Bridgeport Mill "locks" While Drilling..?

[EmilioG]

I was drilling a 3/4" hole in 1/4" aluminum yesterday, when at the point when the drill breaks through to the other side, the spindle just froze, locked in place for a second until I moved the quill.

I was using 700 rpm, r8 collet with an S&D 3/4" drill. I was peck drilling and using enough tap magic (aluminum).

What happened and how can I avoid this? The drill did not slip.
Does something need to be looked at here? It's a BP variable speed J head, doesn't look too old.

Let me know if I should provide any more information. Thanks

 

[P T Schram]

Sounds like it's stalling the motor to me.

 

[Ed ke6bnl]

what I would try would be to use the slow feed wheel prior to breaking through

 

[T Bredehoft]

Logic behind that. As you feed down, there is slack in the feed mechanism. When the pressure is relieved (break through) gravity takes over and the quill drops, taking a much heavier cut, stalling the motor.

 

[EmilioG]

Is this normal or is there something worn or wearing?
I'll try higher speeds, 900 RPM maybe, slow feed and sharper drill.??

 

[P T Schram]

It is a function of a very sharp drill bit-IMO. Although, there might also be a minute fault at the cutting edge where it's hanging up.

Sure, touch it up a bit and see how it goes-use plenty of lube as well.

 

[P T Schram]

I just went back and read the initial post.

I think you would be well served step-drilling up to size rather than "Peck" drilling with such a big twist drill.

 

[brino]

Emilio,

There also may be an optical illusion going on here........let me try to explain.....

When drilling with all parameters correct, the drill bit is rotating but making slow downwards progress. The advance depends on the chip load, sharpness, angles, material, cutting force, etc.

When the drill bit breaks thru the back side of the work it can suddenly "appear" to stand still or stall because the forward (downward) speed temporarily increases drastically due to the lack of back pressure. It almost looks like the drill bit is "standing still" until the slow user (me!) reacts to the lack of back pressure and lets up on the forward feed. Then the drill bit once again is spinning much faster than it's being fed forward (down) and once again appears as a turning blur.

There is also the affect of the drill bit "screwing into" the work. When there is only a thin web of material left at the bottom of the hole, then the bit can try to "thread" onto it like a screw into a nut, or literally act like a wood screw and pull itself thru....this affect is worsened by "down feed backlash". With the power off can you grab the spindle and pull it down without the down feed handle moving? or alternately if you start turning the down feed handle does the spindle not move immediately? This will give you an idea of backlash or "slop" here.

This is also why work (especially thin sheet) must be bolted down or held in a vise, one the drill bit breaks thru it can screw in until the bit is pulled thru to the bottom of the flutes and then starts throwing the work around like a fan blade.

Can you post a video of what you see?

-brino