? Where is your part right now?

Not made yetOne shop: we machine the part and produce this surface in the same order, so the finishing pass is set up on the machine rather than sent away and brought back. The roughness comes from the cutting parameters, not from a separate bath or booth.See the processes we run →Already madeSend the finished parts and say which faces carry the roughness requirement. This is a light finishing pass, so a part that is already to its final size has no stock left for it - tell us the size the parts come to us at.Check if my parts qualify →Still comparing finishesThis is the second rung of the roughness ladder: smoother than as-machined, coarser than fine machining. If the requirement is Ra 0.8 um or a mirror, it is not this rung - the surface is smoother here, but tool marks may still be visible.Which rung do I need? →

SURFACE FINISHING

Smooth Machining Service

Ra 1.6 um Smooth, not mirror Metals and plastics No layer added

A controlled finishing pass that lowers surface roughness to a published Ra 1.6 um, by running a low feed rate and a high cutting speed with sharp tools. The surface is smoother and the risk of corrosion is reduced, but tool marks may still be visible - which is why it suits parts that will be processed further, such as painting or polishing, rather than parts that have to look like a mirror.

Get a quote Which materials does it suit?

Ra 1.6 umThe roughness figure published for this finish

Metals and plasticsThe two material groups in our published table

Smooth finishThe published texture - matte, not a mirror

No filmMachining cuts, so nothing is added to the size

A row of machined pump shafts with an even, low-roughness grey surface after a smooth machining pass — The finish is produced by the cutting parameters, not by a coating: a low feed rate and a high cutting speed with a sharp tool leave a smoother surface while the part keeps its own material colour.

Where it fits

The surface is set on the machine - the finishing pass is the last cut

Smooth machining is nearly the same process as an as-machined surface, with the parameters tightened: a lower feed rate, a higher cutting speed and a sharp tool. Nothing is applied, so what changes is the surface the cutter leaves.

STEP 1Rough and semi-finishThe bulk of the material comes off and the part is brought close to size

STEP 2Leave stock for the finishing passThe last cut removes a light layer, so it needs material to cut

STEP 3Set the parametersA low feed rate and a high cutting speed with a sharp tool lower the roughness

STEP 4Take the finishing passYou are here - the last cut leaves the published smooth finish

STEP 5Deburr or hand onSmooth machining reduces burrs; a separate deburring step may still be needed

The order matters because the surface is created by the last cut. If a later operation scratches or re-clamps a finished face, the roughness is lost. That is also why this finish suits parts that are processed further - painting or polishing can follow it, because what it leaves is a good base rather than a final appearance. See drawing notes.

A CNC cutter removing metal under a stream of coolant during a finishing pass

The surface is set on the machine: the same cutter, feed and speed that remove the material also decide the roughness the part comes back with.

A machinist at the control panel of a CNC machine setting the parameters for a finishing pass — Feed rate and cutting speed are set at the control. Lowering the feed rate and raising the cutting speed is what turns an as-machined surface into a smooth one.

A chart of cutting speed and feed rate values used to control the surface left by a CNC tool — The two parameters that matter most for this finish are the ones the published description names: a low feed rate and a high cutting speed.

Fit check

Can we smooth-machine your part? Three things decide it

The material, the stock left for the finishing pass, and the appearance the part actually needs. The last one is where this rung is chosen by mistake.

Material

Published scope - our table for this process names metals and plastics
The published description adds that the finish is appropriate for metal and plastic parts that will be processed further
Which specific grades are covered is being confirmed - ask before you quote on an exotic plastic
Tell us the alloy or the plastic grade, not just the group name

Send the material with the drawing and we will confirm it against the published scope.

Stock for the finishing pass

This finish is a light cut, so the part must still have material for the last pass
A part that is already to its final size has no stock left - say so on the drawing
The finishing pass can change the measured size, so state whether dimensions are before or after it
See what it changes for the allowance question

Tell us the size the parts come to us at and the size they have to leave at.

