Mark,

I think you will find it worth looking at this previous thread: **LINK**

http://www.model-engineer.co.uk/forums/postings.asp?th=111162&p=1

It's inconclusive I'm afraid … but an interesting discussion.

MichaelG.

My old Drummond M-type has had a honking great 5" (125mm) chuck on it since 1937 and is still going strong. The Drummond's headstock bearings are smaller than the ML7's yet have stood up well, so far.

Consider a thin four jaw, less weight than a standard three jaw. I bought a Zither from ARC and fitted to my SC3 no problems of twelve months use. Consider not only the chuck weight but also the weight and overhang.

I ran a five-inch 4-jaw on my Super 7 for years without problems. But it was a shallow, threaded-body chuck, and therefore light in weight and with not much overhang.

Myford will have calculated the stresses and strains involved and then sized everything up by a safety factor, probably at least x5. You have headroom.

With moderation you can use a heavier chuck. Bear in mind Jason's point about keeping an eye on the total weight – resist the temptation to put extra large, heavy, or unbalanced work in the chuck just because it fits.

Personally I wouldn't overrate anything I care about by more than 1.5x normal for long. Slight overloading will likely take decades to harm a lightly used machine. Routinely overloading a busy lathe will cause early problems. In other words, go for it within reason – just don't take the piss. If you break the lathe it's your own fault.

Dave

I have got a 6" 4 jaw which I use on my ML S7, often intermittent cuts on a chunk of Cast Iron or black mild steel, 2"/3" square x 3" or 4" long, but only at very low speeds. The chuck alone is mighty heavy but I think it's more a test of my weight lifting ability these days. Lathe appears to be able to cope with it OK, no swash – plate effect so far.

John