How to prepare a shaft or bushing for a CNC turning quote

Shafts and bushings look like the simplest parts in the catalogue — and yet they can generate the most questions at the quoting stage. The reason is simple: the price of a rotational part is decided not by its overall size but by the details — fits, grooves, threads, heat treatment and datums. A CNC turning quote goes quickly when those details are settled on the drawing, not in the third round of e-mails.

This post is a practical checklist for rotational parts: what must appear on a shaft or bushing drawing, which gaps most often stall the quote, and what is worth adding so that the first reply already contains a near-final price. We covered the general rules for assembling the files in the post which files to send for a CNC quote — here we go down to the level of one specific part family.

Why rotational parts are quoted differently

With a milled housing, the process engineer mainly looks at the number of pockets, holes and setups. With a turned part, the calculation revolves around different questions:

• how many diameter steps and transitions the part has, and what the coaxiality and runout requirements between them are,
• which diameters are fitted and how tightly — they decide whether turning is enough or whether grinding will also be needed,
• whether the part is slender (a high length-to-diameter ratio) — slender shafts need support from a centre or a steady rest, which changes the machining time and method,
• whether there are any "non-turning" features: keyways, flats, cross holes — added by milling in a separate operation or on a lathe with live tooling,
• whether the functional surfaces must return to tolerance after heat treatment.

Thin-walled bushings are a separate topic: with a small wall thickness, clamping forces can deform the part, so the process engineer must plan for soft jaws or a machining arbor. Information about wall thickness and roundness tolerance therefore belongs to the data that directly affects the price.

Each of these questions changes the process, and therefore the price. That is why a shaft drawing that answers them up front cuts the quoting time from days to hours.

What must be on a shaft or bushing drawing

The drawing does not have to be beautiful — it has to be unambiguous. A correctly dimensioned sketch with a title block is enough, as long as it contains everything from the mandatory list below. Without this information, a reliable quote is either impossible or requires assumptions:

• overall dimensions: total length and largest diameter (they determine the choice of bar or tube stock and the machine),
• all step diameters with their lengths — dimensioned from a consistent datum,
• tolerances and fits of the functional surfaces: bearing journals, sliding surfaces, seats — designated as H7, g6, k6 and the like,
• surface roughness (Ra) of the mating surfaces and a general value for the rest,
• for bushings: the inner diameter with its tolerance, coaxiality with respect to the outer diameter and wall thickness requirements,
• keyways and undercuts: width, depth, position, key standard,
• threads: full designation (e.g. M20x1.5-6g), length, thread relief undercut,
• chamfers and transition radii — especially corner radii at bearing journals,
• material with its grade, plus heat treatment with the required hardness and the surfaces it applies to,
• whether centre holes are permitted (or forbidden) — they are the datums for machining between centres and for grinding.

If you are still settling some of these requirements, have a look at the post tolerances and surface roughness in CNC turning — it helps match the values to the function instead of copying them in "just in case".

Typical gaps that delay the quote

From the estimator's perspective, a few gaps come up over and over:

• material described as "steel" — C45, 42CrMo4 and 1.4301 mean different bar prices, different machinability and different heat treatment,
• a fitted diameter without a tolerance — "Ø25" on a bearing journal says nothing; Ø25k6 says everything,
• no quantity given — without it there is no way to spread the setup across the pieces or to know whether to price the part as a prototype or a series,
• heat treatment mentioned but not specified — "harden" without a hardness and a surface range makes it impossible to plan the sequence of operations,
• a keyway without a standard and angular position — especially with two keyways on different diameters,
• a thread without pitch and tolerance — a fine-pitch M30 and a standard one mean different tools and cycle times,
• no information about how the part mates — a bushing "for installation" without an assembly clearance provokes questions about the fit in its seat.

Chain dimensioning without a stated datum also raises doubts — when the tolerances of consecutive steps add up, a dimensional conflict is easy to create and only shows up at the first piece. If a dimension is critical for assembly, dimension it directly from the datum and mark it as an inspection dimension.

Every such gap means an e-mail with a question, a day of waiting for the answer and a quote burdened with assumptions. For parts on the borderline between technologies, it is also worth stating your expectations about the method right away — the post CNC turning vs conventional turning will help.

An example of a complete enquiry

This is what a message can look like that allows the part to be priced without a single follow-up question. The values are for illustration:

"Please quote the shaft per the attached PDF and STEP. Material 42CrMo4, quenched and tempered to 28-32 HRC before finish machining. Bearing journals Ø30k6 and Ø35k6, Ra 0.4, runout of 0.02 relative to the common axis — to be ground. Keyway 8P9 per DIN 6885. Thread M24x2-6g with undercut. Centre holes permitted. Quantity: 5 pcs now, ultimately 40 pcs per year in batches of 10. First batch deadline: 4 weeks."

Eight sentences and a complete data set: material with heat treatment, fits with surface roughness, additional features with standards, datums, quantities with a forecast and a deadline. A message like this goes straight to the process engineer, not into a queue of questions.

Mandatory vs nice-to-have information

A quick cheat sheet before you send an enquiry about a rotational part:

The "nice to have" column is not bureaucracy. Knowing that the shaft runs in a bearing assembly at specific speeds, or that the bushing is a frequently replaced spare part, lets the process engineer propose changes that lower the price without losing function.

If you are not sure whether a piece of information is needed — include it. An excess of data has never delayed a quote, unlike gaps. Another good practice is one line about what happens to the part after delivery: assembly at the end customer, a spare part for stock, or a component of a larger assembly. That sets the priorities for inspection and packaging.

Summary

A good quote for a rotational part starts with a drawing that settles the details: complete diameter stepping, fits with tolerances, surface roughness of the functional surfaces, grooves and threads with full designations, material with grade, heat treatment and quantity. A complete set like this eliminates rounds of follow-up questions and makes the first price the real price.

Do you have a drawing of a shaft or bushing — or just a worn part to reproduce? Send the documentation via the contact form, and if anything is missing, we will tell you exactly what to add. We return the quote within 48 hours.