One-Off, Prototype and Repeat Work: How the Brief Should Change

A prototype, a part that has to fit an opening already on site, and a batch you will order again can all be made in the same workshop. They should not be briefed as if they were the same job. If the brief mixes them up, one of two things happens. A design that is still being tested is treated as final, or a part you have already corrected is remade from an older file.

The confusion usually starts with the enquiry itself. An email saying "Please quote" arrives with a drawing, and the workshop has to guess what you want. It may be one piece to learn from, one piece that must fit a doorway, or fifty pieces that must match a part you have already approved. That guess changes the plan and the price, which is why a quotation can seem wrong when the real problem was a vague brief. What a fabricator needs to quote explains how quantity, tolerance and finish change the price. This article covers a different question. Tell the workshop which type of job it is, and send the information that belongs to that type, because a prototype needs different information from a repeat.

Name the type of job before you describe the part

These three types of job need the workshop to work in different ways, even though the cutting, forming and welding may look identical on the job sheet. A prototype is a trial, so if it is planned as though every dimension were fixed, it cannot show you what needs to change without a dispute. A site one-off has to fit, so if the drawing is followed without question, the part may not meet the opening. A repeat batch is a copy of a revision you have already approved. If it is planned as though the design were still open, each batch may differ a little more from the approved part.

Put the type of job in the first line of the enquiry: prototype, one-off or repeat. Then send the information that belongs to that type, rather than the same general folder you send for every job.

Prototype: say what is allowed to change

A prototype is how you find out whether the part works. If every dimension is marked critical, the first piece cannot show you anything without turning into an argument about who was wrong. So tell the workshop which features are being tested. Take a guard whose handle position is still a guess, while its bolt pattern must match a machine that is already built. That is an ordinary prototype: the handle can move and the bolts cannot, and that sentence is worth writing on the drawing. If you mark the whole sheet as tight, the workshop will either plan for tolerances you do not need yet, or move the handle and then justify the change.

Tell us whether a second version is likely. "We may want another after we have tried this" is enough. If the workshop knows the first piece is a step rather than the finished product, it will keep notes with the job. These record what you accepted, what you asked to move, and which file holds the change. Those notes are more useful when cutting, forming, welding and finishing stay in one place, which is the practical side of single-source fabrication, because the person who saw the problem can change the next piece.

The notes do not replace a written change, though. A workshop can remember a conversation, but the second version may be ordered three weeks later by someone who was not there. A written note means you do not depend on that memory.

Site one-off: the opening decides the shape

Some parts are made only once because they have to meet a wall, a machine or a steel structure that is already there. A formal drawing helps, but often you do not have one. A sketch with dimensions, photographs of the opening, or the broken part you are replacing is a proper brief, and the quotation article explains how to send them. It mentions only briefly the thing that decides this job: which existing surfaces the new part must touch, and what it must pass through to get there.

A handrail that has to be carried into a stairwell in two pieces is a different object from the same rail drawn as one length. In the same way, a frame that must pass through a doorway cannot be one welded assembly, however neat that looks in the workshop. Tell us this in your first note. If the access problem is found only when the piece is finished, the fault was in the brief, not in the fabrication.

If you are unsure which measurements matter, send the photographs and the sizes you have, and tell us which edges must meet something solid. We will tell you what is missing before anything is cut, because a short list of missing sizes is better than a part that almost fits. If the one-off is a rail, a tube or a plate with a tight bend, read Bending stainless steel without cracking. Send the grade and the thickness with the sketch, because a radius that looks easy on paper can still crack the plate or flatten the tube, depending on the thickness.

Repeat batch: keep to the approved revision

For a repeat, keep the revision, the grade, the thickness, the finish and the dimensions you accepted on the approved piece exactly as they were. What can still change is the quantity, and where the parts should be delivered this time. If a first piece was approved, send that revision, and do not send an earlier file just because it is the one still in the March email. That is exactly how a corrected hole gets made wrong again. The March file was easier to find, and the June marked-up drawing, which was checked against the part in the workshop, was not attached.

"The same as last time", with no revision named, is not an instruction. If something must change, mark it on the approved drawing and state that everything else stays the same. We would rather treat a small revision as a new instruction than assume the old part is still what you want. A request to "make it a bit shorter" will be sent back to you. A dimension with the old number crossed out and the new one written beside it will be made.

When you place the repeat order, attach the approved revision to the enquiry itself. The workshop plans the job from that file, so nobody should have to search old emails for it.

Tell the workshop whether this part is allowed to change. That is the brief.

The type of job also decides the price, which is why the quotation article spends so long on preparation work. A one-off has to carry all of that preparation on a single piece, while a repeat batch shares it across many pieces. A prototype should not be priced as if the method were already settled, and a repeat should not be priced as if the workshop had never seen the part. None of these prices can be worked out if the enquiry does not say which type of job it is.