Cast Iron Accessories: Small Parts That Carry the Assembly

Most of the iron in a machine is not structural. It is the handles, covers, spacers, brackets, stops and mounting pads that make the assembly work and get no attention in the specification. Individually they cost little. Collectively they decide whether a product feels finished or improvised.

What Counts as an Accessory

In a bill of materials, accessories are the parts that are not the main function. A pump has a body, an impeller and a shaft; everything else is an accessory: the foot bracket, the coupling guard, the drain plug boss, the lifting eye, the terminal box lid.

The category is useful because these parts share a set of economics. They are small, they are numerous, they are usually bought in larger quantities than the main casting, and they are almost always cost-driven rather than performance-driven. A buyer will spend a week on the alloy for a valve body and five minutes on the bracket, which is why brackets are where castings are most often replaced by weldments that fit badly.

When Casting Beats Machining From Solid

A bracket machined from a billet is accurate and expensive. The material removal can be 70 percent of the blank, and the machining time dominates the price. Casting wins as soon as three conditions hold.

The shape is not a simple prism. A part with a curved foot, a rib, an offset boss and a slot cannot be cut from plate without several setups.

The section varies. A casting can be 12 mm at the boss and 4 mm at the web, which is where the strength is needed and where the weight is not.

The quantity is above a few hundred a year. Below that, the pattern cost per part is high and machining from solid is often cheaper overall.

Between those conditions, casting typically lands 20 to 40 percent below a machined equivalent at 500 pieces, and the gap widens with volume.

The Shape Freedom You Get for Free

Once a part is cast, features that would be separate operations become part of the mould. A cast-in part number, a locating boss, a cable channel, a stiffening rib, a recess for a label: each of these is free or nearly free in a casting and costs money in a machined part.

This is where design effort pays best on accessories. A customer who sends a simple L-bracket often gets back a suggestion with an added rib, a formed foot and a cable clip, at the same piece price. The part works better and costs the same.

The limit is the mould, not the imagination. Undercuts need a core or a loose piece. Deep narrow slots need draft. Thin isolated sections do not fill. Those constraints are worth knowing before the drawing is frozen, and we will say which features are causing cost when we quote.

Small Parts, Small Tolerances: What Holds

Accessories are usually specified with general tolerances, and that is appropriate. An as-cast accessory holds ISO 8062 CT10 to CT13, which on a 100 mm dimension is roughly plus or minus 1 mm.

Where it matters is the interface. A mounting hole pitch, a spigot diameter, a face that seats against another part: those need machining, and machining brings them to plus or minus 0.05 mm on a face and 0.02 mm on a bore.

The common mistake is specifying a tight tolerance on a dimension that does not touch anything. It adds a setup, a fixture and cost, and it buys nothing. We mark on the quotation which dimensions we plan to machine, and if that list looks longer than the function requires, it is worth a conversation.

Finishing Options That Fit a Catalogue Part

Accessories are often sold as catalogue items, which means the finish has to look consistent across batches and across years.

Paint is the default. A powder coat at 60 to 80 micrometres gives a durable, uniform finish and hides small casting texture, which makes it a good match for a part that will be seen. Wet paint is cheaper and adequate for a part that will not.

Galvanizing is for outdoor and wet service, at 70 to 90 micrometres. It changes the dimensions slightly and closes small holes, so it needs to be in the drawing from the start.

Black oxide or phosphate is for parts that will be oiled or painted later, and for anything where a bare machined surface would rust in storage.

What we do not recommend is leaving an accessory as-cast and bare unless it is inside a housing. A rusty bracket in a crate is the first thing a customer sees when they open the box.

Tooling Families: One Pattern, Five Variants

The biggest cost lever on accessories is not the casting, it is the tooling per part number. A customer with eight bracket variants in a product family does not need eight patterns.

We build a family pattern with interchangeable inserts, so the common body is one tool and the variant feature, whether it is a hole pattern, a boss position or a mounting foot, is an insert. The tooling cost drops by 60 to 80 percent and a new variant can be added in two weeks instead of eight.

The constraint is that the variants must share the same basic envelope and the same parting line. If they do not, it is two families, not one. Looking at a product range with that in mind, before the drawings are finished, is the most valuable thing a buyer can do at the specification stage.

Stock, Reorder and the Cost of Interruption

Accessories are the parts that stop a production line when they run out. Nobody notices a bracket until there are none.

For repeat customers we hold patterns in storage at no charge and can run a batch on two to three weeks' notice for an existing pattern, against four to six weeks for a new one. We will also quote a blanket order with scheduled releases, which fixes the price for a year and lets the customer call off quantities monthly without a new purchase order each time.

For parts with long demand and stable design, we can hold finished castings in our warehouse and ship against release. That is worth more than a small price difference when the alternative is an air freight shipment of brackets.

Kanban, Blanket Orders and the Inventory Question

An accessory that runs out stops an assembly line, and the cost of that stop is usually larger than the annual saving from buying the part cheaper.

Two arrangements work. A blanket order fixes the price and the annual quantity, with releases called off monthly; the customer holds no stock and we hold the pattern. A kanban arrangement goes further, with an agreed minimum stock held at our warehouse and replenished when the customer's bin empties.

Both depend on a stable design and a forecast that is roughly right. An accessory with a design change every quarter is a bad candidate, because the stock becomes obsolete.

What we will not do is hold stock on a part with no order behind it. That ties up capacity and it teaches the wrong lesson about which parts need buffer stock.

For most customers, the practical answer is a blanket order with a quarterly release and a two-week call-off. It removes the purchase order from the critical path without turning the foundry into a warehouse.

Reference Price Range and Minimum Order

Reference price range: US$0.10 to 100.00 per piece, FOB Qingdao. Minimum order quantity: 500 pieces per item. The final unit price is quoted against your drawing and depends on part weight, material grade, moulding process, machining allowance and tooling amortisation. Reference prices are valid until 31 December 2027.

Frequently Asked Questions

What is the smallest accessory you will cast?

Around 100 grams. Below that, gating and handling dominate the price and the part is usually better as a machined or stamped item.

Can you supply accessories painted in our colour?

Yes, to a RAL or Pantone reference, with a sample chip approved before the batch. Colour consistency across batches is held to a Delta E of about 2.

Do you keep stock of common accessories?

For repeat customers with a blanket order, yes. For one-off parts, we cast to order.

Can one pattern really cover several variants?

When the variants share an envelope and a parting line, yes. Send the family and we will tell you how many patterns it actually needs.

How do you handle a part that is currently welded?

Send the weldment and its loads. If the shape converts, the casting is usually lighter and needs less finishing, and we will show the cost comparison rather than assert it.

What lead time applies to a reorder?

Two to three weeks for an existing pattern with no drawing changes, including casting, finishing and packing.