Decent tools of the type mentioned would not be made of a 'rust resisting' steel [stainless?], the manufacturers would be looking for stability & resistance to wear as priorities.

Tony

I use the Burnerd Multisize ones, you have to keep them clean and well lubricated otherwise the blades get stuck in the slots and the collet struggles to self eject from the taper and release the work piece.

Other than that I find them to be very good for general use, and the collet chuck lives permanently on the lathe, it only comes off when the four jaw chuck or face plate is used.

I cannot comment on the Crawford Multibore ones as I have never used them, but they work on a different principle being rubber sprung segments rather than sliding blades.

Phil

With the Crawford type there are lots of size and shapes of these out there in industry so always a good supply to chose from, dont know much about the Burnerd though

Funny that. First hits on ‘goggle’ provided fun-out specs for the Bernerd item. 0.0005 (imperial examples?) and 12 microns (metric versions?) at the nose.

Clearly that is not the whole story. Rotagrip appear to stock both items, so likely a good place to get the specs or comparison.

I would agree that some grades of steel are better than others, but cost is clearly one issue – typically those who buy chinese, buy cheap. Those that invest more dosh in tooling likely don’t leave it around in damp conditions, either. That, of course, does not mean that the high-end products are not made in China. Better quality just costs more.

Burnerd Multisize are specified as having a maximum eccentricity at the spindle nose 0.0005" rather than run out. Similar concepts but not strictly directly comparable.

For practical purposes eccentricity is directly related to collet errors whilst run-out comes from the spindle and mount. Hence run-out is hard to control and specify to close limits unless the collet system is native spindle fitting.

The sliding jaw construction of the Burnerd make is sort-of somewhat akin to a 6 jaw chuck. Theoretically the freely moving jaws cannot settle into a tilt along the axis but any inaccuracy in the jaw construction will produce eccentricity at force balance when the collet is closed one the material.

The Crawford is more akin to a conventional spring collet with rubber inserts to "extend" the spring range. Due to the solid nature of the construction and the elastomeric compression involved in closing the collet its theoretically possible for the force balance when the collet is closed on the material to produce an axial tilt. With more gripping jaws eccentricity is unlikely.

Either way the effect will be very small unless something went seriously awry when the collet was made. In a practical world there is little to worry about unless the material stick out is grossly excessive. Stock quality is likely to give greater errors. Unless you are in inspector meticulous mode with carefully controlled process odds are you will be dealing with a thou or two of error whichever system you choose. Whatever, its going to be rather academic for the likes of us who cannot afford a new , factory fresh, set so will need to cope with unknown errors due to wear. On production work wear will occur.

Being built up from parts neither system can measure as accurate as a classic spring collet which is made in one piece.

Clive

Edited By Clive Foster on 18/04/2020 19:04:31

If you have been reading up on them, you will know how the collet is constructed (i.e. six 'fingers', spring loaded) and that each collet has a relatively large grip range.

If you try to hold something shorter than, let us say, 2/3 of the finger length, the fingers are likely to try to cant in the tapered bore of the closer, pivoting about the back end of the stock. It is much the same issue as with ER collets and short items.

If you want to grip short stuff, you need an 'enveloping' type like a Morse taper, 5C or W-type, with its correspondingly very limited grip range.

Adam

Small errors can drive you nuts.

Assuming sufficiently parallel stock of sufficiently consistent diameter with randomly distributed variations the sliding fingers of a Multisize collet are fully constrained when tightening if both sides of the finger are in full length contact with the closing cone and the stock. Full constraint is necessary for a meaningful and repeatable specification of accuracy. When a perfect collet is tightened down onto perfect stock all forces are in balance with constant loading along the length of the fingers. In this case any error is, theoretically, due to manufacturing tolerances in both collet, its holder and the stock and the results therefore quantifiable.

If the stock is shorter than the fingers the back, unsupported, end of the fingers are in the wind with no compression forces from gripping the stock. Hence the forces are out of balance. Depending on the exact interaction between manufacturing tolerances and the force variation along the finger the nearest approach to force balance may be with the stock slightly out of line. For obvious reasons this is totally unquantifiable as the error is emergent, not deterministic.

Of course any residual out of balance forces need to be absorbed in the finger itself. Which is clearly less than ideal. They are not designed to deal with that. Unlike a chuck jaw which, up to a point and with appropriate care, is. Whether such use actually results in damage must, in the general case, be unknowable. Obviously the shorter the length of stock held the worse the situation but defining where the line between "naughty but OK this one time" and "flagrant abuse" falls is impossible. So Burnerd have to say "Don't do it." Clearly 10 thou short is neither here nor there but having only 1/4" of stock in the collet isn't going to fly. Well actually it might literally fly when tool hits work.

Clive

PS Dennis Turk, SouthBend Guru, lathe restorer extraordinaire  and all round good guy was wont to say "Everything is made of rubber.  Maybe stiff rubber.  But still rubber."

Edited By Clive Foster on 18/04/2020 21:12:03

Edited By Clive Foster on 18/04/2020 21:12:22

The Burnerd Multisize collet chuck sits on my lathe by default, unless I need the 4-jaw chuck or one of the faceplates.

Despite the skepticism I can assure Adam that they simply don't hold short parts. For lengths less than a quarter of the collet length the fingers simply tilt and one can pull the work out by hand. Up to about half collet length the work seems to be held ,but may well come out under cutting forces. More than half the collet length is fine. One solution is to use a short length of material at the back of the collet when using short work. But the spare material must be the same diameter as the work to within a thou or two otherwise the collet won't hold the work securely when it closes.

Andrew

That's how I got mine as well, the set was included with my Harrison L5A when I bought it.

NDIY

What's your dislike of "Moggie Minors" all about ?, they are OK if you look after them and maintain them properly.

Phil

Who said I have a dislike of the venerable moggie? Most certainly not me. I simply used a worn out old car as an example to compare with Rollers (which can also eventually wear out and are hugely more expensive to repair!).

In my younger days I only had one BMC – an 1100 – and soon swapped that for a Ford. Anglias, Cortinas and Escorts were favoured over a slow moggie minor (I drove a girl friend’s moggie only when Inreally needed to&#128578.

Trouble is, these days, many (not all) who have Minors do not have a clue of how to fix them – they are too simple for many owners to understand them.🙂