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Cost Drivers for Machined Motor Housings and End Bells

Written by the sourcing desk that reads your RFQPublished September 21, 2026How we write these

Two shops quote the same motor housing and the prices land 40 percent apart. That is normal, and it is almost never because one shop is greedy. It is because they made different assumptions about the blank, the gauging and the process route. This guide breaks the price of a machined housing or end bell into the six things that actually move it.

1. The bearing fit

This is the callout that moves the price most, and it is the one buyers most often set by habit rather than by need.

A bore at H8 can be finish bored and spot checked. A bore at H7 usually needs a dedicated finish pass, a bore gauge at the machine and a check on every part. A bore at H6 adds temperature control of the part before measuring, because a housing that is still warm from the cut reads differently than a cold one.

The bore limits for each class are in the bearing seat fit reference. The jump from H8 to H7 is a real step in cost. The jump from H7 to H6 is a bigger one. If the bearing maker’s table says H7 is right for your load case, asking for H6 buys nothing and pays for inspection you do not need.

2. The blank

Cutting a housing from solid bar means removing most of the bar. For a 120 mm diameter aluminum housing with a 90 mm bore, that is a lot of aluminum turning into chips, and chips are paid for twice: once as material, once as cycle time.

A casting arrives close to shape. The shop machines only the faces that matter. Cycle time falls sharply and material cost falls with it. Against that you pay for the pattern or die and wait 4 to 10 weeks for the first blank.

A drawn or extruded tube sits between the two. No tooling cost, and the bulk of the material is already gone. For round aluminum housings this is often the answer nobody asked about.

3. Concentricity between features

A bearing seat and a spigot register that have to run true to each other are cheap if they are cut in the same setup, because they are concentric by construction. They get expensive the moment the process route splits them across two operations, because the second operation has to re-clock the part and hold the relationship through the fixture.

This is why a turn mill with a sub spindle quotes some motor parts far cheaper than a lathe and a separate machining centre. The part never loses its reference.

If your drawing has a concentricity callout between two features, ask the shop which setup each one is cut in. The answer tells you whether the tolerance is nearly free or is carrying a fixture cost.

4. Grinding

Turning gets a shaft journal to roughly ±0.02 mm with a finish around Ra 0.8. That is enough for many fan, pump and appliance motors.

Below that, you need a grinder. Grinding is a separate machine, a separate setup and usually a separate queue, sometimes at a subcontractor. It adds cost and it adds days.

So the question on a shaft is not “how accurate can you make it” but “does this journal actually need grinding”. A drawing that calls Ra 0.4 on every diameter because the template said so puts the whole shaft into the grinding queue when one journal needed it.

5. Batch size

Setup cost is paid once per order and spread across the parts in it. On 10 pieces it is most of the price. On 500 it is noise.

The practical shape of the curve for a typical machined motor part:

Batch	What dominates the price
1 to 10	Setup, programming and fixturing
11 to 100	Setup still visible, cycle time starts to matter
101 to 1,000	Cycle time and material dominate
1,000+	Cycle time, material and whether the blank should change

The useful move is not to argue about the unit price on 10 pieces. It is to ask what the price would be at 100 and at 500, and then decide whether to order ahead.

6. Finishing and inspection

Anodizing, plating, impregnation, paint and dynamic balancing each add a process step, a transport leg and a few days.

Inspection is the quiet one. A first article inspection report with every dimension measured and recorded is real work. PPAP is more. If you need them, say so on the RFQ, because a shop that finds out after quoting either absorbs it or comes back to renegotiate.

What you can change, and what you cannot

You can change:

• The fit class, where the bearing table allows it
• Which surfaces carry a finish callout
• The blank, if you are willing to look at tooling cost against volume
• The batch size, by ordering further ahead
• Whether inspection is a certificate of conformity or a full report

You cannot change:

• The material, usually, because the motor design decided it
• The size of the part
• Whether two features have to be concentric, if the bearing arrangement says they do
• Lead time on a casting

How to read a quote that came back high

Ask the shop which single line on the drawing had the biggest effect. A shop that machines motor parts will answer in one sentence: the H6 bore, the Ra 0.4 on all diameters, the concentricity that forced a second fixture. That answer is worth more than a discount, because it is the one you can act on for the next revision and every order after it.

Standards and references

1. ISO 286-1, ISO code system for tolerances on linear sizes. The H6, H7 and H8 bore classes discussed under the bearing fit.
2. ISO 2768, General tolerances for linear and angular dimensions. The general tolerance note that keeps a drawing from over specifying.
3. ISO 1940-1, Balance quality requirements for rotors in a constant (rigid) state. Referenced under balancing and finishing.

Figures on this page are typical ranges drawn from the standards above and from ordinary shop practice. They are not a specification for your part. The shop that quotes you confirms what it can hold on your geometry.

Frequently asked questions

Keep reading

• Bearing Fits, Concentricity and Runout: Tolerances That Matter in Motor Parts
• How to Choose a Machining Supplier for Electric Motor Components