R8 or morse taper 3?

[old martParticipant @oldmart]

I have always tightened up the R8 drawbar before switching on and have never had any slippage. R8 is only prone to slippage when operators fail to tighten properly. The largest shell mill size we have are 80mm Hertel and one that takes APMT16 inserts and they have never had problems. The drill mill still has its pin in the spindle bore, undamaged. Bridgeport where the R8 design originated used 2hp motors, see my last post referring to the South Bend mill.

As for stiffness, in any machine it is an advantage and the point at which the tooling bends is at its narrowest where the tool leaves the spindle. There it no way on earth that MT3 can be as stiff as MT4, and that is the size that equals R8. The smaller MT2 is at an even greater disadvantage which is why Clarkson made a stiffening collar to windup against the end of the spindle.

The ease with which R8 can be removed compared with Morse tapers is obvious, the Morse design was intended to be removed by a cross wedge, it was a poor decision to add a drawbar and make a good drilling machine design into a poor milling machine fitment. Recent threads on this forum have highlighted the problems of stuck Morse tapers in mills.

[JasonBModerator @jasonb]

Dave I doubt you will actually bend an MT tool but you will get more flex and therefor vibration and chatter on a thin MT2 than you would on R8

[Anonymous]

Posted by SillyOldDuffer on 14/06/2020 17:47:23:

• Any evidence that R8 tapers slip?

As you know I don't believe in pussyfooting about.

I've stalled both the Bridgeport and CNC mill without the R8 taper slipping.

Andrew

[Martin ConnellyParticipant @martinconnelly55370]

I've done a quick revision on friction and it has reminded me that it is a function that is independent of area and is a relationship between force normal to the surface and the force causing sliding movement along the surface.

This is the bit where I am open to correction.

The friction producing force in a taper is proportional to drawbar tension x sine of the half angle. So a morse taper with a smaller angle than R8 will have a lower friction producing force in the taper than R8 (for the same drawbar thread and torque applied to the drawbar). As a result R8 will transmit more torque than a Morse taper.

It seems counter intuitive but if the angle is reduced to zero there would be no friction drive because drawbar tension could not occur. If the angle went up to 90 there would be friction drive but poor positioning. There is also the need to squeeze the tool in a collet. The forces for this will be proportional to drawbar tension x cosine of the half angle. So a small angle gives higher clamping forces than a larger angle and at 90 there would be no clamping forces.

There must be some choice in the taper angle for the collet designer that balances torque transmission and clamping forces when designing a tool holding collet. Morse tapers were not originally designed to hold tooling as a clamping collet, the angles were chosen for their self holding feature for drills, drill chucks and reamers, R8 could pick an angle that was not self holding.

Martin C

[Former MemberParticipant @formermember32069]

[This posting has been removed]

[DiogenesIIParticipant @diogenesii]

..what degree of tool height repeatability is available with R8?

[Anonymous]

Posted by DiogenesII on 14/06/2020 21:52:04:

..what degree of tool height repeatability is available with R8?

Depends on how you hold the cutter. In a Clarkson lookalike chuck on my CNC mill I reckon better than a thou. When the Tormach CNC mill came out there was some work done on this (on CNCZone I think) which came to the same conclusion. In a R8 finger collet I've no idea as I rarely use them. But since they're not dead length collets I suspect that repeatability would be poor.

Andrew

[Mike PooleParticipant @mikepoole82104]

Running without a key in the R8 spindle could make removing a finger collet tricky if the drawbar doesn’t unscrew cleanly, I think I would use a tube and the quill or table to force the collet into the taper and hope it holds tight enough to undo the drawbar. Probably unlikely but **** happens.

