Why are plunge cuts more dangerous?

[Windows]

Can someone explain the mechanism that makes plunge cuts with a track saw more likely to produce kickback than non plunge cuts? I think I understand it, but I like to recheck my understanding occasionally and web search is now more or less useless to me because of AI pollution, so a brief human description would be helpful. Thanks.

 

[Malc2098]

If I understand it correctly, with the level cut, the blade rotation approaches the work from underneath while the operator is pressing down from above. With the plunge, the rotation of the blade is trying to approach from above, requiring more effort/control on the part of the operator until the blade reaches the underside and equilibrium, for want of a better expression, is restored.

 

[Mike]

I use my track saw in nearly every project, and @Malc2098 is correct. I can count on one hand the number of times I plunged through the sheet goods when starting the cut. In these cases, I have never experienced kickback because I start the cut slowly and don't rush the initial cut. Once the blade is through the material, I continue with the horizontal cut.

 

[AJB Temple]

Indeed ^^. If you use a tracksaw to plunge into small sheets, it can propel the work off the bench if you are not careful and the blade can snatch. It happened to me when I first got my Mafell several years ago and had never used a tracksaw before. Like Mike I lower the blade in slowly on the rare occasions that I have to plunge. Most of the time I am doing a full cut on sheet good and start the saw off the work like you would with a table saw. Oddly enough I will be doing some plunge cuts this week as I am making some radiator covers.

 

[Windows]

How’s this?

Consider a track saw facing to the right (so that moving it from left to right would cut in the normal way). The rotation of the blade would be anticlockwise.

If the blade were a wheel, the direction of rotation would move the saw to the left (towards the operator), but under normal use the operator pushes the saw to the right with more force, so the saw moves to the right and the operator feels some resistance.

If the operator were to pull the saw towards himself instead of pushing the saw, the forces would combine instead of cancel and the saw would move quickly to the left.

Further, when the saw is moving to the right, the bottom of the wood is cut first. Because the blade is not infinitely sharp, there is some resistance and a force opposing the direction of travel of the blade at the contact point acts on the saw. This force has a downwards component that pulls the saw body into contact with the wood.

But when the saw is moving to the left, the top of the wood is cut first producing an upwards force on the saw. This can result in the saw rising out of the cut and rolling across the surface of the wood with no resistance. This is why we call a cut made in the wrong direction a climb cut.

A plunge cut is at the tipping point between a normal cut and a climb cut. If the saw moves just slightly horizontally, you get one or the other.

If the saw moves forward to the right, you get a normal cut with opposing forces largely cancelling out and the saw pulled down on to the wood.

If the saw moves back to the left, you get a climb cut. Forces can combine to move the saw rapidly towards the operator and the saw can climb out of the cut. Kickback.

A stop positioned on the track directly behind the track saw prevents it moving backwards and helps ensure that a plunge cut doesn’t unexpectedly turn into a climb cut.

If the plunge completes and the saw starts to move forward, the plunge-based danger is over. The cut becomes a normal cut and there is no greater danger of problems from that point because there is no wood directly behind the blade for it to grab on to.

 

[Raymedullary]

Is it not simply that during the plunge the riving knife has little effect?

 

[Ovaloe]

I'm with Mike - probably done ten thousand or more plunge cuts with a saw and rail over a quarter of a century with only the occasional kick back, invariably my own fault. If you plunge in you need to ensure that your blade is sharp (blunt blades can cause kickback) and that the blade is suitable for the work, you need to ensure that the motor is fully up to speed before plunging in (more of a problem on 110 volt toola if you happen to be working at the end of a long cable run because of the effect of voltage drop over distance, and with cordless tools which always seem to take longer to spin up to full speed) and you need to control the plunge so it isn't too quick (nor so slow that you get scorching). So I don't think plunge cuts are more dangerous - they just require a wee bit more care to do

 

[Windows]

Is it not simply that during the plunge the riving knife has little effect?

