Larger drills bits on a smaller lathe

[mickri]

I think that you have to consider if you are what I call step drilling (probably wrong term) where you start off with smaller drills and progressively work up to larger drills. I always start with smaller drills and work up to bigger drills. Usually in 1/8" increments. I typically start boring when I can fit my boring bar in the hole. In the past that was at about 5/8." I now have drills that go up to 1 1/8 inch. I have not used them yet.

 

[brandon428]

Brandon428, yes, if you have a gearbox or a pully system to reduce RPMs (acting as a torque multiplier), you can push a higher diameter. For those without this advantage (ones that use variable speed motors), they will find this limit realistic.

What problems should I be seeing that I'm not?

I use the standard SB9 pulleys, and usually go 700RPM when drilling less than 1/2", and drop to ~400 RPM when I go over that.

 

[addertooth]

To risk repeating myself, that limit is for the fixed gearing folks who have a variable speed motor. For those who can "gear down (or pulley down)" they can have the torque to deal with larger drill bits.

 

[brandon428]

Got it -- assumed most people used a pulley setup rather than VFD.

 

[addertooth]

Yep, mine is a 3 phase brushless motor, and fixed gears. I have done 3/4 inch with a MT2, it worked... but much larger would have been too much.

 

[EricB]

Eric, is the idea that the lathe dog will quickly hit the carriage if it spins to minimize the damage?

The end of the dog is resting on some dunnage on the carriage to keep the bit from spinning. I found this in an old text book after I replaced my tail stock spindle.

 

[Arielht500]

Have been using this method for some years on my Myford ml7. It has a 3 ph motor and vfd and I run it in back gear to keep motor revs up. My largest mt2 drill is 1”.

 

[graham-xrf]

To risk repeating myself, that limit is for the fixed gearing folks who have a variable speed motor. For those who can "gear down (or pulley down)" they can have the torque to deal with larger drill bits.

There is nothing wrong with gear-down, or pully-down, or a bit of both AND a VFD!

Some VFDs are like servos, and deliver near full forward torque, even when stalled, or externally being driven backwards, but most are not like that. They do perform better when up to some comfortable speed.

I think the advice to go up in steps is good, though I would opt for boring the final cuts. Drills follow whatever hole the previous drill made, and can wander off track a bit. Boring brings the reference line back to being determined only by the lathe axis, and the cutting tool.

 

[brandon428]

Tally so far is that 1 person uses a 20N, everyone else uses MT2 bits up to their max size and bores beyond that, and no one is using S&D bits...

 

[RJSakowski]

One must add to the equation, the drive system. My variable speed motor is easily up to the task and I can use the lowest apeed pulley configration for torque multiplication but the 3L belt that drives the spindle is going to slip with excessive load.