Variable speed press drill

[Former Member]

A common approach to this PITA is to get the right belt tension so it does not slip, but you can change the belt position by hand without grabing for tools. My Buffalo DP for example is not intended to have the belt loosened to change it, it would be far too big a job...but its seconds to change by hand



A few will argue this, but all of engineering is stacked against them . An electronic speed controller (unless a servo with feedback) makes a poor speed controller for a machine tool as with a machine tool, you usually (if not always) want torque to increase as the speed goes down. HP is the product of speed and torque. For example, if you have 1/2 hp at 2500, and you use a vector VFD (constant torque) you will have 1/20 of a HP at 250 RPM. Good luck drilling a 1/2" hole with 1/20th HP. With a mechanical transmission (ignoring friction loses) torque would go up as speed went down meaning you'd still have 1/2HP @ 250rpm.

If someone says a VFD works for them for speed reduction, I don't doubt it, but what is really being said is they find the reduced HP at low speeds still adequate for the work they do. Maybe its light work or maybe they aren't reducing the speed very much. Its all good but it doesn't change the physics: that performance is sub optimal as you lose HP as the speed is reduced or that with a 10:1 reduction the lack of HP will be dramatic to the point of not really being unusable for drilling that 1/2" (or pretty much any) hole.

I must say you are some what mistaken here, when the elevator business started with VFD's the biggest issue was that most VFD's did not vary the voltage along with the frequency (purpose is to keep the power constant, P=Voltage x Current) once that happened it was a whole new game.

Most AC motors the speed is determined by the number of poles and the frequency power coming in.

What basically happens is the motor for all intense purposes thinks it is running at the correct speed (which also explains the lack of in rush current) regardless of the frequency.

The issue now becomes so that the motor has the correct power (watts) as frequency changes so does the current draw (sorry don't remember up or down, its first thing in the morning and haven't had a coffee yet) to compensate the voltage must be adjusted to keep the same power (wattage)

Now two factors come into play here, motor winding and insulation (duty factor, such as industrial, farm, elevator and as I understand it now VFD) that allow the increases in voltage without damaging the motor.

The second is the VFD itself, the more expensive (generally speaking) the better the range of both frequency and voltage balance.

Now have been running VFD for about 1-1/2 years (personally) and I find I have more flexibility at the lower end range very little toque lose, however, as I exceed the design frequency at the upper end for the motor, torque drops off dramatically. Spoke to my motor shop guy and he asked what do you expect the motor is not a $$$ VFD rated motor.

So for a range of what works, you are going to have to play around a bit, for me bottom end is about 20hz and the top is about 80hz, ideally I stay +/-20hz. As soon as I get about 80hz the motor can stall out with just a little load. At the lower end I have yet to experience stall under load.

Second item, my while on the pricer side (compared to some on the site) is spec grade, but for that last bit of range more $$$ must be spent.

 

[Former Member]

Yes I tried to wrap my head around this concept and how it would affect my vfd conversion of both my milling machine and drill press. I ended up not removing any pulley or belt combinations that originally came with the machine and kept the ability to do the mechanical speed changes along with the vfd. I really like that combo and I rarely change belt positions. As you stated, the reduced hp at low speeds (with just VFD) has been adequate so far, but when it does become a problem, I'll do a belt change too.

Stick with the same power rating 2hp for 2hp you'll be fine, that's what I did. Nice thing is you'll be able to fine tune cutting speeds by varying up and down for best finish/speed of cut/material removal/cutter size. Surprisingly moving rpm up or down marginally can greatly improve the out come.

 

[PeterT]

I feel like we have discussed this VFD HP/torque/rpm relationship before. Maybe a few times LOL. I still don't get it, but admittedly I am electrically challenged. I wish there was some kind of VFD-101 For Dummies resource out there. And ideally some kind of online sizing calculator, pick this parameter & that parameter & it spits out a motor/VFD combo where you can also download specs. Is that too much to ask? LOL. Not to detract from drill presses but if someone has some VFD resources let me know.