I don’t think it’s just the riving knife. Many plunge saws don’t have a riving knife (Makita, Bosch for example) and modern Festool replaces the riving knife with electronic kickback protection which works whether you’re plunging or not. Would you say that - for these saws specifically - there’s no difference between a plunge cut and a forward cut? I would say there is a difference, because I believe there is a greater chance of kickback during a plunge cut even when the saw has no riving knife. And I’m basing that on how a plunge cut is just one slip away from being a climb cut. Does that make sense?

 

[Windows]

If my reasoning is correct, I would perhaps expect track saw designers to design their plunge mechanisms to move the blade minimally forward in the plunge (to ensure that it does not move minimally backwards), but perhaps this level of care is considered unnecessary because rake of the teeth on the blade could possibly be a larger compensating factor for hand jitter and engineering inaccuracies. Have saw manufacturers ever advertised their techniques for reducing kickback on plunge cuts?

It seems like you could greatly increase kickback by picking the wrong blade.

 

[Trevanion]

Funnily enough, I’ve been doing a few plunge cuts today. I used to do it with a regular circular saw by pivoting off the front of the base, holding the guard back and slowly lowering the blade into the work, never had an accident but the plunge saw is certainly a safer and more accurate way to do it.



It’s a tricky one, basically when you plunge you have a significant amount of the blade engaging with the workpiece, so any misplaced move will cause kickback. Technically, if you were to place the track over the edge and plunge into the piece halfway off the edge with the front half of the blade you would find the plunge very difficult to control because it will want to snatch downwards as it’s technically a climb cut. However, if you plunge with the back half of the blade in the same scenario you might find it easier to control the plunge, but there is a risk if you were to move even slightly backwards that the blade would catch and launch the saw backward.

 

[Windows]

So I don't think plunge cuts are more dangerous - they just require a wee bit more care to do

I love this way of looking at the world. I can’t subscribe to it, but I really do appreciate the optimism of those who do. In my world, care (attention) is a limited resource and unevenly distributed. Any operation that requires more attention than a similar operation to achieve the same level of safety is by definition more dangerous.

 

[Doug71]

I generally use my track saw one handed but use two when making plunge cuts just in case.

DeWalt claim you get less chance of kickback when plunging with their track saw because it has a parallel plunge action rather than the more common pivoting/hinged action used on other saws, don't know if it's true.

 

[Eric the Viking]

Yes, but isn't that the same DeWalt that thought double sided tracks were a smart idea?

Full disclosure: I doubt it's true.

That said, I've had kickback three or four times with a tracksaw. I have a Makita SP6000K with no riving knife, and on at least one occasion having one would have mitigated it.

 

[Ovaloe]

I love this way of looking at the world. I can’t subscribe to it, but I really do appreciate the optimism of those who do.

The "optimism" you describe is the result of nearly four decades of using plunge saws (starting with an Elu MH25, then a Holz-Her Mosquito, neither of which used a guide rail) - and just a few years of college doing my C&Gs. So not optimism at all

In my world, care (attention) is a limited resource and unevenly distributed. Any operation that requires more attention than a similar operation to achieve the same level of safety is by definition more dangerous.

Attention when using tools which posses fast, sharp spinney things attached to them, like saw blades, is an absolute requirement, not an option, I'd say. Even slow sharp stuff like Stanley knives can and do injure the unwary. If you don't know how to use something safely, and you aren't paying attention, you are much more likely to have an accident (look up accident figures to see how many utility knife accidents there are each year, an item we all take for granted)

Most plunge saws are designed to retract when the operator releases pressure on the handle, at which point the blade retracts under spring pressure to inside the housing. That's one of the safety features which makes plunge saws safer to use than conventional portable circular saws.

