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I bought a cheap corded impact drill so that I can use it with a drill stand to drill holes at 90 degrees angle accurately, which I find very hard with a battery powered drill. My battery powered drill doesn't work with the stand for three reasons:

  1. It lacks the 43mm mount needed to fit it to the stand
  2. It lacks a locking function to allow using the drill without continuously pressing on the trigger
  3. It lacks a speed control wheel, that would allow using the drill at reduced speed with trigger locked on

I made sure that the corded drill has all of these three features.

However, I am unsure how the speed control works (electronic speed control, not gearbox). If I run the corded drill at a reduced speed, does it also reduce torque?

Let's say I want to drill a large hole to wood. I probably don't want to use the maximum speed of the drill then. However, I do want all the torque I can get from the drill.

If I set the drill to a low speed, does it mean that I still have all of the torque available I would have with the highest speed with the wood workpiece slowing down the drill?

Or does the low speed setting not only decrease speed, but also decrease torque, so that if I run the drill at 20% of its maximum speed, I also only get 20% of its maximum torque?

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    An impact drill is not a good fit for the use case of larger holes in wood. A drill that's geared low and slow, or has switchable gear options to go low and slow as one option is a far better fit. Impact does squat-diddly for drilling.
    – Ecnerwal
    Nov 12, 2022 at 15:15
  • A Zip-tie works well to lock the trigger...
    – Solar Mike
    Nov 12, 2022 at 17:07
  • Torque will follow the "Juice" the motor gets
    – Traveler
    Nov 12, 2022 at 18:43
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    Why would you want to control torque? It's "feeds and speeds" not "feeds and torques". You want to dial in RPM and advancement rate for your material (you have to do the latter by feel). Torque is a concern when driving screws. Nov 12, 2022 at 22:18
  • "Let's say I want to drill a large hole to wood. I probably don't want to use the maximum speed of the drill then." Yeah, you do. Run a holesaw at less then full speed and it can bind and spin you in a circle. You want it to chip and spit it out, not slice it like a chisel. For metal you want the sweet spot of in between those two; chipping, slicing and spitting. "feeds and speeds" sounds great and all but isn't going to become clear what that means until you know how to machine, because you spent 20y in a shop like our resident aviation accounting lawyer who used to work on high voltage trams
    – Mazura
    Nov 12, 2022 at 23:51

1 Answer 1

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The "speed" setting that is part of the trigger in a drill controls the average voltage delivered to the electrical motor within it.

Power Control - The Trigger Button

This is accomplished by a PWM (Pulse Width Modulation) circuit, which pulses the voltage rapidly, say at several kHz. The position of the trigger button controls average width of the "on" portion of the pulse - the duty cycle-, which in turn determines the average voltage applied to the motor and thus its power setting.

This electrical power translates to mechanical torque.

There are two charts that tell us how to relate trigger setting, RPM and torque to the task at hand, e.g. drilling, hoisting etc....

Torque and RPM

The first chart graphs RPM and Torque for various power settings. Have a look:

enter image description here

The curves above shows two important aspects:

  1. Torque and RPM are related by the power setting. They are not constant, but they are related. Follow any of the blue lines for some power setting, e.g. 60%: if the torque is high, the rpm is low and vice versa.
  2. At a given RPM, the torque can be increased by increasing duty cycle (from say 60% to 100%). This increases the electrical power and thus increases the RPM if the torque is kept the same, as in a load that is being hoisted. Or, it increases torque if the operate keeps the the RPM the same, e.g. by pressing a drill harder into the workpiece while increasing the power setting.

Mechanical Power is Torque and RPM

Mechanical power is determined by the product of torque and RPM. What does that mean in these charts? Select a power setting (select a blue line), determine your RPM, read the torque, and multiply the two.

You can also do this graphically, multiplying RPM -a horizontal distance on the chart- with torque -a vertical distance- means you are calculating the area of a rectangle at that operating point.

That is, select a point one of the electrical PWM duty cycle lines, determine RPM and Torque on your point, and multiply the two. Multiplying two numbers is how you calculate the area in a rectangle. So this power calculation is effectively the area of a rectangle in the curve, and the rectangle's top right corner is determined by the selected operating point.

When you select a higher power setting you are riding a higher blue line which will draw larger rectangles.

When do you get Maximum Power?

When is the mechanical power maximum? Is it at high RPM and less torque, or at high torque and less RPM?

The maximum mechanical power is delivered when the product of torque and RPM is maximum.

(I'll spare you the details, but after some math involving triangles and linear equations, it turns out that the area is maximum when the sides of the rectangle are equal. It's intuitive too, which helps: a thin narrow triangle under the blue line has less area than a square.)

enter image description here

To get the maximum torque you will get the lowest RPM. And to get high RPM you need to relax the torque. Notice how the maximum drill power is not delivered when RPM is maximum or when the torque is maximum.

Drilling or Hoisting - Not the Same Load

What determines the RPM of your drill is however not just determined by the PWM duty cycle as set by the trigger button, but also by the load. At the same power setting (duty cycle), the RPM will vary depending on the torque load: press hard while drilling and the RPM drops because the load increases. For a hoisted weight it's different, the load is the weight of the hoisted object and the weight (obviously) does not depend on the RPM.

These effects are described by "load torque speed characteristics". Different operations have different characteristics. For instance, for drilling, friction increases with RPM and so does the torque load.

Here are some example load characteristics, where "w" is the greek letter omega and it represents the RPM:

enter image description here

The one on the left is typical for friction loads. Notice it starts at zero: with zero RPM there is zero friction (zero load), and Friction increases with RPM.

A hoist load on the other hand is relatively constant: the RPM does not make the load heavier (acceleration aside), but there is a bit of friction in the pulleys or gears that may increase with RPM.

Note that these curves describe how torque and RPM relate under the same load condition; that's for the same drill pressure or same hoist weight.

If you increase or decrease the drill pressure or change the workpiece material, you get a different curve (drawn lower or higher) but with similar shape.

Putting the two together: Equilibrium

To understand what torque and RPM you get for the task at hand, you need to put these two curves together.

If you keep the drill presssure constant (not pressing light or harder on the drill bit) the drill RPM will settle where the load torque from the friction and RPM equals the motor torque delivered at that RPM.

Here's a graph that includes two load characteristics in brown, one for pressing hard and one for pressing lightly.

If you go from pressing lightly to pressing hard (while you keeping the same power setting), the RPM will drop, but the torque will increase. Try to follow this on the curve.

Now you can hopefully visualize what happens with torque and RPM when you increase & decrease the power setting, or increase & decrease the pressing force.

enter image description here

Don't burn your workpiece

As an aside, friction load increases with the square of the RPM. Double the RPM means quadrupling the friction torque. This is in fact why RPM in many tasks is limited. Sure it makes drilling or sawing go faster, high RPM also quickly overheats your work piece or cutting bit.

Overheating the tool will cause it to soften and wear faster. Overheating the workpiece will cause undesirable damage.

Images:

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