Hi Baal - perhaps your instructor is getting mixed up between PUWER or what used to be in e.g. Factories Act 1961 Section 14 and the long since revoked Abrasive Wheels Regulations.

The Abrasive Wheels Regulations recognised that compliance with FA 1961 Section 14 was impractical, BUT required guarding of the "dangerous parts of machinery" so far as PRACTICABLE, but also recognised the risk of an abrasive wheel bursting, hence introducing training etc requirements.

Your sanding disc might not represent the same risk of bursting, but is unlikely not to be a "dangerous part of machinery" (or the form of words used in PUWER). As example there is still likely to be potential for entanglement around its spindle. The level of danger of the sanding disc will depend on its nature of construction. 

If your instructor is happy to show that the edge of the disc doesn't hurt them when they touch it when it is rotating at its normal speed, then may be it isn't in itself dangerous.

P

Sounds contrary to PUWER as you are removing a device fitted for an intended purpose by the manufacturer.

Too frequently "tooling" appears (especially on-line) to alter the designed function of power tools.

Lately with grinders there is a tendency for "toothed" chain wheels similar to a chain saw - all ending up subject to regulator recall / withdrawal from sale.

When you say 4" grinder I am going with a "hand-held" "angle grinder" rather than a bench mounted or pedestal off hand grinder as the latter are normally 6" or above.  Such devices normally use reinforced abrasive disks.

Either way, to remove the guard you would be modifying the safety features of the machine, and unless this is an approved method detailed in the operating manual for the equipment would negate the CE marking, thus putting you in breach of PUWER Reg 10. (I am presuming that you are in the UK, as I can only respond from a UK standpoint.)

You will almost certainly find that sanders designed to use flexible coated abrasive disks have a different design to that of an angle grinder, even if it is only the mounting pad for the disk itself.

If the machine OEM offers a mounting disk for coated abrasives then this this could be an approved use.

If you are looking at an offhand bench or pedestal mounted grinder that normaly uses bonded abrasives such as archetypal rigid aluminium oxide or silicon carbide grinding wheels.

These are often available with a grinding wheel at one end of the spindle and a sanding disc at the other, however, such machines are designed in this manner.

To modify one, again unless sanctioned by the OEM would negate the CE marking and put you in brach of PUWER reg. 10.

To do either is possible, but you would need to design and CE mark suitable guarding as required to be fitted to the converted equipment.

Such guarding falls under Annexe V of the Machinery Directive and the Supply of Machinery (Safety) Regulations, and therefore must be CE marked independently unless provided as a spare part by the OEM.

To do such things safely and correctly, probably negates the cost of procuring the correct equipment for the task.

Though if one does buy equipment, please ensure it is correctly CE marked and most importantly, safe!

The fist security image ended in CE how apt!

Edited by user 06 October 2021 10:09:55(UTC)  | Reason: Comment

"told by abrasive wheels instructor" will not stand up in Court.  Have you received documentation written by the said instructor.

As well as asking him to prove what he says ask for the training notes.