Original BOCHU protective lens D34 × 5 mm for 15 kW-class fiber laser cutting heads. Fused silica, AR coated on both faces for the 1064-1080 nm fiber band, passing over 99% of the beam, sitting below the focus lens where it takes the spatter and fume thrown back up out of the kerf. 34 mm is an easy diameter to get wrong in a workshop that also stocks 30 mm and 37 mm windows, because all three look alike on a shelf and only a caliper separates them. The 5 mm thickness sets a focal displacement of about 1.55 mm that your machine is already calibrated around. GST invoice on every order, with bulk rates for workshops, service engineers and dealers.
BOCHU Protective Lens D34 × 5 mm - Original Window for 15 kW Cutting Heads
The BOCHU protective lens in the D34 × 5 mm size is the sacrificial window that sits below the focus lens in a fiber laser cutting head, standing in the path of everything the kerf throws back up so the focusing optics above it stay clean. It is fused silica, anti-reflective coated on both faces for the 1064-1080 nm fiber band, and passes over 99% of the beam. Supplied as original BOCHU stock for 15 kW-class heads. The part itself is straightforward; the thing worth reading carefully is the pair of numbers, because 34 mm is a diameter that gets confused with its neighbours more than any other.
Why 34 mm is the size that catches people out. Protective windows in this catalogue run 30 mm, 34 mm and 37 mm on the focusing side, and those three sit close enough together that nobody can tell them apart across a bench. Four millimetres in one direction, three in the other. A 30 mm window dropped into a 34 mm seat sits loose and the seal stops closing; a 37 mm window will not enter the seat at all. Neither mistake is subtle once it happens, and both are completely avoidable with thirty seconds and a vernier.
Both numbers are fitment dimensions, and they do different jobs. The 34 mm diameter decides whether the window seats flat and whether the holder closes properly on it - and the clear aperture has to pass the converging beam without clipping it, which is geometry rather than preference. The 5 mm thickness decides something separate: any flat window in a converging beam pushes the focus further from the lens by roughly t(n-1)/n, and with fused silica near a refractive index of 1.45 at 1064 nm that comes out at about 1.55 mm. Your machine was calibrated with that displacement present, so a different thickness moves your focus by the difference.
Sizes belong to the head that takes them. Different manufacturers do not share one size ladder, so a number remembered from another machine on the same shop floor proves nothing about this one. That matters more in India than it might elsewhere, because plenty of fabrication units run machines from two or three different builders side by side and end up with a consumable shelf holding several window sizes that all look identical in the packet. The window you are removing is the specification worth trusting - not the model name, and not the last one you ordered.
How this size gets bought in India. Machines at 15 kW run heavy plate work: structural steelwork, pressure vessel fabrication, heavy engineering and shipyard shops, and job-work lines cutting thick section on continuous shifts. As the focusing-side window this is the fastest-moving optic in the head, so it is held as standing stock in packs sized to consumption rather than ordered one at a time. Every order ships with a proper GST invoice from our Ahmedabad, Gujarat warehouse, and workshops, service engineers and dealers ordering in quantity can ask for bulk rates before placing the order.
Key Features
Original BOCHU protective window, not an unbranded near-size substitute
Fused silica (quartz) substrate, plane-parallel and polished on both faces
AR coating on both faces, centred on the 1064-1080 nm fiber laser band
Over 99% transmittance, so beam power reaches the sheet instead of heating the glass
Exact 34 mm diameter and 5 mm thickness, the fitment specification for this seat
Focusing-side protection, supplied for 15 kW-class cutting heads
Both faces coated identically, so there is no orientation to get wrong on a shift change
Bulk supply across India with GST invoice on every order
Specifications at a Glance
Diameter and thickness decide fitment, and both have to match the seat. Everything else describes how the optic behaves once it is in and the drawer is closed on it.