Rail guided plunge saws are designed to be plunged into the cut at the start. TBH hanging the guide rail over the edge of the work with the saw atop the rail and then plunging the blade before moving the saw into the material is actually less safe than starting a cut by plunging into the material to begin with. This is because the weight of the saw will tend to bend the guide rail which in turn can result in it actually lifting off the work piece at the far end and potentially pivoting the rail. Guide rails can also twist when hung overboard which is even less safe. I have seen both of those things bhappen. Bear in mind that whilst that is less likely you will bend or twist an overhanging guide rail with a corded saw such as the TS55 or MT55c or GKT55CE, it becomes more likely with a similar size cordless model (due to the extra weight of the batteries) and when you get up to the weight of a TS75 or the big Mafell KS85EC l it will definitely happen. Of course you can always clamp the guid rail to the material, but that a is very slow way to work.

As to the other issues which generate kickback on plunging I more ir less stated them above:

(i) blunt or inappropriate blade in use - blunt blades will kickback on any type of saw

(ii) low voltage (overly long main cable or batteries nearly exhausted in the case of cordless tools) will result in the tool not spinning up to the required speed which can result in a stall or kickback when plunged in

(iii) to fast a plunge speed - again can result in a stall or kickback. Just learn to feed the saw into the material smoothly and at the correct rate so that the motor doesn't slow down too much. Easily done by ear. It isn't rocket science

The other thing is that you ideally should use two hands to plunge and NEVER position yourself directly behind the saw whilst cutting. But I'd have thought that was obvious

Compared to freehand plunging a conventional circular saw rail modern rail guided plunge saws are very much safer to use.

I therefore wouldn't agree that plunge saws aren't safe - surely it's other factors which can make a plunge cut unsafe?

 

[Windows]

It also seems to me that plunge saws are safer than circular saws. I’m not trying to suggest that plunge saws are worse than alternatives.

I’m talking about plunge cuts with track saws because that’s the kind of saw I have and use. I’m trying to explore the differences in the physics of a plunge cut vs a forward cut with this kind of saw so that my safe use of the saw becomes automatic. It’s a way for me to reinforce good habits through deeper understanding.

The background context is that Festool training and video content from other pros almost universally suggests that plunging is riskier than forward cuts and that you should in general start with the saw on the rail and off the workpiece unless the plunge is necessary. I’m trying to put this message in context and all the replies have been very helpful.

What you’re saying about the risks of starting the cut on an unsupported part of the rail is a good point. It made me uncomfortable when I first started using the saw so I very quickly switched to supporting the rail fully. I also clamp both ends of the track routinely. I value safety over speed.

All your other advice is really helpful too. I use both corded and cordless track saws so the sometimes unstated risks of cordless that you have mentioned are definitely good to think about.

 

[Windows]

I also agree that attention is a requirement.

My view isn’t that people should choose operations that require less attention so that they can pay less attention.

My view is that humans cannot pay attention at all times so they should choose operations that require less attention (all else being equal) to account for the inevitable attention deficits that occur because we are but flesh and blood.

We don’t build reliable systems that involve humans without accounting for toilet breaks. Same with attention.

 

[Ovaloe]

It also seems to me that plunge saws are safer than circular saws

They most certainly are! As an example of safety, take the case of cutting out a 75mm thick, 3-layer pitch pine mill floor (with sundry embedded nails, screws etc as well as 1mm thick loose tongues between boards - nightmare stuff): this is a task I undertook on one job maybe a four or five dozen times or so over the two plus years of one project. It's the sort of task which takes a big, powerful saw (e.g. 9in/230mm blade) with bags of power and which wrecks blades in no time at all. It is also a task which is riven with the danger and "excitement" of multiple kick backs.

