Original BOCHU protective lens D25.4 × 4 mm for 8 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. 25.4 mm is exactly one inch, which is why the figure looks unusual beside the round metric sizes - record both dimensions on the machine once and nobody has to work the size out again. Fitting the same 4 mm thickness needs no focus recalibration, because the displacement it introduces does not change. GST invoice on every order, with bulk rates for workshops, service engineers and dealers.
BOCHU Protective Lens D25.4 × 4 mm - Original Window for 8 kW Cutting Heads
The BOCHU protective lens in the D25.4 × 4 mm size is the sacrificial window below the focus lens, taking the spatter, fume and fine dust thrown back up out of the kerf so the focusing optics above it stay clean. Fused silica, anti-reflective coated on both faces for the 1064-1080 nm fiber band, passing over 99% of the beam, supplied as original BOCHU stock for 8 kW-class heads. The diameter is exactly one inch - 25.4 mm is the definition, not a rounding - which is worth knowing once and then writing down somewhere it will not be forgotten.
This is a size worth recording rather than remembering. An inch-based diameter sitting among round metric numbers is exactly the kind of figure that gets rounded off in conversation, written as "25" on an order note, and then reordered wrongly six months later by somebody who was not there the first time. None of that is a knowledge problem. It is a record-keeping problem, and it is solved permanently the first time somebody measures the window, writes both dimensions on a label stuck to the head, and puts the same two numbers on the machine's service sheet.
Changing this window does not disturb your focus calibration. That question comes up constantly and the answer is reassuring: as long as you fit the same 4 mm thickness, nothing changes. A flat window displaces the focus by roughly t(n-1)/n, which for fused silica at 1064 nm works out at about 1.24 mm here - and that displacement was already present when your focus settings were established. Same thickness, same displacement, nothing to adjust. It is only when the thickness changes that every programmed focal position moves by the difference.
The corollary matters more. If cut quality is drifting and the window is the cause, recalibrating will not fix it - and a lot of time gets lost finding that out the hard way. A contaminated window absorbs part of the beam, the glass warms, and warm fused silica behaves as a weak lens that pulls the focal position while the machine runs. Calibrate cold and the drift returns as it heats. That is a consumable problem wearing a calibration problem's clothes, and the fix is a new window rather than a new set of numbers.
How this size gets bought in India. Machines at 8 kW cover a wide span of general fabrication - sheet and medium plate, structural work, automotive and component manufacturing, and job-work lines running mixed material 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 lens, 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 25.4 mm diameter - one inch precisely - and 4 mm thickness
Focusing-side protection, supplied for 8 kW-class cutting heads
Same thickness as the window it replaces, so focus calibration is unaffected
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
25.4 mm (one inch exactly)
Thickness
4 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 1.24 mm (calculated for fused silica at 1064 nm)
Laser Power Class
8 kW-class fiber laser cutting heads
Does Changing the Protective Lens Affect Focus Calibration?
This is the question that decides whether a window change is a two-minute job or an afternoon, and the answer depends on exactly one thing: whether the thickness stayed the same. Work down this table before you touch a calibration routine:
Situation
Does Focus Change?
What to Do
Same 4 mm thickness fitted, nothing else touched
No - the displacement is identical
Nothing. Fit it, close the drawer, carry on cutting
A different thickness fitted
Yes - every programmed focal position offsets by the difference
Fit the correct thickness instead. Do not compensate in the program
Cut quality drifting through the shift, recovering when cold
Yes, but thermally - and only while the machine is warm
Replace the window. Recalibrating cold will not hold once it heats
Head dismantled or the focusing optics moved during a service
Possibly - the window is not the only variable now
Re-establish focus as part of the service rather than assuming it held
The third row is the one that costs people time. A contaminated window absorbs part of the beam, the glass warms, and warm fused silica behaves as a weak lens that pulls the focal position while the machine runs. Every symptom points at calibration - quality falls away as the shift goes on, and everything is fine again next morning - so calibration is what gets adjusted. It never holds, because the cause is sitting in the drawer. If your drift follows the machine's temperature rather than the program, change the window before changing any numbers.
Record the Size Once and It Stops Being a Question
An inch-based diameter among round metric sizes is precisely the figure that gets rounded off, mis-transcribed and reordered wrongly. The fix costs five minutes at the machine and lasts for the life of the head:
Measure the window at first fit with a vernier rather than trusting the packet, so the record starts from the part rather than from a label
Write both dimensions - 25.4 and 4 - on a durable label stuck to the head or the control panel, where the next person will see it
Put the same two numbers on the machine's service sheet, so they survive a label being cleaned off
Record the grade and the supplier alongside, so consumable behaviour stays consistent and traceable between orders
Note the seal part on the same record, because it is the item most often remembered halfway through a service
Log the date and machine hours at each change, which turns "how often does this need replacing" into a number you actually know
Never write 25 as shorthand for 25.4 anywhere in that record, since the two are different windows
What "Original" Means on This Listing
Original means genuine BOCHU stock supplied in the brand's own packaging and channel. Where that 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. It also gives you something to write on the record described above - a grade and a channel that can be reordered identically next time, rather than a loose disc of glass whose origin nobody can reconstruct.
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
There is no fixed hour count - life depends on the material, the assist gas, how often the machine pierces and the condition of the nozzle above it. Inspect at every shift change and replace on evidence, holding the window at an angle under a bright light rather than looking straight through it, because damage that is invisible face-on shows clearly as scatter when the light rakes across the surface:
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. After the nozzle, check the seal, because a flattened or perished one lets contamination past continuously rather than in bursts.
