How to Fit a Fiber Laser Protective Window: Seal, Seating and First Cut

To fit a fiber laser protective window, switch the laser off, move the head to a clean spot, pull the window holder, cover the open slot, clean the holder and seal, then drop the new window in with gloved fingers on its edge only. Refit the seal and pressing ring, push the holder home until it reaches its stop and lock it. Before cutting production parts, check the window under a torch once more and run a test cut at settings you know. Most new windows that fail in the first hours fail for one of three reasons: dirt or a fingerprint was sealed in with them, the holder was not seated, or the wrong window went in.

This guide is for anyone who has just received a window, such as the Bystronic 10086368 D34 x 3 mm window, and wants it to last. If you are still deciding whether the old window needs changing, start with our guide on when to replace a fiber laser protective window.

Before you open the head: check it is the right window

Lay the new window next to the old one before you take anything apart. Compare the part number on the packaging, the diameter and the thickness. We list 54 protective windows, and several sizes come in more than one version that looks identical on the bench.

Bystronic's D34 x 3 mm window is a good example. The fused silica version is sold as 10086368 and also as 10071591. Bystronic sells a sapphire window in the same 34 x 3 mm size, 10145862, for higher power heads. Fitting a fused silica window where the head calls for sapphire puts the wrong material in the beam path, and nothing in the holder stops you doing it. If your old window came out of a 10145862 slot, order that number. The same applies to the power rated windows from BOCI, Raytools and Precitec, where one size is sold in several power ratings.

Bystronic also uses a D36 x 5 mm window, 10045746, listed as Glass ARQ1064.50.36. It is the lower window that faces the cut, and the same glass is used in II-VI HIGHYAG BIMO heads.

Set up a clean place to work

Precitec's optics manual says any work on the beam path optics belongs in a clean room or a laminar flow box, and that the beam path should be open for as short a time as possible. Most shops do not have a flow box beside the machine. You can still get close:

• Stop grinding, blowing down and sweeping nearby before you start.
• Have everything ready: new window still in its case, gloves or finger cots, lens tissue or swabs, ethanol of at least 99.5 % or isopropanol, canned air, and protective film or tape to cover the open slot.
• Wear solvent resistant, powder free gloves. Precitec warns that dust, fingerprints and firmly stuck dirt can leave burn marks on optics or destroy them after short operating times.

Two things that seem harmless do real damage. Do not blow on the window or the holder: Precitec states that saliva droplets cause burn marks. Do not use the shop air line either. Precitec says standard industrial compressed air must not be used on optics, because oil residue and dust cause burn marks. Use canned air free of oil, held upright, from at least 15 cm away.

Fitting the window, step by step

Heads differ in detail, so follow your head's manual where it differs. The sequence below follows the Raytools BM115 manual and the Precitec optics manual, and it matches the drawer or cartridge holders used on most fiber cutting heads.

• Switch off and lock out. The laser must be off. Let the head cool and wipe the outside of it with a damp cloth so dust on the body does not fall into the slot.
• Pull the holder. On a Raytools drawer holder, open the buckle and pull the holder out by pinching its two edges. On a Precitec head the window sits in a cartridge or in the sensor insert.
• Cover the slot at once. Put protective film or clean tape over the opening in the head. The focusing lens is directly above it.
• Remove the old window. Take off the pressing ring, lift the window out and set it aside. Never turn a dirty window over and put it back in. Precitec is explicit: dirt that comes off it will burn the lens above.
• Clean the holder and seal. Blow out loose particles with canned air, then wipe the seat and the pressing ring with a swab dampened in ethanol. A dry swab scratches.
• Inspect the seal ring. Replace it if it is flattened, nicked, cracked or burned. Raytools warns against pulling the seal ring by its edge because it tears easily.
• Fit the new window. Hold it by the edge only. On Raytools heads either face can go down.
• Refit the seal and pressing ring. Seat the ring evenly. Do not force it.
• Refit the holder. Remove the film, slide the holder in until it reaches its stop and close the buckle or tighten the nut as the head requires.

