ER-40 Collet Chuck For My Lathe

[Rickabilly]

(09-13-2012, 07:09 PM)starlight_tools Wrote: The reason I asked is that in the instruction book for my 1440V lathe which is 30-2200 RPM it states clearly that "Grey Iron Chucks must NOT be fitted on this high speed lathe. Use only Ductile Iron Chucks"

Walter

Hello All,

This sort of talk apparently started with the Colchester Mascot 1600 in 1965 when as a part of the 600 group of machine manufacturers, Colchester started to recommend the use of Pratt Burnard dynamically balanced ductile iron chucks for all of their larger, new high speed lathes, It is widely accepted that the Mascot was the first of these new "High speed" larger lathes. When one does the sums, it is obvious that Grey Iron is still suitable at many of the max speeds available on these machines, BUT, Pratt Burnard made a lot of money for the 600 group when this specification became standard fare on any high spec machines and Pratt were the only company at the time mass producing chucks out of this material.

Call me suspicious but when there is money to be made I'm not very trusting, an admission by one of the engineers at Colchester said that "the material and max speed rating was of less concern than the dynamic balancing that was standard on these higher specified chucks", dynamic balancing puts less load on the bearings and reduces surface finish defects.

Suffice to say that on lathes swinging 12" or less the speed rating of most chucks will be fine, considering the diameter of most car engine flywheels and the fact that they get up to 6-8000RPM while remaining well within their burst pressure, and for a 6" diameter cast iron chuck adaptor this would convert to many thousands of RPM. given a 12" grey iron flywheel running at 6000 RPM halving the diameter should increase the allowable speed by four times (the inverse square law) which sees a max RPM up around 24,000 RPM so probably suitable for most hobby lathes I would think.

Accurate calculations for burst pressure/speed are near to impossible in most cast iron materials as cast irons are not homogeneous and the effect of casting porosity in the wrong spot could change the strength by a factor of 10 in a worst case scenario,

So how to definitively answer this question, well;
Where grey cast iron is sometimes quoted as having a tensile yield strength up to around 220 MPa and the yield tensile strength of ductile cast iron can start as low as 200MPa which means there is a cross over between the terms grey and ductile iron, with some greys being more ductile than others. Keep this concept tempered with the knowledge that a casting flaw will affect the strength far more than the grade of the material ever will. so keep away from chucks with any visible casting defects, in fact return them immediately where possible

I think it is safe to assume that any factory going to the extra expense of using ductile iron will also be more focused on quality than an equivalent group happy to use grey iron, I think that the only way to be certain is where the manufacturer states a maximum speed rating, where the manufacturer does not provide this use 2/3-3/4 of the figure stated for a similar sized chuck from a different manufacturer and you can be relatively confident, but regardless when test running a chuck at high speed for the first time do so from a safe location make sure no one is standing in the burst zone which is approximately 30 degrees either side of a line from the centre of the chuck and perpendicular to the spindle axis. It is also a good idea to at least test the static balance of a chuck prior to running it in a high speed application.

Sorry for the head ache Ed, (Hmmm, EDache?) I hope this has been of value to some one out there.

One last question though, In my 30 or so years around machine tools I have never heard a first hand account of a burst Chuck from overspeed, has anybody else heard a first hand account? to be valid the breakage must have been seen by yourself or someone you trust completely. I have seen many grinding wheels and milling cutters burst, maybe even explode, also I had a hammer head split into many shards, despite Mythbusters saying that it is impossible, Over hard cold chisels do it all the time, but still I've not seen a chuck of any kind break up or even crack, except for one being thrown off when a camlock system was left insecure this cracked the back edge of the outer diameter, where the casting is very thin.

Best regards
Rick