I have that little M12 cutoff tool (brushless), and one advantage it has over a small angle grinder is it’s reversible, so you can have the sparks throw where you want, or have the blade pull the workpiece against the shoe, etc. My understanding is that it’s very easy to make brushless tools reversible.
So why aren’t all brushless tools reversible, speciflcally the angle grinders? Wouldn’t having that option be a good thing?
I can think of a few reasons. First off cost. If it’s not absolutely needed including it makes the tool more expensive. Second, in the case of an angle grinder: safety. A small cutoff tool is easy to make reversible because it has very little angular momentum, if you accidentally reverse it – no big deal. An angle grinder has a huge amount of angular momentum, if you accidentally reverse that while on – big deal.
You could have either an electronic or physical interlock where you can’t reverse the tool unless it is stopped, but see point one.
I believe there is no cost. It just requires enabling a change in the control electronics, and that’s software. No new transistors or resistors, etc.
Safety? Well obviously I would make reversing the direction impossible while the tool is running. That’s the case for the M12 cut-off tool.
This is all just a few lines of code, apparently.
You would have to put some kind of additional physical switch to reverse the tool. They haven’t invented mind control yet.
I agree with Koko here that there should be no appreciable cost, it’s really just changing code in the electronics which govern the speed control of the motor.
Now as for an angle grinder? That’s easy: Most grinding wheels or other tooling is simply screwed onto the arbor. If you run the tool backwards and apply any load on it then the grinding wheel comes off. The M12 cutoff tool doesn’t have this concern because it has a bolt to secure the cutting disc rather than it threading directly onto the spindle.
it’s the gears and bearings. An angle grinder uses bevel gears to make the 90 turn from motor axis to wheel hub axis.
bevel gears that more than likely have an helic or angled tooth profile optimized for the load/ and RPM conditions. Likewise the bearings for both one set has to tollerate movement in the axis not just the rotation. Think end stop.
So turning the motor the other direction would mess up that axial load and if the gear is optimized with an angle it would mess up the backlash.
Meanwhile your small M12 cutoff tool is using direct drive motor so there are no other concerns. Note here that drills, impact drivers, impact wrenches all reversible - different gear case.
Both MechaMan and Napalm make total sense.
The threaded arbor should be easy to fix by changing a part or two without much added cost, but the helical gears create that lateral thrust, as you say. You could use thrust bearings on both ends, but that’s more cost. You could use straight gears, but they are noiser and not as smooth (though I guess that’s how reversible drills do it?) You could use two skinny helical gears with opposed twists, or double helical gears, but that’s more cost.
I don’t really see how backlash is an issue?
Incidentally, I guess before they were brushless, reversible drills actually did use a separate set of brushes or something.
Bevel gears are only an issue for a gearbox that makes a 90 degree (most commonly) turn. You could use less efficient and nosier straight gears–in fact these were standard in angle grinders many years ago. I remember watching my older cousin repair grinders back in the late 80’s and pack new grease into the gearbox head. They had straight gears. Or you could use helical gears and accept reduced efficiency in reverse–as in any car with a longitudinal engine driving in reverse. Depending on the tool that could be acceptable.
It’s not a concern for a standard drill because there is no need to change angle, the motor is inline with the chuck.
Brushed DC motors will run backwards if you simply reverse the polarity. How well that works depends on the timing of the motor (the rotational position of the brushes relative to the magnets in the body or “can” of the motor). If the motor has neutral timing then it will perform equally in either direction. If the timing is advanced then it will perform better in one direction than it will in the other, but it will still reverse with the polarity swapped.
Thanks again, MechaMan! I didn’t know that about brushed DC motors. My brushed reversible drills seemed to work equally well either in forward or reverse, but I never measured them.
I believe some drills actually had the chuck offset from the motor shaft slightly. My dad’s old Black and Decker with the metal case was like that. But come to think of it, it wasn’t reversible.
Would helical gears result JUST in lower efficiency in reverse? I thought Napalm’s point was that for two bearings on a shaft, one will be made to accept thrust along the shaft, and the other bearing might just be an oil bushing (AvE took apart a Festool track saw that was built that way.) If operated in reverse, the bushing might be forced out of its sleeve and the gears lose contact. Or am I wrong about that?
Not backlash I meant thrust loads. it came to me when I read that. Yes thrust loads and the gear or arbor moving in and out.
but yes it’s the angle grinder gear box that is why you don’t see one reversible. it could be done as mentioned above but there is no compelling reason too - that I can see. If you jammed up an angle grinder you 're probably running it wrong.
I mean might as well look into reversible other tools like a jig saw, recip, circular, sanders, routers, etc. It’s not that the motor won’t run backwards. efficient or slightly inefficient.
I thought of the threads on the grinder after I replied. Metabo even uses this property in their M-Quick release grinders. You press in the spindle lock as the grinder is slowing down and the inertia loosens the nut. Here’s a video: The Metabo M-Quick System (English) - YouTube
I have had grinding wheels come off unintentionally due to inertia before. It’s always been the large 7" wheels on a big 15 amp grinder. If you don’t get them tight when installing a wheel they can fly off when you let go of the trigger.
