Plastic shims have been a standard part of slitting arbor setups for decades. They fill the small gap between what your standard metal spacers can achieve and the exact pocket width required. But they come with well-known problems: compression under clamping force, creep during long runs, thickness variation between batches, and the inability to maintain tight tolerances over time.
Fraction kit spacers offer a fundamentally better approach — and modern setup software makes them practical to use.
The Problem with Plastic Shims
Plastic shims typically range from 0.05mm to 0.5mm in thickness. While they're convenient for fine-tuning, they introduce several issues:
- Compression: Under arbor clamping force, plastic shims compress by 0.01-0.05mm, changing the effective pocket width during production
- Creep: Over extended runs, shims continue to deform, causing progressive width drift
- Temperature sensitivity: Shim thickness changes with arbor temperature, which rises during cutting
- Inconsistency: Shim thickness varies between manufacturers and even between batches
- Handling: Thin shims tear, fold, and jam during arbor loading
For operations targeting ±0.05mm width tolerance, these issues are not just theoretical — they're the direct cause of width variation and out-of-tolerance strips.
What Are Fraction Kit Spacers?
Fraction kits are precision-ground metal spacers in two resolution sets:
- F-0.10 set: Spacers at 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90mm
- F-0.01 set: Spacers at 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09mm
By combining standard spacers (whole millimeters) with F-0.10 spacers (tenths) and F-0.01 spacers (hundredths), you can build any pocket width to 0.01mm resolution using only metal parts.
For example, a pocket width of 127.73mm would be built as: 100 + 20 + 5 + 2 (standard) + 0.70 (F-0.10) + 0.03 (F-0.01) = 127.73mm exactly.
Why Shimless Is Better
| Factor | Plastic Shims | Fraction Kits |
|---|---|---|
| Material | Plastic / polymer | Hardened steel |
| Compression under load | 0.01-0.05mm | Negligible |
| Temperature stability | Poor | Excellent |
| Repeatability | ±0.02-0.05mm | ±0.005mm |
| Lifespan | Limited (wear, tear) | Thousands of setups |
| Setup complexity | Low (just slide in) | Moderate (need right combination) |
| Software support | Not needed | Recommended for efficient selection |
The Calculation Challenge
The reason plastic shims remained dominant for so long is simple: finding the right fraction kit combination manually is tedious. When you have standard spacers in 20+ widths and fraction kits in 18 sizes, the number of possible combinations per pocket is enormous.
This is exactly the problem OptiStack Pro's shimless solver was built to handle. The algorithm:
- First solves using standard metal spacers to get as close as possible
- Calculates the remaining gap (e.g., 0.73mm)
- Finds the optimal F-0.10 + F-0.01 combination (0.70 + 0.03)
- Verifies all parts are available in inventory with sufficient quantity
- Generates the complete load sequence for the shop floor
The entire process takes under 1 second.
When to Switch to Shimless
Shimless operation makes the most sense when:
- Your customers require ±0.05mm or tighter width tolerances
- You're experiencing width drift on long production runs
- You process materials where edge quality is critical (automotive, electrical steel)
- You're tired of plastic shims tearing, folding, or jamming during setup
- You want repeatable setups regardless of which operator is on shift
Getting Started
Transitioning to shimless operation requires two things: a set of fraction kit spacers (ask your tooling supplier), and software that can calculate the combinations efficiently.
OptiStack Pro includes dedicated shimless mode with fraction kit support. Try it free for 14 days to see how it works with your specific inventory and setup requirements.