Machining accuracy refers to the degree of conformity between the actual geometric parameters (size, shape, position) of a part's surface after machining and the ideal geometric parameters specified in the drawing. Ideal geometric parameters are: for dimensions, the nominal size; for surface geometry, absolute circles, cylinders, planes, cones, and straight lines; for positional relationships between surfaces, absolute parallelism, perpendicularity, concentricity, and symmetry. The deviation between the part's actual geometric parameters and the ideal geometric parameters is called machining error.

Basic Size Segmentation (mm)	IT01	IT0	IT1	IT2	IT3	IT4	IT5	IT6	IT7	IT8	IT9	IT10	IT11	IT12	IT13	IT14	IT15	IT16	IT17	IT18
≤ 3	0.3	0.5	0.8	1.2	2	3	4	6	10	14	25	40	60	100	140	250	400	600	1000	1400
>3–6	0.4	0.6	1	1.5	2.5	4	5	8	12	18	30	48	75	120	180	300	480	750	1200	1800
>6–10	0.4	0.6	1	1.5	2.5	4	6	9	15	22	36	58	90	150	220	360	580	900	1500	2200
>10–18	0.5	0.8	1.2	2	3	5	8	11	18	27	43	70	110	180	270	430	700	1100	1800	2700
>18–30	0.6	1	1.5	2.5	4	6	9	13	21	33	52	84	130	210	330	520	840	1300	2100	3300
>30–50	0.6	1	1.5	2.5	4	7	11	16	25	39	62	100	160	250	390	620	1000	1600	2500	3900
>50–80	0.8	1.2	2	3	5	8	13	19	30	46	74	120	190	300	460	740	1200	1900	3000	4600
>80–120	1	1.5	2.5	4	6	10	15	22	35	54	87	140	220	350	540	870	1400	2200	3500	5400
>120–180	1.2	1.8	3	4.5	7	12	18	25	40	63	100	160	250	400	630	1000	1600	2500	4000	6300
>180–250	1.6	2.4	4	6	8	14	20	29	46	72	115	185	290	460	720	1150	1850	2900	4600	7200
>250–315	1.9	2.9	4.5	6.5	9	15	23	32	52	81	130	210	320	520	810	1300	2100	3200	5200	8100
>315–400	2.2	3.4	5	7	10	16	25	36	57	89	140	230	360	570	890	1400	2300	3600	5700	8900
>400–500	2.5	3.9	5.5	8	11	18	27	40	63	97	155	250	400	630	970	1550	2500	4000	6300	9700

• Dimensional Accuracy: The degree of conformity between the actual size of a machined part and the center of the part's dimensional tolerance zone.

• Form Accuracy: The degree of conformity between the actual geometric shape of a machined part surface and the ideal geometric shape.

• Positional Accuracy: The actual positional difference between relevant surfaces on a machined part.

• Interrelationship: When designing machine parts and specifying their machining accuracy, form errors should generally be controlled within the positional tolerance, and positional errors should be smaller than the dimensional tolerance. That is, for precision parts or critical surfaces, form accuracy requirements should be higher than positional accuracy requirements, which in turn should be higher than dimensional accuracy requirements

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3. Adjustment Methods
 

3.1 Adjusting the Process System

• Adjustment by Trial Cutting: Cutting a trial piece - Measuring the size - Adjusting the tool's depth of cut - Cutting - Repeating the trial cut. This process iterates until the required size is achieved. This method has low productivity and is mainly used for small-batch and single-piece production.

• Adjustment Method: Obtaining the required size by pre-setting the relative positions of the machine tool, fixture, workpiece, and tool. This method has high productivity and is primarily used for mass production.

3.2 Reducing Machine Tool Errors

Improving the Manufacturing Accuracy of Spindle Components
* Improve Bearing Rotation Accuracy:
* Select high-precision rolling bearings.
* Use high-precision multi-wedge hydrodynamic bearings.
* Use high-precision hydrostatic bearings.
* Improve the Accuracy of Components Matched with Bearings:
* Improve the machining accuracy of the housing bore and spindle journal.
* Improve the machining accuracy of surfaces matched with bearings.
* Measure and adjust the radial runout range of relevant components to compensate for or cancel out errors.
Applying Appropriate Preload to Rolling Bearings
* Eliminates clearance.
* Increases bearing stiffness.
* Averages out rolling element errors.
Preventing Spindle Rotation Errors from Affecting the Workpiece.

3.3 Reducing Transmission Chain Error

1. Fewer transmission components and shorter transmission chains lead to higher transmission accuracy.
2. Using speed-reducing transmission (i < 1) is an important principle for ensuring transmission accuracy. The transmission ratio of the transmission pair closest to the end should be the smallest.
3. The accuracy of the end component should be higher than that of other transmission components.

3.4 Reducing Tool Wear

Tools must be resharpened before reaching the severe wear stage of dimensional wear.

3.5 Reducing Force-Induced Deformation of the Process System

* Improve System Stiffness: Especially the stiffness of weak links in the process system.
* Reduce Loads and Their Variation:
* Improve System Stiffness:
* Reasonable Structural Design:
1. Minimize the number of connection surfaces.
2. Prevent localized low-stiffness sections.
3. Select reasonable structures and cross-sectional shapes for base components and supports.
* Improve Contact Stiffness of Connection Surfaces:
1. Improve the quality of mating surfaces between components in machine tool assemblies.
2. Apply preload to machine tool assemblies.
3. Improve the precision and reduce the surface roughness of workpiece locating datum surfaces.
* Adopt Reasonable Workholding and Locating Methods.
* Reduce Loads and Their Variation:
* Select appropriate tool geometry and cutting parameters to minimize cutting forces.
* Group workpieces (blanks) to ensure uniform machining allowances during setup.

3.6 Reducing Thermal Deformation of the Process System

1. Reduce Heat Generation from Sources and Isolate Heat Sources:
* Use smaller cutting parameters.
* Separate roughing and finishing operations for high-precision parts.
* Isolate heat sources from the machine tool whenever possible to reduce machine thermal distortion.
* For inseparable heat sources like spindle bearings, lead screw nuts, and high-speed slideways, improve friction characteristics (structure, lubrication) to reduce heat generation, use insulating materials.
* Employ forced air or liquid cooling for heat dissipation. 
2. Balance the Temperature Field.
3. Adopt Reasonable Machine Component Structures and Assembly Datums:
* Use thermally symmetric structures – Symmetrically arrange shafts, bearings, and gears in gearboxes to ensure uniform temperature rise in the housing walls and minimize deformation.
* Select appropriate assembly datums for machine tool components.
4. Accelerate Reaching Thermal Equilibrium.
5. Control Ambient Temperature.

3.7 Reducing Residual Stress

1. Add heat treatment processes to relieve internal stresses.
2. Arrange the process sequence rationally.

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4. Influencing Factors