Technical whitepaper · WP-01 · 15 pages

Deep Draw Design Handbook

Design rules, draw scheduling and defect avoidance for deep drawn sheet metal parts - written for the engineer who signs the drawing and the buyer who releases the tool. Free PDF, no form.

Download the 15-page PDF

What is in it

01 Why deep drawing - when the process wins and when it is the wrong answer
02 Material and anneal - grades, work hardening, and when an anneal has to be planned in
03 The design rules - nine rules that decide whether the part can be drawn at all
04 Blank and draw schedule - blank development, draw ratios and a worked example
05 Tooling essentials - die materials, blank holder pressure, lubrication, die life
06 Defects and remedies - what fails where, what causes it, what to change
07 Tolerances and inspection - what is achievable as drawn, and what a second operation buys
08 The drawing we need - the checklist that turns a drawing into a quotation

When deep drawing is the right process, and when it is not

Deep drawing turns a flat blank into a seamless shell in one press stroke or a short series of them. It is the cheapest way to make a thin-walled, closed-bottom part at volume, and the most expensive way to make it when the geometry fights the process. The handbook opens with the two lists that matter: it wins when the part has a closed bottom, the wall is thin relative to the diameter, volume is high enough to amortise a die, a weld seam is unacceptable for pressure tightness or magnetic symmetry, and the material work hardens. It loses when the batch is in the tens, when the wall is thick relative to the diameter, when the part is a deep rectangular box with sharp internal corners, or when the bottom has to be flat to a few hundredths without a coining operation. Deep drawing moves cost from the machine to the die, so volume changes the unit price faster here than in any other forming process.

The nine design rules

Rules 1 to 4 are geometry the tool has to follow. Radii decide everything: punch nose radius normally 2t to 4t on the first draw and 0.8t to 1.5t on later draws, die shoulder radius 4t to 8t on the first draw and 2t to 4t after that - too small tears, too large wrinkles. Clearance per side sits at 1.1t to 1.3t; tighter thins the wall deliberately, looser costs roundness. A wall thinner than the blank means an ironing operation, which is a separate die and less remaining elongation. Bottom flatness in the hundredths needs coining or machining, because as drawn the bottom of a cup is slightly domed. Rules 5 to 9 are limits and features: stay inside the draw ratio for the material, specify grade plus temper plus surface condition, keep pierced features at least 2t from the wall and pierce after drawing where the hole has to be true, give the tool room a trim allowance because a drawn rim is never level, and state the datum and the measuring height for every diameter.

Blank development and the draw schedule

Two numbers decide the tooling: the blank diameter and the number of draws. For a plain cylindrical cup the handbook uses D = root(d squared + 4 d h), where d is the mean cup diameter and h is the cup height, with 5 to 10 percent added to the height before calculating because the rim is trimmed afterwards. The worked example runs a 40 mm inside diameter, 1.0 mm wall, 78 mm deep cup in DC04: mean diameter 41.0 mm, blank 118.7 mm, first draw at a ratio of about 1.85 to 55.9 mm, second draw at 1.36 to 41.0 mm, then a sizing pass to hold the diameter and the roundness. The chapter then shows the same part written as an operation schedule with an op number, a diameter, a height and a ratio per step - because the schedule is what turns a drawing into a quotation, and a part that needs two draws cannot be made in a single-station die without a transfer.

Where a drawn part fails, and what to change

Almost every draw failure appears in one of a handful of places, and naming the location is most of the diagnosis. Wrinkling in the flange or wall comes from blank holder pressure that is too low or clearance that is too large, and it is answered by pressure, clearance or an extra draw. Tearing at the punch radius comes from radii that are too small or a reduction that is too large, and it is answered by increasing the radii, splitting the reduction or annealing. Earing - an uneven rim - is anisotropy in the rolled sheet and is answered by trimming, rotating the blank or changing to a grade with less directionality. Wall thinning above the radius is a clearance problem. Orange peel is a grain or limit problem that cannot be polished out of a drawn wall. Bottom bulge is residual stress released when the part leaves the die. Rim height that varies around the part is earing combined with a trim referenced to the outside rather than the bottom.

What a drawn housing can hold as drawn

The handbook gives typical as-drawn results for a cylindrical housing, and the operation that buys each one tighter. Inside diameter lands at IT9 to IT11 as drawn and is bought tighter with a sizing or ironing pass. Roundness is typically 0.03 to 0.10 mm and improves with a sizing pass. Coaxiality between inside and outside diameter is typically 0.05 to 0.15 mm and needs a tool designed for it. Wall thickness varies by about plus or minus 5 percent of nominal and is tightened by ironing. Overall height holds plus or minus 0.2 to 0.5 mm with a trim fixture referenced to the seating face. Bottom flatness is 0.10 to 0.30 mm and needs coining or machining to go tighter. Where a drawing is silent we work to ISO 2768-1 class m plus these as-drawn values, and any feature that is functional has to be stated, because a general note will not protect the assembly.

The drawing checklist

The last chapter is the list that lets a supplier answer with a schedule and a price instead of a list of questions. On the drawing: finished inside and outside diameters with the measuring height, wall thickness as drawn and after any ironing, height and whether it is measured over the dome or to the trimmed rim, corner and radii requirements including the bottom radius, concentricity and roundness call-outs with their datums, flatness of the bottom if it is functional, holes and slots with position tolerances, and surface finish with any post-treatment. Around the drawing: material grade, temper and surface condition, annual volume and expected batch size, any magnetic or pressure tightness requirement, which dimensions are functional, whether a first article and PPAP are required and at what level, and destination and packaging because they change the unit price.

Frequently asked questions

Is the handbook free, and do I have to give an email address?

It is free and ungated. The download link goes straight to the PDF. We publish it that way because a document that is worth citing by an engineer or an assistant should be readable without a form.

Are the numbers in it specific to Balford?

Every number is labelled by where it comes from. Ranges marked as planning values come from our own tool room - the radii, ratios and clearances we lay a first schedule out with. Standards such as ISO 2768-1 and the material designations are published reference material. No customer measurement and no customer data appear in the document.

Can I use it to check a part before I send a drawing?

Yes, and that is the intended use. Compute the draw ratio - blank diameter divided by cup diameter - and compare it with the table for your material. If it exceeds the first draw limit, the part needs more than one draw and the tool needs more stations. If nobody does that arithmetic before the RFQ, the first quote will be wrong.

Does the handbook replace a DFM review?

No. It tells you what the process can generally hold and what has to be written down. A DFM review turns those ranges into a specific blank size, a specific schedule and a specific set of tolerances for one part.

Download

WP-01 Deep Draw Design Handbook (PDF, 15 pages). Companion documents: WP-02 Drawn Housing Tolerances and WP-03 Sourcing Drawn and Stamped Parts.

Have a part that breaks one of these rules?

Send the drawing through rfq.balford.net and we return a DFM note: the draw schedule we would use, the radii we would change, the call-outs the process cannot hold as written, and what each of those costs if the requirement stays. See also deep draw metal stamping capability.