Guide
How should dowel pin alignment be laid out so two machined parts reassemble without binding?
Dowel pin alignment is the practice of locating one machined part on another with two precision pins, so the parts come back to the same position every time they are separated and reassembled while the bolts only clamp. When it is laid out well, the parts drop together by hand; when it is laid out badly, they bind, need a mallet, gall the pins or seat in a slightly different place each time.
By Saola Manufacturing engineering team · Published · Updated
Most binding dowel joints are not caused by a bad machinist. They are caused by a layout that asks two round pins to fit two round holes in a second part whose hole spacing can never match the first part exactly, or by a slip-fit hole class that is actually a transition fit with the pin that goes into it. Both are decided on the drawing. This guide covers how to recognize the problem, the causes in the order they usually show up, how to measure which one you have, and the layout, tolerances and notes that stop it from coming back.
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How do you recognize a dowel pin alignment problem?
The parts start onto the pins and then stop, need tapping or levering to seat, come apart only with a puller, or seat fine but end up in a slightly different position after each reassembly.
Binding and poor repeatability are the two faces of the same problem. Binding means the joint is overconstrained: the pins are asking for more accuracy than the holes can deliver, so something has to bend, scrape or gall for the parts to meet. Poor repeatability means the opposite: the slip-fit holes are so loose that the bolts, not the pins, decide where the part ends up. A good layout sits between the two, with enough clearance to assemble under all tolerance combinations and little enough to locate the part where the function needs it.
Look for evidence before changing anything. Bright rub marks on one side of a pin, a scored bore, a pin that has walked partway out of its press-fit hole, or raised metal at the hole entry tell you where the contact is. Note whether the parts bind at first entry or only near full depth, whether every part in the lot binds or only some pairings, and whether a part that binds on one mate goes on freely to another. Those observations separate a hole-size problem from a hole-position problem, and both from a perpendicularity problem.
What causes dowel pins to bind on reassembly?
The cause to rule out first is two round pins engaging two round holes, which overconstrains the joint along the line between the pins. The others are slip-fit holes that are really transition fits, tilted pins, holes dimensioned from different datums on the two parts, and coatings or burrs in the holes.
The usual causes, in the order worth checking, are these.
- Two round pins in two round holes. A part located by one pin in a hole has only one remaining freedom, rotation about that pin, and a second pin in a hole removes it. Any difference between the pin spacing on one part and the hole spacing on the other then has nowhere to go. A metrology article from FARO describes two pins in two holes as leaving the location impossible to determine and creating high stresses from overconstraint, and notes that the holes must then be machined to very tight diameter and position tolerances (FARO, using slots in precision location).
- The slip-fit hole is a transition fit. Hardened dowel pins are ground slightly over nominal, and a hole reamed to the class many drawings default to can be smaller than the largest pin. The table below shows that this is not a corner case.
- Pins that are not square to the joint face. A press-fit hole that is drilled or reamed at a slight angle sends the pin off line, and the error grows with the length of pin standing proud, so a long engagement in the removable part binds near full depth.
- Different datum schemes on the two parts. If one part dimensions its pin holes from an edge and the other from a bore, each can be inside its own tolerance and still miss the other.
- Material in the hole. Anodize, plating or paint in an unmasked slip-fit hole, burrs at the hole entry, or no lead-in chamfer will each turn a correct fit into a tight one.
- Temperature and material mix. Steel pins in aluminum plates, or parts machined warm and assembled cold, change the effective pin spacing on a large part.
| Hole class (limits, µm) | Fit with m6 pin (+6 to +15 µm) | Fit with a pin ground to h6 (−9 to 0 µm) | What it means for a removable part |
|---|---|---|---|
| M7 (−15 to 0) | 6 to 30 interference | 15 interference to 9 clearance | Press fit for the part that keeps the pin |
| H7 (0 to +15) | 15 interference to 9 clearance | 0 to 24 clearance | Can bind with a standard hardened pin |
| G7 (+5 to +20) | 10 interference to 14 clearance | 5 to 29 clearance | Still can bind with an m6 pin |
| F7 (+13 to +28) | 2 interference to 22 clearance | 13 to 37 clearance | Almost always free; a worst-case pair can still touch |
| E7 (+25 to +40) | 10 to 34 clearance | 25 to 49 clearance | Always a slip fit with an m6 pin |
How do you diagnose which cause is making the parts bind?
Measure the actual pin diameters and hole sizes first, then the pin spacing on one part against the hole spacing on the other from a common datum, then the squareness of the pins. Then remove one pin and try the assembly again, which separates an overconstrained layout from a fit problem.