Parameter
Specification
Brand
BOCHU (original)
Optic Type
Flat protective window (sacrificial consumable)
Diameter
34 mm
Thickness
5 mm
Substrate
Fused silica (quartz)
Coating
Anti-reflective, both faces
Design Wavelength
1064-1080 nm
Transmittance
Over 99%
Position in Head
Focusing-side protective window, below the focus lens
Optical Displacement
Approximately 1.55 mm (calculated for fused silica at 1064 nm)
Laser Power Class
15 kW-class fiber laser cutting heads
34 mm Against Its Neighbours - Why a Caliper Beats an Opinion
These are the three focusing-side sizes most likely to be sitting on the same shelf in the same workshop, and they are close enough that eyeballing them is not a method. The numbers below are worked from the dimensions themselves, so you can check them:
Size
Clear Aperture vs 34 mm
Optical Displacement
In a 34 × 5 mm Seat
30 × 5 mm
About 22% less area
About 1.55 mm
Undersized - sits loose and the seal stops closing
34 × 5 mm - this window
Reference
About 1.55 mm
Correct fit
37 × 7 mm
About 18% more area
About 2.17 mm
Will not enter the seat, and would shift focus 0.62 mm if it did
The 30 mm case is the dangerous one, because an undersized window still drops in and the machine still runs. What it stops doing is sealing - and a seat that no longer seals draws shop-floor dust straight past the window and onto the focus lens above it, which is the exact failure the protective window exists to prevent. The 37 mm case announces itself immediately by not fitting, which is inconvenient rather than expensive. Measure before you fit, not after.
Running a Consumable Shelf When the Shop Has More Than One Machine Brand
Cross-fitting is not really a knowledge problem - most operators know the sizes differ. It is a storage problem. Windows arrive in similar packets, get decanted into a drawer, and six months later nobody can tell which loose disc came from which box. A few habits remove the failure mode entirely:
Keep one bin per machine, labelled with the head it serves and the size in millimetres, rather than one bin for "protective lenses"
Leave windows in their original packaging until the moment of use, so the size marking stays with the glass
Keep a vernier at the store rather than at the machine, so the check happens before the part is carried to the head
Write the size on the machine itself - a label on the head or the control panel outlasts any memory of the last order
Never decant part-used packs together, because two similar sizes in one bin is how a wrong fit eventually happens
Check both dimensions on receipt, since a delivery is far easier to query than a stripped head is to reassemble
Order per machine rather than per shop when the shelf holds several sizes, so quantities stay traceable to a head
What "Original" Means on This Listing
Original means genuine BOCHU stock supplied in the brand's own packaging and channel. Where that specifically earns its place is in the paperwork rather than in the optics: a machine running under a service agreement that names genuine parts, or a workshop that needs brand-marked packaging for internal traceability or audit. If either applies to your machine, this is the listing that satisfies it without a conversation.
What decides whether any protective window works in your head is narrower than the branding, and worth knowing whichever grade you buy. The diameter and thickness have to match the seat so the window sits square and the holder closes properly. The substrate has to be fused silica so it passes 1064-1080 nm without absorbing. The anti-reflective coating has to be intact and present on both faces. All three are checkable at a bench in a couple of minutes with a vernier and a bright light, and it is worth doing on receipt rather than with the head already open.
When to Replace It, and Why Consumption Sometimes Jumps
There is no fixed hour count. Life on a focusing-side window depends on the material, the assist gas, how often the machine pierces and the condition of the nozzle above it, so inspect at every shift change and replace on evidence. Hold the window at an angle under a bright light instead of looking straight through it, because damage that is invisible face-on shows clearly as scatter when the light rakes across the surface. Replace on any of these:
Pinpoint pits, burn craters or bonded spatter anywhere in the clear aperture - cleaning cannot restore a damaged coating
Haze or film that survives a wet pass with a lint-free tissue, since what remains is in the coating rather than on it
Patchy or rainbow discolouration across the face, which indicates the coating has been thermally degraded
Cut quality that degrades as the machine warms through the shift and recovers after a window change
Piercing that turns inconsistent on settings which were working, pointing to transmission loss
Any chip or crack at the edge, replaced before running rather than watched
When consumption suddenly climbs, check the nozzle before ordering more glass. A nicked, oval or off-centre nozzle tip stops clearing molten material down through the kerf and starts flicking it upward at the optics instead, and every extra shift run on a damaged nozzle eats window life. Replacing the nozzle usually returns consumption to normal on its own, which is a far better outcome than buying your way through the symptom. After the nozzle, check the seal - a flattened or perished one lets contamination past continuously rather than in bursts.
