One of the strengths of the Tormek system is that you aren't limited to a single grinding wheel. Depending on what you sharpen and the kind of edge you're trying to produce, you can choose from three traditional grindstones and three diamond wheels.

That flexibility is useful, but it also raises an obvious question: which Tormek grinding wheel should you actually buy?

For most people, the answer is simpler than the six-wheel lineup makes it appear. You don't need every wheel, and you may not need to replace the standard Original Grindstone at all.

If you want the short version:

• Original Grindstone: the best general-purpose choice for most Tormek owners.

• SB-250 Blackstone Silicon: a specialist wheel for HSS and other harder, high-alloy steels.

• Japanese Waterstone: a very fine finishing wheel for exceptionally polished bevels.

• Diamond Wheel Coarse: best for repair, reshaping and heavier steel removal.

• Diamond Wheel Fine: the best all-around choice if you want a diamond wheel.

• Diamond Wheel Extra Fine: a fine diamond finishing wheel for users who already have a coarser sharpening stage.

The right choice depends less on which wheel is "best" and more on what you sharpen, how much material you need to remove, and what kind of finish you want.

Tormek Grinding Wheels at a Glance

First Decision: Traditional Grindstone or Diamond?

Before comparing individual wheels, it helps to divide the Tormek lineup into two groups.

Traditional Tormek Grindstones

The Original Grindstone, SB-250 Blackstone Silicon and Japanese Waterstone are bonded abrasive stones. As you sharpen, the stone gradually wears and its diameter gets smaller.

That isn't necessarily a drawback. They can be refreshed with the SP-650 Stone Grader, while the TT-50 Truing Tool will restore a wheel that has become uneven.

The biggest advantage of the Original Grindstone and Blackstone Silicon is that their sharpening characteristics aren't fixed. By changing the surface with the Stone Grader, you can make the same wheel cut more aggressively or leave a finer surface.

That makes the Original Grindstone in particular remarkably versatile.

Tormek Diamond Wheels

The Diamond Wheel Coarse, Diamond Wheel Fine and Diamond Wheel Extra Fine use diamond abrasive bonded to a steel wheel.

Unlike a traditional grindstone, they maintain their diameter and stay flat throughout their working life. That has several practical advantages. Jig settings remain consistent between diamond wheels, the wheels don't require grading or truing, and you can sharpen on the side of the wheel when using the Multi Base to create a flat bevel rather than the slight hollow grind produced by the circumference of a wheel.

Diamond also makes these wheels attractive for harder steels, high-carbide steels and other materials that can be more challenging for conventional aluminum-oxide stones.

There are tradeoffs. Diamond wheels cost more than a conventional grindstone, and technique is different. Tormek specifically recommends grinding with low pressure because excessive pressure can damage the diamond coating. When the wheels are used wet, Tormek also recommends adding ACC-150 Anti-Corrosion Concentrate to the water.

For many buyers, the real choice is therefore:

Do I want the versatility and value of the Original Grindstone, or the consistency and material advantages of a diamond wheel?

Original Grindstone: The Best General-Purpose Choice

The Original Grindstone is the standard wheel supplied with the Tormek T-8 Original and Tormek T-4. There is a good reason for that.

It is an aluminum-oxide grindstone designed to balance efficient sharpening with a relatively fine finish. Using the SP-650 Stone Grader, its sharpening characteristics can be altered from approximately 220 grit to as fine as approximately 1000 grit.

That makes one wheel capable of doing several jobs.

Grade it coarse when you need more material removal. Grade it fine when you are approaching the finished edge. For everyday knives, chisels, plane irons and many other common tools, that's often all the grinding wheel you need.

This versatility also shows up in feedback we get from our customers. Long-term owners repeatedly describe the Original Grindstone as a capable all-around wheel, and we generally recommend that beginners spend time learning how to use the Original Grindstone and Stone Grader properly before assuming they need to upgrade.

Where the Original Grindstone works best

The Original Grindstone is particularly well suited to:

• kitchen and pocket knives made from conventional steels
• chisels
• plane irons
• carving tools
• general woodworking tools
• users who sharpen a wide variety of tools and want one wheel to cover most of them

The limitation of the Original Grindstone

The Original Grindstone isn't equally efficient on every steel.

