R8 spindle advice required

I am currently converting a Tom Senior light vertical head from MT2 to R8.

R8 spindles were designed with a pin projecting into the bore to stop the tooling from slipping when the drawbar is tightened. There are mixed opinions as to whether the pin is needed, or just a liability. I have the choice at this stage, "to pin, or not to pin, that is the question".

I look forward to your comments.

My VMB has the pin, it hasn't sheared off yet! I think for normal cutters and materials it probably isn't needed as long as you tighten the drawbar well; but if you're using beefy face cutters or shell mills it might be handy (but also might shear off!). One issue I had with the R8 collets I bought, some of them the slots had tight spots which took some stoning off (I probably used a diamond hone) to fit easily. For a "light vertical head" maybe you can dispense with the pin?

I think the word ‘designed’ is the operative word. The mixed opinions are likely not from the designer or designer team, who actually knew why it was designed that way?

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I concur

The pin should never take any driving load ... if it does, the accident has already happened !

That said: if you are prone to overload machines it may be expedient to omit the pin.

MichaelG.

I like a pin,

Colin

My Bridgeport has lost its pin. No problem with tightening the collet., but if the collet slips the pin I understand can cut a groove in the collet and lock it in place

My Sieg sx2p cnc has a pin but also has a very effective spindle lock so its there, but never needed

It all depends on providing an effective spindle lock. The Bridgeport has built in brake shoes, while the sieg has a block fitted to the base of the head with a hole through the front which matches the tommy bar pin hole in the spindle .

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I am genuinely confused, Zan

Can you please explain how locking the spindle can affect the ability to tighten a collet into its taper socket in the spindle

Thanks

MichaelG.

My Naerok (which is probably nearer in scale to your Senior than a Bridgeport) didn't have a pin. The drawbar had a hex on the bar as well as the nut. To tighten the collet run the drawbar into it until it has plenty engagement then tighten the nut whilst holding the bar from turning, no spindle lock required

True Michael hadn’t thought of that but I can assure you it works . Perhaps me holding the tool in a gloved hand stops it rotating.

The point I’m making is that Not having the pin doesn’t seem to make any difference in what is probably the biggest miller found in a me workshop

There is no way that the pin could prevent slippage, the taper being tight does that. The drill mill at the museum had to have the bottom bearing replaced, and that spindle has two pins, one directly under the inner bearing race. I just set them to allow easy tool fitting. One or two of the R8 tool shanks I bought needed their pin slot relieving slightly before they would fit.

 A spindle lock won't stop the tool being slightly prone to slip while the drawbar is tightened, unless the tool shank has a spanner flat on it. I would hope that smartly seating the tool and holding it while the drawbar is tightened should suffice. The drill mill has no spindle lock, we just hold the top pulley and give the drawbar tool a couple of whacks with the hand. The biggest cutter that is used is 80mm, we have never had a problem, although we refrain from taking big cuts.

The new spindle will project enough to put spanner flats of 32mm without loosing too much wall thickness, there is no other way of holding the spindle due to the design of the mill.

Edited By old mart on 28/07/2019 13:47:20

Thanks, Zan

I do have another 'take' on the pin or no pin dilemma, but I will save it for later ... there are domestic chores to be done.

MichaelG.

That's strange Duncan as my naerok RD350 mill has the pin, i also have the 2 nuts at the top of the draw bar. The top nut pinned through the drawbar & second nut adjustable, my milling chuck along with drill chuck arbor have the appropriate groove for the pin & you have to rotate the chucks to line the pin & groove up to get the arbors fully up into the R8 taper..

My VMC has a pin and it is probably only useful for the initial insertion and running down the drawbar, once the drawbar starts to tighten then the taper provides all the friction needed to stop the collet turning.

Mike

My Herbert mill was converted from Herbert collet taper to R8 by John Stevenson and it has never had a pin. You can use one or not use one but if it has one fitted you better make damn sure it's in the slot of the collet every time before you pull up the drawbar.

On my Naerok mill the draw bar is no where near the threaded chuck arbor until the pin is in the groove & the chuck & arbor are lifted up to reach the draw bar. So it is not possible to thread the drawbar into the arbor & doing any damage to the pin or arbor.

I notice that Pete Rimmer mentions that John Stevenson did not include the pin when he made the custom R8 spindle. It's a great pity that John is no longer with us to explain why, he was a great engineer.

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O.K. here goes: Whilst the pin is of some benefit when closing the collet [or just fitting a solid R8 shank] it should not generally be neccessary; because the draw-bar thread should be clean and well-maintained.

BUT in the unfortunate event of a tool dig-in, sans pin, the taper could slip and the draw-bar become overtightened ... In that case, the absence of a pin would make removal more difficult. [*]

Just a thought, to fuel the discussion

MichaelG.

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[*] Note: I'm sure that JS would have simply cut the top off the drawbar, to release the tension, and made a replacement within the hour.

Edited By Michael Gilligan on 28/07/2019 18:31:58

There were a few reasons why JS did not include the pin...

1. Depending on who you got the R8 collet from, the slot width and depth could be different. This issue has been debated over time. The obvious issue being.. will the pin in the spindle allow the R8 collet to fit. If not, some modification needed to be made to the pin or the collet. All of this had been experienced by him, us and others over the years.

2. In certain mills, depending on 'the event' of how and when the pin sheared, the back part of the pin could fall inside the spindle assembly and get lodged into a moving part such as the bearing, and in-turn create further damage.

3. According to John, the original Bridgeport R8 design with a slot was designed as such, to be a fail safe. The pin could/should shear as a fail safe to avoid damage to other parts of the spindle assembly, if a heavy duty cut jammed, for example, and allow the collet to slip.

JS came across quite a few 'Bligeports'/clones over the years, many of which no longer had they original pins.. possibly sheared/breaking as a result of impatience by the user when introducing an R8 tool into the spindle.

It is true also as per MichaelGs observations that in such an event, sometimes the whole assembly could still go tight, making it more difficult to take R8 tooling out... although we have not seen this happen too often.. thankfully.

Ketan at ARC.

Edited By Ketan Swali on 28/07/2019 19:44:29

I am about to pull the pin.

On my BP I have some heavy attachments that clamp to the quill. Trying to support the device and rotate it to get the key aligned is a hassle. The pin has to go. then I can just shove it into place and hold it with the drawbar.

The pin is only to ensure that the tools are placed in a repeatable radial position. Especially when they are made from bored in position collets. If your quill is near perfect, the pin wont make much difference.