Burnerd Multisize collets vs Crawford Multibore collets

I wonder which type is preferred, particularly in terms of better results for runout and which is manufactured with better materials resistant to rust. Any views much appreciated

Edited By Adam Harris on 18/04/2020 15:20:27

Edited By Adam Harris on 18/04/2020 15:22:12

Are their no specifications on the net? Keeping/storing tools in a dry place - and protected with a suitable product (oil/grease?) - would avoid rusting, which is likely for any ferrous item if stored under damp conditions without adequate proection.

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 lubricate my Multisize collets, keep them in their case and should be surprised if rust were to be a problem.

Don't know about the Multibore but see no reason why they should be rust-prone either.

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

used to use crawford collets collets on a Emimec Sprint auto lathe, would run all day 7 days a week never a problem

you can get some with grippers in the bore to stop bar push back, but they all rust if you let them.

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

There are detailed specifications in the official Crawford Multibore brochure (eg for a 16mm diameter bar, measured at 38mm from end of collet, runout up to 0.03mm) but I have not managed to find any such detail for Burnerd Multisize, despite being part of the same "600 Group". This makes me wonder if perhaps Burnerd Multisize are not so accurate and so 600 Group are not falling over themselves to provide such data, which should be normally of interest to buyers of any expensive collet system. Regarding resistance to rust and corrosion, obviously everyone knows what are the best methods and conditions to keep tooling rust free, but nevertheless some manufacturers do seem to produce tooling that is remarkably resistant to rust (eg in my experience particularly swiss makers like Schaublin and Aciera) whereas others rust remarkably easily (eg in my experience Myford collets, some chinese ER collets).

Edited By Adam Harris on 18/04/2020 18:27:40

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.

For what it’s worth ...

When I was persuaded to sell my Burnerd Multisize set [and yes, it was something I will always regret], it had been stored, indoors in the warm and dry, for about 25years.

There was black ‘veining’ visible all over the chuck body ... a form of corrosion strangely characteristic of high quality steels.

MichaelG.

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

Thanks Clive - so no runout spec for Multisize to usefully compare with the Multibore. If the Multibore is using rubber inserts presumably they are perishable in which case it sounds to me that the Multisize could be better especially as I am always thinking buy used. Yes I was viewing the Multisize EC collets as a type of 6 jaw chuck (and MC as a type of 8 jaw chuck) but am disappointed to read that not recommended for holding short ends of bar which I though might be a particularly useful extra benefit over a 3 jaw chuck...is this so and if so why?

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.

But I don't see how the fingers can cant because the finger is trapped hard between the workpiece and the conical bore of the collet chuck - there is logically no way for it to cant in my opinion. Perhaps one could imagine the finger could slide in its slot if not tightened up enough, but even then the finger is a continuous piece of metal and it would have to deform/squash around the workpiece in order for the workpiece to come loose.  I think the ER "fingers" that don't run in a slot but are all spring together could twist out of position and in doing so release the bar .

Edited By Adam Harris on 18/04/2020 20:23:29

Edited By Adam Harris on 18/04/2020 20:27:12

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

As a matter of interest, what lathe and what condition is it in? Little No point in having ‘“Rolls Royce” tooling holders if it is to run in an old, worn out “Moggie Minor” machine. The value of the tool-holding will be more than swamped by the other systematic errors.

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

Thanks everyone and thanks for the specifics on short parts Andrew. Andrew, why did you choose the Multisize system over the Multibore system?

Edited By Adam Harris on 18/04/2020 22:12:27

Thanks Clive

I didn't, it chose me. When I bought my lathe and vertical mill from a dealer he threw in the collet chuck and collets as a deal sweetener. I didn't know any better, but I sure wasn't going to say no. As it turns out it was a good deal as I use the collet chuck by default.

Andrew