Drill Press motor choice

[shamrock12]

I have been given the choice of 1/2 or 3/4 HP Baldor motor to install onto Buffalo No. 15 drill press. I also have the option of selecting between 1725 and 1140 RPM. So there are four possible motor configurations to choose from.

This drill press will allow me to have five different spindle speeds, which will be dependent on the motor RPM I select.

Here are the spindle speeds for the 1725 RPM motor:
500
950
1725
3280
6000

And for the 1140 RPM motor:
330
615
1140
2165
3965

I am a serious hobbyist and would often be drilling mild steel with hole diameter of 1/2" and less, but would occasionally be drilling larger diameter up to 3/4" from time to time. I do not plan to do power tapping with this drill press.

Under what condition would it be beneficial for me to upgrade to 3/4 HP? Just remember it will be an American made Baldor motor, so the HP rating will be true to its size, unlike those cheap China/Taiwan motors. Also given those spindle speeds above, which motor RPM would be more beneficial? It is my understanding that lower RPM means more torque, but I am sure there is a reason (or two) for having higher RPM motor too.

Please advise, thanks!

 

[bsaint]

Can you give us the model numbers of the two you are looking at?

 

[catalytic]

1140 RPM, 3/4hp for sure. Also make sure it's TEFC & made for vertical orientation. Note that not all motors are made to be run vertically -- this will mess up the bearings on some motors, so it's worth double checking.

I have a 1140RPM, 3/4hp Baldor on my 15" Clausing, and it's fantastic -- I deliberately sought out this hard to find slow speed model. 1140RPM motors are offered as the slow speed (and more $$) option on many 14-17" drill presses.

Basically, you're never going to use more than 3000RPM (if you do, it's for a tiny drill and your Buffalo doesn't have the precision to use it properly anyways, and if you do decide to do it you can just feed more slowly). However, if you drill anything hardened or want to swing an oversized drill or some other tooling, then the lower speed range is exactly what you need.

Power-wise, there's no reason not to go 3/4hp. When you swing bigger drills, you definitely want the extra hp, and the 3/4hp is only drawing like 10amps so you can still run it and something else on your smallest 15a household circuit. In contrast, I can bog a 1/2hp motor pretty easily on a drill. With 3/4hp, you tend to encounter belt slippage on the sheaves before you really bog it down, which is good -- it means 3/4hp is roughly the maximum useful horsepower on a small 15-17" drill.

 

[Davefr]

^^^ Agree. The ability to go down to 330 RPM will be more useful then the need to go to 6000 for the average shop DP.

 

[Outlawmws]

1140 RPM, 3/4hp for sure. Also make sure it's TEFC & made for vertical orientation. Note that not all motors are made to be run vertically -- this will mess up the bearings on some motors, so it's worth double checking.SNIP.

 

[shamrock12]

Thanks everyone for the inputs. Just curious, how much more torque is the 1140 RPM motor as compared to the 1725 RPM with the same HP rating?

bsaint - sorry, I don't have the model numbers of the motors ... just the basic specifications.

 

[A_Pmech]

Most all modern motors with rolling element bearings will run in any orientation. The restriction comes from the days when plain bearing motors were common. Running a plain bearing motor in the vertical orientation will wear the thin hat of the flange bearing on the "down" side of the motor.

1140 RPM 3/4 HP: 3.5 ft. lbs. FLT

1725 RPM 3/4 HP: 2.3 ft. lbs. FLT

Breakdown torque of an induction motor is usually ~ 3x the Full Load Torque.

 

[shamrock12]

A Pmech - that is some good information there. Just curious, what would be the full load torque of 1/2 HP motors in those RPMs? Just am wondering how much of an improvement 3/4 HP would be. Thanks!

 

[A_Pmech]

A Pmech - that is some good information there. Just curious, what would be the full load torque of 1/2 HP motors in those RPMs? Just am wondering how much of an improvement 3/4 HP would be. Thanks!

A 1/2 HP motor would make 67% of the 3/4 HP torque, speed being equal.

 

[uart]

A Pmech - that is some good information there. Just curious, what would be the full load torque of 1/2 HP motors in those RPMs? Just am wondering how much of an improvement 3/4 HP would be. Thanks!

Hi Shamrock. The formula for the torque in ft-lbs is approximately 5244*HP/RPM

 

[shamrock12]

Alot of good information here. It's interesting where the 1140 RPM number came from. We know that 3600 RPM motor is really 3450 RPM, and that 1800 RPM motor is really 1725 RPM. So I'm curious why 1200 RPM motor is said to be really 1140 RPM instead of 1150 RPM? Any explanation for that?

A Pmech - that is what I thought too ... just wasn't 100% sure

uart - I knew there was a formula for this but just couldn't remember - thanks for refreshing my memory!

 

[A_Pmech]

Taking your first example, 3,600 RPM is the synchronous speed of the motor. Calculated as: 120 (f) / n poles where f is the power supply frequency in Hz and n is the number of motor poles. In other words, synchronous speed is the speed at which the magnetic field rotates within the motor core.

In an induction motor, mechanical power is produced when the rotor field slips relative to the core field. For this to happen, the rotor must spin at some RPM less than the synchronous speed. Rated RPM is measured at rated FLT and is always less than synchronous speed in the case of induction motors.

As the motor is loaded beyond FLT, the shaft torque and slip increases until breakdown torque is reached. At which point increasing load stalls the motor and slip rises rapidly to 100%.

Alot of good information here. It's interesting where the 1140 RPM number came from. We know that 3600 RPM motor is really 3450 RPM, and that 1800 RPM motor is really 1725 RPM. So I'm curious why 1200 RPM motor is said to be really 1140 RPM instead of 1150 RPM? Any explanation for that?

 

[uart]

Alot of good information here. It's interesting where the 1140 RPM number came from. We know that 3600 RPM motor is really 3450 RPM, and that 1800 RPM motor is really 1725 RPM. So I'm curious why 1200 RPM motor is said to be really 1140 RPM instead of 1150 RPM? Any explanation for that?

Ok, I see where you're going with that. You're wondering why the ratios of ideal (synchronous) speed to actual rated speed are very slightly different for the two motors. (95.8% versus 95.0%)

The simple fact is that there is no fixed ratio between rated and synchronous speed which is applicable to all induction motors. Larger more efficient motors tend to have rated speed closer to synchronous (that is, low slip) and smaller low power motors tend to have somewhat higher rated slip. The actual ratio however depends on many factors of the machine design, the relative starting and running torque as just one example. So we definitely shouldn't expect those speed ratios to always be identical from one machine to the next.

BTW shamrock, I have a question for you. Just wondering if you've seen the physical size of the motors. My expectation is that the slower speed motor would be a slightly larger physical size (for the same power). Do you know if that's the case here?

 

[Carla]

I'll concur with those who said 3/4 hp and 1140 rpm, since you'll be drilling steel.

Personally, I'd pick an 'open drip' meaning 'open drip-proof' motor over a 'TEFC' 'totally enclosed fan cooled' for a metalworking machine.

The open motors will stay cool better under load, and there's little point in paying for for the TEFC configuration where it isn't needed.

The TEFC motors are really only needed on woodworking machines where an open motor would quickly fill with sawdust.

cheers

Carla