Titanium Disc 1300 mm: Flatness After Cutting

Titanium Disc 1300 mm: Flatness After Cutting

A disc looks like the simplest titanium part on a price list, and it is the one where a buyer most often gets a round plate that will not sit flat. The reason is geometry: a disc is thin relative to its diameter, so it is stiff in its own plane and flexible across it, and any imbalance between the two faces shows up as a bow or a dish once the part is off the machine. Diameter and thickness on this page are a cutting envelope, from 150 to 1300 millimetres across and 35 to 550 millimetres thick, and the way a blank is made inside that envelope decides whether the finished part is flat.

Up to about 100 millimetres thick, a disc is normally cut from plate: the plate is rolled to thickness and the disc is sawed or water jet cut from it, so thickness is whatever the mill rolled and only the faces are machined. Above that, a 400 or 550 millimetre thick disc cannot come from plate, and the blank is sawn from a forged or rolled block, which means the thickness is a sawing decision and both faces have to be machined true.

That is why the same diameter can be a plate blank or a block slice, and why the two behave differently after machining. A plate carries its own rolling texture and a modest internal stress; a block slice carries the stress of the whole forging, released on one side when the first face is cut. A disc sawn from a block and machined on one face only will bow towards the cut face, every time.

When metal is removed from one face of a stressed blank, the balance of the remaining section changes and the part curves to find a new equilibrium. The cure is procedural rather than metallurgical: remove similar amounts from both faces in the roughing stage, leave enough allowance for a finish pass on each side, and where the blank is thick enough to carry real stress, interpose a stress relief anneal between roughing and finishing in a vacuum or inert atmosphere.

The order of operations matters more than the size of the last cut. A finish pass of a few tenths on a disc that is already bowed simply copies the bow into a smoother surface, and the part measures correct in thickness and wrong in flatness. That is the failure that passes a dimensional inspection and fails on the machine table.

Thickness is easy to measure and easy to hold; flatness is the one that decides whether a disc can be clamped, lapped or bolted. On a disc of half a metre across, a bow of two or three tenths of a millimetre is invisible to the eye and enough to rock on a surface plate. Flatness should be specified as a number, measured on a plate or by a dial indicator on a rotating table, and stated over the full face rather than at the rim only.

Parallelism between the two faces is the companion tolerance, and it is the one that lets a disc sit in a fixture without shimming. A disc that is flat but wedge shaped will clamp at one edge and open at the other, which matters wherever the part is used as a spacer, a backing plate or a target.

The polished silver finish on this range is a surface condition, and it is worth being clear about what it does and does not do. Polishing improves appearance, makes the part easier to wipe down and removes the fine tool marks that hold contamination, which is useful in clean service. It does not remove a bow, and a mirror polish over a dished face gives a very convincing looking part that still will not sit flat.

Polishing also comes after the last dimensional operation, so any polishing that removes measurable metal can take a disc out of thickness tolerance at the rim. The safe sequence is to face, check flatness and thickness, then polish with a controlled removal, and to name on the order which faces are polished and to what finish.

Weight grows with the square of the diameter and in proportion to thickness, and at the top of this range it dominates the job. A disc 1300 millimetres across and 550 millimetres thick is roughly three quarters of a tonne of titanium, which needs lifting points, a crate built for it and a machine tool that can turn it. Even a 1300 by 35 millimetre disc is over two hundred kilograms.

It is worth quoting the mass with the part, both for freight and for the workshop, and worth deciding early whether the disc is a blank to be machined further or a finished part. A blank needs extra thickness and diameter as machining allowance; a finished part needs tolerances that a blank would not be held to.

This range is supplied as round titanium alloy blanks, polished and packed in plywood cases, with diameter and thickness to requirement and the quantity quoted per size. Because the page gives a range rather than a grade, the grade has to be settled on the order: a commercially pure grade for corrosion duty, an alloy where strength per unit mass matters, and the UNS number written next to it so that the certificate and the part agree.

A complete order reads: diameter and its tolerance, thickness and whether it is nominal or machined, flatness, parallelism, roundness, which faces are polished and to what finish, edge condition, grade with UNS number, and whether the disc is a blank or a finished part. Add the service and the temperature if the grade is still open. Those few lines decide whether what arrives can be clamped and used, or has to be re machined before it earns its place.