooth profiles run into each other. Within the same drive train the rack and pinion must be on the same diametral pitch; one on imperial and one on metric will not match.
There are also several key omissions on the drawings: the material, heat treatment, and hardness columns on both sheets are blank; the rack’s face width (the third dimension of the section) is not marked, and no section view is provided; the accuracy grade is not given. The pinion has another dimension where only the hub is drawn on the sheet with no value marked, which has to be back-calculated from the drawing’s vector coordinates.
Verifying the basis cannot rely on the impression that “these two modules are different” — evidence must be kept. The rack drawing gives the most direct evidence: the sheet marks an overall length of 900.00 mm, and at a 1:2 scale the rack solid’s length measures exactly 450.00 mm; the center-to-center spacing of the flat tooth-top segments converts to a circular pitch of 6.2830 mm, exactly equal to π × 2.0, i.e. m2; over the 900 mm length about 144 teeth can be counted (m2 should give 143.2 teeth, whereas at 12DP there would be only 135 teeth, a difference of 10 teeth, which cannot be miscounted).
The pinion was cross-verified two ways at 1:1 and 2.834645 pt/mm: the top-view outer circle measures 57.07 mm, and the standard outer diameter of a 12DP · 25-tooth gear is 57.15 mm, a match; the root circle measures Ø47.6, standard value 47.625 mm; the circumferential tooth tops are exactly 25 evenly distributed, 360 ÷ 25 = 14.4°, which checks out. The conclusion is settled: the rack is indeed m2, the pinion is indeed 12DP, and the conflict is in the drawings, not in the reading.
One, parse item by item, then verify against drawing coordinates. First list the items that can be read directly — module, tooth count, pressure angle, outer diameter, root circle — then back-calculate them once from the drawing coordinates; if the two routes do not agree, the drawing itself has a problem.
Two, put the two closure paths to the end user to choose. The first is to make the whole set to 12DP, with the rack also reissued to 12DP at 900 mm (about 135 teeth); the second is to change the whole set to m2, with the pinion recalculated to m2. Which sheet is the basis must be made explicit by the end-user factory; the manufacturer does not decide on the customer’s behalf. This kind of cross-system basis check is exactly what drawing engineering review should complete before quoting.
Three, work out the metric alternatives in advance. If the end user is willing to switch to metric, the selectable metric gear parameters and dimensional changes are as follows:
| Option | Teeth | Pitch diameter | Outer diameter | Difference from original drawing |
|---|---|---|---|---|
| Original 12DP | 25 | 52.92 mm | 57.15 mm (measured 57.07) | Basis, module unchanged |
| Change to m2 | 26 | 52.0 mm | 56.0 mm | Pitch diameter close to original |
| Change to m2.5 | 21 | 52.5 mm | 57.5 mm | Outer diameter close to original |
Four, ask for all the missing items in one go. Material, heat treatment and hardness; accuracy grade (DIN 3962 or ISO 1328); rack face width and section dimensions; quantity basis (how many pieces per 900 mm single bar); whether the pinion hub diameter Ø46 and keyway 6 × 2.5 are final values. If these items are not clear, no price can be issued.
| Item | Content |
|---|---|
| Pinion (customer drawing) | 12DP (converted module 2.117), 25 teeth, pressure angle 20° |
| Pinion measured check | Outer circle measured Ø57.07 (12DP·z25 standard Ø57.15); root circle Ø47.6 (standard 47.625); hub Ø46.04; face width 25.40 |
| Pinion structure | Bore Ø20 through hole; keyway width 6.0, bore bottom to keyway bottom 22.50, depth about 2.5 (DIN 6885) |
| Rack (customer drawing) | m2 (circular pitch 6.2830 = π × 2.0), about 144 teeth, overall length 900 mm single piece |
| Rack section and holes | Tooth height 19.72 (nominal 20); total tooth height 4.52; 5 × Ø6.6 through holes |
| Basis conflict | 12DP circular pitch 6.652 mm ↔ m2 circular pitch 6.283 mm, difference per tooth 0.37 mm (5.6%) |
Data comes from the end user’s production drawings provided by the customer in September 2026 and the item-by-item review records; final parameters are subject to the formal drawings confirmed by the end user.
First, it digs out the basis conflict before material is released. If a 5.6% pitch difference is only discovered after the parts are made and will not assemble, the loss is not a matter of one drawing; at the review stage, putting the two drawings side by side and measuring the circular pitch exposes the problem the same day.
Second, the measurements leave evidence. The circular pitch 6.2830 mm, the 144 teeth, the two-way cross-check of outer diameter and tooth count — these are all numbers that can be re-checked from the drawing, not “I think.”
Third, the missing items are laid out openly, without guessing on the customer’s behalf. If the material, heat treatment, accuracy, and face width columns are blank, a list is drawn up asking the end-user factory to fill them in, rather than quoting a price against some default value and muddling through — if the uncertain items are not made clear at the drawing review stage, every one of them can later turn into rework.
Send the drawings over, and Geyon Transmission’s engineering team will first conduct a drawing review, checking module, tooth count, tolerances, and meshing relationships item by item, then reply with the process route, inspection items, and price. Ground tooth accuracy can reach DIN 3, and reports on pitch, profile, helix, hardness, and carburized case depth can be issued to DIN / ISO / AGMA standards. For the rack specification system see precision racks , and for the matching pinions see precision spur gears .
Upload a 2D/3D drawing (STEP, DXF, PDF) — we reply with a quote and lead time.