Sharpening HSS or modern high-alloy steels can result in slower cutting or more frequent need to refresh the stone surface. That's exactly the kind of sharpening that led Tormek to develop the Blackstone Silicon and, later, its diamond wheels.

The other tradeoff is maintenance. The wheel wears, its diameter changes, and periodically it needs grading or truing.

Those disadvantages are real, but they should be kept in perspective. For someone sharpening ordinary knives and woodworking tools, they don't necessarily justify replacing an exceptionally versatile wheel.

Who should buy the Original Grindstone?

Most Tormek users.

If you don't have a specific problem you are trying to solve, the Original Grindstone is usually where we'd start.

If your T-8 or T-4 already came with one, we would recommend learning what it can do before buying a specialty wheel.

SB-250 Blackstone Silicon: For HSS and Harder Steels

The Blackstone Silicon uses silicon carbide and was specifically developed for HSS and other high-alloy steels. Tormek also highlights its usefulness for tools with large sharpening surfaces or situations where grinding pressure is relatively low, including planer blades and certain woodturning tools.

Where the SB-250 works best

The Blackstone makes the most sense for:

• HSS turning tools
• harder or highly alloyed steels
• planer blades
• tools with large bevel surfaces
• users who routinely find the Original Grindstone too slow on the steels they sharpen

Woodturners are one of the clearest audiences for this wheel because HSS turning tools combine hard steel with relatively large bevels.

Is the SB-250 Blackstone Silicon simply a better Original Grindstone?

No, the Blackstone is a specialist wheel, not a universal upgrade.

We've found that some Tormek users are enthusiastic about the Blackstone Silicon on HSS, while others continue to prefer the Original Grindstone for ordinary steels or would now choose the Diamond Fine wheel instead of buying a SB-250 Blackstone Silicon.

If the Original Grindstone already handles everything you sharpen efficiently, you shouldn't expect the Blackstone Silicon to transform your sharpening simply because it is a different wheel.

Who should buy the SB-250?

Someone who routinely sharpens HSS or other difficult steels and wants to remain with a traditional Tormek grindstone.

If you are considering a Blackstone Silicon as an upgrade to your Original Grindstone simply because you want "something better," we'd recommend looking at the Diamond Wheel Fine instead.

Japanese Waterstone: For an Exceptionally Fine Finish

The Japanese Waterstone is very different from both the Original Grindstone and Blackstone Silicon.

It is a 4000-grit finishing stone intended to produce an extremely smooth, highly polished bevel. Tormek specifically recommends it for tools such as knives, carving tools, scissors, plane irons and chisels where minimal material needs to be removed and a particularly refined finish is desired.

The Japanese Waterstone isn't intended for shaping, repairing damage or establishing a bevel. Tormek recommends doing that work on another wheel and using the Japanese Waterstone only for final refinement.

What makes the Japanese Waterstone special?

Its finish.

Among experienced Tormek users, this is probably the least controversial characteristic of any wheel in the lineup. Owners regularly praise the Japanese Waterstone for its ability to produce a highly polished, mirror-like finish.

That can matter for carving tools and woodworking edges used for very clean cuts. It can also matter aesthetically if a polished bevel is something you value.

But it isn't necessary for achieving a genuinely sharp tool.

A properly sharpened edge from the other wheels, followed by appropriate honing, can also be extremely sharp.

Where the Japanese Waterstone works best

Consider the Japanese Waterstone if:

• you sharpen carving tools
• you want highly polished bevels
• you sharpen premium knives where appearance matters
• you want a very fine final sharpening step
• you enjoy taking an edge considerably beyond what is necessary for ordinary working sharpness

What to know before buying one

The Japanese Waterstone is intentionally fine and therefore removes very little steel.

It also deserves some care. Tormek recommends cleaning its surface periodically with the fine side of the Stone Grader or a sponge, and specifically warns against using the coarse side of the Stone Grader on the Japanese Waterstone.

The Japanese Waterstone is much softer than the Tormek's other grindstones, therefore its recommended to sharpen edge trailing, with the Japanese Waterstone spinning away from the edge you're sharpening. 

And if you are coming from a much coarser wheel, remember that the Japanese Waterstone still needs to remove the scratch pattern left by the previous abrasive. Experienced users sometimes find very large grit jumps inefficient.

Who should buy the SJ-250?

Someone who already has the basic sharpening covered and specifically wants an exceptionally fine, polished finish.

