RAS vs mitre saw technique

[Guineafowl21]

One for the likes of Trevanion, perhaps.

https://www.ukworkshop.co.uk/threads/ra ... st-1696449

My contention is that a RAS is pulled towards the user (with stiff arm technique), whereas a SCMS is lifted over the work, brought down, and pushed away. Climb and conventional cutting, respectively.

The chap I’m discussing this with, asserts that the RAS and SCMS are used in the same way, ie climb cutting.

Now, he is right that negative rake blades are often specified for both machines, whereas I thought that only the RAS needed these.

Thoughts?

 

[Just4fun]

I am absolutely NOT a power tool expert. I rarely use power tools. However, I had a recent unpleasant SCMS experience that your question has made me re-think, so I will chip in.

I was cross-cutting a 50mm thick board, about 170mm wide. The board had one sawn edge which I placed against the fence. The other edge (the one nearest me) was a live edge, but I don't suppose that has a bearing on anything.

I pulled the blade towards me, started the saw, lowered it and then pushed it away from me. I assumed that was the correct procedure but your post made me realise that at least someone disagrees. I had not even thought about pulling the blade towards me to cut.

Anyway, I got a kick back. Why? I don't know. I had clamped the board in place with the toy screw-down clamp thingy on the saw and I was pleasantly surprised to see it did its job and held the board in place. I instinctively pulled my hands back, releasing the trigger and stopping the saw. No harm done but it was a bit of a scare.

Would this have been different had I been pulling the blade to cut rather than pushing it? No idea, but it occurs to me that pushing against the fence might be a factor. My thought is that this helps to hold the work in place and improves safety but is that incorrect? Does holding/pushing the work against the fence cause it to bind somehow? I am not qualified to say. On the other hand, if you pull the saw towards you are you relying on that itsy-bitsy screw-down clamp to hold the timber square to the fence? That doesn't seem right (especially with larger boards) and it doesn't seem safe.

On balance I think you should push, not pull, the saw to cut but I repeat that I really don't know what I am doing.

 

[Craig Salisbury]

Another option featuring pull and push is to pull towards you with a scoring depth, then push down and back through making sure you dont get top or bottom tearout.

 

[AJB Temple]

The manual for my Bosch SCMS (12" with the lever arm thing) makes it crystal clear that you push towards the fence. It also says that the saw should be brought back past the work, started and then lowered (ie not lowered down on top of the work if possible). This gives the cleanest cut. Mine scores in the same direction (using a score stop) but I never do that.

Asking for trouble to cut towards you with a SCMS in my view. The fence is there for a reason.

 

[Trevanion]

Despite doing much the same task, both machines are quite different and there are different techniques for them. My personal opinion on the subject would be that the radial arm saw used as it typically would be is a climb-cutting machine, whereas the mitre saw is typically used as a conventional-cutting machine cutting against the rotation of the blade relative to the workpiece.

You can climb-cut with a mitre saw when the workpiece is wide enough to permit that, but for 90% of cuts you will be conventional-cutting as the saw blade is lowered upon the piece, for example anything narrower than 6” on my 300mm mitre saw will be cut in one drop of the blade without utilising the slider, as the blade is being plunged into the wood it is not climb-cutting, its conventional-cutting, this is evident as you have to put effort into cutting the piece, a climb cut would pull itself into the cut. With a climb-cut, you are putting almost as much effort into resisting the action of the saw pulling itself through the workpiece as you do pulling the saw head into the workpiece, it’s a fine line.

To climb-cut on a mitre saw, you would first have to plunge into the workpiece and draw the saw unit towards yourself, that initial cut is a conventional cut and then becomes a climb-cut as you draw the unit towards yourself, this can be quite unwieldy to control as a mitre saw isn’t as rigid as a radial arm saw and the saw unit is hinged and moves in two axes, rather than fixed on a sliding arm and moving in a single axes. When climb-cutting on a mitre saw, there is a much higher likelihood of the saw unit climbing the workpiece and ejecting the blade out of the top as the saw head is hinged, where with a radial arm saw it will pull itself through the timber as it cannot go upwards.

Best practice especially when cutting wide and thick boards, and example being a board 12” wide and 3” thick on the mitre saw is to draw the saw unit towards yourself and commence the cut on the top corner, cutting about 1” deep and push it through to the back of the fence, then draw the saw unit back towards yourself, drop down another 1” and push it back to the fence and repeat until you’ve cut all the way through. This method helps prevent tension in the board from pinching the blade as can happen when trying to cut a full-depth cut. You shouldn’t try to bring the unit all the way towards you and drop it full-depth and proceed to cut backwards as this can cause the workpiece to lift up, particularly on lighter and smaller pieces. Instead of thinking about cutting from the front or behind, with a mitre saw you are cutting the top surface.

