Surface roughness converter

Because they measure different things about the same surface, and how far apart they land is decided by the shape of the profile rather than by arithmetic. Take a perfectly periodic profile — a sine wave, which is what an idealised turned surface at a fine feed approaches. Its Rz works out at exactly π times its Ra, about 3.14. Now take a random profile with a normal height distribution, which is close to what grinding leaves: its Rz comes out around seven times its Ra. The same two parameters, more than double the ratio.

Real surfaces sit between those two shapes. That is why the trade quotes a band of four to seven whenever nothing more is known, and why this converter asks which process made the surface before it estimates anything. Naming the process does not make the answer exact — it narrows the band.

Direction matters too. Going from Ra to Rz, at least the Ra is a measurement. Going the other way you are dividing by an uncertain number, so the converter marks the Ra itself as an estimate and everything downstream of it with it.

The consequence for a drawing is simple: if both the average and the extreme matter, specify both. A surface that meets an Ra limit can still carry a valley deep enough to leak past a seal or start a crack, and only an Rz or an Rt limit catches it.

The roughness grade numbers N1 to N12 come from ISO 1302 and are shorthand for twelve Ra values, from the finest polish at N1 to a rough cut face at N12. Each grade is close to double the one below it, which is what lets them be drawn as an even ladder on a logarithmic scale.

A grade number is a ceiling, not a target. A surface measured just under the N7 value is an N7 surface; a hair over it and it is N8. That is also why this converter never rounds a measurement down into a finer grade than it has earned.

Current practice writes the Ra value on the symbol rather than the grade number, because the value says the same thing without a lookup and can carry a parameter name and a rule with it. The grades survive on older drawings, in casting and forging specifications, and in conversation — which is reason enough for a converter to keep them.

The symbol is a tick with one long leg, touching the surface it applies to. On its own it means the surface has a texture requirement and the method is open. A horizontal bar across the top means material must be removed, so the surface has to be machined. A circle in the vee means material must not be removed: the surface stays as cast, moulded, forged or drawn.

Read down the ladder above and the order is the familiar one. Flame cutting and sawing leave the coarsest surfaces on it; rough turning and milling sit a band finer; finish turning and milling reach the middle of the scale. Grinding takes over below that, honing below grinding, and lapping and diamond polishing hold the fine end on their own.

Spark erosion spans an unusually wide band, because its roughness is set by the discharge energy rather than by a tool: one machine produces a coarse texture and a fine one depending on how it is set. Wire erosion runs finer than die-sinking for the same reason, on a smaller discharge.

These bands are a general engineering reference for what the processes are capable of, on ordinary materials, on equipment in reasonable condition. They say nothing about what any particular machine, steel or supplier holds on any particular part — that belongs in the quotation and the control plan, not on a ladder.

Two things the bands do not show. Reaching the fine end of a band costs disproportionately more than reaching its middle, and the finish a surface can hold depends on the material as much as on the process: a clean hardened tool steel polishes to a level a soft or inclusion-rich one never will.

Both specify a mould surface by how it was produced, not only by a number. SPI, the twelve-grade system published by the American plastics industry association, groups its grades into four families: diamond buffing, abrasive paper, abrasive stone and dry blasting. VDI 3400 is the European system for spark-eroded surfaces, numbered from 0 to 45 in steps of three.

The SPI families overlap on the roughness scale on purpose. A-3 and B-1 cover the same Ra, and so do B-3 and C-1 — the same roughness reached by buffing and by paper, or by paper and by stone. They are not interchangeable. The buffed surface is glossy and without direction, the papered one carries a faint direction, and the moulded part shows the difference at a glance.

VDI 3400 is a series rather than a list. Its Ra doubles every six reference numbers, which makes a texture easy to step up or down predictably — one reason erosion shops and texture houses work in it rather than in roughness values.

Because both systems are indexed by Ra, any value entered here can be read against both, and the converter shows both. What it will not do is translate an SPI grade into a VDI number: they are not the same kind of surface, even where their Ra values meet.

Three things at once: how the part looks, how easily it leaves the mould, and how it behaves afterwards.

Appearance is the obvious one. A polished cavity gives gloss, a stoned one matt, a blasted one a texture. The finer the polish, the more honestly the surface reports everything underneath it — a mirror finish shows weld lines, sink marks and steel defects that a matt finish would have hidden.

Release is the one that surprises people. A mirror-polished wall is not the easiest to release from; with a soft or tacky material it can be the hardest, because the contact is nearly complete and there is nothing for air to get behind. A fine blasted texture often releases more readily for exactly that reason. What always helps is working the surface in the direction of draw: a tool mark running across the direction of ejection is a mechanical lock.

Texture also interacts with draft. A deep texture on a side wall needs more draft than a smooth one, or the part drags on ejection and the texture is scored off the wall along with it. Texture suppliers publish a recommended draft angle per texture depth, and it is worth reading before the wall angles are frozen.

Name the parameter, not just a number. Ra 0,8 is a specification; 0,8 beside a tick is an invitation to argue about which parameter was meant. If the extreme matters as well as the average, add a second requirement — an Rz limit under the Ra one.

Say where it applies. A general texture requirement inherits to the whole part unless it is placed on a particular face, and a mould insert almost never wants one requirement everywhere: the moulding surface, the parting face and the back of the plate have nothing in common.

For a mould surface, cite the finish system as well as the roughness. An SPI grade or a VDI reference number tells a toolmaker how to get there, which an Ra value on its own does not. Where appearance is the point, a physical reference sample beats any number — two shops can both hit a stated Ra and hand back two surfaces that do not match.

And state which standard the drawing works to, in the title block. The symbol has outlived several editions of the standards behind it, and the default rule for an unadorned value has not always been the same.