The appearance you need

This rung - smoother than as-machined, but tool marks may still be visible
A requirement of Ra 0.8 um or finer belongs to fine machining, not to this finish
A mirror or a highly reflective surface belongs to polishing
The published texture is a smooth finish, which is a matte surface rather than a shine

If the drawing calls for a mirror or for Ra 0.8 um, say so - we will point you at the right rung.

Specifications

Smooth machining: a published Ra 1.6 um with no added thickness

Every value below is what we publish for this process. Where the shop has not confirmed a figure, the line says To confirm rather than a guess.

Surface roughness

Ra 1.6 umThe roughness figure published for this finish

Materials (published table)

Metals, PlasticsThe two material groups named in our table for this process

Texture

Smooth finishAs published - a matte surface, not a mirror

Colour

Not applicableA machined surface has no colour of its own to apply, so the part keeps its material colour; the published table shows no colour value for this process

Thickness

Not applicableMachining removes material rather than depositing a layer, so this finish has no film thickness

Standards

To confirmThe published table gives no standard for this process; the standard it is run to is being confirmed

Effect on later processes

Published claimThe description states the finish is appropriate for parts that will be processed further, such as painting or polishing

Corrosion comment

Published claimThe description states the finish minimises the risk of corrosion; no test standard is published

Roughness on a specific material

To confirmRa 1.6 um is the figure published for the finish; whether it holds across every metal and plastic is being confirmed

The line that matters most: Ra 1.6 um is the only roughness figure published for this finish, and it is a target rather than a promise on every material. It is finer than as-machined and coarser than fine machining at Ra 0.8 um, which is what makes this the second rung of the ladder. If the drawing needs a number tighter than 1.6, this is not the rung - see the rungs either side.

What it changes

Machining cuts rather than coats - allow stock for the pass, then nothing is added

Unlike a plating or a coating, this process does not grow the part. It removes a light layer on the last pass, so the only dimensional question is how much stock that pass needs.

Where it acts	What changes	Why it matters	What to do
Free surfaces	A light layer is cut away	The finishing pass removes material to lower the roughness, so the part has to arrive with stock for it.	Leave stock for the final pass.
Threads	No coating to bridge the flanks	Nothing is added, so a thread still takes its nut - unlike a plated or a coated finish.	No masking needed for fit.
Press fits and bearing seats	The measured size can shift on the last pass	If a fit is cut by the finishing pass, the size has to be set after it, not before it.	State whether dimensions are before or after the finishing pass.
Sharp edges and burrs	Fewer burrs than a rough cut	Smooth machining reduces burrs, but it is not a deburring operation in its own right.	Specify deburring separately if the edge break is critical.
Internal bores	A tool has to reach the face	A bore the finishing tool cannot reach keeps the roughness of the cut that made it.	Say which bores carry the roughness requirement.
Appearance	Smoother, but not a mirror - tool marks may remain	This is a matte, low-roughness surface; it is not a cosmetic mirror.	Choose the finer rung or polishing if the look is the requirement.

The published finish is dimensional only through the stock it removes. What it cannot do is create a mirror, add a protective layer, or hold a roughness value the last cut was never set up to reach.

Tell us the size the parts arrive at and the size they must leave at, and we will set the finishing pass around it.

Get a quote

Next rung

Smooth machining is the second rung of four - the rung below and the rung above

The roughness ladder runs from as-machined, through this finish at a published Ra 1.6 um, to fine machining at Ra 0.8 um and a mirror polish. This rung is chosen for the middle of that range, and the last card - when it is the wrong rung - matters as much as the others.

A machined metal face left as-machined, with visible tool marks across the surface

Coarser - as-machined or sanding

The rung below

If the drawing carries no roughness callout, an as-machined surface is the default: the surface the cutter leaves, with visible tool marks. Sanding reaches a similar band from the other direction, rubbing an abrasive across the surface to lay down a random texture rather than a cut one.

