A Comprehensive Guide to Selecting and Prepping Stock for Lathe Work

Successful lathe work begins long before the first chip is cut. The careful selection and meticulous preparation of your stock material are paramount to achieving accurate dimensions, desired surface finishes, and ensuring safe operation. This guide outlines the key considerations for choosing the right stock and the essential steps to prepare it for turning.

Selecting the Right Stock

The material you choose will depend entirely on your project's requirements, including its intended use, desired aesthetics, strength, weight, and of course, your machining capabilities.

1. Material Type:

• Metals:
  • Aluminum: Often a good choice for beginners due to its relatively soft nature and ease of machining. Various alloys offer different properties; 6061 is a common and versatile choice.
  • Steel: A broad category with varying hardness and machinability. Low-carbon steels like 1018 are easier to machine than higher-carbon or alloy steels. Stainless steel offers corrosion resistance but can be more challenging to turn.
  • Brass: Machines very well and produces a nice finish. It's often used for decorative parts, fittings, and bushings.
  • Plastics: Acrylic, Delrin, and Teflon are commonly turned plastics. They offer properties like electrical insulation, corrosion resistance, and low friction. Their machining characteristics differ from metals, often requiring sharper tools and different cutting speeds.
• Wood:
  • Hardwoods: (e.g., Maple, Walnut, Cherry, Oak) Generally more durable and can achieve finer finishes. They require sharper tools and potentially slower speeds than softwoods. Some hardwoods, like oak, can contain tannins that may corrode lathe beds if turned green.
  • Softwoods: (e.g., Pine, Cedar, Fir) Easier to cut but can be prone to Tear-out and may not hold fine detail as well as hardwoods. Turning green (wet) softwood can be easier but requires proper drying afterward to prevent cracking and warping. Cedar contains oils that can affect finishes.

Considerations for Material Selection:

• Machinability: How easily can the material be cut and shaped? Softer materials are generally easier but may produce stringy swarf. Harder materials require more rigid setups and sharper tooling.
• Strength and Durability: Will the finished part need to withstand significant loads or wear?
• Weight: Is the weight of the final product a critical factor?
• Corrosion Resistance: Will the part be exposed to moisture or harsh environments?
• Appearance: Does the material's natural look or its ability to accept finishes matter?
• Cost: Material costs can vary significantly.
• Availability: Can you easily source the desired material in the required form and size?

2. Stock Form and Size:

Stock for lathe work typically comes in various forms, most commonly round, but also square, hexagonal, and occasionally rectangular bars or billets.

• Round Bar: The most common form for turning, as it requires less initial material removal to achieve a cylindrical shape.
• Square and Hexagonal Bar: Useful when the finished part requires a non-round feature at one end or needs to be driven by a wrench. The excess material must be turned away to create cylindrical sections.
• Billets or Castings: Can be used for more complex shapes or when starting with a near-net shape is advantageous.

Choosing the Right Size:

Select stock that is slightly oversized for your largest required diameter. This allows for material to be removed during the turning process to achieve the final dimensions and a clean surface finish, free of the original stock's surface imperfections. Having slightly oversized stock also provides material for facing the ends square.

Consider the capacity of your lathe, specifically the "swing" (the maximum diameter of a workpiece that can be rotated without hitting the bed) and the distance between centres (the maximum length of a workpiece that can be mounted between the headstock and tailstock).

Prepping Your Stock for Turning

Once you've selected your stock, proper preparation is crucial for accurate and safe machining.

1. Cutting to Length:

Cut your stock slightly longer than the final desired length of your part. This excess material will be removed during the facing operation to ensure the ends are square to the centreline.

• Methods:

  • Horizontal Bandsaw: Ideal for cutting metal stock accurately and relatively quickly.
  • Chop Saw (Abrasive or Cold Saw): Can be used for cutting metal, but abrasive saws can leave a rougher edge. Cold saws provide cleaner cuts.
  • Hacksaw: Suitable for smaller diameters and provides a good workout. Use a guide or a vice with square jaws to help ensure a reasonably square cut.
  • Wood Saw (Band saw, Table Saw, or Hand Saw): Use appropriate blades for wood. Aim for as square a cut as possible.

