Cut screen slots in PVC casing with a fine hacksaw blade held in a depth stop and guide, making slots 0.5-1 mm wide and 50 mm long in rows 10 mm apart, with no more than about half of any row’s circumference removed so the pipe keeps its strength. On 110 mm pipe with a 4 mm wall, a saw cut 10.5 mm deep from the top of the pipe opens a 50 mm slot through the wall, and three slots a row gives 300 slots and about 150 cm² of open area per metre.
Part of Deep Wells
Slot cutting is the hand-sawing of narrow openings in plastic pipe to make a well screen. The slot width, length, pattern and depth of cut set both how well the screen filters and how strong it stays.
A Straight Saw Cut Across A Round Pipe Makes A Slot Of Known Length
Hold the pipe still and saw straight down across it, with the blade in a plane at right angles to the pipe’s axis. A saw blade going down to a depth d below the top of the pipe cuts a straight line across the circle. The cut opens right through the wall only where the line passes below the inner surface, so the length of the finished slot depends on how deep you saw.
For a pipe of outer radius R, wall t and inner radius r = R − t, a cut to depth d below the top of the outer surface makes a slot at the inner surface with length
L = 2 × √(2 r x − x²), where x = d − t
Worked example: 110 mm pipe, 4 mm wall (illustrative; measure your pipe). R = 55 mm, t = 4 mm, r = 51 mm. To open a 50 mm slot, solve 50 = 2 × √(2 × 51 × x − x²), so x² − 102 x + 625 = 0 and x = (102 − √(10,404 − 2,500)) ÷ 2 = (102 − 88.9) ÷ 2 = 6.55 mm. Then d = t + x = 4 + 6.55 = 10.55 mm. The cut as seen on the outside is longer: 2 × √(2 × 55 × 10.55 − 10.55²) = 2 × √1,049 = 64.8 mm, about 65 mm. A depth stop set 10.5 mm below the top of the pipe gives the 50 mm slot of the Deep Wells guide; a stop at 10 mm gives 48 mm through the wall and 63 mm outside.
Open Area And Strength Pull In Opposite Directions
Open area. One slot of 1 mm × 50 mm is 50 mm². With rows 10 mm apart there are 100 rows per metre. At three slots a row that is 300 slots, 15,000 mm² or 150 cm² per metre of screen. The surface of a metre of 110 mm pipe is π × 0.110 × 1 = 0.346 m² = 3,460 cm², so the open area is 150 ÷ 3,460 = 4.3%.
Strength. Each row removes slots from the circumference, and the solid strips between slots carry the pipe’s load. The inner circumference is 2π × 51 = 320 mm. Three slots of 50 mm remove 150 ÷ 320 = 47% of it. This guide’s working rule, a judgement and not a tested figure, is to remove no more than about half of any row, and to rotate alternate rows by half the slot spacing (60° for three slots per row) so that the slots do not line up into a continuous weak line. Cut a test length and load it before trusting a pattern for a deep well.
Slot count and length. The screen must supply the open area needed for the yield (Screen Section): 111 cm² for 20 L/min at 0.03 m/s needs 111 ÷ 150 = 0.74 m of this pattern.
What You Need
- A fine-toothed hacksaw and a spare blade — the blade’s thickness sets the slot width
- A feeler gauge or thin shim — to check the width of the cut
- A jig — a V-block or wooden cradle to hold the pipe, a guide to keep the blade square, and a stop to limit depth
- A marked template — a strip with row positions every 10 mm and the slot positions round the pipe
- A bottle brush or stiff wire — to clear the slots
- Eye protection and gloves
Method: Mark, Clamp, Cut, Check, Clean
Step 1: Measure the pipe. Measure the outside diameter and the wall with calipers or a drilled test piece, then work out the cut depth with the formula.
Step 2: Measure the blade. A slot is as wide as the blade’s kerf, usually its thickness plus any set of the teeth. Measure a test cut with the gauge; the Deep Wells guide wants 0.5-1 mm. If the cut is wider than 1 mm, change to a thinner blade.
Step 3: Mark the rows every 10 mm along the pipe, and the slot positions round it, rotating alternate rows.
Step 4: Clamp and cut. Clamp the pipe in the jig, set the depth stop, and saw down to the stop. Keep the blade in the plane at right angles to the axis.
Step 5: Check. Slip the gauge into the first ten slots and into one from each row after that. Reject any wider than 1 mm.
Step 6: Clean. Knock out the cuttings, scrub every slot, and flush the pipe with clean water, since plastic swarf left in the slots clogs them.
Common Mistakes
- Cutting too shallow. The saw makes a groove on the outside but the slot does not go through the wall.
- Cutting too deep. The slot lengthens, the strength falls, and the saw can nick the opposite wall.
- Slots wider than 1 mm. That is the limit in the Deep Wells guide; wider slots erode the margin over the smallest pack grains (3 mm) and pass sand where the pack is thin.
- Aligned slots. A vertical stripe of slots is a crack line.
- Time not counted. Time ten slots and multiply by 300 per metre before committing to a length.
Safety
Sharp blades, plastic dust and fumes
Clamp the pipe in the jig and cut away from your hand. Wear eye protection: a snapped blade or a flying chip can blind. Plastic dust is irritating to breathe; cut outdoors. Never burn PVC offcuts, because burning PVC gives off hydrogen chloride and other toxic fumes.
What’s Next
- Learn next: Gravel Pack — the filter that goes round the finished screen
- Combine with: Casing Materials — choosing the pipe to cut