Fitting and Handling
Hold it by the rim in lint-free finger cots or clean gloves, because skin oil on the clear aperture burns in permanently
Confirm both dimensions with a vernier on receipt rather than assuming the packet is right
Inspect the new window at an angle under a bright light before fitting, not after the head is back together
Clean a fume film with a lint-free tissue wetted with isopropyl alcohol or acetone, drawn across the face once and then discarded
Never dry-wipe a gritty window, as bonded spatter dragged across the glass cuts the scratches that become the next absorption sites
Check the seal and the seating face every time the window is out, since a compromised seal lets dust reach the focus lens continuously
Seat it squarely and close the retainer firmly without forcing it against the glass
Both faces carry the same AR coating, so there is no orientation to worry about during a fast change
Store spares sealed in their original packaging, with the size marking kept alongside the glass
Features and Benefits
Features
Original BOCHU fused-silica protective window, 34 mm diameter by 5 mm thick, AR coated on both faces for 1064-1080 nm with over 99% transmittance. Supplied for 15 kW-class cutting heads, with the exact dimensions the holder and its seal are built around.
Benefits
Takes the spatter and fume that would otherwise reach the focus lens, seats correctly so the seal keeps doing its job, and holds the 1.55 mm optical displacement your machine was calibrated around instead of shifting it. Replaced on evidence, it keeps the focusing optic above it out of the maintenance budget.
How to Order the Right Window First Time
Take out the window you are replacing and measure it, or read the size off it if it is marked. The old part is the most reliable specification available.
Measure both numbers with a vernier - across the flat face for diameter, across the edge for thickness, at two or three points each.
Do not order from memory if the shop runs more than one machine brand, because 30 mm, 34 mm and 37 mm windows are indistinguishable by eye.
Choose the grade deliberately. Original suits a machine under a service agreement naming genuine parts, or where brand-marked packaging is needed for traceability.
Check the seal while the head is open and order a replacement with the window if it is flattened or perished.
Order in a pack sized to your consumption rate, ask for bulk rates if you run several machines, and request a GST invoice so the input credit is documented properly.
Heavy plate fabrication, structural steelwork and pressure-vessel manufacturing
Shipbuilding, railway and heavy engineering fabrication shops
High-throughput job-work lines cutting thick section on continuous shifts
Steel processing units and coil-to-plate cutting lines
Workshops running machines from several builders and stocking multiple window sizes
Laser machine service engineers and spare-parts dealers supplying high-power users
Why Buy This Window From SparesZone
Original BOCHU optics - not unbranded glass cut to a similar size and sold under a borrowed name
The exact D34 × 5 mm size, stocked as its own line rather than approximated from a neighbouring one
Several head brands supplied from one place, so a mixed-machine workshop orders from a single supplier
Compatibility confirmed on WhatsApp before you order, by someone who handles fiber laser consumables daily
Protective windows, lenses, nozzles and seals on one invoice, so a head service arrives in one box
Bulk supply for heavy fabrication units, service engineers and spare-parts dealers
GST invoice on every order - GSTIN 24AFSFS3212F1Z2, credit-ready documentation with each dispatch
Dispatched from Ahmedabad, Gujarat to workshops, OEMs and service engineers across India
Frequently Asked Questions
It is the sacrificial window below the focus lens in a fiber laser cutting head, taking the spatter, fume and fine dust thrown back up out of the kerf so the focusing optics above it stay clean. It is the only optic in the head designed to be consumed, and that trade is deliberate: a routine consumable is replaced regularly so a high-value focus lens is not replaced at all. It also seats against a seal, so a correctly sized window is part of keeping contamination out of the head as well as off the glass.
