Two Parts, One Nominal Dimension, Three Different Outcomes

A drawing that says the shaft is 0.500 in and the bore is 0.500 in has specified nothing. Depending on where the two parts actually land inside their tolerance bands, that pair can spin freely, be a firm hand push, or require an arbor press and never come apart again. The nominal dimension is shared. The fit is what you actually design, and it lives entirely in the tolerances.

Getting this right is one of the cheapest ways to make a mechanism work. Getting it wrong produces the two most common mechanical complaints in a first product: a part that wobbles, and a part that will not go together on the assembly line.

The Three Families of Fit

Clearance fit. The bore is always larger than the shaft across the full tolerance range, so there is guaranteed play. Use it anywhere parts must rotate, slide, or come apart by hand: a shaft in a bushing, a dowel locating a serviceable cover, a fastener through a clearance hole.

Transition fit. The result may be slight clearance or slight interference depending on where each part lands. Use it for accurate centering with the parts still separable by light pressing, such as locating dowels or a gear hub that is also keyed so the fit is not carrying the torque.

Interference fit, also called a press or shrink fit. The shaft is always larger than the bore, assembly requires force or a temperature difference, and the joint transmits load by friction alone. Bearing inner races, bushings pressed into housings, keyless gear hubs.

How to Read H7/g6

The ISO system encodes a fit as a letter and a number. The letter gives the position of the tolerance band relative to nominal, uppercase for holes and lowercase for shafts: H means the hole starts at nominal and can only be larger, g means the shaft is slightly under, and p, r, and s put the shaft above nominal, which is where interference comes from. The number is the IT grade, which sets band width and scales with feature size. On a 1 in feature, IT6 is roughly 0.0005 in of total tolerance, IT7 about 0.0008 in, IT11 about 0.0043 in.

So H7/g6 is a hole at nominal-to-slightly-over with a shaft slightly under: a close running fit. The combinations worth memorizing:

  • H7/h6: sliding fit, close but assembles by hand. Good for parts that must be located but removable.
  • H7/g6: close running fit for a precisely located rotating shaft.
  • H8/f7: normal running fit, the general-purpose choice for a plain bushing at moderate speed.
  • H11/c11: loose clearance for rough parts, weldments, and anything that just has to go together.
  • H7/k6: transition, light press, needs a small arbor press.
  • H7/p6: light interference for a bushing in a housing.
  • H7/s6: heavy interference for a torque-carrying hub.

How Much Interference Do You Actually Need

A rough starting point for steel-in-steel press fits is 0.0005 to 0.001 in of interference per inch of shaft diameter. On a 1 in shaft, that is half a thousandth to one thousandth. It sounds like nothing. It is enough to hold real torque, and it is also enough to yield the outer part if the housing wall is thin.

Check three things. Hoop stress in the outer part, which must stay well below yield. Assembly force, which runs into hundreds or thousands of pounds and determines what press the factory needs. And torque capacity, which is contact pressure times contact area times friction coefficient times radius. If the answer is marginal, add a key, a pin, or a retaining compound rather than increasing the interference; an FEA check is cheaper than a cracked housing found at pilot build. Heating the outer part or chilling the shaft turns a heavy press into a drop-in assembly that locks as temperatures equalize.

What Nobody Chose Is Still a Choice

When a drawing carries only a title-block default of plus or minus 0.005 in on both parts, the fit is undefined. Some assemblies will be loose, some will bind, and the ones that bind get forced together on the line, damaging both parts. The failure looks random, which makes it expensive to diagnose, and the accumulated effect across a multi-part assembly is what tolerance stack-up analysis exists to catch. Call out the fit explicitly on every mating pair that matters and leave the default on everything else: a drawing where five dimensions are tight and forty are loose is cheap, and one where everything is tight is not.

What a Tight Tolerance Costs

Tolerance is the main lever on machining price. Moving from plus or minus 0.005 in to plus or minus 0.001 in adds a finish pass and inspection time. Going to plus or minus 0.0002 in means grinding or honing, controlled-temperature inspection, and a scrap rate priced into the part. The full breakdown is in CNC machining cost. A useful discipline: for every tight tolerance on the drawing, be able to say in one sentence what fails if it is not held. If you cannot, loosen it. That single pass is one of the highest-return items in a design for manufacturing review.

Plastics, Temperature, and What the Table Does Not Tell You

The ISO fit tables assume metal, and plastics break most of their assumptions. Thermal expansion of a typical engineering plastic is five to ten times that of steel, so a fit correct at room temperature can go loose at 120 degrees F or bind at 20 degrees F. Hygroscopic plastics grow with humidity on top of that, and all of them creep, so an interference fit in plastic relaxes over weeks and loses much of its grip.

So use a metal insert wherever a press fit has to hold long term, calculate the fit at both temperature extremes, and add a locking feature such as a flat, a pin, or a knurl instead of relying on friction. Remember too that a molded bore is rarely round; the shrink pattern in wall thickness for molded parts shows up as ovality that eats your clearance. Printed parts are looser still, and printer-specific, which is why 3D printing tolerances deserve a test coupon before you trust a fit on a printed prototype.

What Has to Reach the Manufacturer

Put the fit intent on the drawing: explicit upper and lower limits or the ISO code, surface finish on both mating surfaces (a press fit into a rough bore shears the peaks and loses most of its interference), a roundness callout if the fit depends on it, and a note stating the assembly method. Those belong in the drawing set inside your manufacturing data package so the shop quotes the right process the first time.

Get the Fits Specified Properly

Projects House produces mechanical drawings with fits, GD&T, and assembly notes that shops can quote and hold. If your parts are binding, wobbling, or coming apart in the field, describe the assembly through our contact form and we will look at the tolerances behind it.