Mike

Edited By JasonB on 15/06/2020 06:54:46

[Michael GilliganParticipant @michaelgilligan61133]

The math gets a bit heavy for me, but this introductory paragraph **LINK** helps with the ‘cone clutch’ interpretation of R8 vs MT

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https://books.google.co.uk/books?id=HsN_j4i90zoC&pg=PA453&lpg=PA453&dq=cone+clutch+angle&source=bl&ots=Y4r4-Q6HES&sig=ACfU3U1FzseMCWI9QbbJD2bEp1HmqXAV4Q&hl=en&sa=X&ved=2ahUKEwin34OasYPqAhWPasAKHaonCroQ6AEwIXoECAkQAQ#v=onepage&q&f=false

MichaelG.

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Edit: ___ in the quoted text:  I presume that ‘changes’ should read ‘chances’

Edited By Michael Gilligan on 15/06/2020 09:46:38

[not done it yetParticipant @notdoneityet]

IMO, these discussions of MT v R8 are fairly academic for most hobbyists. My machines are one horse power jobbies and I don’t set out to over-stress them. A couple thousand rpm max, generally, so taking my time with MT2 id perfectly good enough for me.

As I see it, the only real advantage is changing tools – and I am not in a great hurry on that score (usually)

Simply work with what you have and be satisfied with your results – or get a larger machine. Somehow I doubt the rigidity of some of the cheaper chinese machines is sufficient to make any overall difference to the finished article as that is often down to the user, not the machine.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 15/06/2020 09:48:09:

IMO, these discussions of MT v R8 are fairly academic for most hobbyists. My machines are one horse power jobbies and I don’t set out to over-stress them. A couple thousand rpm max, generally, so taking my time with MT2 id perfectly good enough for me.

As I see it, the only real advantage is changing tools – and I am not in a great hurry on that score (usually)

[…]

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An entirely reasonable analysis, if I may say so

I wish you continued happiness, and the avoidance of stuck Morse Tapers.

MichaelG.

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Edit:

[quote]

The R8 Tool Holder

The Bridgeport Machine Company in Bridgeport, Connecticut developed the R8 tool holder to respond to the need for quicker tool changes. 

[/quote]
 

Ref. __ https://www.sierraamerican.com/r8-tool-holder-versus-morse-taper/

Edited By Michael Gilligan on 15/06/2020 10:15:28

[Anonymous]

Posted by not done it yet on 15/06/2020 09:48:09:

………….or get a larger machine.

Done.

Andrew

[Nigel McBurney 1Participant @nigelmcburney1]

Before dowsizing I had an large Elloitt turret mill with 30 int spindle,which was totally inadequate for a large mill,particularly when boring,and with the tooling I had the 3 mt drill holder had to to be removed from the spindle to release a drlll, My other horizontal mill will with 40 int spindle the morse taper holder did allow access to the drift slot while it was still in the spindle but projected a long way out of the spindle,also int 30 tooling is not readily available second hand. So for drilling I bought a 3mt Meddings, now I only have a smaller Elliott omnimill with vertical and horizontal spindles both 3 mt ,the 3 mt does have a big advantage to me,my colchester,the meddings and the omimill all have the same spindles and tail stock. I have never used a Bridgeport ,though I can see the advantage of the R8 taper allowing collets to be so close to the spindle bearings,to improve rigitity of cutters. On my mill I use er 32 and 40 holders ,the big advantage of the ER system is thet each collet will grip over a 1mm range,ideal when holding drills. I have not had an ER collet slip yet,though I prefer to use a Clarkson on really tough materials. I suppose the ER system came about when plain shank solid carbide cutters became availble and they could not be gripped in a Clakson collet. On the big mill I had the Bristol Ericson collets, I found that these would slip,the ER system is far superior.

[old martParticipant @oldmart]

The Clarkson threaded shank system will not slip, but I have heard of smaller cutters snapping their thread during pile ups. The only carbide cutters which fit the Clarkson system are inlaid with steel shanks. A thread in solid carbide would not be strong enough. You can buy MT 2 and 3 collets to fit the taper directly like the R8 ones which help with rigidity and work height for mills with limited Z dimensions.

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