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Author Topic: BT30 spindle from scratch - with power drawbar and ATC of course  (Read 408523 times)

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Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #250 on: October 07, 2012, 06:42:46 AM »
Yes, I have an Emco F1 bench top mill and it uses BT30 holders without the flats and the spring force is sufficient to hold the holders not allowing them to slip.

It would be helpful to know the application.  There is a lot of distance between driving 1/4 roughers thru aluminum and pushing a big face mill across an interrupted cut on stainless.

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #251 on: October 07, 2012, 06:46:29 AM »
all the denford atc triacs after 1990 use either bt35 or bt30 toolholders without the drive dogs, so they rely solely on the taper for drive.

That could be because someone determined that the dogs were not needed (say if the mills are all low HP spindles),  . . . . -or- . . .  it could be that they were unable or unwilling to provide the required indexing for the spindles in order to use the drive dogs.


Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #252 on: October 07, 2012, 06:54:11 AM »
something i've just remembered, many years ago i was operating a 40 int mill and i checked to see if the lugs were actually doing anything. After the job the gap between the drive lugs and the toolholder was still there, so this job didn't cause the toolholder to use the drive lugs.

Let me first say that I am merely speculating when I comment that it occurs to me that in a perfect world the friction would always be sufficient for the job. There are those extreme cases where perhaps a tool spins or even dislodges from the spindle, and then the momentary slip where the friction was overcome momentarily.

I suspect that the latter is far more common and would include things like tool crashes. The damage to the spindle taper could be significant if the holder were allowed to spin, so perhaps the dogs have always been intended as 'arrestors' rather than 'drivers'.
Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #253 on: October 07, 2012, 06:59:24 AM »
The dogs may be overkill for this but they are used as an orientation device when you need to orient the tooling such as when back boring.
We never have the time or money to do it right the first time, but we somehow manage to do it twice and then spend the money to get it right.

Offline Dan13

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #254 on: October 07, 2012, 07:05:10 AM »
Yes, I have an Emco F1 bench top mill and it uses BT30 holders without the flats and the spring force is sufficient to hold the holders not allowing them to slip.

It would be helpful to know the application.  There is a lot of distance between driving 1/4 roughers thru aluminum and pushing a big face mill across an interrupted cut on stainless.

8mm roughing mill full diameter 4mm deep in aluminium. I am sure I could do twice that depth and the tool wouldn't slip. Don't use face mills on that small machine. Do use slitting saws and had a few times it jamming, but again it's not really a reference as it was stalling the undersized spindle motor (around 5Nm at the spindle).

Dan

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #255 on: October 07, 2012, 07:05:27 AM »
Interesting, fellas, thanks for chiming in.

So how much force are we talking about?

Thanks,
Dave

R8 requires upwards of 2500 # drawbar tension for max retention.  And, BTW, the set-screw in the slot on the side of an R8 has nothing to do with preventing the tool from slipping, except when the drawbar is being tightened.  The friction created in the taper FAR exceeds the feeble ability of that screw to prevent slippage.  

I will deny I ever said this, but many, many moons ago, we used the pin and a loose adapter as a red neck floating tap holder. For emergency use only, of course . . . .   :-[

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Spec for 30-taper is about 1300#, 40-taper (IIRC) is about 1800#, 50-taper I'd guess upwards of 2000#.  Keys are not necessary for small machines (up to 3-5 HP), above that, the keys seem, if not necessary, at least worthwhile.

It seems logical to me that with only a couple of steps in adapter size and a wide range of spindle HP, that both the drive dogs and the drawbar tension would be specified for the machine and not as a set number per taper.

Does anyone know if there are published HP ranges for each taper?

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #256 on: October 07, 2012, 07:15:45 AM »
Yes, I have an Emco F1 bench top mill and it uses BT30 holders without the flats and the spring force is sufficient to hold the holders not allowing them to slip.

It would be helpful to know the application.  There is a lot of distance between driving 1/4 roughers thru aluminum and pushing a big face mill across an interrupted cut on stainless.
8mm roughing mill full diameter 4mm deep in aluminium. I am sure I could do twice that depth and the tool wouldn't slip.

CNCcookbook has a speed/feed calcuator that includes (IIRC) the HP required and the MRR as part of the data for each calculation. This would be good info to have available for machines that are short on power.

Currently, I have 7HP peak and I have not slipped an R8  . . that I know of . . .  , but then I am changing tools by hand and cranking on the drawbar nut with a wrench. I would guess that I am easily putting over 2,000 lbs tension on the drawbar. The spindle is small and skinny and not made from very good material so it gets a killer monkey grip on some of the tooling and needs serious persuasion sometimes to release the tools.

Offline Chaoticone

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #257 on: October 07, 2012, 11:32:43 AM »
Quote
Let me first say that I am merely speculating when I comment that it occurs to me that in a perfect world the friction would always be sufficient for the job. There are those extreme cases where perhaps a tool spins or even dislodges from the spindle, and then the momentary slip where the friction was overcome momentarily.

I suspect that the latter is far more common and would include things like tool crashes. The damage to the spindle taper could be significant if the holder were allowed to spin, so perhaps the dogs have always been intended as 'arrestors' rather than 'drivers'

Quote
The dogs may be overkill for this but they are used as an orientation device when you need to orient the tooling such as when back boring.

I actually think them being arrestors (cheap insurance) were more an added benefit rather than a feature by design.  Most I have seen use tiny bolts to attach them to the face of the spindle.  All though they are in a shallow groove an empty tool holder allowed to spin at spindle start up could easily shear them off.  Totallyrc hit the nail on the head as to why most I have seen use keys.  I hate to mention these last 2 because I really don't like either one but.........  I have seen them use keys for a fine orientation adjustment, an alignment tool.  These were on a belt driven spindle and I'm just guessing this was their motovation.  The outside edges were ground on an angle. This would only allow them a tiny bit of misalignment but between the tool holder rack and some other low end features it needed it.  They did however run for many years like this with good results............. most of the time.  ;D  I think my least favorite use of a key on a spindle was to actuate a release on the tool change arm.

Brett 
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!

Offline BR549

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #258 on: October 07, 2012, 11:48:45 AM »
If you were looking for grip/slip value per spindle type you would use the available TORQUE value not HP.  

Just a thought, (;-) TP





Offline BR549

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #259 on: October 07, 2012, 12:01:01 PM »
Bret that key with tiny bolts will take a LOT more load than you think the key would be in shear mode  in the slot.

Cat holders use offset keys to locate the tool holder to the spindle in ONLY one spot. I have seen a few LOAD the tools backwards with not so good results when it came time to toolchange. Some machines would actually LOAD  the tool holder jammed offset in the spindle. You can guess the results.

(;-) TP