Negative rake sawblade!

[lastminute]

...correct me if I'm wrong, but negative rake blades are usually fitted to mitre saws...
...is it ok and safe to use one in a table saw?

 

[Alexfn]

id advise against it, some people do use them with a power feed

ive never tried it but from my understanding the wood will want to lift and it could be dangerous, for the sake of the 25 quid it will cost id buy a new blade that was designed for the job

https://www.youtube.com/watch?v=KI9adIab6eI

 

[Steve Maskery]

Don't do it.

On a mitre saw the blade is above the table and so pushes the workpiece down. The negative rake reduces the "grab", but it still pushes down.

On a TS the blade is below the workpiece. With a positive rake the tip of the tooth pulls the workpiece down onto the table, but a negative rake mean that the heel of the tooth makes contact fist, pushing the workpiece up off the table and into your face.

So don't do it.

 

[meccarroll]

You are correct in saying that negative hook saw blades are usually used in mitre saws. Table saws usually use positive hook saw blades.

A negative hook saw blade will help reduce, grabbing action, when used in an overhand saw such as a mitre saw or radial arm saw and is therefore safer to use when crosscutting on overhand machines. Positive rake saw blades are better for ripping wood but if the hook is reduced on a positive saw blade it can be better for hardwood ripping.

There are many, many, different saw blade tip designs and sharpening profiles for the tips on saw blades. It is better to swap/change the saw blade to the correct one for the job when cutting different materials on a machine for sawing wood. If you are unsure just check with the manufacturer of the saw blade to obtain their advice regarding the best blade to use in your machine for the type of work you are doing. Using the wrong saw blade could cause it to overheat and become dangerous to use so always check to ensure you have the correct blade for the job.

unless you have specialist knowledge for the work you are doing a general guide would be to use negative hook saw blades in overhead saws (radial arm,mitre saws etc) and positive hook saw blades in panel and bench saws.

Mark

 

[lastminute]

Thankyou all...much clearer now...glad I asked.

Gerry

 

[andersonec]

Don't do it.


On a TS the blade is below the workpiece. With a positive rake the tip of the tooth pulls the workpiece down onto the table, but a negative rake mean that the heel of the tooth makes contact fist, pushing the workpiece up off the table and into your face.

So don't do it.

Don't see how that can happen Steve, if you look at the geometry of the tooth when it is coming into contact with the wood, unless it is approaching the wood from underneath, the tooth is contacting the wood on a downward trajectory and unless that rake is something like minus 45 degrees the tip of the tooth must touch first.

I know negative rake teeth should not be used to rip but that is only because of waste removal, rip blades teeth are further apart and are shaped so that the gullet can remove longer and larger lengths of waste because the waste is coming off in lengths due to the direction of the fibres of the wood, negative rake needs to remove smaller pieces of waste as it is crosscutting and the teeth being closer together will not produce such a tearing action in the wood resulting in a finer cut. Negative rake teeth cannot remove waste efficiently when rip cutting and could clog the blade and cause it to jam, especially when cutting thicker sections.

Let me know what your thoughts are.

Andy

 

[Steve Maskery]

Hi Andy.

Lets say that you have the blade adjusted properly, so that just the tooth sticks out over the top of the workpiece. That tooth will make contact with the workpiece at just about TDC. The negative rake would mean that the heel makes contact before the tip, sliding underneath the board like a wedge. The board will be lifted and pushed back at you.

You are right about the gullet and sawdust removal, but that is a function of the number of teeth, not the tooth form.

 

[andersonec]

Hi Andy.

Lets say that you have the blade adjusted properly, so that just the tooth sticks out over the top of the workpiece. That tooth will make contact with the workpiece at just about TDC. The negative rake would mean that the heel makes contact before the tip, sliding underneath the board like a wedge. The board will be lifted and pushed back at you.

You are right about the gullet and sawdust removal, but that is a function of the number of teeth, not the tooth form.

Right, point taken.

Andy

 

[Steve Maskery]

I've been thinking about this. You'll be pleased to know that I am right

The way that the saw makes contact with the wood means that the actual cut path is circular, exactly the same diameter as the blade itself. Each tooth cuts a little bit, but because of that radius, each tooth is still presenting itself heel-first, all the way around its cutting arc, no matte where on that arc the tooth is.

So don't do it.

 

[meccarroll]

I've been thinking about this. You'll be pleased to know that I am right

The way that the saw makes contact with the wood means that the actual cut path is circular, exactly the same diameter as the blade itself. Each tooth cuts a little bit, but because of that radius, each tooth is still presenting itself heel-first, all the way around its cutting arc, no matte where on that arc the tooth is.

So don't do it.

