The Rail That Didn’t Sit Flat
We mounted a profile rail (article 122) on a machined aluminum base plate. The plate was milled (flat). On the bench, the rail sat flat. On the floor, after a month, the rail wasn’t straight (the slide bound at one point). The problem: the base plate wasn’t flat enough. The plate was milled to ±0.1 mm (flatness). The rail (which is straight to ±0.02 mm) sat on a plate that wasn’t flat. The rail bowed (the plate deflected). The slide bound. We either re-machined the plate (to ±0.02 mm flatness) or scraped (hand-scraped) the surface. The rail sat flat. The mistake was specifying a flatness looser than the rail’s straightness.
Base plate tolerance (flatness) matches the plate to the component. This article covers the spec.
The Flatness Requirement
A machined base plate (article 107) has a flatness tolerance (how flat the surface is). The flatness must be better than (tighter than) the component mounted on it. If the plate is looser (not flat), the component bows.
| Component | Mounting Flatness |
|---|---|
| Profile rail (precision) | ±0.02 mm (or better) |
| Ball screw support (article 61) | ±0.02 mm |
| Non-critical bracket | ±0.2 mm |
| Guard frame | ±0.5 mm (no precision) |
Step 1: Specify the Flatness
On the drawing, specify the flatness (e.g., “flat within 0.02 mm over 500 mm”). The machine shop machines to that. Don’t leave it unspecified (the shop uses a default, which is loose). Specify it.
For a rail mount (precision), specify ±0.02 mm (or per the rail manufacturer’s recommendation). For a non-critical bracket, ±0.2 mm is fine (cheaper to machine).
The base plate rule: Specify the flatness. The rail that bowed sat on a ±0.1 mm plate (looser than the rail). Specify ±0.02 mm for a precision rail mount. The plate’s flatness must be tighter than the component’s straightness. Don’t leave it unspecified.
Step 2: Scraping (Hand-Scraping)
For the highest precision (a machine tool way), the surface is hand-scraped (scrape the high spots with a scraper). Scraping gives a very flat surface (±0.005 mm) with oil pockets (for lubrication). But it’s expensive (manual). For a standard automation rail, milling (±0.02 mm) is enough (no scraping).
Step 3: Stress Relief (Article 146)
A welded base plate (article 146) warps after machining (residual stress). Stress-relieve before machining (article 146). For a cast iron plate, it’s already stress-relieved (casting). For a welded steel plate, relief it.
Step 4: Mounting Holes (Article 137)
The mounting holes (for the rail, the screw support) must be positioned accurately (article 137). The holes locate the components. Specify the hole positions (±0.05 mm) on the drawing. Don’t rely on the rail’s adjustable mounting (the rail has some adjustment, but the holes must be close).
A Base Plate Checklist
- What is mounted? (Rail? Screw support?)
- Specify the flatness? (±0.02 mm for rail?)
- Is the flatness tighter than the component?
- Is the plate stress-relieved? (Article 146?)
- Are the mounting holes positioned? (±0.05 mm?)
- Is the surface milled? (Or scraped?)
- Does the rail sit flat? (No bow? Article 122?)
- Does the slide bind? (Check?)
- Is the flatness measured? (Indicator?)
- Is the spec on the drawing? (Not default?)
The Bottom Line
Base plate tolerance specifies the flatness. The rail that bowed sat on a loose plate. Specify ±0.02 mm for a precision rail mount. Stress-relieve (article 146). Position the holes (±0.05 mm). The plate that held the rail straight wasn’t the thickest one — it had a specified flatness.
