how to SG

[Brucey]

SG means to SpinGrind

eh?

well I [and other folk] have been doing it for years, but it seems it isn't widely known.

All you have to do to SG something is to be able to turn it quickly [say~1000rpm using an electric drill] whilst using an angle grinder on it. Any of the operations [the exception being screw-cutting] on the outside of the workpiece you would normally use a lathe for are possible, but this is much faster and it allows you to work on hard steel. Better yet,the contact forces can be very low which means it is relatively easy to fixture something. Since the bearings are not that good in most drills I recommend using a drill chuck mounted in a separate shaft supported by two decent bearings

[Brucey]

btw most Hoffmann-style [deep groove] bearings can be ground to make them a bit smaller using a method I call PSG or Passive SpinGrinding. In this, you bring the AG wheel to bear at first tangentially, to spin up the workpiece, then once it is rotating move the AG to about 45 degrees to remove material whilst maintaining spin. Note that it is usually necessary to remove the seals and the grease from the bearing/use light oil otherwise it will be too draggy to spin nicely. Other than that it is quite easy, all you need too do is fixate the bearing centre.

[Cyclothesist]

I'm sure that's very safe in your hands Brucey but I can well imagine in mine it would be a high speed shrapnel flinging hazard.

[Brucey]

I think it is probably safer than using a conventional lathe but I agree it probably isn't for everyone.

[rjb]

Don't forget the goggles. I was using a wire brush in a drill recently when a wire strand came loose and flew into my arm. Far worse than the covid jab.

[Brucey]

incidentally, the best (and simplest) way I know to fixate the work is to use plywood. A small piece of plywood can be mounted and held on a length of studding, eg using epoxy resin. The plywood can be SG'd to the exact same diameter as you wish to hold. Then it is simply a case of wrapping a steel strip around both the plywood and the workpiece, and clamping it in place, eg. using hose clips. This kind of tooling would never be good enough to allow the work to be turned in a lathe; the tool would very quickly 'dig in' if you tried. However, the contact forces can be so low during SGing that it is usually possible to employ improvised tooling of this sort.
I have occasionally used a small angle grinder in a lathe, but this can be no fun at all, since you are usually showering the expensive bedways with abrasive crud. By contrast, SGing normally showers the ground, harmlessly, with abrasive crud. Using the right setup, it is possible to do surprisingly accurate work this way eg regrinding of bearing surfaces etc.
I have also found it easier to work some materials this way. For example, it is not normally considered possible to grind Al alloys using standard wheels because they clog too easily. Yet often, they are more amenable to being SGed for some reason.
If you are a seasoned tinkerer, you probably (as I did) already have a lathe. However I soon found myself doing many more things via
SGing. There are exceptions of course, but there is very little that cannot be done by someone armed with just a drill, angle grinder, dremel tool, and welding set, if they know how to use them.

[Biospace]

This reminds me of seeing brake discs being 'cleaned up' in situ at a roadside workshop in Africa. There was a queue of cars waiting for the 5 minute operation in which the vehicle was jacked onto stands, engine started, gear engaged and the angle grinder would be offered up to the rotating braking surfaces.

Another day on the same section of road, a taxi was observed being stripped to a bare shell, before being welded (under an umbrella, it was drizzling) then repainted. The following day it was being reassembled and had been driven off by the end of the day.

[Brucey]

........the vehicle was jacked onto stands, engine started, gear engaged and the angle grinder would be offered up to the rotating braking surfaces.

oddly enough I have been thinking about this kind of thing myself, lately. You can buy an attachment which allows you to remachine discs in situ, but it is fiercely expensive. Central to its operation is the fact that it takes a cut from both sides at once. I think this greatly lessens the chances of leaving unacceptable DTV (disc thickness variation). The tolerances for DTV are typically 1/10th of those for disc runout. Typically a vibration is felt through the brake and the conclusion is soon reached that 'the discs must be warped', whereas in fact it is far more likely that DTV is to blame and any warpage is quite incidental to this.
Now nearly all angle grinders have a threaded (often M10) fitting on the gearbox, for a handle. My scheme is to use these fittings to mount two angle grinders in a single pivoted frame, thus allowing both sides of the disc to be dressed at the same time. This should allow discs to be dressed in situ, whilst keeping the spectre of DTV firmly at bay. The hub can be rotated using an electric drill if necessary.

[plancashire]

I remember my metalwork teacher blanching when he saw a pupil approach a lathe with a piece of emery cloth. He explained that all abrasives must be kept away from a lathe to avoid abrading it.

How do you keep the abrasive away from the drill? Or doesn't it matter as the only moving part is fairly well protected, unlike the sliders and screws of a lathe.

[Brucey]

when using the SG method it important to note that the peripheral speed of the grinder (yes, even a dremel) is well in excess of that offered by the drill. So the drill might be going at 1000rpm but the wheel of a small angle grinder is both larger @ ~100mm as well as faster, typically 10000-12000 rpm, than a typical workpiece. This means the debris from grinding flies off tangentially, pretty much as normal. In practice it is not difficult to ensure most of the debris goes on the floor and not into the drill.
The drill typically creates a peripheral velocity in the workpiece that is (in m/s) in single figures; by contrast, the grinding wheel is usually doing 300-3000 m/s at the edge. This means the grinder will always remove material, regardless of how the drill is set. However, there can be little doubt that the material is best removed by maximising rather than minimising the relative velocity of wheel to workpiece. This often means (if you are right-handed) it is best if the workpiece is rotated ACW, since this usually offers the highest relative velocity allied with the best access to the workpiece.