Same scene most weeks: three quotes for the same 1,000 mm HGR25 rail, prices 30% apart, nobody able to explain why. We've been making linear motion parts in Zhejiang since 2010, so we sit on the other side of those emails daily. Here's how we'd check linear guide manufacturers out if it were our own money on the table.
Is your linear guide rail manufacturer a factory or a trading company?
You don't need an audit - one email asking for something only a producer can produce is enough.
- The inspection record, not the certificate. Ask for the sheet for that batch. A factory sends measured numbers - height tolerance H, hole pitch, running parallelism, inspector's stamp. A reseller sends a PDF with a logo.
- Can they change a drawing? Send a rail with a non-standard hole pitch. In our workshop that turns into questions about the E dimension at the ends and whether the holes still clear the plate; a trader just forwards your drawing and returns a price.
- Ask about grinding in numbers. "Which surfaces do you grind in-house, and what's the longest rail you can grind in one pass?" Anyone who grinds answers immediately. Anyone reselling says "we have a professional factory partner".
- Whose part number is on the quote? If it's a generic catalogue code, ask who machines the rail and who assembles the block. If nobody can map it back, you can't cross-check later.
A trader isn't always wrong - for odd quantities and mixed orders, a good one earns its margin. Just don't pay factory expectations on reseller answers. Vet linear guide manufacturers on paperwork, not on the price list.
Which accuracy grade - and the exact words to ask it in
Ask these three close to word for word:
- "Which grade are you quoting - C, H or P - and is the running parallelism figure per 1,000 mm or over the full length of the rail?"
- "What's the height tolerance H on the blocks in this batch, and do the blocks come from the same lot as the rails?"
- "If I order grade H now and need P next quarter, what changes besides the price?"
| Grade | Running parallelism over a 1,000 mm rail (typical) | Where it usually lands |
|---|---|---|
| C - normal | around 0.025 mm | Routers, woodworking, plasma tables |
| H - high | around 0.015 mm | General CNC milling, laser cutting, engraving |
| P - precision | around 0.008 mm | Grinders, measuring axes, precision machine tools |
Most machines our rails go into don't need P: on a 1,300 × 2,500 mm router the gantry is the bigger error source, and grade H on a flat frame beats grade P bolted to one that isn't. For a benchtop router or engraver, the EGR low-profile rails (EGR15 to EGR30) usually fit better - the block height matches the plates people design. Heavier gantry: HGR rails from HGR15 to HGR35.
Preload and runout decide the result, not the grade on the label
The codes are Z0 (0–0.02 C, effectively none), ZA (light, roughly 0.03–0.05 C) and ZB (medium, 0.06–0.08 C). Two rails, same grade and length, one Z0 and one ZA, behave like different products. Z0 on a slightly deflected rail gives a free carriage and chatter on a finish pass. ZB on a mounting surface that's out by 0.05 mm loads one row of balls harder, and the block runs warm after an hour. The question that gets a real answer: "This axis carries X kg at Y m/min - Z0 or ZA?"
Runout is second, and the cumulative figure matters more than the nominal grade. Buy a pair of 2,500 mm rails and one that's fine in the first metre can wander near the ends. Whether the two blocks under a single carriage plate sit within a couple of hundredths of a millimetre of each other is nowhere on the datasheet - ask for it on the inspection record before the PO.
The same applies up the axis: a grade H rail matched to a screw with sloppy axial clearance is money in the wrong place, so our DFU ball screws are judged on lead accuracy, preload and end machining the same way.
Lead time and MOQ: separate the stock range from the custom range
To avoid a vague answer, force a split: "If I place the PO today, which day does it ship, and what part of that is waiting on stock or on a setup?"
- Stock. Standard cut lengths of common sizes - HGR20 and HGR25 at 1,000 / 1,500 / 2,000 mm, Z0 or ZA - leave in 3–7 days, and one rail is a valid order.
- Cut to length. A non-standard length or end dimension E, or a matched pair, runs 10–20 days: it's waiting for a grinding slot, not for steel.
- Custom. Special hole pitch, extra drilling and tapping, jointed rails, special preload - 15–25 days, and here MOQ is real. A setup costs the same whether it runs 5 pieces or 50, which is why suppliers ask for 20–50 pcs or a tooling charge.
When a supplier says "fast delivery" with no number, ask for the shipment date on the PI.
What should be in the box with the rail
Ask for these at quotation stage, not at shipping:
- Material certificate naming the rail steel (S55C-class) and the rolling-element steel, plus raceway hardness, HRC 58–62.
- A precision inspection record with measured values - height H, hole pitch, E dimensions, running parallelism - not a pass/fail stamp.
- A batch number on both the rail's laser marking and the box label, so a re-order traces back to the same process.
- A packing list saying which blocks are paired with which rails, at what preload class - with two blocks under one plate, that pairing decides whether the axis runs smooth or rocks.
If a supplier can't put a batch number on paper, nothing can be proven later - the real cost of the cheapest quote.
Validating a sample: the order we'd follow
- Appearance, first five minutes. Raceway free of dents and rust, marking legible, seals seated. Slide the block the full length by hand - even light drag is right. Gritty means stop here.
- Dimensions. Measure rail width, height H, hole pitch and end E against the drawing, and the block holes against your plate.
- Break-in and runout. Run the block over the full stroke 20–30 times, then check it with a dial indicator against the mounting datum. Drag should match at the ends and the middle - a change near the ends is straightness, not lubrication.
- Machine test cut. Cut a pocket or a square, then check for step marks at reversals, chatter on a finish pass, and repeatability after an axis reversal. Run it two to four hours and touch the block: noticeably warm means too much preload or an uneven mounting surface, and both are cheaper to fix on a sample.
None of these six checks needs a factory visit. They need a supplier willing to put a batch number, an inspection record and a lead time in writing before the deposit goes out.
If you want to run them on us, send the rail size, the quantity and the accuracy grade you need through the inquiry page. We will answer all six in one reply, including the ones that cost us the order.
The questions we get asked most weeks
Can I buy a single rail, or do I have to take a pair?
One is fine. Stock lengths ship as single rails, and we pair the blocks to the rail at the preload class you order. A single rail with custom hole pitch still needs a setup, and sometimes the setup costs more than the rail.
Two suppliers quoted the same grade and the prices are 25% apart. What am I missing?
Usually preload class, or whether end machining and dust seals are in the price. Ask for a line-by-line breakdown - the cheap quote is often grade C with Z0, compared against your grade H with ZA request.
Can rails from two different batches go on the same machine?
Two rails on one gantry, generally yes, if height H matches and you re-check parallelism after mounting. Mixing blocks between batches is where people get into trouble - keep each block with the rail it was matched to.
What if a rail runs rough after two weeks on the machine?
Check mounting surface flatness and bolt torque before blaming the rail - we see more roughness from a bowed surface than from the rail itself. If it is the rail, the batch number plus a photo of the raceway tells a producer what happened.
If you're sizing a new machine, or replacing a rail whose marking you can't read any more, send the drawing, the stroke length and the load per block. That's usually enough for us to come back with a grade, a preload class and a shipping date the same day - through the inquiry form, or browse the linear guide range for sizes first.