- Size. Measure the pins with a micrometer and the slip-fit holes with gauge pins or a bore gauge. If the largest pin is bigger than the smallest hole, the problem is the fit class, and no amount of position accuracy will fix it.
- Spacing. Measure the center distance between the two pins on the pinned part and between the two holes on the mating part. Each pin can shift by half its diametral clearance, so if the difference is larger than half the sum of the two diametral clearances, two round pins cannot assemble without force. For example, an E7 hole with an m6 pin leaves at least 10 µm of clearance per ISO 286-2 in the size range of the table above, so with that fit at both holes the pin spacing and hole spacing must agree within that same amount.
- Squareness. Check each pressed pin for perpendicularity to the joint face over the length that engages the mating part, not just at the face.
- Datums. Compare how each drawing locates its pin holes. Different origins on the two parts are a drawing problem, not a machining problem.
- Hole condition. Look for coating, burrs or a missing chamfer at the hole entry, and check whether a blind hole is trapping air that pushes back on the pin.
- One pin at a time. Assemble with one pin removed. If the part then drops on freely, the joint is overconstrained; if it still binds on one pin, the problem is size, squareness or hole condition at that pin.
When the parts already exist, fix them in order of cost, and change the drawing to match so the next lot does not repeat it.
- Overconstrained layout. Replace the second round pin with a diamond (relieved) pin of the same nominal diameter in the existing press-fit hole, oriented as described below, or open the second slip-fit hole in the removable part into a short slot along the line of centers. Neither touches the round-pin location.
- Fit class too tight. Ream or bore the slip-fit holes in the removable part up to a clearance class from the table, such as F7 or E7 for an m6 pin, rather than grinding the pins.
- Tilted pins. Replace pins that were pressed crooked; if the press-fit hole itself is out of square, the hole has to be corrected, not the mating part.
- Coating or burrs. Deburr and chamfer the hole entry, and ream coated slip-fit holes back to size only where the drawing allows bare metal there.
- One-off pair. If the parts never need to interchange, match-ream both together in assembly and serialize them, as explained below.
A useful way to see why spacing matters is the fixed-fastener relationship in the positional tolerancing appendix of ASME Y14.5: for a pin fixed in one part and passing into a clearance hole in the other, the two position tolerances together may not exceed the hole size at maximum material condition minus the pin size at maximum material condition. Take the size range in the table above. Per ISO 286-2, an m6 pin there is at most 15 µm over nominal and an E7 hole at least 25 µm over nominal, so per ASME Y14.5 the pin location on one part and the hole location on the other share only 10 µm of diameter between them. With an H7 hole the difference is negative, which means two round pins can bind even if both parts are positioned perfectly.
How should the pins be laid out so the parts assemble freely and still locate?
Use exactly two pins, as far apart as the part allows, with a round pin in a round hole as the primary locator and a diamond pin or a slot for the second pin so the spacing error between the parts is free to float. Press both pins into one part and slip-fit the other.
The round pin fixes both translations in the plane of the joint. The second pin only has to stop rotation about the first, so it should constrain the part in one direction only, square to the line joining the two pins. A diamond pin does this by relieving its sides along the line of centers, leaving two narrow cylindrical lands that touch the hole across that line. A slot does it the other way round, with the slot width controlling rotation and the slot length, along the line of centers, absorbing the spacing error; FARO notes that the slot length is the feature that can take the largest tolerance. Orient the diamond’s lands or the slot’s width wrongly and the joint is overconstrained again, so show the orientation on the drawing.
Spacing matters as much as the pin type. For a given clearance at the second pin, the angular play of the part falls as the distance between the pins grows, so place the pins at opposite ends or diagonally across the joint, not side by side. Keep pins away from the bolts so the bolt holes can stay as clearance holes and never act as a third locator.
| Situation | Recommended layout | Watch out for |
|---|---|---|
| Parts separated in service, accuracy needed | Round pin + diamond pin, both pressed in one part; round hole + matching hole in the other | Diamond orientation shown on the drawing |
| Thin plate or sheet part where a slot is easier to cut than a diamond pin is to fit | Round pin in a round hole + second round pin in a slot along the line of centers | Slot width tolerance controls rotation |
| Assembled once, never separated | Two round pins acceptable; ream both parts together in assembly | Parts become a matched pair; mark them |
| Large part, mixed steel and aluminum | Round + diamond, pins far apart, diamond relief generous along the line of centers | Spacing changes with temperature |
| Blind pin holes | Vented or flatted pins, or pull-out dowels with an internal thread | Trapped air pushing the pin back |
| Coated parts | Mask pin holes, or finish-ream after coating | Coating build-up inside slip-fit holes |
Matched reaming, drilling and reaming both parts together after they are bolted in position, removes the spacing error completely, and it is the classic fix for a one-off fixture. Its cost is that the parts are no longer interchangeable: a spare top plate will not fit a different base, so use it only where interchangeability does not matter, and serialize the pair.
