Early Bridgeport M head looking quite at home on a 1950 Van Norman 22L-U Universal Mill The M head, unlike its successor, the J head, doesn't have back gears or power spindle downfeed. That could be a deal-killer to anyone who wants to bore out a lot of cylinder blocks. The M head is certainly much easier to move around and get into odd positions than the far bigger and heavier J head. Back in the days when I'd never even heard of the M head, I didn't realize how handy it is to be able to move the head around so easily. So a prospective owner shouldn't let the fact that it's little, old, and not as high-powered as its more fashionable descendant put him off. On the other hand, he can't expect to find much in the way of factory parts. These things are old - when the last M head was made, the US flag had only 48 stars on it. But the prospective owner might very reasonably be discouraged by The Collet Problem. Based on the samples I've myself encountered, perhaps 75% of all M head spindles take collets with a Morse #2 taper (hereinafter 2M). Most of the rest take collets with a Brown & Sharpe #7 taper (or 7B&S). A very few take B-3 collets. All three collets are small (1/2" diameter max), much smaller than the R8's capacity. And to answer the obvious question - no, the M head spindle can't be reground with an R8 taper. It's just too small. B-3 collets look like scaled-down R8 collets. The Bridgeport C head used B-3 collets, standard. M heads using B-3 collets are so rare I won't discuss that size further, except to note that they are still available from Hardinge. BEWARE - B-3 is not the same as 3B. 3B collets were used in some vertical and horizontal mills made circa 1900 by Becker (Hyde Park, MA). Although both 2M and 7B&S collets are still available new, they are more expensive than some of the R8 collet sets. And just identifying which fits in any particular M head can be a real challenge. They are almost identical to visual inspection, and the Bridgeport heads are not marked in any way to indicate which they take - an annoying oversight by the normally meticulous little elves at Bridgeport. Here is some info stolen from the Bridgeport pages of Tony Griffith's invaluable site, www.lathes.co.uk. These are the same dimensions published by Bridgeport in the early 1950s, in their ATTACHMENTS catalog. 
And here are the relevant bits from Hardinge Workholding General Catalog 2348 (get the whole thing as a pdf file from www.hardingeworkholding.com/w1collets.html to see just how many oddball collet sizes are available, if you're willing to pay for them) - UPDATE - They don't seem to have enough to do at Hardinge, so they move the pages around. Try hardinge.com/index.asp?pageId=259 FURTHER UPDATE - It's still a moving target. As of November 2007, it's linked from hardingeus.com/index.asp?pageId=141 
NOTE - There are some minor errors in Tony G.'s data (for instance, the best numbers I have for the 2M taper is 0.59941 inches per foot, and for 7B&S it's 0.50147 inches per foot), and maybe with Hardinge's. I'm working on verifying the given dimensions (as of 29april04), but that's all I have for now. COLLET SETS I know of no mill currently in production which uses 2M or 7B&S collets. Fortunately, even though the machines aren't being made, the collets are. The basic 7B&S or 2M set, as used in vertical mill spindles, is seven collets ranging from 1/8" to 1/2" diameter, by 1/16" increments. The thread for the drawbar is 3/8-16 UNC. VARIATIONS Intermediate sizes - When used as workholders, more in-between sizes are needed. Both tapers are available in 1/64" increments. Fortunately, these intermediate sizes are not needed to hold mill cutters. Most high-speed steel or cobalt cutters have 3/8" and 1/2" shanks (or larger, but those won't fit in the M head spindle, not nohow), so you could whittle out an ark with just those two collet sizes. Solid carbide cutters come in a greater range of shank sizes, and the full set of seven would then be useful. Metric - Occasionally I see collets in metric sizes, 2mm to 12mm. These wouldn't be too useful to most of us in the USA. Drawbar threads - The standard thread is 3/8-16 UNC. However, I have seen these collets advertised with 3/8-16 BSW threads. BSW is British Standard Whitworth. The thread angle is 55 degrees, unlike the 60 degree thread angle of the American Standard thread form. The other Whitworth thread dimensions are very close to the Unified standard, but whether you would want to use a BSW collet with a drawbar with a 3/8-16 UNC thread is a question I am going to dodge. In England, Chronos has 2M collets with these Whitworth drawbar threads. COLLET MANUFACTURERS & IMPORTERS Hardinge Inc. lists its collets at Hardinge Workholding, and sells them at Hardinge Tooling. B&S tapers are well covered, but the Morse listing is oddly skimpy. Phase II lists 2M but not 7B&S. However, I know that they do in fact make (or market) 7B&S collets - apparently they just don't want to admit it. Another big collet manufacturer, Royal, lists neither 7B&S nor 2M collets. Well, phooey on them. The Lyndex site is similarly disappointing. And 'way back when, Bridgeport sold these collets, marked, appropriately enough, "Bridgeport". Nice old tired specimens regularly turn up on eBay. RETAIL
The only item in the above list that I've myself bought was a 1/2" 7B&S collet from Meridian. Meridian's site didn't specify the manufacturer or importer, but it came in Phase II packaging. The only mark appearing on the collet itself is 1/2. The little plastic tube it came in is marked phase II+ 1/2" Brown&Sharpe #7 Collet 224-322 
The new Phase II collet is 3.170" long, from the shoulder of the business end up to the threaded end. The same dimension of the old collet is 2.875", as we would expect from the Hardinge specifications and the unidentified specs I cribbed from Tony Griffiths. So what's with the longer Phase II dimension? The tapered bearing surface of the new collet is much longer, too. From the wear patterns on both collets, I would say they fit the spindle bore the way they should, so I don't seem to have misidentified the spindle taper. That was a possibility, as neither the M head spindle nor the old collet are marked in any way to indicate the taper. The old collet has no brand or size markings, either. I really don't know why manufacturers have to be so cryptic, as 7B&S and 2M are very similar, and easily confused. In fact, Gold Machinery had misidentified the spindle taper of my mill - when I bought it, I thought that it had a 2M taper (that misidentification is my only complaint with Gold, by the way). I held off on buying collets until I had picked up the mill and checked the spindle taper for myself, though, so at least I didn't get stuck with a set of 2M collets which wouldn't fit. 7B&S collets are more expensive than 2M, and tooling in that taper (such as boring head shanks) is gawdawful rare. But we can work around that problem - no need to abandon the poor orphan M head because of that. These are actually outstanding collets, functionally, as those gradual tapers grip like grim death - much tighter than the more leisurely taper of R8. SO WHICH TAPER DOES YOUR M HEAD SPINDLE HAVE? A good way to tell if a spindle takes collets with a 2M or 7B&S taper is by checking how something with a known 2M taper fits the spindle. If 2M doesn't fit exactly, then it must be 7B&S. 1. Remove drawbar Loosen the nut at the top of the spindle with a 1" wrench. It is a left-hand thread, so turn the wrench to the left to unscrew the nut from the top of the spindle.2. Put some sort of layout dye on a test taper Here is a dead center for the tailstock of a 10" lathe. These are very commonly 2M tapers, and make good test tapers for our purposes.3. Put test taper in spindle Do this immediately, before the Sharpie ink dries. Don't bang it in or it will be impossible to move. Rotate it slightly with your fingers.4. Remove test taper and examine mating marks The Prussian blue (or Sharpie ink) will have been scraped off where the test taper touched the spindle bore. If it looks like this - To The Engine Room
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