This link/plot 'kinda' suggests constant torque throughout the low frequency (rpm) range which I think is what we are typically after in a drilling mode? So the formula byproduct dictates that HP is varying from low to high/some max rating? What electronic switching magic that is occurring behind the scene in terms of varying motor voltage or system current I may never understand, but as long as my wall socket is capable & smoke stays in, I'm happy.

https://controltrends.org/wp-content/uploads/2010/10/VFFundamentals.pdf

 

[DPittman]

Stick with the same power rating 2hp for 2hp you'll be fine, that's what I did. Nice thing is you'll be able to fine tune cutting speeds by varying up and down for best finish/speed of cut/material removal/cutter size. Surprisingly moving rpm up or down marginally can greatly improve the out come.

Yes that's what I did, I've had my machines converted for a while now and am quite happy with them.

 

[Mcgyver]

This link/plot 'kinda' suggests constant torque throughout the low frequency (rpm) range which I think is what we are typically after in a drilling mode?

No, look at the chart you presented. At 10Hz, 1/6 of line, you will get about 16% of your HP. Constant torque means the slower you go the less HP you will have. As I wrote above, a 1/2 HP at 2500 will give you 1/20 HP at 250....and that won't be enough to drill a half inch hole. (that is half the HP of small watchmakers lathe motor).

All it requires is to buy into:

HP = RPM x Torque (written right in that chart)

Below the line frequency, 60 Hz, VFD's do not put out anything more than constant torque (the flat red line to the left of 60Hz on the chart)

This is all in the context of using VFD to slow things down, not speed up. Where constant torque works (where its ok for HP to go down as speed goes down) is with say a conveyor. Then, running at half the speed, you are doing half the work and it only needs 1/2 the HP. Not true for machining where we slow down because the diameter increased but we sill want to do the same amount of work (use the same amount of HP)

 

[Dabbler]

When I talk to someone about converting to a VFD, I suggest the same HP for a mill but 1.5 times the HP for a lathe.

On a mill you always have the option to put in the back gear, which gives you either 3X or 6X torque, depending on the mill.

On a lathe, you have very large start up inertia, which is hard on the VFD and motor windings at off-speed starts. The extra HP headroom really helps there.

 

[Former Member]

When I talk to someone about converting to a VFD, I suggest the same HP for a mill but 1.5 times the HP for a lathe.

On a mill you always have the option to put in the back gear, which gives you either 3X or 6X torque, depending on the mill.

On a lathe, you have very large start up inertia, which is hard on the VFD and motor windings at off-speed starts. The extra HP headroom really helps there.

Hate to say you are wrong here, elevators the max torque is required at low speeds to pull a car off the safeties should they be set. Low rpms max torque. VFD's make it easy and no we change belt ratios, EVER! Not an option.

 

[Susquatch]

You will note that I advocated a VFD solution to reduce the need for belt changes but DID NOT ADVOCATE removing the ability to do belt changes. Give yourself the ability to do either one and be happy happy!

This question of HP and torque comes up over and over. It's been a while since we last flogged it to death. That also means I've had some time to think about how to explain it better. Please bear with me and give me a chance to try.

I think it helps to look at the situation differently.

Seperately from the equations (with which I am intimitely familiar). It is useful to understand that it is the torque that cuts the metal and it is the hp that determines how quickly that cut metal can be removed. Without relating them to each other just yet, torque is by definition the foot-pounds of force applied to turn the tool - which is the cutting force. You can apply that cutting force either slowly or quickly or someplace in between. It doesn't matter which. The force required to cut the material remains the same.

However, doing it slowly requires less horsepower than quickly because hp is by definition the rate at which the cutting is done - in other words, how much material is removed in a given amount of time.

In my opinion, there is some magic there that is easy to overlook. As we turn our drill press slower, we are removing less material in the same amount of time. We still need constant torque to be able to keep cutting, but we do not need constant hp since we are not removing as much metal in the same amount of time anymore. In other words, that means we need less horsepower.

I think the reason this gets confusing is that bigger diameter cuts require slower speeds to keep the surface cutting speed where it needs to be for the material and tool diameter. But a bigger tool removes more material per unit time which requires BOTH more force AND more hp. Since we cannot have both we are in trouble.

So it's this surface speed thing that messes us up. But surface speed is not the same as RPM. YES, they are related by a simple formula, but they are not the same. The solution to this issue is simple. Use slower feed rates to go with the increased tool size and material being removed. This way you can keep the torque up to keep the tool cutting, the tool can be turned at a lower speed to keep the surface speed where it needs to be, and the HP can be balanced with the reality of bigger tools and bigger chips by keeping the rate of producing those bigger chips down by making less pounds of them per unit time.