Best for: parts where the surface is not a specified requirement

Choose it when: nothing on the drawing calls the roughness out - there is no reason to pay for a finishing pass.

A machined component with an even, matte surface produced by a smooth machining finishing pass

This finish - smooth machining (Ra 1.6 um)

The rung you are on

A low feed rate and a high cutting speed with a sharp tool lower the roughness to the published Ra 1.6 um, while tool marks may still be visible. It reduces the risk of corrosion and leaves a surface that further finishing - painting or polishing - can build on.

Best for: metal and plastic parts heading for painting or polishing

Not for: a mirror finish, or a requirement tighter than the published Ra.

A CNC machined cylindrical component with a fine, low-roughness turned surface

Finer - fine machining (Ra 0.8 um)

The rung above

Fine machining creates the minimum roughness by controlling the same two parameters - feed rate and cutting speed - with sharper tools. Its published roughness is Ra 0.8 um, one step below this finish, and it can prevent burrs, chips and highly visible tool marks. It is used where the surface itself is the requirement.

Best for: parts that need the lowest roughness this shop machines

Not for: a mirror - a machined surface still shows a direction, even at Ra 0.8 um.

An aluminium part with a reflective, mirror-polished surface next to a duller machined face

Mirror - polishing

The rung that is not this process

Polishing physically rubs the part to remove or minimise scratches and marks and leave a smooth, reflective surface. It is a different operation from smooth machining: it does not cut with a tool, and it is rarely used on plastics - which is one more reason this rung should not be specified for a mirror.

Best for: parts whose appearance or reflection is the requirement

Not applicable to this process: if the requirement is a mirror or a reflective surface, smooth machining cannot deliver it - choose polishing. If the requirement is a protective layer or a colour, choose a coating or a plating, because this process adds nothing to the surface.

Tell us the roughness the drawing calls for and what the part has to do next, and we will confirm which rung of the ladder fits.

Get a quote

Pick or avoid

When smooth machining is the right call - and when it is not

Pick smooth machining when

The drawing calls for a surface finer than as-machined but does not need Ra 0.8 um or a mirror.
The part is metal or plastic - the two groups in our published table.
The part will be processed further, such as painting or polishing, and needs a good base surface.
A smoother surface with no added layer is wanted, so threads and fits stay as cut.
Reducing the risk of corrosion at the surface matters, without a coating.

Avoid smooth machining when

The requirement is Ra 0.8 um or finer - that is fine machining. See fine machining.
The requirement is a mirror or a reflective surface - that is polishing. See polishing.
A protective layer, a colour or a corrosion barrier is needed - this process adds nothing. See all surface finishes.
The drawing carries no roughness callout - an as-machined surface is enough, and there is no reason to pay for the extra pass. See as-machined.
A random, non-directional texture is wanted rather than a cut surface - that is sanding. See sanding.

Not sure which rung the drawing asks for? Send it with the roughness callout and what the surface has to do next, and we will confirm the process.

Get a quote

Design constraints

What decides whether the smooth-machined part comes back at Ra 1.6 um

Four variables: the material, the stock left for the finishing pass, the parameters, and what happens to the part after the cut.

Material

The published scope is metals and plastics, and the same roughness figure does not behave the same way on all of them. Some plastics cut cleanly and some smear, and the alloy changes how a tool leaves the surface. Name the material and grade so the finishing pass can be planned.

Stock left for the finishing pass

This finish is a light final cut, so the part has to arrive with material for it. A part that is already to its final size cannot be smoothed without going undersize - say so on the drawing and we will work around it.

The cutting parameters

The published description names the two that matter: a low feed rate and a high cutting speed, with sharp tools. These are set on the machine, which is why this finish is produced in the same order as the machining rather than as a separate step.

What happens after the cut

The surface is made by the last pass, so any later operation that scratches or re-clamps a finished face spoils it. If the part is going on to painting or polishing, the smoother surface helps - but the finished face still has to be protected until then.

