To sandblast lettering into a granite headstone or plaque through a stencil, a 3 mm nozzle uses around 500 to 600 l/min and a 4 mm nozzle around 900 to 1,000 l/min, at 6 or 7 bar. With the margins, an engraving cabinet with a 3 mm nozzle runs on a 7.5 kW 4-in-1 screw compressor such as the K 75 (1.1 m³/min at 8 bar). A 4 mm nozzle needs the K 110, which delivers 1.7 m³/min.
This article deals with sandblast engraving. For all the air used in a workshop, hammers, polishers and vacuum pads included, see our page on the compressor for stonemasons.
How is stone engraved by sandblasting?
The memorial mason sticks a stencil onto the polished stone, a rubber film or a photoresist film, in which the lettering, motifs or portrait have been cut out. The mason then fires a hard abrasive, often aluminium oxide or silicon carbide, inside an engraving cabinet. The abrasive cuts into the stone where the stencil is open and bounces off the film elsewhere. The longer it strikes, the deeper the engraving.
Compressed air does all the work: it carries the abrasive and gives it its speed. If the air flow falls short, the pressure drops at the nozzle, the abrasive slows down and the engraving stops progressing. If the pressure is too high, the abrasive tears the stencil and the edges of the letters crumble.
How much air does an engraving nozzle use?
Consumption depends first on the nozzle diameter and rises with the square of that diameter. Values given as a rough guide, to be checked in your blaster's manual:
| Nozzle diameter | Typical use | Indicative consumption |
|---|---|---|
| 2.5 to 3 mm | fine lettering, portraits, plaques | around 400 to 600 l/min |
| 4 mm | memorial lettering, motifs, hard granite | around 900 to 1,000 l/min |
| 5 mm | large areas, decoration, frosting | around 1,500 l/min |
Two things make these figures vary.
- Pressure. A nozzle fed at 4 bar uses noticeably less than at 7 bar. On soft marble and limestone, engraving is often done at lower pressure to spare the stencil.
- Wear. The nozzle widens with every hour of engraving. A 3 mm nozzle worn to 4 mm uses almost twice as much air: check its diameter and change it in good time.
What pressure to engrave granite, marble or limestone?
As a rough guide: granite and hard stones are commonly engraved at around 5 to 7 bar at the nozzle, marble and limestone rather at around 3 to 5 bar. The right setting is the one that cuts without lifting the stencil: run a test on an offcut of the same stone.
The compressor is set to around 8 bar, and a regulator at the station brings the pressure down to the value you want. Without this reserve, the pressure at the nozzle drops as soon as another tool starts on the network. Place the cabinet close to the compressor or the air receiver, and use a hose of adequate bore: our pressure drop calculator shows what an over-long hose costs you.
The step-by-step calculation
We follow the method of our configurator: the nozzle is counted at 100% because it blows continuously during engraving, the blow gun at 50%, then a 25% safety margin and 20% for expansion.
Case 1, 3 mm nozzle.
- 3 mm nozzle: 600 l/min.
- Blow gun to dust off the piece: 150 l/min × 50% = 75 l/min.
- Subtotal: 675 l/min.
- Safety margin: 675 × 1.25 = 844 l/min.
- Expansion margin: 844 × 1.2 = about 1,010 l/min (around 36 CFM).
The K 75 delivers 1,100 l/min at 8 bar: it is enough, provided you keep an eye on nozzle wear.
Case 2, 4 mm nozzle.
- 4 mm nozzle: 950 l/min.
- Blow gun: 75 l/min.
- Subtotal: 1,025 l/min.
- With both margins: 1,025 × 1.25 × 1.2 = about 1,540 l/min.
You need the K 110, 1.7 m³/min at 8 bar. If a wet polisher runs at the same time in the workshop, add it: with 600 l/min more, the total with margins rises to about 2,440 l/min and exceeds the 2.2 m³/min of the K 150. The S 220 PM, 3.6 m³/min, is then the right fit.
Which compressor for sandblast engraving?
| Setup | Recommended model |
|---|---|
| Engraving cabinet, 3 mm nozzle, few other tools | K 75, 1.1 m³/min at 8 bar, 420 L air receiver |
| Engraving cabinet, 4 mm nozzle | K 110, 1.7 m³/min at 8 bar |
| 3 mm engraving and polishing at the same time | K 150, 2.2 m³/min at 8 bar |
| 4 mm engraving and polishing, or two cabinets | S 220 PM, 3.6 m³/min at 8 bar |
The S and K series are variable speed: between two engravings, while the next stencil is being laid, the compressor slows down instead of running idle. For a 5 mm nozzle or larger, used for stripping rather than engraving, see our page on the compressor for sandblasting.
Why must the air be dry for engraving?
The abrasive must flow freely in the blaster. If it picks up moisture, it forms lumps, blocks the metering valve and the nozzle, and the jet becomes irregular: the engraving shows gaps, and you have to go over it again at the risk of damaging the stencil. Moisture also makes stone dust stick to the film.
A refrigerated dryer at the compressor outlet is the basis; it is built into the K series. Add an automatic condensate drain on the air receiver, and a water separator or filter as close as possible to the cabinet. Our article on dry air for soft blasting goes into the choice of dryer in detail; the logic is the same for engraving.
The pitfalls of sandblast engraving
- A worn nozzle. It is the first cause of air shortage. A carbide nozzle lasts longer than a steel one; measure it regularly.
- Cabinet dust. Engraving gives off a fine dust of stone and abrasive. The cabinet must be connected to a dust extractor, and the compressor installed elsewhere, with an air intake on the clean side. See our article on silica dust and the compressor.
- The start-up peak. When the nozzle opens, the air receiver supplies the peak. A receiver that is too small makes the pressure drop for a few seconds. The air receiver size calculator checks the volume.
- Marks on the stone. On a pale stone, blown oil or water can stain. A fine filter on the finishing blow gun prevents this defect.
Enter your nozzle and your other tools in the configurator, or send us your cabinet's data sheet with a quote request.
For your trade: Compressor for stonemasons, memorial masons and worktop fabricators.