It is a luxury wheel for many sharpeners, but a very useful specialist wheel for the right person.

Tormek Diamond Wheels

The three diamond wheels share the same construction and advantages but perform distinctly different jobs.

The easiest way to understand them is as a progression:

Diamond Coarse = heavy work
Diamond Fine = general sharpening
Diamond Extra Fine = fine finishing

You don't necessarily need the entire progression.

Diamond Wheel Coarse (DC): For Repair and Reshaping

The Diamond Wheel Coarse is a 360-grit wheel and the coarsest of Tormek's three diamond options.

Tormek designed it for rapid repair of dull or damaged edges, efficient steel removal and tool shaping. Tormek also recommends following it with a finer wheel when a smoother surface is required.

Where the Diamond Coarse works best

The Diamond Wheel Coarse earns its keep when you regularly:

• remove chips
• repair heavily damaged edges
• establish completely new bevels
• change bevel angles substantially
• reshape turning tools
• sharpen hard steels while removing a relatively large amount of material

Experienced owners similarly tend to describe the DC primarily as a reshaping wheel rather than an everyday sharpening wheel.

Is the Diamond Wheel Coarse extremely fast?

"Coarse" needs some context.

The Diamond Coarse is much more appropriate for major steel removal than the Fine or Extra Fine, but a 360-grit wheel running at Tormek's slow speed is still a controlled sharpening tool rather than an industrial grinder.

If you are doing major reshaping that substantially changes the edge geometry, that still take time.

That's not necessarily a disadvantage. The Tormek system's great strength is controlled, cool grinding. But someone expecting the Diamond Coarse to behave like an 80-grit high-speed bench grinder may be disappointed.

Who should buy the Diamond Wheel Coarse?

Someone who frequently repairs damaged tools or does significant reprofiling.

For ordinary resharpening, we'd choose the Diamond Fine first.

Diamond Wheel Fine (DF): The Best All-Around Diamond Wheel

If you want one Tormek diamond wheel, the Fine is where we would start.

At 600 grit, it sits in the useful middle ground between heavy material removal and fine finishing. Tormek describes it as its all-around diamond wheel because it combines efficient sharpening with a comparatively smooth surface finish.

Customers often arrive at much the same conclusion. Experienced users frequently identify the Diamond Wheel Fine as the most versatile of the three diamond wheels, and its our go-to recommendation when someone asks which diamond wheel they should buy first.

Where the Diamond Wheel Fine works best

The DF is a strong choice for:

• general knife sharpening
• woodturning tools
• hard and high-alloy steels
• carbide tools
• repeat sharpening where maintaining the same wheel diameter is useful
• users who want diamond but don't want to own three different wheels

Its 600-grit surface is fine enough to produce a very good working edge yet coarse enough to handle ordinary resharpening efficiently.

Who should buy the Diamond Wheel Fine?

Someone who wants the advantages of diamond in a single, versatile wheel.

For many buyers, the most meaningful Tormek wheel decision isn't between six products.

It's the Original Grindstone versus Diamond Wheel Fine.

We'll come back to that comparison shortly.

Diamond Wheel Extra Fine (DE): For Fine Diamond Finishing

The Diamond Wheel Extra Fine is Tormek's finest diamond wheel at 1200 grit.

It is intended for minimal steel removal and a smoother finish, making it particularly suitable for knives, carving tools and other edges that are already in good condition.

Think of it primarily as a finishing wheel.

If a tool is chipped, extremely dull or needs its bevel changed, the Extra Fine isn't the wheel you want to start with.

Where the Diamond Wheel Extra Fine works best

The Extra Fine makes sense when:

• you already sharpen with a Diamond Wheel Coarse or Fine
• you want a finer finish before honing
• you sharpen hard or carbide-rich steels and want to keep using diamond through the finer stages
• very little metal needs to be removed

Some experienced Tormek users use the Diamond Wheel Extra Fine after the Fine when they want a smoother final surface, while others find a carefully finished edge directly from the Diamond Wheel Fine sufficient for much of their sharpening.

That illustrates the Extra Fine's position quite well.

It's useful, but it isn't automatically necessary just because you own the DF.

Who should buy the Diamond Wheel Extra Fine?

Someone who already has an effective general purpose sharpening wheel and wants a finer diamond finishing stage.

