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.

Interesting, fellas, thanks for chiming in.

So how much force are we talking about?

Thanks,
Dave

Depends on the taper, but quite a lot.  A number of folks tried with R8's and it wound up taking more than they expected.  Life seems to begin at 1000 lbs or more.  Best numbers I've seen on it come with the docs for drawbar force gages if you want to Google around.  Some are 3000 lbs or more.

Best,

BW

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.  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.

Regards,
Ray L.

Thanks, fellas, and yes, Ray, most guys I know usually yank that set screw/key out as soon as they get a mill. More of a nuisance if anything.

So Steve, how much force will be used with this unit?

Dave

It would be interesting to know the formula used to come up with the draw bar pull force required.  It undoubtedly uses surface area/vs tool load. Another good thing to point out here I think is the importance of keeping the spindle taper and tool holders clean and free of surface rust and dings. Nothing will undermine this like an unseated tool spinning inside the spindle.............. that makes for a bad day.    I have seen a lot of routers in particular that seem to be abused in this way.

Brett

That's a good point, and I like when I see the air blast to clean the tooling during toolchanges, however, sometimes it ends up blowing chips/dust right into the critical surfaces.

Dave

Brett I think Most of the values were derived from calculations AND trail and error. I have seen values from 900# up to almost 5000#  30-50 taper.

(;-) TP

Drawbar tension has been argued ad-nauseum, so instead, how about some physics and 'why everyone can be correct even with vastly different numbers'. First; there is no linear relationship between drawbar tention and the friction generated. The way tapers work is a very large topic, but in a nutshell, the force on the taper (pull on the drawbar in this case)  is generating a press fit. If you want to calculate the torque transfer, you would use similar formulae. The inner part of the interface (the adapter) will be considered incompressible and thrown out for the purpose of this discussion. So the amount of interference fit depends primarily on the material of the outer part of the interface (in this case, the spindle itself) and the amount of drawbar force. That may seem contradictory, but the same drawbar force will create different interference with different materials because the material stretch is vastly differnet from one material to the next.
 
This is where the 'trial and error' would come into play as was mentioned in the last couple of posts. By way of example; my current mill has an X3 spindle which is just soft mild steel. May have been Toyota frames in a past life, who knows. As a guess, I would (generously) give it 50k - 70k yield. My new Mega-Duty 4th axis spindle and the BT30 spindle are 110k yield. A deeply hardened 4000 series steel spindle could easily hit 190k. It is going to take a lot more tension to spread the taper on that last spindle than on my crappy X3 spindle. It would not surprise me if the drawbar tension for a VMC spindle would literally exceed the yield strength of an X3 spindle and permanently deform the taper. A big hole in the nose and the adapter would be up inside the spindle somewhere . . . .LOL!


To address another question: Surface area of the taper is a consideration, of course, but the gorilla is the taper angle.

So before anyone argues that the drawbar alone is retaining the adapter, consider that if the taper angle is very low (i.e. MT, JT, etc) the leverage is great (lots of stretch for not so much force) and the grip is tremendous for that reason. Note that these tapers do NOT need continuous drawbar tension once the initial force has caused the interference fit. It would be a decidedly bad idea to remove the drawbar from R8 for example after the adapter/collet was seated, but the effect of the pseudo press fit is evident when you have to hammer on the holder (thru the drawbar) to free it from the spindle.

In a later post, I  will comment on some other topics and show the completed iteration #2 ATC claw . . working perfectly and will be the final design.

 

Surface FINISH also comes into play. The drawbar force IS important to maintain force on the taper it is NOT like a morse #2 where the taper is so long it has a lot of holding power from the force exerted on the taper by the pin(tool holder).  Remove the tension from the BT30 holder and it WILL fall out under cutting loads. Let it get light and it will chatter the toolholder in the spindle under heavy loads

I do not KNOW of a STANDARD value or calculation to estimate the force needed.  Each Manf I have dealt with has their OWN NUMBER working range they want you to set the drawbar tension to. They know the basic values needed then add in a fudge factor from in field experinces over the years.

Most are similar for each taper.  THICKER spindle shafts may need more, harder spindle shafts may need more.

Call a spindle grinder or repair shop and you will get the genaric values used by most OR they tell you to call the manf for the correct values for the machine.

The standard taper used for all the sizes is not used by accident it it a compromise of torque transfer  AND being able to release it from the spindle

Just a thought, (;-) TP