Machining Tolerances

This application help designers and engineers while designing the parts for any type of mechanical device.
The application will give you a tolerance range, equal bilateral, for a especific manufacturing process selected. Turning, Grinding, Broaching, Reaming... up to 12 different types.

  Machining Tolerances.

As a designer of mechanical components, here are a couple of recommendations I do always consider when assigning tolerances:

• It's very important to define a correct dimension composition in your drawing. Avoid having accumulation of lineal dimensions. They will only add variation in your part and in your assembly.
• Tolerance values below ±0.013 are difficult to maintain under regular conditions, even when talking about bearing bores or bearing supports, requesting values below those 0.013 mm tends to be a problem for manufacturing and quality. If you can live with something avove it, your supplier or manufacturing collegue will appreciate it.
• Face to face location in case of bearings supports (shafts or pinion gears) could be in values around ±0.05 mm. 
• Think always in those features that would need to be machined after heat treatment. In general, it is common to use a grinding operation to compensate deformation and to obtain the desired roughness. Depending on the size of the feature, values below ±0.013 can be achieved adding more cost to the part.(again, cpk requirements could be an issue for quality and for the supplier)
• Try to not assign geometric tolerance values lager than feature tolerances, so think twice about the values you define in your parts.
• Use equal bilateral tolerances in your drawings. It will help you with your stackup calculations (comming soon application) and if you are using 3D models, and your manufacturing does also, your parts will be machined centered always.
• Cost of the tolerances:

• General Tolerance values: This this the tolerance table intended to simplify drawing indications and specifies general tolerances in four tolerance classes. It applies to the dimensions of work pieces that are produced by metal removal or are formed from sheet metal. It contains three tables and an informative annex with regard to concepts behind general tolerancing of dimensions. 

         General tolerances for linear measures according to DIN ISO 2768-1

         Table one - Permissible deviations for linear dimensions except for broken    

         edges (external radius and chamfer heights) 

Designation	Description	Permissible deviations for basic size range in mm
		from 0.5# upto 3	Over 3 upto 6	over 6 upto 30	over 30 upto 120	over 120 upto 400	over 400 upto 1000	over 1000 upto 2000	over 2000 upto 4000
f	fine	±0,05	±0,05	±0,10	±0,15	±0,20	±0,30	±0,50	Nil
m	medium	±0,10	±0,10	±0,20	±0,30	±0,50	±0,80	±1.20	±2.00
c	coarse	±0,20	±0,30	±0,50	±0,80	±1.20	±2.00	±3.00	±4.00
v	very coarse	Nil	±0,50	±1.00	±1.50	±2.50	±4.00	±6.00	±8.00

Table two - Permissible deviations for broken edges (external radius and chamfer heights)

		Permissible deviations for basic size range in mm
Designation	Description	from 0.5# upto 3	over 3 upto 6		over 6
f	fine	±0,20	±0,50		±1.00
m	medium
c	coarse	±0,40	±1.00		±2.00
v	very coarse

Table three - Permissible deviations for angular dimensions

		Permissible deviations for ranges of Lengths in mm of shorter side of the angle concerned
Designation	Description	upto 10	over 10 upto 50	over 50 upto 120	over 120 upto 400	over 400
f	fine	±1.00°	±0.500°	±0.333°	±0.166°	±0.083°
m	medium
c	coarse	±1.500°	±1.000°	±0.500°	±0.250°	±0.166°
v	very coarse	±3.00°	±2.000°	±1.000°	±0.500°	±0.333

General tolerances for form and position DIN ISO 2768-2 is for simplifying drawing and fixes general tolerances in three tolerance classes for form and position. By choosing a special tolerance class exactly the precision level common in workshops should be taken into account.

