You really can't generalise about cheap inserts bought from somewhere like ebay because you have no idea why they are cheap. For example:
- Likely to be Rubbish
- Poorly made for the hobby market with low tungsten in a weak matrix, perhaps made with worn tooling
- Counterfeit – as above but with a fancy brand-name and price tag
- Mixed Quality
- Factory rejects
- Old stock (early carbide is often inferior to modern)
- Wrong type of insert (fits holder but rake etc. not as expected)
- Mid-range carbide, for example inserts not made to the highest standards, or with an old process.
- Cost reduced Inserts intended for short-life production.
- Likely to be good quality
- Surplus from a production over-run
- Customer Surplus to release warehouse space or storage costs
- Customer surplus due to change of business
- Bankruptcy sales
- Ex-government
- Stolen goods
I'm afraid we live in a world where you can't trust brand-names, or that cost guarantees quality, or that Far Eastern made goods are automatically poor, or that cheap inserts are always rubbish.
Andrew has it right: it's about managing cost, If messing with tool changes in a commerical workshop wastes time and money it's sensible to de-risk by buying known inserts from a reputable supplier. Conversely, if you're a hobbyist on a budget with time to spare, it's equally sensible to look for bargains. But as always with bargains, recognise you're taking a gamble and be prepared for the occasional disappointment.
The inserts I've bought from advertisers on this forum have all been acceptable. I have not – so far – been disappointed by ebay either, though I haven't bought many. The worst inserts I own came from a show : I should have looked more carefully because they are factory defects.
The other suspect is the holder. Commercially made holders are expensive. A home-made holder in mild-steel works pretty well, so why are the real things big money. The reason seems to be that real holders are made from a more suitable alloy : one that's more resistant to bending and vibration. It's possible that Robin's cheap holder rather than the inserts is behind his disappointment. Be interesting to try the inserts in an expensive holder to see if their performance improves.
Dave