• Achieving Square Cuts: While not absolutely critical at this stage, a reasonably square cut makes the subsequent facing operation easier and faster. Using a good saw and proper technique will help. For hand sawing, marking a line around the stock using a square can serve as a guide.

2. Inspecting the Stock:

Before mounting the stock in the lathe, visually inspect it for any defects:

• Cracks or Splits: Especially important for wood, but can also occur in some metal alloys.
• Deep Scratches or and Dentures: These may not clean up during turning and could affect the final part.
• Excessive Runout or Bends: While some runout is expected, severely bent stock can be difficult and dangerous to turn. Minor bends in metal can sometimes be straightened.
• Inclusions or Foreign Material: Look for anything embedded in the surface, particularly in wood.
• Rust or Corrosion: Surface rust on metal can often be turned away, but severe corrosion might indicate deeper issues.

3. Facing the Ends:

Facing is the process of machining the ends of the stock flat and square to the rotational axis. This is a critical step for establishing accurate lengths and providing a clean reference surface.

• Process:
  • Securely hold the stock in the lathe chuck. Ensure a sufficient length extends from the chuck for the facing operation and initial turning.
  • Mount a facing tool in the tool post and set its height to the centre of the workpiece.
  • Bring the tool close to the end of the stock.
  • Engage the carriage feed (or use the handwheel carefully) to move the tool across the face of the stock, removing a thin layer of material.
  • Take successive cuts, moving the tool slightly towards the centre with each pass, until the entire face is clean and square. For a smooth finish, take a final light pass with a fine feed rate.
  • Repeat for the other end of the stock if necessary, after reversing it in the chuck or supporting it with a tailstock centre.

4. Finding and Drilling Centres (for Between-Centres Turning):

If your part is long or requires machining along its entire length, you will likely turn it between centres. This requires creating a precise centre hole in each end of the stock.

• Finding the Centre:

  • Centre Finder Tool: A specialized tool that uses a cone or V-shaped guide to help scribe lines intersecting at the centre of round stock.
  • Combination Square: For square or rectangular stock, draw diagonal lines from corner to corner; the intersection is the centre. For round stock, you can make several scribe marks from the edge using the square to approximate the centre.
  • In the Lathe (for Round Stock): Mount the stock in the chuck and bring a pointed tool or wobbler close to the end while the lathe is spinning slowly. The tip will appear to stand still when it's on the rotational axis. You can then use this to mark the centre.

• Drilling the Centre Hole:

  • Centre Drill: Use a combination centre drill and countersink bit to create a small, precise starting hole with a chamfer. This helps guide the drill bit and provides a bearing surface for the lathe centre.
  • Drill Press: For accurate centre holes, use a drill press with the stock held securely in a vice or a V-block.
  • In the Lathe (using a Tailstock Chuck): Mount a drill chuck in the tailstock and align the drill bit with the centre mark on the stock. Advance the tailstock ram to drill the centre hole.

5. Checking for Runout:

Runout is the amount of wobble or deviation from the true axis of rotation. Excessive runout can lead to inaccurate parts, poor surface finish, and increased vibration.

• Checking in the Chuck: After securing the stock in the chuck, rotate it by hand and observe the end or a section near the chuck. You can use a dial indicator against the surface to measure the runout accurately. Minor adjustments to the chuck jaws (if using an independent jaw chuck) can help minimize runout. Self-centring chucks have a fixed amount of runout.
• Checking Between Centres: Once mounted between centres, rotate the stock and use a dial indicator on the surface to check for bends or inaccuracies in the centre holes.

By carefully selecting your stock and performing these preparation steps diligently, you establish a solid foundation for successful and enjoyable lathe work. Taking the time to prepare properly will save you frustration, improve the quality of your parts, and enhance the safety of your machining operations.