Troubleshooting Sheet Metal Forming and Fit-Up
Find out why formed parts come out wrong, from bad blanks and drifting angles to cracks, oil-canning, twist and parts that will not fit, and fix the real cause without guessing.
Table of contents
When a formed part does not fit, the temptation is to adjust the brake and try again. Sometimes that works. Often the real problem is in the blank, the material or the bend order, and adjusting the machine only hides it. A mid-level fabricator measures first, finds out which step went wrong, and fixes that step. This work assumes you can already run a brake, read a bend chart and calculate a flat pattern.
What You Will Be Able to Do
- Measure a bad part in an order that points to the cause.
- Spot blank and cutting problems before they reach the brake.
- Diagnose angle problems along a bend and from part to part.
- Prevent cracks, oil-canning, twist and bow.
- Check squareness, and correct a blank from a first article.
Measure Before You Adjust
Work through these steps in order. Each one rules out a whole group of causes.
- Measure the thickness. Use a micrometer. Sheet can differ from the nominal gauge, and a new lot may differ from the last one.
- Measure the blank. If you still have one, check it against the flat pattern. If it is wrong, stop there.
- Measure the angle. Check each end and the middle of every bend.
- Measure the flanges. Check each flange at both ends.
- Check flatness and twist. Set the part on a flat table and look for rocking or gaps.
- Compare it to the first good part. Ask what changed: lot, die, operator, or bend order.
Change one thing at a time, and write down what you changed.
Start With the Blank
A bad blank cannot be saved at the brake. Before you blame forming, check how the part was cut.
| Cut problem | Likely cause | Fix |
|---|---|---|
| Heavy burr on a sheared edge | Blade clearance wrong, or dull blades | Reset clearance, about 5 to 8 percent of thickness |
| Rough, torn sheared edge | Too little clearance | Open the clearance slightly |
| Strip bows or twists after shearing | Narrow strip, stress in the sheet | Shear wider and trim, or level the sheet |
| Blank is not square | Sheet not seated on the gauge or squaring arm | Reseat it and check the first cut |
| Plasma sparks blow back toward you | Moving too fast | Slow down until sparks exit the bottom |
| Heavy dross under a plasma cut | Speed wrong for the thickness | Check the chart, adjust speed |
| Warped parts from a laser or plasma nest | Parts nested too close, heat builds | Add spacing, change the cut order |
Burrs matter more than they look. A burr on the stretched side of a bend can start a crack, so put the burr side on the inside of the bend or deburr it first.
Angle Problems
The first question is whether the angle is off everywhere, in one spot, or only on some parts.
| What you measure | Likely cause | What to do |
|---|---|---|
| Open the same amount on all parts | Springback, not enough ram depth | Add depth and check a test piece |
| Open in the middle, closed at the ends | Bed not crowned for the load | Adjust crowning, check tonnage |
| Tighter at one end | Ram out of parallel, worn tools, part off center | Check machine setup and tooling |
| Changes from part to part in a batch | Thickness or hardness varies | Measure the stock, separate the lots |
| Changes when the part is turned around | Grain direction or uneven tool wear | Keep blanks the same way round |
| Drifts during a long run | Tools or hydraulics warming up, or the lot changed | Spot-check parts as you run |
Small angle errors have a big effect at the end of a flange. The tip of a flange moves by its length times the tangent of the error:
- 1 degree on a 3 inch flange: 3 x 0.0175 = 0.052 inch
- 1 degree on a 6 inch flange: 6 x 0.0175 = 0.105 inch
- 2 degrees on a 6 inch flange: 6 x 0.035 = 0.210 inch
So a part can be perfectly sized at the bend and still miss its mating part by over 1/8 inch at the far edge. Before you change a blank to fix a fit problem, measure the angle.
Warning: Do not reach into the die area to check, hold or adjust a part while the machine can cycle. Follow your lockout procedure before you change or adjust tooling, and never bypass a guard or light curtain to get a better look.
Length Problems
If every flange is off by the same amount on each bend, the bend deduction is wrong. If one flange is off and the others are fine, look at your backgauge, your bend order, or whether the part sat fully against the gauge.
| Pattern | Likely cause |
|---|---|
| All flanges long or short by the same amount | Bend deduction wrong for this die or lot |
| Only the last flange is off | Errors stacking, or gauging off a formed flange |
| Flange length differs end to end | Blank not square, or part skewed against the gauge |
| Length changes with each new sheet | Thickness varies from sheet to sheet |
Worked Example: Correcting a Blank From a First Article
An enclosure side has three bends. The flat pattern is 20.000 inches with a bend deduction of 0.130 inch per bend. The first article comes out 0.045 inch long overall, and every flange is a little long. The angles are correct at 90 degrees, and the thickness is right.
Step 1: Find the error per bend. 0.045 / 3 = 0.015 inch per bend.
Step 2: Correct the deduction. The real bend deduction is 0.130 + 0.015 = 0.145 inch.
Step 3: Correct the blank. 20.000 - 0.045 = 19.955 inches.
Step 4: Move the bend lines. The first line moves by half the error, and each line after it moves by a further full error. If you only shorten the blank, the bend positions are still wrong.
Step 5: Prove it. Run a new first article, measure it, and update the shop chart for that die and material.