Confirm it against the window you are removing rather than against a head model, because protective window sizes are specific to the head that takes them and manufacturers do not share a single ladder. Take the old window out, read the size off it if it is marked, and measure it with a vernier if it is not - across the flat face for diameter and across the edge for thickness. If the old window is already broken or missing, send us a photo of your head label on WhatsApp and we will confirm the size before you order.
No, and the two errors behave differently. A 37 mm window simply will not enter a 34 mm seat, so it announces itself straight away. A 30 mm window is the dangerous one: it still drops in and the machine still runs, but it sits loose and the seal stops closing, which draws shop-floor dust past the window and onto the focus lens above - exactly the failure the protective window exists to prevent. Thickness matters too, since a 7 mm window displaces the focus about 0.62 mm further than a 5 mm one.
Because the diameter is not a fit-anywhere measurement - it is what makes the window seat flat against its seat and lets the holder close on the seal. Undersize it and the seal has nothing to close against. Oversize it and the window will not enter at all, or loads against the retainer as the drawer shuts. In clear-aperture terms a 30 mm window gives about 22% less area than a 34 mm one and a 37 mm gives about 18% more, so these are not small differences even though they look like small numbers.
Treat it as a storage problem rather than a knowledge problem, because most operators already know the sizes differ - they just cannot tell two loose discs apart in a drawer. Keep one labelled bin per machine rather than one bin for protective lenses, leave windows in their original packaging until they are fitted, write the size on the head or the control panel, and keep a vernier at the store so the check happens before the part is carried to the machine. Never decant part-used packs together.
Original means genuine BOCHU stock in the brand's own packaging and channel, and where it specifically matters is in the paperwork - a machine under a service agreement that names genuine parts, or a workshop needing brand-marked packaging for traceability or audit. Where it matters less is in the physics: what decides whether a protective window works is that the dimensions match the seat, the substrate is fused silica, and the coating is intact on both faces. Check those on receipt whichever grade you buy.
It already has, and that is fine. A flat window displaces the focus by roughly t(n-1)/n, which for fused silica at 1064 nm gives about 1.55 mm at 5 mm thickness - and your focus settings were established with that displacement present. So as long as you keep fitting 5 mm, nothing changes and there is nothing to adjust. The moment the thickness changes, every programmed focal position is offset by the difference, which is why "any window that fits" is a more expensive idea than it sounds.
There is no fixed interval - life depends on the material, the assist gas, pierce frequency and nozzle condition, so inspect at every shift change and replace on evidence. Replace on pitting, burn craters or bonded spatter, on haze that survives a wet clean, on patchy coating discolouration, on any edge chip, or when cut quality drifts through the shift and recovers after a change. At this power the margin for running one shift too long is small, because damage in a coating feeds itself once it starts.
Check the nozzle first. When window life collapses, the window is the symptom rather than the cause: a nicked, oval or off-centre nozzle tip stops clearing molten material down through the kerf and starts flicking it upward at the optics instead. Every extra shift run on a damaged nozzle eats window life, and replacing the nozzle usually returns consumption to normal on its own. After the nozzle, check the seal - a perished one lets contamination past continuously instead of in bursts.
Yes. Protective windows are a routine consumable, so they are supplied in packs and bulk quantities to fabrication units, service engineers and spare-parts dealers, with a GST invoice on every order under GSTIN 24AFSFS3212F1Z2. Workshops running several machine brands usually order per machine so quantities stay traceable to a head, and add nozzles and seals to the same invoice. Send your head details and quantity on WhatsApp and we will confirm the size and the bulk rate together before you order.
Original BOCHU fused-silica protective windows for 15 kW-class cutting heads, supplied to heavy fabrication units, service engineers and dealers across India with a GST invoice on every order. Send the size off your old window, or a photo of your head label, and we will confirm the right part before you buy - and put the nozzles and seals on the same invoice if the head is due a service.