Thanks, Napalm.
The reason I could see for reversible angle grinders is so you can control, kinda, where the sparks fly, and also you can use the rotation of the wheel to force the workpiece against the shoe. Maybe regular users of angle grinders don’t care about that, but when I use the M12 cutoff tool, it’s pretty handy.
A brushless motor running in reverse can be just as efficient as running forward. It’s all about when the electronic brain chooses to flip the electromagnets, right?
A reversible jigsaw, recip, or sander doesn’t sound useful, and in a circ, it might be dangerous. But a reversible router could be useful, because the rotation direction determines how you feed the work to the tool (or the tool to the work.) But you’d need left-handed router bits, which to my knowledge don’t exist, probably because reversible routers don’t exist (yet).
But grinder wheels don’t care about direction. It’s just the tool.
oh I was just being a horses rear on the other comments.
actually not true on the brushless motor thing - if doing it right the windings are set and the axle magnets are set with angular momentum and rotation in mind so yes some brushless motor do work better in one direction than the other - the are physically made that way. It’s not an new thing either - most brushed motors are that way too. Some are indeed uniform - or non directional. but many are directional.
Take for example the motor that runs your windows up and down - that motor actually made to make more torque in one direction than in the other - now you don’t notice because the weight of the window helps it in the other direction.
Look at how many motors will live life spinning in one direction - then you might as well optimize them for that task. so many are. Also I suspect but cannot prove right off that drill motors happen to be like this too. and you know impact drivers and wrenches are. more removal torque than they have install torque. well it’s wound to be stronger in that direction.
No problem. You know, I’m not sure how brushless motors would be optimized for one direction for the other. The timing of the field reversals is all controlled electronically, with an actual digital processor, I believe. That’s how the tool can, for instance, maintain a steady speed regardless of load (though I know not all brushless tools have that ability.) The processor varies the magnet timing using feedback (I’ve learned) from a Hall effect sensor measuring the actual rotation speed. It could certainly do the same running “backwards.” No?
I don’t understand how brushless motors would be optimized that way either. Because there are no brushes all the timing is handled by the microprocessor controlling the motor, it ought to be able to run equally well in both directions. It’s not like a brushed motor where the angle between the brushes and the static magnetic field in the motor’s housing could be varied to make the motor favor one direction over the other. Winding, as in the number of turns of wire in the armature of a brushed motor or the stator of a brushless motor, has nothing to do with the directional bias of a motor. A brushless impact wrench which has higher torque in reverse than forward must either be using a hammer mechanism which is reverse-biased mechanically, or simply has the microprocessor within limit it’s forward power.
I think the application of this reversing technology is quite limited and could certainly lead to issues and injuries with more powerful/torquey tools. Nearly Anything with a toothed blade wouldnt work for a couple reasons.
Ive had times i wish a grinder would throw sparks the other way but not sure if its worth a proprietary disk fixing system.
it’s the windings and magnetic fields - they have a directional component to them. So you can for example setup the windings to where they have more pull in one direction or the other.
Meanwhile the magnets used are not exactly like your fridge magnet - they are in some cases custom magnetized. When I worked as a child labor engineer it was at a plant that made auto motive electric motors. So we had these devices that magnetized - the magnets put in the motor were a ceramic/matrix composite thing that had no magnetism when in the crate. Put motor in the box - hit a button - 10 seconds later those magnets where quite strong. Point is the field is not always uniform.
So it’s possible to make a motor pull harder in one direction than in the other - not by the controller circuits but by the windings and magnets and their fields. Does that help - I need to find a drawing.
I have to say, it’s hard to see how a brushless motor could have more pull in one direction than another. The “gearing” for the motor (speed vs. torque) is set by how far ahead of the rotor poles the coils are switched on or reversed (or how much juice the coils get), to attract or repel the rotor poles. So no matter how the permanent magnets are oriented, the coils can work on them, and direction doesn’t matter. I imagine even the wave form of the electrical flow can be controlled, the way a car’s camshaft profile can be tweaked.
Or maybe you’re saying there’s a gradient in the magnetic field of the permanent magnets? Like it’s denser (more intense) at N than at S?
If the coil windings were asymmetric in shape then that could bias the motor in favor of one direction relative to the other because the magnetic field wouldn’t be uniform. I have seen that exactly once, many years ago, in a pancake-style motor from an ancient computer hard drive. However I am not aware how you could make asymmetric windings in a cylindrical motor can. Because the windings have to pass through slots in the stator (or in the armature, in the case of the “outrunner” style motors) it really isn’t possible to make them asymmetric. It was only possible on that pancake motor because of its unusual construction, it was just flat coils of copper wire straight on a fiberglass PCB. The coils were D-shaped. But like I said that kind of thing isn’t possible with standard brushless motor geometry, inrunner or outrunner.