Edit:Luckymfc Technology Click:34 Date:2026-08-24
A tighter tolerance doesn't always mean a better part.
This comes up quite often when we review drawings for CNC machining.
A customer may specify ±0.01 mm on a dimension that could potentially work perfectly well at ±0.05 mm. From a design perspective, the difference looks small. From the machining side, it can change quite a lot.
The tighter the tolerance, the smaller the process window becomes.
That can mean more careful setups, additional machining passes, tool wear becoming more critical, more frequent measurements, and more attention to temperature and process stability.
None of that is necessarily a problem when the tolerance is required.
The problem is when it isn't.
For engineers and purchasing teams, this is worth looking at before sending a drawing out for a CNC machining quote.
A typical machined component can have hundreds of dimensions, but only a small number of them usually determine whether the part actually works.
A bearing bore may be critical.
A locating hole may be critical.
A mounting surface may need controlled flatness.
But an outside dimension that has no effect on assembly may not need the same level of precision.
We sometimes see drawings where ±0.01 mm has been applied to a large number of dimensions simply because the designer wants the part to be "accurate."
That sounds reasonable until you look at what happens during production.
A machinist isn't just trying to hit a number once.
The real requirement is to produce the dimension consistently across the entire batch.
That's a very different problem.
Take a simple dimensional example.
With a nominal dimension of 20 mm:
±0.05 mm gives a range of 19.95–20.05 mm.
±0.01 mm gives a range of 19.99–20.01 mm.
The second requirement gives the manufacturing process only 0.02 mm of total room.
That's a much smaller process window.
Tool wear, material behavior, machine condition, thermal changes, workholding and even the way the part is measured can become more important.
For a single prototype, this may be manageable.
For 500 or 5,000 parts, process stability becomes much more important.
That's one reason tight tolerance machining can have a noticeable effect on production cost.
There are plenty of situations where tight tolerances are completely justified.
For example:
If a ±0.01 mm requirement is controlling the performance of the assembly, it should be there.
We don't recommend relaxing a tolerance simply to make a part cheaper.
The question is different:
What is this tolerance doing for the part?
If the answer is clear, keep it.
If nobody can explain why the tolerance is required, it's worth reviewing.
There is another point that sometimes gets overlooked.
Making one dimension extremely accurate doesn't automatically make the whole assembly more accurate.
Consider two mounting holes.
You might specify a very tight diameter tolerance for each hole, but if their positions aren't properly controlled, the mating component may still not fit.
This is where GD&T and tolerance stack-up become important.
For an assembly, the relationship between features can be more important than the absolute size of an individual feature.
A good CNC machining supplier should be able to identify this during drawing review.
Sometimes the right answer isn't "make everything tighter."
It is "control the right features."
This is something purchasing teams see all the time.
Three suppliers receive the same drawing.
Supplier A quotes $18.
Supplier B quotes $25.
Supplier C quotes $34.
The first reaction is usually:
Why is Supplier C so expensive?
There can be many reasons, but tolerances are often part of the answer.
A drawing with many tight tolerances may require:
A supplier who properly accounts for those requirements may quote higher than someone who simply calculates machine time and hopes everything goes well.
So when comparing a CNC machining quote, don't only compare the number.
Ask what the supplier believes is driving the cost.
Another common situation is when a designer tightens a tolerance because an assembly isn't working properly.
For example, a shaft doesn't align with a mating hole.
The solution becomes:
"Let's tighten this dimension."
Sometimes that works.
Often it doesn't address the real problem.
The issue may actually be:
Adding tighter dimensional tolerances can make the part significantly more expensive without solving the actual assembly problem.
This is where an experienced manufacturing engineer can provide useful feedback before production.
If a CNC supplier tells you that a part is expensive because of the tolerance requirements, ask a few simple questions.
Which dimensions are driving the cost?
Are those tolerances functionally necessary?
What inspection method is required?
Can any non-critical dimensions use standard tolerances?
Is there a different machining approach that would reduce the cost?
A good supplier should be comfortable having this conversation.
They shouldn't simply say:
"Tight tolerance = expensive."
They should be able to explain where the additional manufacturing work comes from.
That information is useful when you're managing a CNC machining RFQ or comparing suppliers.
Design for Manufacturability isn't always about changing the geometry.
Sometimes it's about removing requirements that don't add value.
A practical DFM review might identify:
These changes can sometimes reduce CNC machining cost without changing the finished product at all.
That's a much better cost reduction strategy than simply asking a supplier to lower their margin.
When a drawing comes to us for quotation, we don't look only at the material and overall dimensions.
We also look at how the part is going to be made.
For example:
How will it be held?
How many setups will it need?
Which dimensions are actually critical?
Can the tool reach the feature?
Will the material move during machining?
How will the critical dimensions be inspected?
Does the specified surface finish make sense for the application?
These questions can have a bigger impact on cost than the raw material price.
For customers developing precision components, early communication between the designer and precision machining supplier can save a lot of unnecessary work later.
This is probably the most important point.
A precision part isn't necessarily a part where every dimension has a ±0.01 mm tolerance.
A precision part is one where the right dimensions are controlled to the level required by the application.
That's a very different philosophy.
If a bearing seat needs tight control, we'll focus on that.
If a mounting surface needs flatness, we'll control flatness.
If a hole pattern determines assembly alignment, we'll pay attention to position and datum relationships.
But if a dimension has no functional significance, making it unnecessarily tight doesn't make the part better.
It simply makes it harder and more expensive to manufacture.
At Luckymfc Technology, we work with customers on custom CNC milling and turning projects where tolerance, surface finish and repeatability matter.
Our engineering team can review drawings before production and discuss requirements that may affect manufacturability, inspection and cost.
We support precision CNC machining, prototype parts, and small- to medium-volume production for customers in the US, Europe and other international markets.
You can learn more about our:
CNC machining capabilities:
https://www.luckymcn.com/
Machining materials:
Materials for CNC Machining
If you have a drawing you'd like us to review:
Request a quote:
Contact Luckymfc Technology
Before putting a new part into an RFQ, take a few minutes to look at the tolerances again.
Ask:
Does this dimension affect function?
Does it affect assembly?
Is the tolerance based on an actual requirement or simply carried over from another drawing?
Could a standard tolerance work here?
Are the critical features clearly identified?
Those questions can sometimes save more money than negotiating the final quotation.
Because in CNC machining, cost reduction doesn't always start with the supplier.
Sometimes it starts with the drawing.
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Jason
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