I personally prefer to use positive take blades in mitre saws, as a negative rake is largely unnecessary and introduces a lot of resistance to the cut, there are occasions where a negative take blade is of use for cutting plastic and metals, but I mostly use a 40-tooth blade with a 15-degree positive rake. This changes with the radial arm saw as the scraping cut of the negative blade helps to prevent a “snatch” where the saw unit tries to pull itself through the timber, but often in industry most radial arms I have seen were set up with positive rake blades, particularly on saws where a fine end-cut was required.

 

[Guineafowl21]

Reading the above, and thinking about this further, I’d summarise by saying there are two main differences between the machine types:

1. The SCMS freely pivots in the ‘Z’ axis, so in the middle of a climb cut, the reaction force from the teeth is upwards (for a thicker piece), which can be hard to control.

2. The RAS blade starts behind the fence, so is always a climb cut, whereas the SMCS has to plunge first, switching from conventional to climb. This requires a bit more mental effort, and may be second nature, but could catch out a novice.

 

[Cabinetman]

I had a radial arm saw 40 years ago and have had nothing since I bought my Sedgwick table saw which has a wonderful crosscut feature on it, here in the US I have acquired a mitre saw.
At the age of 68 I am a novice with the machine so thank you Trevanian that was most informative, thank goodness it was pretty much what I had thought anyway! The bit that I have found difficult to get used to is trying to line it up for a cut and then dealing with the vicious kick upwards as the saw is turned on.
It’s a Dewalt, do they all do it? The feeling of not being in control of a moving blade is most unpleasant.
Perhaps I need a slow start saw?
Ian

 

[Trevanion]

The bit that I have found difficult to get used to is trying to line it up for a cut and then dealing with the vicious kick upwards as the saw is turned on.
It’s a Dewalt, do they all do it? The feeling of not being in control of a moving blade is most unpleasant.
Perhaps I need a slow start saw?

That’s exactly what a soft start saw is for, I believe the Festool Kapex is one with this feature. Mitre saws for the most part are aimed at worksite users where absolute accuracy isn’t necessary and speed is valued more, having to wait for a blade to spin up even though it takes only a couple of seconds adds a lot of time onto a job where there are thousands of cuts to be done.

Some people have wired in their own “soft start” modules into mitre saws and other aggressively starting equipment, I can’t remember where I saw someone doing this, might have been on UKW or even here not so long ago.

 

[Eric the Viking]

Another option featuring pull and push is to pull towards you with a scoring depth, then push down and back through making sure you dont get top or bottom tearout.

I use the scoring function on my rail saw (AKA "plunge saw") a lot, and it works beautifully, but I realised to my annoyance recently, that my SCMS cannot do the same, since the blade doesn't reach the front of the fence, unless it is fully at the bottom of the cut (and only just reaches, at that point).

Of course I could use a false fence, but that messes with other things such as length of cut. My recent 'discovery' (I know, RTFM) was doing a task that needed slightly more than maximum length of cut, so it was a nuisance at both ends: if you kiss the surface, so as to make a scoring cut, it stops well short, and the thicker the stock, the more this happens.

Now the SCMS blade has worn a bit, I get more obvious tearout on the top and back end of the cut, which is a nuisance. For what it's worth, the blade rake is nearly zero (Festool 250mm). If there is negative rake it's pretty much imperceptible.

I've no experience of using a RAS, but although it was once something I coveted, I'm now glad I never bought one. I can probably do 95% of anything I ever wanted to with rail saw and SCMS, and the rest can almost always be done another way, for example with a router.

As I've got older, I've grown increasingly risk averse, and whilst big blades don't terrify me, I do look for less scary ways of doing stuff nowadays.

E.

PS: I've had nasty kickback on the rail saw at least twice, but in both cases I've been doing something stupid. That said, if I was buying again it would be a TS55 instead of the Makita SP6000 - I love the saw, but the Festool has a sprung-loaded riving knife...

PPS: My Kapex has a sort-of soft start, but sounds like a bag of nails as it gets up to speed. Apparently they all do that sir. It's interesting how many popular YouTube channels mute the sync sound when the hero goes over to use their Kapex... ;-)

 

[AJB Temple]

My Mitre saw supposedly has a soft start, but there is still a bit of a kick at the beginning and end of a cut. It bothered me when I first got the saw (a few years ago now) but literally thousands of cuts later I'm totally used to it.

Track saw (Mafell) has a scoring function and it is brilliant. The device on the Bosch mitre saw is rudimentary and really meant for trench cuts. Eric is spot on re need for false fence. I just use am extra bit of wood.