Steve if you wanted to cut aluminium on a saw bench/panel saw, a negative hook saw blade is recommended for that material. The tips of saw blades often do have several angles for cutting, including, but not exclusively, the hook/rake angle. I agree that in most cases a positive saw blade Hook/Rake saw blade is commonly the type used for most cutting operations involving wood material on panel/table saws but sometimes negative hook/rake are better suited to a particular material. Best to consult the manufacturer of the saw blade for specific machine and material use first to avoid mistakes.

Mark

 

[Steve Maskery]

I must admit that, being a woodworking forum, I did assume that we were talking about using a tablesaw to cut wood.

 

[meccarroll]

I must admit that, being a woodworking forum, I did assume that we were talking about using a tablesaw to cut wood.

Yes this is primarily a woodwork forum Steve but the OP's question in this thread is about the, safety, of using negative hook/rake saw blades in a table saw.

Negative hook/rake saw blades may be used, and are sometimes recommended, for cutting certain materials on a table saw including, but not exclusively related to, aluminium, melamine faced boards, plastics etc.

I used to work in a Yacht building yard a while back which had metalwork and joinery shops with table saws, the joinery shop used to cut a lot of aluminium and plastic coated materials using negative hook/rake saw blades. The yard had a lot of specialist knowledge in the selection and correct use of saw blades for different materials types. Choosing the best saw blade for a specific task can be quite challenging and if you don't have specialist knowledge it is better to consult the saw blade manufacturer first.

Mark

 

[Steve Maskery]

Fair enough.

 

[Eric The Viking]

In the absence of any other information, it's _sensible_ to assume it's a woodworking question on a woodwork forum.

And there is a crucial difference between wood and other materials in this context, which has been hinted at, but needs adding to the discussion (IMHO), and that's grain and the related propensity to warp.

If you cut nothing but man-made materials, a negative-rake blade would be less dangerous in a tablesaw (but not risk free entirely). But natural timbers add risk. It's mostly with regard to ripping, and the ability for the workpiece to catch a tooth in the wrong part of its rotational travel - but neg-rake teeth increase the propensity to kickback in this context, as they move the danger point further to the front, and reduce the abillity of the correctly-working teeth to act to hold the workpiece down.

Simply put, with a neg-rake blade in a mitre saw:

• gravity is working with you,
• you cut against a fence,
• kickback mostly throws the workpiece away from you.

With a neg-rake blade in a table saw:

• gravity works against you,
• there is no fence (usually),
• kickback usually comes right at you.

I appreciate that keeping a selection of blades for specific purposes is very sensible, but those elements above don't change, apart from the fact that man-made materials have no grain and thus usually don't distort when cut (although they can).

Arguing from a specific (specialist blades in a high-precision application, used by well-trained staff) to a general case is always risky. And anyway the simple fact that a high-end boatbuilder would keep a selection of specialist blades is tacit acknowledgment of the risks of using the wrong one in the wrong context - even if at best that only means a poor quality cut.

I'm not picking a fight, but it would be wrong for someone to skim-read this and get the impression that it doesn't matter to have a neg-rake blade in a tablesaw.

For most of us, in most circumstances, it's probably less safe than a normal one.

 

[meccarroll]

In the absence of any other information, it's _sensible_ to assume it's a woodworking question on a woodwork forum.

And there is a crucial difference between wood and other materials in this context, which has been hinted at, but needs adding to the discussion (IMHO), and that's grain and the related propensity to warp.

If you cut nothing but man-made materials, a negative-rake blade would be less dangerous in a tablesaw (but not risk free entirely). But natural timbers add risk. It's mostly with regard to ripping, and the ability for the workpiece to catch a tooth in the wrong part of its rotational travel - but neg-rake teeth increase the propensity to kickback in this context, as they move the danger point further to the front, and reduce the abillity of the correctly-working teeth to act to hold the workpiece down.

Simply put, with a neg-rake blade in a mitre saw:

• gravity is working with you,
• you cut against a fence,
• kickback mostly throws the workpiece away from you.

With a neg-rake blade in a table saw:

• gravity works against you,
• there is no fence (usually),
• kickback usually comes right at you.

I appreciate that keeping a selection of blades for specific purposes is very sensible, but those elements above don't change, apart from the fact that man-made materials have no grain and thus usually don't distort when cut (although they can).

Arguing from a specific (specialist blades in a high-precision application, used by well-trained staff) to a general case is always risky. And anyway the simple fact that a high-end boatbuilder would keep a selection of specialist blades is tacit acknowledgment of the risks of using the wrong one in the wrong context - even if at best that only means a poor quality cut.

I'm not picking a fight, but it would be wrong for someone to skim-read this and get the impression that it doesn't matter to have a neg-rake blade in a tablesaw.

For most of us, in most circumstances, it's probably less safe than a normal one.

Eric, if you are not sure which saw blade is safe to use then why not consult the saw blade manufacturer for their advice?

Mark