Do You Need All Three Tormek Diamond Wheels?

Probably not.

The three diamond wheels form a logical sharpening progression, but that doesn't mean every Tormek owner needs to buy the complete set.

Think about the type of work you actually do.

Diamond Wheel Fine Only

This is the setup we'd recommend to many people who simply want to switch to diamond.

The Fine can handle ordinary resharpening while leaving a good working finish.

Best for: general sharpening.

Diamond Wheel Coarse + Fine

This pairing covers major material removal followed by normal sharpening.

Use the Coarse to repair damage or establish geometry and move to the Fine for the finished edge.

Best for: people who frequently repair, reshape or reprofile tools.

Diamond Wheel Fine + Extra Fine

This is a better pairing for someone who usually starts with an edge that is already reasonably close to its intended geometry.

The Fine performs the sharpening and the Extra Fine refines it further.

Best for: maintaining tools and knives where finer edge quality matters more than major repairs.

Diamond Wheel Coarse + Fine + Extra Fine

The complete set lets you move through coarse, medium and fine diamond sharpening while retaining the advantages of constant wheel diameter.

It's an excellent system.

It's also more than most casual Tormek users usually need.

Best for: high-volume sharpeners, specialists and enthusiasts who routinely need the full range.

Original Grindstone vs. Diamond Wheel Fine: The Most Important Choice

If someone asks us which single grinding wheel they should use on their Tormek, this is the comparison we'd spend the most time discussing.

Both can serve as a main everyday sharpening wheel.

The difference is in what you prioritize.

Choose the Original Grindstone if you value:

Versatility.
The Stone Grader gives one wheel a wide range of sharpening characteristics.

Value.
One relatively affordable wheel that can cover coarse sharpening through fine sharpening.

Learning.
Traditional stone wheels are generally more tolerant of pressure and beginner technique than diamond wheels.

Common tool steels.
For knives, chisels and plane irons made from conventional steels, the Original Grindstone is extremely capable.

Choose the Diamond Wheel Fine if you value:

Constant diameter.
The wheel doesn't become smaller as it wears.

No grading or truing.
There is less wheel maintenance.

Hard-material capability.
Diamond is especially attractive for hard steels, carbide and ceramics.

Repeatability.
Fixed wheel geometry simplifies repeated setups.

Flat bevel capability.
With the Multi Base, the side of the diamond wheel can be used to produce a flat bevel.

Neither is universally better.

For someone new to Tormek, sharpening ordinary knives and woodworking tools, we'd lean toward the Original Grindstone.

For someone deliberately choosing diamond, especially for hard steels or highly repeatable sharpening, we'd lean toward the Diamond Wheel Fine.

SB-250 Blackstone Silicon vs. Diamond Wheels

This is a common question because the SB-250 Blackstone Silicon and Tormek's diamond wheels overlap in one important area: both are good choices when you regularly sharpen HSS, harder steels, and other materials that can be slower going on the Original Grindstone.

The main difference is not simply which one cuts harder steel better. It is how you want your sharpening system to work.

Choose the SB-250 if:

• you primarily sharpen HSS turning tools or other hard steels
• you prefer the feel and behavior of a traditional grindstone
• you want a wheel that can be graded to change its cutting characteristics
• you do not need the fixed diameter or side-grinding capability of diamond
• you want a lower-cost specialist wheel for harder steels

The SB-250 is especially well suited to woodturners and others who sharpen a lot of HSS. It retains the familiar Tormek grindstone experience, including the ability to refresh and alter the surface with the Stone Grader.

Choose a diamond wheel if:

• you want a wheel that maintains a constant diameter
• you would rather avoid grading and truing
• you sharpen a wider range of hard steels, carbide, or other difficult materials
• you want the option to sharpen on the flat side of the wheel
• you value repeatable setups and easy movement between different grits

For most general sharpening, the DF-250 Fine is the diamond wheel we would consider first. If you regularly reshape tools or remove significant damage, the DC-250 Coarse may also be useful.

Diamond wheels do require a somewhat different technique. They work best with light pressure, and when used wet Tormek recommends adding ACC-150 Anti-Corrosion Concentrate to the water.

Which is better for HSS?

Both can be very good choices.

