Gear Head Operation w/ VFD

Orphanbrg

Orphanbrg

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Hello All,

I am again looking for my first home shop lathe and leaning towards buying used American vs an import. While I am somewhat familiar with VFDs and the principles on how they work (HVAC equipment), I am trying to wrap my head around how you operate a gear head lathe using a VFD. On the face of it, I assume that the most often used practice is to set the lathe on it's highest gear and make speed changes using the potentiometer on the VFD (or remote) to change speeds. Given motors do not like to run in a low speed for extended periods, is it possible to set your VFD to provide full power to the motor and then regulate the speed of the lathe using it's gear box? Are there pros or cons to running a machine in this way?

Thanks for the help.

Rick
 
Best bet with a gearhead lathe plus VFD is to set the lathe gears for the lowest speed that is high enough, then turn it down as needed for your part. For example, my Cadillac lathe has 6 gear speeds, 83, 155, 275, 550, 1020, and 1800 RPM. Suppose I have a 2" part and I want to use carbide at 200 SFPM. My handy cardboard slide rule says that is 382 RPM. So I'd set the gears for 550 and run the VFD at about 70% of full speed to get 382 at the spindle. That maximizes the available HP and avoids running the motor at very low speeds where it isn't happy.

My lathe has roughly 2:1 steps between geared speeds. That means I'll never need to run the motor at less than about 50% of full speed.

Also, for anyone who hasn't seen the handy slide rule I'm referring to, it was made by Warner & Swasey. Set the diameter across from the desired SFPM on one scale and read the appropriate RPM on the other scale. Very handy. Still readily available on ebay for $10-20. Here is one seller (no connection to the seller, just the first hit when I searched): https://www.ebay.com/itm/364004633929
 
If you want to vary speed with a knob, you'll need a VFD. Plenty of guys on here running them with great success. It's not always power that you need to optimize, speed control is what folks are after. VFDs produce a percentage of the motor's rated torque at low speeds, but that might not be an issue when torque is at an excess, which is most of the time on the lathe.

If you want 3-phase and run the machine's controls as they were intended without chopping up your electrical box, RPC is the way to go. RPC can always supply the needed power, and they are transparent to the machine's operation. I would rather VFD a drill press because it's simple. A mill or lathe with lots of electrical controls, mag switches, interlocks, and other goodies loomed into its control box might be a messy conversion to VFD, in those cases a RPC is the far better choice.
 
Orphanbrg said:
Hello All,

I am again looking for my first home shop lathe and leaning towards buying used American vs an import. While I am somewhat familiar with VFDs and the principles on how they work (HVAC equipment), I am trying to wrap my head around how you operate a gear head lathe using a VFD. On the face of it, I assume that the most often used practice is to set the lathe on it's highest gear and make speed changes using the potentiometer on the VFD (or remote) to change speeds. Given motors do not like to run in a low speed for extended periods, is it possible to set your VFD to provide full power to the motor and then regulate the speed of the lathe using it's gear box? Are there pros or cons to running a machine in this way?

Thanks for the help.

Rick
Click to expand...
You said in your post that you were at least considering an older American lathe. One of the things to know is that many of these old lathe manufacturers went to great lengths to isolate the oil reservoir from the headstock gearing. Thus, eliminating the detrimental effects of churning the oil. They often had headstock clutches and oil pumps that run off of that input shaft. It is important when considering using a VFD for speed control what is the maximum and minimum speed that this shaft turns and what was the maximum spindle speed that they ran for that machine.

As an example, my Pratt & Whitney 12c has a 3 hp, 2-speed motor of 900 and 1,800 rpm. Pratt & Whitney ran these lathes in either 1,000 or 1,500 rpm with only a motor pulley change. My machine has a maximum of 1,000 rpm. This means I could run a single 5 hp 1,800 rpm motor with the original pulleys from 30 hz to 90 hz and achieve a variable speed range from 3 rpm to 1,500 rpm and still keep the input shaft speed in the original rpm range of 500 to 1,000.

I applaud your willingness to consider an older American machine. Whether this is a good decision for you is dependent on many factors such as experience and shop capability. These old machines like Pratt & Whitney and Monarch have great value even if you never own one. They’re like blue prints for what was important in design characteristics such as the oil system or ratios such as swing-to-bed-width and many more.

Many times people looking for their first machine want to know which machine in terms of make or model. I have come to believe that the better question maybe what makes a good machine of the size and type one is considering. The first question is easier, and I understand that, but the second question will lead to knowledge that will help you make a better selection.
 