Maybe that helps, maybe it doesn't. But I do hope it does.

 

[Susquatch]

Hate to say you are wrong here, elevators the max torque is required at low speeds to pull a car off the safeties should they be set. Low rpms max torque. VFD's make it easy and no we change belt ratios, EVER! Not an option.

I think it's a mistake to compare elevators with lathes and mills.

You are right about torque being what matters most in an elevator. More horsepower would just shoot the elevator cab and passengers up into space.

But the situation in an elevator is different than in a mill or a lathe.

I believe what @Dabbler is talking about is the space shot component of operating a lathe. Lathes have lots of gears and big heavy masses that must be brought up to speed quickly. Mills have very little spinning mass by comparison but they still must be spun up quickly. On the other hand, the elevator car can (and must) start slowly and accelerate slowly too. In fact, as you know, that's exactly what you want. People don't like being accellerated up or down very fast......

I have the 3HP VFD for my lathe already. I am only looking for the right motor now. My current motor is a 2Hp, and I want a 3Hp for it (actually I'd be happier with 2.5 but I doubt I'll ever find one.) I don't want a bigger motor because I want to make deeper cuts and/or to remove more metal faster, but rather because I want to balance the spin up hp requirements and high current loads that go with using a VFD without overloading the motor during the start up.

Because the spinning mass of my mill is low, I stayed with the same 2hp size motor as I had before I added the VFD.

I am totally thrilled with the end results on my mill.



Hopefully I'll find the right motor for my lathe before I get too much older.

@PeterT - I wish you luck finding the technical resource you seek. There are very very few engineers who are both good mechanical engineers and good electrical engineers. I had to literally teach the support engineer at techo about this hp/torque stuff. He knew his wires, but he sure didn't understand the mechanical side. Thankfully, he admitted that right up front. He had a very open mind so it was easy to teach him and he was genuinely grateful. For my part, he taught me a lot about high frequency noise in the power lines. If you ever do find a good resource, please share. I'd love to see it too.

@Marc Moreau - your drill press is most like a mill. You can add a VFD to reduce belt changes, but I recommend you keep the pulley system intact for those times when you also need the torque multiplication capabilities. You can, so why not?

 

[whydontu]

Maybe another analogy? It’s not perfect, and leaves out a bunch of math. And ignores back EMF, that has a huge bearing on electric motor torque.

My mill is 3/4 HP variable speed gear drive, with two ranges 0-1000 and 0-2000. My lathe is 3/4 HP fixed 1750 RPM motor with six speed gear box.

With the mill in low range, speed control set at 200 RPM, I can grab the chuck and slow down the quill. No way in hell I’m going to grab my lathe collet chuck at 200 RPM, it will break my wrist.

Best of both worlds? As @Susquatch says, VFD motor with gearbox or stepped pulleys. Use the mechanical advantage and torque multiplication of the gears or pulleys to give you adequate torque, and the VFD to fine-tune SFM.

 

[Former Member]

I think it's a mistake to compare elevators with lathes and mills.

You are right about torque being what matters most in an elevator. More horsepower would just shoot the elevator cab and passengers up into space.

But the situation in an elevator is different than in a mill or a lathe.

I believe what @Dabbler is talking about is the space shot component of operating a lathe. Lathes have lots of gears and big heavy masses that must be brought up to speed quickly. Mills have very little spinning mass by comparison but they still must be spun up quickly. On the other hand, the elevator car can (and must) start slowly and accelerate slowly too. In fact, as you know, that's exactly what you want. People don't like being accellerated up or down very fast......

I have the 3HP VFD for my lathe already. I am only looking for the right motor now. My current motor is a 2Hp, and I want a 3Hp for it (actually I'd be happier with 2.5 but I doubt I'll ever find one.) I don't want a bigger motor because I want to make deeper cuts and/or to remove more metal faster, but rather because I want to balance the spin up hp requirements and high current loads that go with using a VFD without overloading the motor during the start up.

Because the spinning mass of my mill is low, I stayed with the same 2hp size motor as I had before I added the VFD.

I am totally thrilled with the end results on my mill.