If HSS turning tools are the main reason you are looking beyond the Original Grindstone, the SB-250 remains a very sensible option. It was developed specifically with harder, highly alloyed steels in mind and gives you that capability without requiring a move to a more expensive diamond system.

Diamond becomes more attractive when you want benefits beyond simply sharpening HSS: fixed wheel diameter, no grading or truing, side grinding, and the ability to choose between coarse, fine, and extra-fine diamond surfaces.

So we would not describe the diamond wheels as a straightforward replacement for the Blackstone.

Original Grindstone vs. Blackstone Silicon: Which Traditional Grindstone?

This comparison becomes much easier if you stop thinking in terms of grit.

Both wheels have adjustable sharpening characteristics ranging from approximately 220 through 1000 grit.

The real question is steel type.

Choose the Original Grindstone if:

• you sharpen a broad variety of ordinary knives and tools
• versatility matters most
• you want the standard general-purpose Tormek experience
• the SG already cuts the steels you use effectively

Choose the Blackstone Silicon if:

• you sharpen HSS frequently
• you work with harder, highly alloyed steels
• you sharpen lots of turning tools or planer blades
• the Original Grindstone routinely feels inefficient on your tools

For most people, the Original is the default and Blackstone is the specialist.

Diamond Wheel Extra Fine vs. Japanese Waterstone: Two Different Kinds of Fine

This comparison is easy to misunderstand because both wheels occupy the fine end of Tormek's lineup.

But they aren't interchangeable.

The Diamond Wheel Extra Fine is a 1200-grit diamond sharpening wheel.

The Japanese Waterstone is a 4000-grit traditional waterstone designed specifically for polishing.

Choose the Diamond Wheel Extra Fine if:

• you want a refined working edge
• you're already using diamond
• you sharpen high-alloy or carbide-rich steels
• consistency and fixed wheel diameter matter

Choose the Japanese Waterstone if:

• you specifically want a highly polished bevel
• you're sharpening carving tools or fine woodworking edges
• appearance matters
• you're willing to add a dedicated polishing stage
  

Choose both if:

• you want a complete fine-finishing progression
• you regularly sharpen tools where both edge refinement and bevel polish matter
• you want the Diamond Wheel Extra Fine to remove the coarser scratch pattern before moving to the Japanese Waterstone
• you sharpen enough premium knives, carving tools, chisels, or plane irons to justify two dedicated finishing wheels

Used together, the Diamond Extra Fine and Japanese Waterstone complement rather than duplicate each other. The Diamond Extra Fine gives you a refined 1200-grit diamond finish, while the Japanese Waterstone takes the bevel considerably further toward a highly polished surface.

Which Tormek Wheel Is Best for What You Sharpen?

For Your First Tormek

Original Grindstone

Learn on the Original Grindstone before assuming you need anything more specialized.

For General Knife Sharpening

Original Grindstone or Diamond Wheel Fine

Choose the Original for versatility and value. Choose Diamond Wheel Fine if you specifically want the advantages of diamond.

For Chisels and Plane Irons

Original Grindstone for most work.

Consider the Diamond Wheel Extra Fine or Japanese Waterstone as additional finishing wheels when a particularly refined surface is useful.

For Woodturning Tools

Blackstone Silicon or Diamond Wheel Fine

HSS makes the SB especially relevant, while the DF provides the advantages of a diamond abrasive and is widely liked by Tormek users for turning tools.

For Major Edge Repair

Diamond Wheel Coarse

That's the job for which the coarse diamond wheel was designed.

For Hard and High-Alloy Steels

Diamond Wheel Fine, with the Blackstone Silicon also worth considering.

Which one makes more sense depends partly on whether you prefer a traditional grindstone or diamond wheel.

For Carbide

Diamond wheels.

Tormek rates the diamond wheels for carbide as well as steel and ceramics.

For a Mirror-Polished Bevel

Japanese Waterstone

This is the specialty that distinguishes it from the rest of the lineup.

Still Not Sure Which Tormek Grinding Wheel Should You Buy?

If you're still unsure, feel free to contact one of our Sharpening Specialists. We have experience using all of Tormek's Grinding Wheels and can help match your specific needs to the best wheel for you.

Don't worry about being confused. There is no single Tormek grinding wheel that is best at everything. That's precisely why Tormek makes several of them.

Frequently Asked Questions About Tormek Grinding Wheels

Can I skip grits when moving between Tormek wheels?

