3MT vs R8

[Gavin Newman]

Probably a dumb question here......

My "new" 20 year old mill came with a few 3MT bits and pieces. I've noticed that the manual for the unit that says the spindle is 3MT/R8.

Are 3MT and R8 tools interchangeable or would this have meant that the unit came in either R8 or 3MT flavour? If the two types are interchangeable what are the advantages (disadvantages) of R8 over 3MT?

[Gra]

MT stands for Morse Taper, so 3MT would be No 3 Morse taper an engineering term for the taper on the end of the tool that mounts in the mill.

I am guessing R8 is another description of the same (Maybe american or european, not sure never heard of it)

[Big Shed]

R8 is a taper/collet system originally designed for the Bridgeport mill and is different from a Morse Tape 3.

See here (http://en.wikipedia.org/wiki/Machine_taper#R8).

A lot of mills on the market today, including my X2 mill, are available with either an R8 spindle or a MT3 spindle (which is what mine has). I use an MT3 chuck (http://cgi.ebay.com.au/ER25-MT3-M12-COLLET-CHUCK-KEY-CNC-MILLING-LATHE-A69_W0QQitemZ110302793931QQcmdZViewItem?hash=item110302793931&_trkparms=39%3A1%7C66%3A2%7C65%3A1%7C240%3A1318&_trksid=p3286.c0.m14)to hold ER25 collets (http://cgi.ebay.com.au/NEW-15PCS-Precision-ER25-Spring-collet-From2MM-16MM-AU_W0QQitemZ160294021484QQcmdZViewItem?hash=item160294021484&_trkparms=39%3A1%7C66%3A2%7C65%3A1%7C240%3A1318&_trksid=p3286.c0.m14) to hold the end mills etc.

[Grahame Collins]

Are 3MT and R8 tools interchangeable

No

or would this have meant that the unit came in either R8 or 3MT flavour? If the two types are interchangeable what are the advantages (disadvantages) of R8 over 3MT?

R8 refers to the type of collet which I read somewhere was designed by the Bridgeport mill people.
.
I suspect your female socket in the machine spindle is 3MT.

Somewhere there is a male 3MT fitting with the fitting to take the R8 collet.The end is threaded and pulled up into place by a bar from the head of the mill. The bar has a name but its in the grip of alzeihmers for the moment.

If I have not got it right no doubt the machinists here will weigh in with more and better info.

The attached pic shows the R8 collet. I googled a bit more and found the same R8 form that was used in the big Hafco at school. The drawing shows the form of the R8 taper on the RH Side .Note the thread in the end of the R8 taper for the drawbar-(thats the bloody word).

The one at school had the parallel shank with a little keyway cut in it.One had to check- with fingersi that it was lined up during tool changes.

cheers



Grahame

[Gavin Newman]

Thanks all for the quick response, the consensus is that 3MT and R8 are dimensionally different beasties - I'll stick with buying 3MT kit for my mill.

Rgds - Gavin

[Woodlee]

Grahame has it right .R8 is a bridgeport proprietry fitting but their machines could be ordered with MT spindles as well. The MT is is a much better mounting taper because of its bearing length ,the R8 has just a short bearing surface for the taper.
You can get a MT3 to er series collet chuck and collets for holding cutters and the collet sets are available on Ebay
Personally I would not bother with R8 collets.

The bar you are looking for is called a draw bar Grahame.

Edit :remembered a post on uk metalworking news group so I pasted some comments here

It's a system that seems to me to be strong enough for the small machines
>and small tools it runs on. It's nothing like as sturdy as the bigger INT
>series but then it isn't designed for taking the sort of cuts that machines
>fitted with those will be doing.
>
>If I had a grumble it's that concentricity is not as good as with collet
>systems with a longer engaged length and I've never seen a machine that
>still had a location pin left in it. They snap off the first time a cutter
>jams. They seem to do bugger all anyway though so it's no great loss.
>
>Theoretically concentricity is meant to be provided partly by the tapered
>end of the collet which is only about 3/4" long and partly by the top end
>being a tight fit in its sleeve 4 inches away. Unfortunately as the sleeve
>and collets wear this end doesn't do much and the bottom end can rock as
>it's tightened. That's the silly part of the design if you want to find one.
>You can't beat a long, narrow angle taper like a Morse for concentricity
>which is why later collet systems than the R8 look more like those. The R8
>is unnecessarily long which wastes space but to no good effect if it's worn.
>You ideally need at least double the R8's length of tapered fit for good
>concentricity. A 3" long collet with 1.5" of taper and the rest for drawbar
>thread would save space and work better. Oh, hey, I've just designed an
>INT30.
>




Kev.