SW09 – Kickback

kick-1There are all sorts of risks in the workshop.  But the one that really seems to give woodworkers the willies is kickback.

When a kickback occurs, it is always pretty startling, you find yourself with the deer in the headlights look, and once your wits start returning, you start looking for claret.

I’ve had some pretty impressive kickbacks, and I can guarantee you something in every single case.  User error.  (Of course I do have that router bit that seems to kickback no matter what I try, but let’s ignore that here).

I’ve had kickbacks from tablesaws, routers (table mounted primarily), thicknessers, bandsaws, lathes, drill press…….  Pretty much anything with a blade can kickback.  That list isn’t a badge of honour.  What is then, is that I still have intact bodyparts.  One kickback I had from a tablesaw a few years ago gave me a bruise/mark that took a year to fade.

So just what is a kickback? I tend to think a kickback is when the cutting surface (be that a tooth, a blade or whatever) fails to do its intended task, and instead propels the work instead (which more often than not is back towards the operator!)

I’m not covering prevention here- that will be tomorrow’s article.  This is looking at causes of kickback.

Tablesaw:

A kickback on the tablesaw can be a frightening experience.  This can be a 3HP machine, with a large diameter cutter (10″+) and given the speed they run, the blade tip speed can easily exceed 200 km/hr.  If a piece of work doesn’t cut, and instead is propelled, then that is the sort of speed it is going to come flying out at.

There are  couple of places on the blade that are most likely to kickback – the side (which then lifts the work to the top of the blade where it gets propelled with a large degree of rotation), and the back edge (where you get massive acceleration of the workpiece).

Tablesaws have been known to propel a piece of timber so fast back towards the operator that it has punched right through the wall of the shed.  You don’t want to be standing in the line of fire.  Ever!

A rear kickback is when the workpiece is lifted by the rapidly rising teeth, and propelled over the top of the blade.  This can happen when an offcut works its way into the back of the blade, or when a workpiece is not adequately restrained and floats (lifts) because of those rapidly rising teeth.

A side kickback is often when a piece is not restrained properly as it passes the blade, and twists, (which obviously is then wider than the gap between blade and fence (measure across the diagonal).  The blade then chucks the piece.  It can also be when the fence is not parallel to the blade, and angles towards the rear of the blade.

It can also be caused when the operator pulls the material back towards him on completion of the cut (or when the piece becomes trapped because of the non-parallel fence), and is pulling it in the same direction as the blade is trying to throw it.

Router Table:

A kickback here is generally parallel to the fence, and to the right of the table.  Less likely to be directly at the operator (unless you happen to be standing down there).

Causes are many and varies again.  Incorrect feed direction is a biggie.  Feeding from the left, (or feeding in from the right but to the wrong side of the router bit), so the feed direction is in the same direction as the cutter is spinning.  A router loves to accelerate and propel pieces, like a small spinning wheel (but one that is going up to 20000 RPM!).  Tip speed of the router bit tends to be around the 150 km/hr.  Climb cutting is rarely a good idea.

I’ve had some other instances when a kickback has occurred, even though everything seems to be correct, and these are freaky, as they are completely unexpected.  Some I still haven’t been able to determine the cause, and have put them down to the router bit itself (one that I have chosen to retire).

Feeding too aggressively is also possible, or when starting a cut, engaging the router bit at the wrong angle can all be causes.

What is really scary about a router bit kickback, is its tendency to pull the operator’s hand towards the cutter.  A few kickbacks I’ve had on the router table, I’ve come away ashen faced, checking the hand to see if it is all still there. (The shock of the impact is quite explosive, and can leave the hand feeling numb, so you can’t actually tell if you’ve been cut or not).  I’ve not, but they have been very strong reminders that extreme care is definitely needed on these machines – the consequences of getting it wrong can be dramatic.

Thicknesser:

Not a common machine to kickback, but it is possible.  I’ve only managed it once – cutting a resawn piece of timber where I had gotten the resaw cut way to uneven, and the blade had managed to catch, rather than cut.  It is also possible when cutting something too thin, which can twist, and rise (especially the leading edge) into the cutter, hitting it at the wrong angle.

Thicknessers tend to be pretty good – some have antikickback fingers, and there is that leading roller that helps constrain the timber so it is cut rather than propelled.

Bandsaw:

Has teeth, can kickback.  However, the cutting direction is vertically downward, straight into the table so the result is pretty much a non-event (from a danger to operatior point-of-view).  The highest risk then is breaking the blade.

Lathe:

This is unusual, in that the workpiece is rotating, and the tool is not moving (the chisel).  It can still kickback though, particularly if you get the angle of attack wrong (or the one I had was……the skew chisel).  Instead of the workpiece flying at you, the lathe propels the cutting edge towards you (mainly down).  The main one I had was the skew, and it was like a firework going off with a loud bang.  Took me a few seconds to realise just what had happened, and I couldn’t feel a thing – my hand and arm were so numb from the shock it took quite a while to complete checking to see if I was in one piece.

Drill press:

Again, a bit unusual – the drill bit can grab the workpiece, so instead of cutting the workpiece becomes a helicopter.  As it rotates around rapidly, it will hit anything in its path, including the operator.

So that is just a brief overview of kickback, and some of the machines that can cause it.

One of the scariest events that can happen for a woodworker, because it is so fast, and dramatic (and there are definitely potential consequences!)

The next article will look at prevention.

SW09 – Damage

Spent a bit of time out in the shed this evening, and given it was safety week, did a quick VCE (visual check of extremities) to see just how much damage I managed to do with a typical shed visit.

3 scratches on one arm, a decent friction burn on the other hand, and a small hole in one foot (from a nail (and yes, I am up-to-date with my tetanus shots)).  So a pretty normal day then.

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