Yes, but very large jumps can make the finer wheel work unnecessarily hard.

Each finer abrasive has to remove the scratches left by the previous one. Moving from the Diamond Wheel Coarse at 360 grit directly to the Japanese Waterstone at 4000 grit, for example, asks the very fine Japanese Waterstone to remove relatively deep scratches. It can eventually do that, but it is not an efficient progression.

You do not need to use every available Tormek wheel, but a sensible progression should avoid asking an extremely fine finishing wheel to perform substantial material removal.

Does the Japanese Waterstone make an edge sharper, or just more polished?

Potentially both, but the difference is not as simple as "higher grit equals sharper."

A properly sharpened edge from the other wheels can already be extremely sharp. The Japanese Waterstone further refines the bevel and removes the coarser scratch pattern left by earlier sharpening stages.

For some tools, that additional refinement can improve cutting performance. For others, the biggest difference may be the smoothness and appearance of the bevel.

Do I still need to hone after using the Japanese Waterstone?

Usually, yes.

The Japanese Waterstone refines and polishes the bevel, while the honing wheel deals with the burr. Those are related but different jobs.

Can Tormek diamond wheels be used dry?

Yes. Tormek diamond wheels can be used dry or with water.

Wet grinding has several advantages, including keeping the sharpening process clean and carrying away removed steel. When the diamond wheels are used with water, Tormek recommends adding ACC-150 Anti-Corrosion Concentrate to protect the steel wheel from corrosion.

Do I really need ACC-150 with the diamond wheels?

If you use a Tormek diamond wheel with water, Tormek recommends it.

Unlike the other grindstones, Tormek's diamond wheels have a steel core. Water left in contact with that steel creates a corrosion risk. ACC-150 is mixed into the sharpening water to help protect the wheel.

That additional consumable is worth considering when comparing the ongoing cost and convenience of a diamond wheel with a conventional grindstone.

Why does a new diamond wheel seem to become finer after I use it?

A new diamond wheel typically feels more aggressive during its initial break-in period. Some of the most prominent diamond particles are gradually leveled during the first sharpening sessions, after which the wheel settles into its normal cutting characteristics.

Because of this, Tormek recommends particularly light pressure with a new diamond wheel. Pressing harder does not speed up the break-in process in a helpful way and can damage the diamond coating.

Can I press harder on a diamond wheel to make it cut faster?

You should not.

Diamond wheels work best with light pressure. Excessive force can shorten the life of the abrasive coating rather than dramatically increasing material removal.

If a job requires much more steel removal than your current diamond wheel can comfortably provide, the better solution is to use a coarser wheel rather than apply more pressure. For example, major reprofiling is better suited to the Diamond Wheel Coarse than trying to force the Fine or Extra Fine to work faster.

Can I sharpen on the side of Tormek wheels?

Yes, but only light sharpening unless you're using the Diamond Wheels. 

One of the advantages of Tormek's diamond wheels is that the side of the wheel can be used for sharpening when paired with the Multi Base.

Sharpening on the circumference of a wheel creates a very slight hollow grind. Sharpening against the flat side produces a flat bevel instead.

This can be particularly appealing for chisels and plane irons, although for most tools the slight hollow produced by a 250 mm Tormek wheel is perfectly acceptable.

Does the Blackstone Silicon clog or glaze?

It can.

Like other bonded abrasive wheels, the Blackstone Silicon surface can become glazed and lose cutting efficiency. Users sharpening hard steels sometimes report needing to refresh the surface with the Stone Grader to expose fresh abrasive.

Stone grading is part of normal use with both the Original Grindstone and Blackstone Silicon.

If you routinely sharpen difficult steels and would prefer to avoid grading and truing altogether, that is one reason to consider one of Tormek's diamond wheels instead.

How often do I need to grade or true the Original Grindstone?

There is no fixed schedule.

Use the Stone Grader when you want to change the cutting characteristics of the wheel or when the surface starts to feel glazed and stops cutting as efficiently as it should.

Use the Truing Tool when the wheel is no longer flat, round, or running true.

How frequently either is needed depends on what you sharpen, how much steel you remove, the pressure you use, and how often you change between coarse and fine sharpening.

The practical rule is to respond to the condition of the wheel rather than sharpening a predetermined number of tools between maintenance sessions.