Fits and Tolerances for Machining Overview Explanation – Purpose and Why it is Useful Different Types of Tolerances and Application Interference and Clearance Fits Tolerances in Machining Presenting Fits and Tolerances on an Engineering Drawing How this Applies to the Duke Machine Shop Tolerance Overview Engineering Drawings are a way to communicate your design ideas Tolerances can make or break a design More precision means more machining time, the use of special tools, and increased manufacturing cost Types of Tolerances Geometric Dimensioning and Tolerancing Clearance and Interference Fits Standard Sizing, Spacing, Angles, Positions for Drawings Machining Tolerancing Geometric Dimensioning and Tolerancing -Extensive and Detailed Dimensional Tolerancing for a Wide Range of Features -Add to title block and Individual Dimensions Clearance and Interference fits (shaft and hole) -Use the chart below to choose the type of fit you need (clearance – hole larger than shaft; interference - shaft larger than hole) -Many type of fits, choose the best for your application -Example – Sliding Fit would be H7/g6, this designation will define the dimensions you need to make the hole and shaft to achieve a sliding fit Identify Tolerance Grade - Based on type of fit chosen, letter and number designation will direct you to the proper data column - Example: Sliding fit is identified as “H7/g6, the numbers represent tolerance grade from the chart below, if we are identifying a sliding fit for a one inch shaft the first number represents the hole, IT7 = 0.0008”, second number the shaft IT6 = 0.0005”) Identify Fundamental Deviation -Depends on Size and Letter Designation from Fit Type -Example – sliding fit for a 1 inch Basic size, for the hole h=0, for the shaft “g” = -0.0003” Use Fundamental Deviation and Tolerance Grade to Determine Sizes -Use information obtained from chart to find your target hole size D = Basic Size of Hole d = Basic Size of Shaft δu = upper deviation δl = lower deviation δF = fundamental deviation (from table) ΔD = tolerance grade for hole (from table) Δd = tolerance grade for shaft (from table) For Hole: 𝐷𝑚𝑎𝑥 = D + ΔD Dmin = D For Shafts With Clearance Fits (c,d,f,g,h) dmax = d + δF dmin = d + δF – Δd For Shafts With Interference Fits (k,n,p,s,u) dmin = d + δF dmax = d + δF + Δd Example 1 – Sliding Fit for 1 Inch Shaft (H7/g6) D = Basic Size of Hole = 1 inch d = Basic Size of Shaft = 1 inch δF = fundamental deviation (from table) = 0 (hole – H), -0.0003” (shaft – g) ΔD = tolerance grade for hole (from table) = 0.0008” (hole – IT7) Δd = tolerance grade for shaft (from table) = 0.0005” (shaft – IT6) For Hole: 𝐷𝑚𝑎𝑥 = D + ΔD = 1.0000” + 0.0008” = 1.0008” For Shafts With Clearance Fits (c,d,f,g,h) dmax = d + δF = 1.0000” – 0.0003” = 0.9997” dmin = d + δF – Δd = 1.0000” – 0.0003” – 0.0005” = 0.9992” Dmin = D = 1.0000” What does this mean? Key information, for H7/g6 sliding fit, 1 inch basic size Hole: 𝐷𝑚𝑎𝑥 = 1.0008” Dmin 1.0000” Shaft: dmax = 0.9997”, dmin = 0.9992” So the minimum clearance between shaft and hole is Dmin - dmax = 0.0003”, and the maximum clearance between shaft and hole is Dmax - dmin = 0.0016” Example 2 – Medium Drive Fit for ¾ Inch Hole/Shaft H7/s6 designation (from chart) D = Basic Size of Hole = 0.75 inch d = Basic Size of Shaft = 0.75 inch δF = fundamental deviation (from table) = 0 (hole – H), 0.0014” (shaft – s) ΔD = tolerance grade for hole (from table) = 0.0008” (hole – IT7) Δd = tolerance grade for shaft (from table) = 0.0005” (shaft – IT6) For Hole: 𝐷𝑚𝑎𝑥 = D + ΔD = 0.7500” + 0.0008” = 0.7508” For Shafts With Interference Fits (k,n,p,s,u) dmin = d + δF = 0.7500” + 0.0014” = 0.7514” dmax = d + δF + Δd = 0.7500” + 0.0014” + 0.0005” = 0.7519” So the Maximum Interference between shaft and hole is 0.0019”, Minimum Interference is 0.0006” Dmin = D = 0.7500” Purchased Components – How to Adjust Fit Calculations Perform Calculations as Usual to Determine Tolerance Range for basic size (minimum and maximum interference or clearance ranges) If size of off-the-shelf component differs from your shaft or hole size, “slide” the range to adjust the part you will be machining. Purchased Part 1 – Dowel Pin Purchased Part 2 – Undersize Dowel Pin Adjustment to Size Hole for Pins – Medium Drive Fit for ¾ Basic Size For Pin 1 the shaft size has already been determined for you: dmin = 0.7500”, dmax = 0.7502” Use the range you determined in tolerance calculations to determine the hole size (max interference = 0.0019, min interference = 0.0006) Dmin = dmax - max interference = 0.7502 – 0.0019 = 0.7483” Dmax = dmin - min interference = 0.7500 – 0.0006 = 0.7494” Similarly, for Pin 2 - dmin = 0.7498”, dmax = 0.7500” Dmin = dmax - max interference = 0.7500 – 0.0019 = 0.7481” Dmax = dmin - min interference = 0.7498 – 0.0006 = 0.7492” Machining Tolerancing Best workable tolerance at our Student Shop for spacing, dimensions, features etc. on a mill or lathe is about +/- 0.002 inches Tighter tolerances on Holes can be achieved by using a reamer Drill Hole first with closest undersize drill bit Then finish hole with reamer, not designed to remove a lot of material (0.005” to 0.01” at the most) Reamers are made to provide tight tolerance holes Machining Tolerancing Continued Provide Tolerance on Title Block, include any variations directly on drawing Use inch dimensions at machine shop (metric is possible, all tools and end mills, etc. are set up for inch sizes) See example below for how to present hole sizes on drawing, use option C for shop work Make sure to limit precision on dimensions to 3 decimal places (inch) or 2 places (metric), unless you are specifying a reamed hole Example Drawing 1 Example Drawing 2 Machine Shop – Real Capabilities and “Shop Talk” tips The methods presented here are the proper way to size fits and present sizes on an engineering drawing In reality, due to capabilities and precision of the Duke Student Shop, you will likely be dealing with the closest thousandth of an in (0.001”) instead of ten thousandth (0.0001”) Due to a limit in the available sizes of reamers, you will also likely have to choose the closest size to your specifications instead of the exact tool When speaking about hole sizes or dimensions at a machine shop “tenths” is the accepted way to refer to ten thousandths of an inch (0.0001”), NOT actual tenths of an inch (0.1”) All machines are set up in inches, all available tools and hardware are also in inches so use them whenever possible, you can switch to metric but try to avoid this. If you prefer to draw in metric that is fine, but when you are ready to machine use the “dual dimension” feature to present both on your drawing