I found the conventional Makita 9in (portable) rip saw supplied by the firm I was working for to be downright frightening to use as it was necessary to start each cut by making "drop cut" (i.e. plunging the spinning blade down into the material using the front of the baseplate as the pivot point). After a dozen or so rather hairy kick back I binned that idea and brought in my own big saw, a Hilti WSC85 (230mm blade, 85mm depth of cut, 1600 watt motor). Whilst it isn't a rail guided saw, it does feature a novel (lever operated) plunge and retract mechanism which means it is possible to minutely control the rate of plunge (pretty much what you can do with a Festool, Makita, Mafell, etc plunge saw) and the saw itself is far less likely to kick back, although it isn't as good as the rail guided Msfell KS80EC and guide rail I had on loan for a few days. That was down to the rail stopping the tendency on some cuts to pull to one side and turn in the cut - which can elicit a kickback (another reason rail guided saws are safer that unguided saws). I'd have bought the KS in a heartbeat, if only the ori e tag wasn't so high (£1k+! Yikes!)

Of course the other big plus of those unge saws with the all-encompassing blade guard is that they have far better dust extraction (even though you do need to close the side opening in the blade guard with a piece of gaffer tape) - better for you, better quality cut with less risk of scorching, less cleanup at end of day etc, etc.

Does it show that I'm a convert?

 

[Eric the Viking]

I have rail clamps,which I bought with the SP6000K years ago, thinking, "might as well...". I probably now use them 60% of the time.

Makita saws and rails have an anti-tip-over feature, which can also used when the saw isn't tilted. This is a big plus over Festool, and, IMHO, better implemented than on other brands.

On the other hand, Festools have an actual riving knife, and waste-side splinter guards.

I have yet to find my ideal feature set in one saw!

By the way, a National Trust card, cut roughly to shape and secured with Frog tape, covers the blade-change hole perfectly. Other woke establisments' cards* are also suitable...;-)

E.

* English Heritage, Bristol library cards, my university alumnus card, my old prescription pre-payment cards, etc...

 

[Trevanion]

Makita saws and rails have an anti-tip-over feature, which can also used when the saw isn't tilted. This is a big plus over Festool, and, IMHO, better implemented than on other brands.

On the other hand, Festools have an actual riving knife, and waste-side splinter guards.

I bought the Makita years ago and I recently acquired a secondhand Festool TS55 that had barely been used by a hobbyist, 2010 model so I’m not sure if there’s been any changes since but it looks largely the same as what’s on offer now.

The main reason I bought the Festool was because I’ve never owned anything Festool so I wanted to see what all the fuss is about (I also bought a OF1400 from the same place). There are some things I like on the Festool, and some things I like on the Makita, it’s difficult to say whether there’s any preference between them, I haven’t really used the Festool enough to have a definitive answer but I would say I marginally prefer the Makita, though I will say perhaps if I was doing finer work with veneered panels that the Festool might edge ahead.

 

[Mike]

When I changed my shop location from the garage to the basement, it didn't take me long to realize the Bosch tools I had been using for a few years were not suited for indoor work. Despite using a large Kärcher vacuum with the router and circular saw, cleanup was nearly as time consuming as using the tools. At the time, I knew about Festool, but had not been interested in the product line because the Bosch Blue tools were good enough as long as the work could be moved outside.

I was able to have hands-on experience with several Festool machines during an open house sponsored by a Festool distributor in Mannheim. I changed my mind and began replacing every powered tool with Festool, as well as expanding my tool inventory as the need arised. I didn't have a problem finding a new home for the displaced tools, as they were top of the line for their respective brands. Had I known how great the dust collection was with the Festool, I would have started with them, but this was part of my learning experience.

I have the TS55 and TS75 and bought them later in my Festool migration. I used comparable plunge saws from Bosch and Makita and was equally happy with the dust collection and cutting. If I didn't have as many Festool machines at the time, I might have bought a different brand of plunge saw, but being able to use the rails and vacuum hoses with every machine is a bonus. The favored customer discount from my vendor made the decision much easier to continue with Festool.

I always clamp the rail to the sheet goods and plunge the blade before making contact with the wood. I was also concerned about the saw overhang affecting the rail, but even with the larger TS75; however, I have not observed any deflection at the start of the cut.