Reachability

A face the finishing tool cannot reach keeps the roughness of the earlier cut. A deep bore, a tight pocket or an undercut has to be called out on the drawing if it carries the roughness requirement.

Appearance versus measurement

Ra is a measured number; a mirror is an appearance. This finish is measured against the published Ra 1.6 um and may still show tool marks, so it should not be specified as a cosmetic finish.

Drawing notes

What to write on the drawing so the surface is what you meant

The roughness callout and the stock allowance are the two lines that decide whether the part arrives with enough material for the finishing pass.

1.  SURFACE ROUGHNESS ON MARKED FACES: Ra ______ um    (PUBLISHED FIGURE FOR THIS FINISH: Ra 1.6 um)2.  MACHINE ALL MARKED FACES TO THIS ROUGHNESS.    UNMARKED FACES: AS-MACHINED.3.  DIMENSIONS ON MARKED FACES ARE    BEFORE / AFTER THE FINISHING PASS  (DELETE ONE)4.  LEAVE STOCK FOR THE FINISHING PASS:    ______ mm ON MARKED FACES.5.  DEBURR SHARP EDGES ______  (SPECIFY IF CRITICAL)6.  NEXT OPERATION AFTER MACHINING:    PAINTING / POLISHING / NONE  (DELETE AS APPLICABLE)7.  PROTECT FINISHED FACES BETWEEN OPERATIONS -    NO RE-CLAMPING OR HANDLING ON MARKED FACES.

Examples only - replace every blank with your own requirement. The roughness callout and the stock line matter most: a part with no stock left cannot be smoothed without going undersize.

Which materials

What we smooth-machine - the published groups, and what belongs elsewhere

Our table for this process names two material groups rather than a list. The rows below set out what that covers, and which process the other requirements belong to.

Material	Smooth machining	Notes	Use instead
Metals	Yes	One of the two groups in our published table. Name the alloy so the finishing pass can be planned.	—
Plastics	Yes	The other group in our published table. The published description states the finish is appropriate for metal and plastic parts that will be processed further.	—
A specific metal alloy	With the grade named	The table names a group, not individual alloys; the grade decides how the tool leaves the surface.	—
A specific plastic grade	To confirm	The table names plastics as a group; which grades are covered is being confirmed.	—
Parts needing Ra 0.8 um or finer	Next rung	A tighter roughness belongs to fine machining, which creates the minimum roughness.	Fine machining
Parts needing a mirror	Different process	Smooth machining leaves a matte surface with tool marks; a reflective surface is polishing.	Polishing
Parts needing a protective layer	Different process	Machining adds nothing to the surface, so it cannot provide a corrosion barrier or a colour.	All finishes

Send the drawing with the material and the roughness callout - if this is not the right rung, we will say which one is.

Get a quote

Applications

Where a smooth-machined surface is doing the work

The published description ties this finish to parts that will be processed further, such as painting or polishing. These are the cases behind that.

A machined shell casting with an even, low-roughness surface after a smooth machining pass

Parts heading for paint

A smoother surface is a better base for paint: a rough surface shows through a thin coat, and the published description names painting as one of the operations this finish is appropriate before.

A matte-finished ABS plastic part with a uniform, low-sheen surface

Plastic parts

Plastics are one of the two groups in our published table, and the published description states the finish suits metal and plastic parts that will be processed further.

An operator measuring a machined part on the machine against a roughness requirement

Parts with a roughness callout

Where the drawing names a roughness, the finish is measured rather than judged by eye. Ra 1.6 um is the figure published for this finish.

A CNC machined medical device component with a smooth, low-roughness surface

Medical and instrument parts

A smoother machined surface is easier to clean and inspect than a rough one. Where a component has to be clean and measured, this finish is chosen as a controlled surface rather than as a look.

QA and documentation

What you get back with the parts

Purchasing usually needs paper as well as parts. Here is what we can state today and what is still being confirmed.

Available

Surface inspection

Machined faces are checked against the roughness requirement stated on the drawing, where one was stated.

