Bridgeport Drawbar Torque

Canus

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Mar 19, 2018
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Is there a proper torque for a Bridgeport drawbar. Don't want to over or under tighten.
 
The suggested torque for a 7/17-20 bolt is 41 ft. lbs. Since the drawbar is merely a 7/16-20 threaded rod I would think this should be sufficient torque. I tighten the drawbar on my Bridgeport with a 3/4" combination wrench. The few times I've checked the torque with a torque wrench it's been in the 40 ft. lb. range plus or minus a couple pounds. It seems to work well and in 20 years I haven't ruined a collet, had an end mill slip in a collet, or stripped the drawbar.
 
I just tighten to a tug on the wrench.

The drawbar needs almost no torque because all it does if hold the R8 tool holder in the R8 taper in the spindle.
There is a key in the spindle to transmit motor torque to tool itself.
Anything that would hold a bicycle wheel to a bicycle frame is tight enough.
 
I just tighten to a tug on the wrench.

The drawbar needs almost no torque because all it does if hold the R8 tool holder in the R8 taper in the spindle.
There is a key in the spindle to transmit motor torque to tool itself.
Anything that would hold a bicycle wheel to a bicycle frame is tight enough.

I wouldn't depend on the "key" which is actually just a set screw to transmit torque to the collet or tool holder. The purpose of the setscrew is to locate the collet in the same orientation each time it's installed. I have seen a number of them damaged by not tightening the drawbar sufficiently. In some cases they trap the collet and make it extremely difficult to remove. In other cases a damaged setscrew can make it difficult to install and properly seat another collet.

Many professional shops remove the setscrew so they don't have downtime because a collet or tool holder spun in the spindle. The setscrew in my mill was damaged when I bought it. It was a pain to remove since the threads got damaged. I replaced mine and have been careful to properly tighten the drawbar and not overload the collet with too aggressive a cut or an oversize end mill.

If I recall correctly the setscrew has a 1/4-24 thread, so replacements aren't usually found at the local hardware store
 
I wouldn't depend on the "key" which is actually just a set screw to transmit torque to the collet or tool holder. The purpose of the setscrew is to locate the collet in the same orientation each time it's installed. I have seen a number of them damaged by not tightening the drawbar sufficiently. In some cases they trap the collet and make it extremely difficult to remove. In other cases a damaged setscrew can make it difficult to install and properly seat another collet.

Many professional shops remove the setscrew so they don't have downtime because a collet or tool holder spun in the spindle. The setscrew in my mill was damaged when I bought it. It was a pain to remove since the threads got damaged. I replaced mine and have been careful to properly tighten the drawbar and not overload the collet with too aggressive a cut or an oversize end mill.

If I recall correctly the setscrew has a 1/4-24 thread, so replacements aren't usually found at the local hardware store

Just replaced one last week. Real easy to do, see:
 
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