What should the drawing and RFQ say so dowel pin alignment does not recur?
Make the pin holes the datums, call out position at maximum material condition from the same datum frame on both parts, specify the fit class of each hole separately, and show the diamond or slot orientation. A drawing that says only “for dowel” leaves the fit to guesswork.
- Datums. Use the joint face as datum A, the round-pin hole as datum B and the diamond-pin hole or slot as datum C on both parts, so they are built from the same reference, as described in the datum reference frame guide.
- Position at MMC. Tolerance the pin holes with position at maximum material condition so bonus tolerance applies as holes depart from their tightest size; the MMC bonus tolerance guide explains the arithmetic.
- Two different hole classes. Give the press-fit holes and the slip-fit holes their own classes, for example M7 for the part that keeps the pins and a class chosen from the table above for the part that comes off. Name the pin standard and its class.
- Perpendicularity. Control the press-fit holes for perpendicularity to datum A, and consider a projected tolerance zone over the pin length that engages the mating part, which ASME Y14.5 provides for threaded and press-fit holes.
- Orientation and engagement. Show the diamond pin relief or slot direction, the pin protrusion, and a lead-in chamfer on the slip-fit holes.
- Finish. State “mask pin holes” or “ream after finish” on any coated part.
- Verification. Ask for measured hole sizes and positions on the first article, and a trial assembly with the mating part or a gauge representing it.
On the RFQ, say which part keeps the pins, whether the parts must be interchangeable across lots, and whether the pins are supplied by the shop or by you. Those three answers decide whether matched reaming is allowed and how the holes are inspected.
Who does what on a Saola order with doweled parts?
Pin holes in turned and milled parts are drilled, reamed or bored and checked at our lead CNC plant in Vietnam. Any casting, stamping or surface finishing is done at partner plants we manage, and each quote names the production site and country of origin.
Doweled plates, housings and brackets are typical work for CNC milling at our lead CNC plant, T.K Viet Nam Precision Company Limited, which runs machining centers and CNC lathes and checks parts with go/no-go gauges, a height gauge and a 2D vision measuring machine, as listed on the factory page. When a drawing calls for two round pins in two round holes on parts that must interchange, we flag it in the manufacturability note that comes with the quote and suggest a diamond pin or slot rather than quietly tightening the hole positions. If the parts are anodized or plated, that step goes to a finishing partner we manage, so the masking or ream-after-finish note on the drawing travels with the order. ISO 9001 certification is in progress, not held. Send the drawings for both mating parts through the quote form.
Frequently asked
Why not just use three or four dowel pins for extra accuracy?
Every pin after the second adds constraint, not accuracy. Two pins fully locate a part in the plane of a joint; a third or fourth pin can only fight the first two, so it makes binding more likely unless all holes are reamed together in assembly.
Which way should a diamond pin face?
Its relieved sides face along the line joining it to the round pin, and its two cylindrical lands sit across that line. That lets the spacing between the pins float while the lands stop the part from rotating about the round pin.
Should the dowel holes be reamed in both parts together?
Only when the parts will always stay together as a pair. Matched reaming removes spacing error but makes the parts non-interchangeable, so spares and replacement parts will need the same treatment.
Can bolts or shoulder screws replace dowel pins?
Not for repeatable location. Bolts pass through clearance holes and clamp; they let the part settle wherever friction leaves it. Shoulder screws can locate light-duty parts, but their fit is usually looser than a dowel and they also carry the clamp load.
Is an H7 hole always right for a dowel pin?
No. Per ISO 286-2 and the m6 class used for hardened dowel pins, an H7 hole is a transition fit that can be slightly tight, so it suits a light locating fit but not a removable part that must slide on by hand.
Sources
- ASME Y14.5-2018, Dimensioning and Tolerancing, ASME, 2018 (reaffirmed 2024)
- ISO 286-2:2010, Geometrical product specifications (GPS), ISO code system for tolerances on linear sizes, Part 2: Tables of standard tolerance classes and limit deviations for holes and shafts, ISO, 2010
- ISO 8734:1997, Parallel pins, of hardened steel and martensitic stainless steel (Dowel pins), ISO, 1997
- ASME B4.2, Preferred Metric Limits and Fits, ASME, 1978 (reaffirmed 2020)
- GD&T in precision engineering: using slots in precision location, FARO, accessed October 2026