Available

Roughness where specified

Where the drawing calls a roughness out, the figure checked is the published Ra 1.6 um for this finish.

To confirm

Measured roughness record

A per-batch roughness record from a profilometer, against the published Ra 1.6 um, is being confirmed.

To confirm

Standard and test method

The published table gives no standard for this process; the standard it is run to, and the roughness measurement method, are being confirmed.

The roughness ladder, and where this rung sits

This finish is one rung of four. These are the pages either side of it, and the surface finishing hub.

As-machined or deburring - the rung below

The default surface when the drawing carries no roughness callout: the surface the cutter leaves, with visible tool marks. Choose it when the surface is not a requirement.

Sanding - the coarser, random-texture route

An abrasive is rubbed across the surface to lay down a random, non-directional texture. It reaches a similar roughness band from the other direction, with a grain rather than a cut.

Fine machining - the next rung up (Ra 0.8 um)

The same control of feed rate and cutting speed with sharper tools, taken to the minimum roughness this shop machines, published at Ra 0.8 um. Choose it when the surface itself is the requirement.

Polishing - the mirror rung

Physically rubbed until scratches and marks are removed, to leave a smooth, reflective surface. This is the rung for a mirror, and the one this process cannot reach.

All surface finishes

How this finish compares with coating, plating, anodizing and the rest - useful when the requirement is a layer or a colour rather than a roughness.

CNC machining services

The machining step that produces this surface in the same order, so the finishing pass is set up with the part rather than after it.

FAQ

Smooth machining FAQ: the questions engineers ask us

What is smooth machining?

It is nearly the same process as an as-machined surface, with the parameters tightened: a low feed rate and a high cutting speed with sharp tools. The result is a smoother surface and a reduced risk of corrosion, and the roughness figure we publish for it is Ra 1.6 um.

How smooth is it?

The figure published for this finish is Ra 1.6 um. Tool marks may still be visible, so it is smoother than as-machined but coarser than fine machining at Ra 0.8 um. See the rungs above and below.

Is it a mirror finish?

No. The published texture is a smooth finish - a matte surface - and the description states that tool marks may still be visible. A reflective surface is a polishing job, not this one.

Which materials does it cover?

Our published table names metals and plastics, and the published description states the finish is appropriate for metal and plastic parts that will be processed further. Which specific grades are covered is being confirmed. See the material table.

Does it change the size of the part?

It adds nothing - this is machining, not a coating - but the finishing pass does remove a light layer, so the part needs stock for it, and the drawing should say whether dimensions are before or after the pass. See what it changes.

Why choose it over fine machining?

Because it is enough for the drawing. Fine machining at Ra 0.8 um is the step below this one in roughness; if the requirement is tighter than Ra 1.6 um, this finish is the wrong rung. See the next rung.

Why choose it over as-machined?

Because the drawing asks for a surface smoother than the cutter leaves by default, or because the part is heading for painting or polishing and needs a better base. If nothing on the drawing calls the roughness out, as-machined is enough.

Can it protect against corrosion?

The published description states the finish minimises the risk of corrosion. That is a smoother-surface effect, not a barrier: no layer is added, and no test figure is published, so specific corrosion performance is being confirmed.

How do I get a quote?

Send the drawing with the material and the roughness callout on it, and say what the part will be processed with next - painting, polishing or nothing. Contact the team with the part details and we will come back with a quotation.

Get your smooth machining quote

Upload a STEP, IGES or PDF drawing. Tell us the material, the roughness the surface has to meet, and what the part is going to next, so the finishing pass and the stock allowance are set together.

Get a quote Machine it first

What to send us

Drawing - 2D and 3D files (STEP, IGES or PDF).
Material - the metal, or the plastic grade.
Roughness callout - the Ra the marked faces have to meet.
Stock allowance - how much material is left for the finishing pass.
Next operation - painting, polishing or none.
What happens next - assembly or storage, so the hand-over state fits.

All uploads are secure and confidential. Engineers review and reply with a quotation and lead time.