The seal ring and why the holder must be fully home

The window holder is part of the pressure seal of the head. The window is cooled by the cutting gas that flows around it, and the seal stops that gas and fume getting past it. Precitec's manual names what happens if the window frame is not screwed in firmly to its stop: the window has free play, the cutting gas pressure makes it vibrate, and the focal position is wrong, which gives bad cuts.

The opposite mistake cracks windows. Precitec warns not to press the fitting tool too hard when seating the O-ring, or the window can break. Let the holder's stop set the clamping force.

Check the seal every time the holder is out. A flattened ring no longer seals, and a burned one shows that fume has been reaching it. Keep a few spare rings with your windows. For Raytools heads we stock the 32.2 x 24 x 3.55 mm ring for 27.9 x 4.1 mm windows and the 42.5 x 4 x 3.2 mm ring for 37 x 7 mm windows in our laser seal range, plus the rings for five handheld welding guns.

One Precitec detail catches people out: Precitec now supplies its protective windows without O-rings. If you have relied on a new O-ring arriving with every window, order the rings separately or order Precitec's O-ring versions (P0595-132986 for the ProCutter 2.0 8 and 20 kW glass, P0595-132985 for the D37 x 7 mm 30 kW glass), and fit a new ring when the old one shows wear.

First start checks

A new window that was fitted well should give the same cut as the last good window. Before you put production parts on the table:

• Look once more. Pull the holder again and check the window under a torch at a low angle against a dark background. A fingerprint or a speck you missed is easier to remove now than after it has burned in.
• Check for alarms. Many heads monitor the protective window or the head pressure. Clear any window or pressure alarm before cutting rather than resetting past it.
• Run a test cut. Cut a part you know, in material you know, at your normal settings. Compare the edge, dross and pierce time with what the machine did before the window started to degrade.
• Recheck after the first hour. A new burn spot this early points to a cause that will keep killing windows.

If the test cut is worse than before, do not raise the power to compensate. Pull the window and check it, then check that the holder is fully seated and the seal is in place.

Mistakes that shorten the life of a new window

When a new window burns or cracks within days, the window is rarely the cause. Look for these first:

• Dirt sealed in at fitting. A fingerprint, a hair or a dust speck on the window or the holder seat absorbs the beam and burns in.
• A holder that was not fully home. The window vibrates under gas pressure and the focus position shifts. Seat the holder to its stop every time.
• A worn or missing seal ring. Fume gets past the window to the lens above and the head loses gas pressure.
• Oil or water in the assist gas. Precitec lists contaminated gas from the compressor as a cause of lens damage, and oil residue on optics as a cause of hot spots. If you cut with compressed air, the air filtration in front of the laser matters as much as the window. Our guide to fitting compressed air line filter elements covers the order and the grades, and the elements are in our in-line filter range.
• The wrong window. A fused silica window in a sapphire slot, or a lower power rating than the source, fails early however carefully it was fitted.
• Wrong cleaning materials. Precitec warns against spectacle wipes, because anti fog residue damages coatings, and against camera and contact lens cleaners, which can contain water.

Keeping spares ready

Keep spare windows in their cases in a clean, closed box with a few seal rings. Our Bystronic D34 x 3 mm window is sold in packs of 5 at see current price and packs of 10 at see current price. The full list of sizes for Raytools, Precitec, Trumpf, BOCI, Bystronic and Mazak heads is in our protective window range. If you cannot find your part number, send it through our quote request page and we will match it.

Frequently asked questions

Which way up does a protective window go?
A flat protective window is coated on both faces, and the Raytools BM115 manual states it can be fitted either way up. Follow your own head's manual if it marks a direction.

Can I blow the holder clean with the shop air line?
No. Precitec says standard industrial compressed air must not be used on optics because oil and dust in it cause burn marks. Use oil free canned air from at least 15 cm away.

Why did my new window crack in the holder?
The usual causes are too much force when seating the seal or pressing ring, a twisted or wrong size seal, or dirt under the window. Let the holder's stop set the clamping force and clean the seat before fitting.

Is the Bystronic 10071591 the same window as 10086368?
Yes. Both numbers cover the same D34 x 3 mm fused silica window. Bystronic's sapphire window in that size is a different part, 10145862.