Hopefully I'll find the right motor for my lathe before I get too much older.

@PeterT - I wish you luck finding the technical resource you seek. There are very very few engineers who are both good mechanical engineers and good electrical engineers. I had to literally teach the support engineer at techo about this hp/torque stuff. He knew his wires, but he sure didn't understand the mechanical side. Thankfully, he admitted that right up front. He had a very open mind so it was easy to teach him and he was genuinely grateful. For my part, he taught me a lot about high frequency noise in the power lines. If you ever do find a good resource, please share. I'd love to see it too.

@Marc Moreau - your drill press is most like a mill. You can add a VFD to reduce belt changes, but I recommend you keep the pulley system intact for those times when you also need the torque multiplication capabilities. You can, so why not?

Again wrong, it not about the passengers and acceleration when the highest load is encountered, but when you set the safeties and then having to pull the elevator off the safeties which requires massive torque from zero rpm, very similar of getting a heavy mass to initiate rotation on a lathe (if not more so as the are jammed on) Again the advantage of the VFD is that motor "thinks" it is at full speed regardless of the frequency. So at ultra low speeds (ie start up) under load VFD's set up correctly with proper voltage compensation do extremely well.

There are other tricks with good VFD's such as DC injection for braking and locking the motor in place.

The little VFD's we use are no comparison to what can be done.

FYI, I sold the first VFD modernizations in Canada in the 90's.

One other tidbit, unless the elevator is load past about 50% of its rated capacity on a geared or gearless elevator the motor prevents you from shooting into the sky and pushes you down to get to lower floors.

The Scotia Tower Freight elevator accident suffered from shaft failure and the construction elevator was calculated (based on the info from the one survivor) to have hit the overhead at an excess of 4500ft per minute without the motor running.

 

[Marc Moreau]

For my motor HP P= vi volt multiply by amp divise by 745 = 1.932 2 hp

 

[Susquatch]

Again wrong, it not about the passengers and acceleration when the highest load is encountered,

Sorry, I totally misunderstood the point you were trying to make. I certainly agree that elevators are great example of what a VFD can do.

My points still stand but not as they relate to your point, only as they relate to my own discussion about torque and hp. I thought the elevator was actually a good example to help explain the difference between hp and torque. But not if you start adding all that other stuff into the discussion. I'd much rather try to simplify than complicate.

 

[Susquatch]

For my motor HP P= vi volt multiply by amp divise by 745 = 1.932 2 hp

Wow Marc - that's a pretty big motor for a drill press. Your machine must be a lot bigger than the average drill press. How big is that machine of yours? Pictures?

 

[Marc Moreau]

For for finding my motor hp that was the only way to calculate ? they don't say the HP on the motor plate how do you find it ? if this motor is a 2 hp a 1 hp won't do the job. It's a Rexon from Busy bee 12 speed 5/8 chuck but on the motor no info about hp ? so if I buy a smaller motor that's not gone work this is like put a 4 cylinder in place of v8 not the same torque .

 

[Susquatch]

For for finding my motor hp that was the only way to calculate ? they don't say the HP on the motor plate how do you find it ? if this motor is a 2 hp a 1 hp won't do the job.

Shoot us a photo of the whole press from the right angle so the motor is visible in the photo.

 

[Marc Moreau]

I try tomorrow with the kid I am not very good on computer , I will try to put a lot but this motor is big and never stop the belt slip but the motor never fail.

 

[DPittman]

For for finding my motor hp that was the only way to calculate ? they don't say the HP on the motor plate how do you find it ? if this motor is a 2 hp a 1 hp won't do the job.

Just know that the HP label that the manufacturer puts on the motor is often a poor indicator of what work it is capable of. Even amp ratings are misleading as I have learned, it depends on how the manufacturers test their motors. As discussed torque is a relevant factor.

 

[Marc Moreau]

Maybe but how could we find the write HP with no info ? sometime like you say company put there rate higher than the reality .

 

[Susquatch]

Just know that the HP label that the manufacturer puts on the motor is often a poor indicator of what work it is capable of. Even amp ratings are misleading as I have learned, it depends on how the manufacturers test their motors. As discussed torque is a relevant factor.

You said a mouthful there! I agree completely.

If you can get a decent photo @Marc Moreau we can probably tell you roughly what size it is.