Cracks on the Bend
Cracks appear on the outside of the bend, where the metal stretches. Work through these causes:
- Radius too tight. The inside radius should be at least equal to the thickness on mild steel. Harder materials need more.
- Bending along the grain. Bending across the grain lowers the risk of cracking.
- Hard material. Aluminum 6061 and hard tempers crack on bends that softer alloys handle easily.
- Rough edges. A burr or a nick at the edge of the bend can start a tear.
- Scribe marks. A scribed line on the outside of a bend can start a crack.
- Die too narrow. A narrow die forces a tighter radius and more tonnage.
Fix the cause, then run a test piece. Do not grind a crack out of a formed part and ship it. The bend is already weak there, and a repair needs approval.
Oil-Canning and Flatness
Oil-canning is when a flat panel pops in and out like the bottom of an oil can. It happens when thin sheet has more metal in the center than at the edges, or when stress is trapped in it. The panel looks wavy, and it can snap from one side to the other when you press on it.
Common causes and fixes:
| Cause | Fix |
|---|---|
| Large unsupported panel | Add a bead, stiffener or cross break |
| Stress from welding on the panel | Use short welds, skip weld, and limit heat |
| Stress in the sheet from the mill | Use leveled sheet, or a heavier gauge if approved |
| Rivets or screws pulled too tight | Fix the spacing and fit, and use the right fastener |
| Hole pattern or cutouts near the center | Add stiffening, and check how it was cut |
A heavier gauge or a formed stiffener changes the design, so check with your lead before you make that kind of change.
Twist and Bow
Twist is when one end of a part rotates compared to the other. Bow is a curve along its length. Long, narrow parts are the most likely to do both.
- Uneven flanges. A tall flange on one side and a short one on the other pulls unevenly. Check the design and the blank.
- Bend line not parallel to the edge. A skewed blank bends into a twist.
- Stress released by cutting. Cutting a long, thin part from a sheet can let it move.
- Welding on one side. Weld shrinkage bows the part toward the weld.
Fit-Up Problems
A part that will not fit its mate is usually a problem of angle, stack-up or squareness. Check both parts before you decide which one is wrong.
Checking Square With Diagonals
Measure both diagonals of a rectangular frame or opening. If they match, the shape is square.
Example: An opening is 24 by 18 inches. The diagonal should be the square root of (24 x 24 + 18 x 18), which is exactly 30 inches. You measure 30.125 inches one way and 29.875 inches the other. The opening is out of square, and the difference is 1/4 inch. Rack it into square before you tack or rivet it, or check why the parts were out of square in the first place.
Fit-Up Rules
- Do not force parts into place. If you need to clamp hard to close a gap, you are locking in stress that can show up later as distortion.
- Do not slot holes without approval. Slotted or enlarged holes can weaken a joint or break a rule on the print.
- Check the whole assembly. One part may be right by the print and still not fit, because its mate is wrong.
- Build a test assembly. If you have a new job, fit one set together before you make the batch.
Welding Sheet Metal Assemblies
If a part fit before welding but not after, heat is the likely cause. Thin sheet buckles easily, and each weld shrinks. Use short welds, skip around to spread the heat, and use a copper or aluminum backing bar if your procedure allows it. Clamp the part while it cools. Keep welds to the size on the print, and tack the assembly before you commit to the long seams.
Marks and Finish Problems
| Problem | Likely cause | Fix |
|---|---|---|
| Lines along the bend | Sharp die shoulders, or dirt on the die | Clean the die, use a protective film |
| Dents on the panel face | Chips on the table or the tooling | Clean the work surfaces |
| Rust spots on stainless later | Iron picked up from steel tools | Keep stainless tooling separate |
| Scratches from handling | Sliding sheets over each other | Lift sheets, keep protective film on |
Common Mistakes
- Adjusting the brake before measuring the blank. You hide the real problem and it comes back.
- Fixing a fit problem with blank size when the angle is wrong. The tip of a long flange moves a long way.
- Assuming the new lot matches the old lot. Measure the thickness of every new lot.
- Shortening a blank without moving the bend lines. The total is right, but the part is wrong.
- Forcing parts together. The stress can show up as distortion or a failed joint.
- Skipping a first article after a die change. The radius changes, and so does your deduction.
Check Your Understanding
Your bends are open in the middle of a long part and correct at the ends. What do you check first?
The crowning. Without enough crowning, the bed and ram separate under load in the middle, so the bend comes out more open there.
A 5 inch flange ends up 0.090 inch out at the tip, but the bend position is right. What is the likely problem?
The bend angle. A 1 degree error on a 5 inch flange moves the tip about 0.087 inch, so check the angle before you change the blank.
A rectangular frame has diagonals of 41.500 and 41.750 inches. What does that tell you?
The frame is out of square, because the diagonals do not match. Bring it into square before you fasten it.
A batch of parts comes out at the wrong angle after a new skid of sheet arrives. Nothing else changed. What is the first thing to measure?
The thickness of the new material, and then its hardness if you can. A change in the stock changes both the angle and the bend deduction.
Key Takeaways
- Measure the thickness, blank, angle and flanges before you adjust anything.
- A bad blank cannot be fixed at the brake, so check the cut first.
- A small angle error causes a large error at the tip of a long flange.
- Correct a blank from a first article, and move the bend lines too.
- Prevent cracks with radius, grain direction and clean edges.
- Check both diagonals for square, and do not force parts to fit.