The method taught by the Festool factory representatives here is to plunge the saw near the leading edge of the sheet, squeeze the trigger to start the saw, make the cut while standing beside the saw, release the trigger when the blade clears the sheet, allow the blade to retract after it stops.

 

[Windows]

I have a Lidl Parkside 240V and the cordless Festool TSC 55 at 10x the price. I like the Festool better. (I DIYd a project to provide/justify the budget for the Festool). No regrets on either purchase.

The Parkside has a riving knife. The Festool TSC 55 doesn’t have a riving knife, but does have a springy bar for detecting when the saw lifts off the rail which is coupled to the brake. I’ve not experienced kick back with either saw yet.

I have a little “workshop” in two locations at the moment, so there’s one saw in each location. I’d like to upgrade the Parkside with something that produces less dust, fits on Makita rails, and doesn’t have a power cord, but it’s pretty hard to justify.

I have duct tape over the blade change hole of the Parkside, but I spent a couple quid on a plastic cover for the Festool. Both noticeably reduce the dust. It’s amazing that Festool doesn’t provide a cover as standard.

 

[toolsntat]

Am I the only one, where possible, to clamp a stop to the workpiece directly the saw when plunging?
Cheers, Andy

 

[Mike]

Am I the only one, where possible, to clamp a stop to the workpiece directly the saw when plunging?
Cheers, Andy

No, you are not the only one. I have the Festool Limit Stop FS-RSP (part number 491582) that attaches to the guide rail. The FS-RSP can be attached to the rail behind the saw to limit kickback when plunging into the wood, or ahead of the saw to provide a stop block.

 

[Windows]

No, you are not the only one. I have the Festool Limit Stop FS-RSP (part number 491582) that attaches to the guide rail. The FS-RSP can be attached to the rail behind the saw to limit kickback when plunging into the wood, or ahead of the saw to provide a stop block.

I also use the Festool stop block attached to the rail. A lot of people are producing stop blocks to attach to the saw tracks and selling on eBay, but not all of them have the lip of the Festool block which goes over the edge of the saw base to discourage vertical movement.

 

[Ovaloe]

I bought the Makita years ago and I recently acquired a secondhand Festool TS55 that had barely been used by a hobbyist, 2010 model so I’m not sure if there’s been any changes since but it looks largely the same as what’s on offer now.

The Festool has changed somewhat over the years - dual position depth indicator (shows depth of cut both on and off the guide rail), scoring setting (something Makita had first AFAIK), thinner blade and on one of the latest versions a separate scoring blade in front of the main blade. TBH the Bosch GKT55CE is much the same as the earlier TS55s whilst the bigger Festool TS75 is more or less an early TS55 on steroids, although the base plate is a bit wider so it won't run on the Makita guide rails whereas the TS55 and GKT55CE both will.

 

[vaj]

To the original question. The problem is not kick back per se - more that the saw can push itself up out of the work if handled incorrectly - or presumably if a cut was for whatever reason to close tightly on the trailing edge of the blade.

The leading edge of the blade as you cut forward in thin material with a track saw is rising/cutting upwards - so that the cutting force pulls the saw down tight on to the work.

The trailing edge is falling/cutting downwards. If you accidentally pull the saw back towards yourself while plunging (or the cut closes on the rear of the blade) it will be the trailing edge that mostly cuts - and the cutting forces will tend to push the saw up out of the work.

The other potential difficulty is that the bottom of the blade when it first touches the work in a plunge is cutting forward - which means that the cutting force will tend to push the saw backwards if it's not firmly held. This backwards force disappears quickly as the plunge breaks through in thin sheet material, but will remain present in cuts in thicker material - for the duration in a blind slotting cut or if the saw doesn't break through very far. This opens the possibility that this force if the saw if not firmly held may propel the trailing edge of the blade hard against the back of the cut with similar consequences to the above.

Given focused attention, a firm grip, downward pressure, a stationary plunge and a small saw it isn't necessarily too big a deal, but it could get messy if not.

As ever it's important to think through what's going on in a given situation and to stay focused...