Fitting and Handling
Measure both dimensions with a vernier on receipt, because this size cannot be identified by eye against its neighbours
Hold it by the rim in lint-free finger cots or clean gloves, since skin oil on the clear aperture burns in permanently
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 so the size marking stays with the glass
Features and Benefits
Features
Original BOCHU fused-silica protective window, 25.4 mm diameter - one inch exactly - by 4 mm thick, AR coated on both faces for 1064-1080 nm with over 99% transmittance. Supplied for 8 kW-class cutting heads, in the brand's own packaging and channel.
Benefits
Takes the spatter and fume that would otherwise reach the focus lens, and holds the same 1.24 mm optical displacement as the window it replaces - so a change is a two-minute job with no recalibration. A branded channel also gives you a grade and a source you can record and reorder identically next time.
How to Order the Right Window First Time
Take out the window you are replacing and measure it, rather than relying on how it was described last time.
Measure both dimensions with a vernier - across the flat face for diameter, across the edge for thickness, at two or three points each.
Write 25.4 rather than 25 on the order. The two are different windows and the shorthand is where the error usually enters.
Match the thickness exactly, because keeping 4 mm is what lets you fit the new window without touching focus calibration.
Choose the grade deliberately. Original suits a service agreement that names genuine parts, or a workshop needing brand-marked packaging for traceability.
Check the seal while the head is open, and order in a pack sized to your consumption rate with a GST invoice so the input credit is documented properly.
Structural steelwork and general engineering shops
Automotive and auto-component manufacturing lines
Electrical enclosure, switchgear and control panel manufacturing
Job-work lines running mixed material on continuous shifts
Laser machine service engineers and spare-parts dealers supplying end 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 D25.4 × 4 mm size held as its own line, not approximated from a 25 mm or 24.9 mm window
Both grades stocked in this size, so the same machine can be supplied either way as its paperwork requires
Compatibility confirmed on WhatsApp before you order, by someone who handles fiber laser consumables daily
Protective lenses, focus and collimation lenses, nozzles and seals on one invoice, so a head service arrives in one box
Bulk supply for 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
Not if you fit the same thickness. A flat window displaces the focus by roughly t(n-1)/n, which for fused silica at 1064 nm comes to about 1.24 mm at 4 mm thickness - and that displacement was already present when your focus settings were established. Same thickness, same displacement, nothing to adjust. It is only a change of thickness that moves the focal position, and the right response to that is to fit the correct thickness rather than to compensate in the program.
Change the window first. Drift that follows the machine's temperature rather than the program is the signature of a contaminated window: the glass absorbs part of the beam, warms, and behaves as a weak lens that pulls the focal position while the machine runs. Recalibrating cold will not hold once it heats up again, so the numbers get adjusted repeatedly and the problem keeps returning. If quality falls away through the shift and is fine again next morning, that is a consumable problem wearing a calibration problem's clothes.
Possibly, because the window is no longer the only variable. If the focusing optics have been moved or the head has been apart, re-establish the focus position as part of the service rather than assuming it held - and refit the same window thickness so that at least one variable stays fixed while you do. Recording the window size and grade on the machine before the head goes back together makes the next service considerably shorter.
It means writing the decimal, every time. 25.4 mm is exactly one inch - a definition rather than an approximation - and an inch-based figure sitting among round metric sizes is the one most likely to be shortened to "25" on an order note or a stores requisition. A 25 mm window is 0.4 mm smaller and a 24.9 mm one is 0.5 mm smaller, and neither seats and seals the way this one does. Put 25.4 on the order, on the label and on the service sheet, and the shorthand never gets a chance to cause a problem.
Measure the window at first fit, then write both dimensions - 25.4 and 4 - on a durable label attached to the head or the control panel, and put the same numbers on the machine's service sheet so they survive the label being cleaned off. Add the grade, the supplier and the seal part to the same record. It takes five minutes once and removes the size question from every future order, which matters most on a machine that several different people service.
Original means genuine BOCHU stock supplied in the brand's own packaging and channel. Where it earns its place 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. It also gives you a grade and a source worth recording, so the next order can be identical rather than approximate. What decides whether any window works, though, is still the dimensions, the substrate and an intact coating on both faces.
Yes, and you do not need it. Fitment is decided by diameter and thickness, so the window you are removing identifies the part on its own - measure it with a vernier, across the flat face and across the edge, and those two numbers are the whole specification. That is more reliable than a model name in any case, because protective window sizes belong to the head that takes them and manufacturers do not share a single ladder. If the old window is broken or missing, send us a photo of the head on WhatsApp and we will work it out with you.
Both listings carry the same fitment specification, so it is a sourcing decision rather than a technical one. Buy original if a service agreement with your machine supplier names genuine parts, or if brand-marked packaging is needed for internal traceability or audit. Buy OEM if you maintain the machine yourself and check optics on receipt rather than relying on the box. Whichever you choose, record it - consistency between orders is worth more than the choice itself.
Measure it rather than adopt a general figure, because the drivers vary too much between shops for anyone else's number to be useful. Log the date and machine hours at every window change for a few weeks, keeping the job types separate if your shop swings between thin sheet and thick plate. What comes out is a rate that belongs to your machine and your work, and that is what a reorder level should be set against. Until you have it, inspect at every shift change and replace on evidence rather than on a calendar.
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 holding several near-identical sizes 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, the grade and the bulk rate together before you order.
Original BOCHU fused-silica protective lenses for 8 kW-class cutting heads, supplied to fabrication units, service engineers and dealers across India with a GST invoice on every order. Send the measured 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.
