Fitting grinding wheels

Hi, I dont know if I'm in the right group for this - but here goes. I gave recently taken up the hobby of lapidary (gem cutting and
polishing etc.). I purchased a grinder polisher and grinding wheels etc. but the machine "Cabmaster" although new, came with no instructions. There is a 20 mm diameter shaft to take the wheels, which have what appears to be polythene or similar inserts in their bores. There is a pair of flanges - one of which, presumably, goes either side of the grinding wheel. Now my problem is this. I can slide one flange on to the shaft, then slide the grinding wheel on (which is a tight fit) and then slide the secon flange on but what stops the wheel from 'wandering up and down the shaft? There are no grubscrews in the flanges. I dont want to turn the thing on in case I do damage to machine or myself. Is it now standard to have a polythene bore wheel simply slide on? I know I sound a bit of a div' but in my days there was collets with grubscrews, and large nuts and washers etc. things seem to have changed. I have e-mailed the manufacturer but so far no answer. Anyone got any ideas?
Thanks for reading anyway, Bob
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