General tolerances for straightness and evenness in mm

		Permissible deviations for basic size range in mm
Designation	Description	upto 10	over 10 upto 30	over 30 upto 100	over 100 upto 300	over 300 upto 1000	over 1000 upto 3000
H		0.02	0.05	0.10	0.20	0.30	0.40
K		0.05	0.10	0.20	0.40	0.60	0.80
L		0.10	0.20	0.40	0.80	1.20	1.60

General tolerances for straightness and evenness in mm

		Permissible deviations for basic size range in mm
Designation	Description	upto 100	over 100 upto 300	over 300 upto 1000	over 1000 upto 3000
H		0.20	0.30	0.40	0.50
K		0.40	0.60	0.80	1.00
L		0.60	1.00	1.50	2.00

General tolerances for symmetry

		Permissible deviations for basic size range in mm
Designation	Description	upto 100	over 100 upto 300	over 300 upto 1000	over 1000 upto 3000
H		0.50
K		0.6-		0.80	1.00
L		0.60	1.00	1.50	2.00

Source : www.huaxing.com
Links to Information:

•  Typical Tolerances
•  Machining Tolerances
•  Cost of Tolerances

Involute Spline Calculation App

This app calculate the shear stress, contact stress and safety factors of a involute spline shaft under a defined torque. This calculation gives a good direction on design and will help to define sizes and lengths of shafts, and therefore the parts that are assembled on it. Bearings, Seals, snap rings sizes will, in most cases, be defined by the spline torque capacity.
The calculation is based in the document released by Darle W. Dudley, and used in many different engineering books.
The geometry of the inner diameter and the outer diameter is approximated for a quality 6 spline.
This calculation can be used for splines designed under the standard ANSI B92.2M or ISO4561.
This tool is defined for giving you directions and not definitive or complete designs. For that particular purpose, It is reccomended to run more accurate analysis and of course validate your designs with physical parts.

    Involute Spline Torque Calculation

Aditional Information about Splines:

• The function of a spline can be defined as fixed or sliding. Sliding one is oftenly used in synchronizers, shift gears and other similar machine elements.Fixed ones, are designed for not allowing any axial movement, therefore a minimun clearance, side or outer diameter is desired.A good practice on fixed splines is te use of pilot diameters for centering the load distribution and avoid excesive tilting in the connection.
• Splines work better when it is lubricated. Grease or oil will avoid premature failures due to fretting.
• Fillet root splines resist better the torque load, even when the minimun diameter is smaller compared with a flat root spline.
• A minimun taper angle on the outside diameter of the spline, along the overall effective length could help when torque twist the shaft and the contact distribution runs to the front or rear area of the spline. FEA analysis can help yo to determine the amount of taper needed.
• Splines failures would happen due to:
• Wearing of the spline teeth due to high contact stress and poor lube.
• Broken shaft due to high torsional stress.
• Spline fretting due to vibration, axial movement and poor lube
• Bending failure due to a small section under the tooth. Hollow diameter to big.
• If Faiure happen due to shear stress, shock load may be the reason.
• As general rule, use quality 6, according ANSI B92.2, for general purposes splines. Quality 7 allows more geometrical deviations and can be used for low spec requirements or when the safety factors are really high. Quality 4 and 5 are choosen when the spline is grinded and the contact distribution must be controlled.
• If the teeth number is low, you will require the tooth to tooth spline locatin to have more  accuracy to asure that all teeth share a similar torque level. It is more easy for a spline with smaller module and high teeth number to share equally the load.
• It is reccomended not to have parts in contact with a different in hardness > 8.
• Splines and Shaft materials:
• SAE 1010 & SAE 1020  Low cost material, under low solicitations and where part of the shaft is heat treated (carburized)
• SAE 1045  Common steel used for shafts. Low cost material. Can be heat treated and provide high load resistance. It is not a good selection if the fatigue strength is a mayor requirement on your design.
• SAE 4340 Cr-Ni Steel, bonified (Quenched and Tempered) high toughness and tension resistance.
• SAE 4140 Cr-Mo Steel that is commonly bonified and induction hardened. This steel is used for applications under high load requirements.
• Shaft Design (Link).
• Failure modes (link)
• Load Distribution (Link)