I have a VFD on a 1944 Monarch 12"CK, 3HP motor, which has a clutch with brake. I run the VFD over a fairly narrow speed range 45-90Hz, so I pick gears appropriately and use the VFD to tweak the speed. Given the clutch/brake mechanism I went with a VFD that doesn't use a braking resistor (Teco L-510). I also went from a 1200 RPM motor to an 1800 RPM motor, so at 90Hz I am at double the original top RPM of 550 to 1100. The other big advantage of the VFD is 3 phase output on single phase input. If I had to do it again I would have put in a 5 HP motor instead of the 3HP ( I replaced the motor to get an inverter rated motor). Note that both 5HP, and a 1000 RPM top end were options on this model, so I'm not exceeding the original operating range of the design. My use is pretty limited hobby stuff compared to the original industrial market for this machine.

My original lathe was a small modern lathe with a large VFD range. I find that having to shift gears makes me do a better job of thinking about what RPM (speeds and feeds) I should be using, rather than just firing it up and fiddling with the dial. I can still adjust the RPM enough to tweak things easily. I haven't found the gear shifting to be an issue compared to the larger VFD range of the original lathe.

As this was an old lathe with an on/off switch and a contactor, the VFD conversion was straightforward. The original on/off switch had mechanical rods running under the headstock to what looked very much like the old style dual pushbotton light switches used in home wiring. I managed to preserve the original front panel buttons and rods and used a couple of momentary contact switches in the back wired to the VFD, and eventually parallel those with a control panel I added to the top which includes the pot for speed control and an e-stop.


Just things to think about if you do your own VFD.
 
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As I said in my original post, this will be my first metalworking lathe. My experience to this point is 1 year of basic MTT at the local community college and that was 10 years ago. I trained on a Leblond Regal and a series 1 Bridgeport. I would very much like to find a small 12" 1ph machine to play with and move up to something larger although I have dreams of buying a Leblond both for the familiarity and the fact that my grandfather built them back in the 40s - 50s. With a 1ph service, it comes down to a RFC or VFD if I wind up purchasing a 3ph unit. Sounds like the RFC would be the easiest install and operation.
 
One thing to consider is you can pick up a LeBlond for less than a 10" benchtop from Precision Matthews. Of course, tooling in general costs more, but there is a funny effect as your lathe gets larger- equipping it can actually cost less than smaller, hobby-size tooling on the used market. For example, a 14" Bison chuck might actually be found costing less and in better condition than a 6" Bison chuck, because there are so many more hounds sniffing the net for a 6" chuck, and relatively few looking for the big stuff. Same is true for lathe cutting tools and such. The drawbacks usually come down to metric threading capability and lack of experience keeping people from buying a lathe that may need some repairs or work to get going. Truth is, if you can rebuild a transmission, you can handle disassembling and cleaning up a lathe. They are pretty simple, after all. Just arm yourself with research going in, and avoid the lemons. It helps to have a lemon farmer to bring along to help with the task if you're not ready to make the decision on your own.

I learned a lot on my Atlas, but it did not take long to realize that my daydreams would outstrip the Atlas' abilities quickly. I replaced it with a 17x60, which is more suited for my purposes and desires. The best advice I can give to someone who wants to get a lathe is to shoot for a 14x40 or so. That's big enough for most of what a serious hobbyist would do, yet still has big lathe features like an electric brake, expanded gearing, and separate lead and feed spindles for the carriage. That includes LeBlonds, Caddillacs, Takisawas, Republics, Nardinis, BP-Romis, and three dozen different Clausings, all of which are excellent machines that can be wrangled for a few grand, often with chucks and tooling included. It's eye opening to look around and consider what your dollars and some effort can procure. It just takes a little bit of surveying and research to build confidence, along with a bucketful of patience to get the best possible deal.
 
So that brings me to another question, which is off the original topic but since it's my post, I guess that's OK. As a newcomer, it's sometimes difficult to reconcile when I veteran hobby machinist says that you can pick up a quality lathe for a few grand when looking around the machines you are finding are much higher in price. A couple of years ago, I contacted an ebay seller about 2 hours away to do an inspection on a Clausing 6300 he had posted for $2000. Being the first lathe I had inspected, I educated myself on the Clasings operation and what to look for when buying a used lathe and wound up knowing more than the owner about how it worked. While the lathe looked good and ran well, he had been running it without oil in the sight glass, not even knowing it had sight glasses or that he had to put oil in it. Since he was firm on his price, I took a pass. To the novice's eye, given that the Cincinnati area produced several brands of machine tools, the area is rather dry of findings. CW Wood is the only local machine dealer that I've run across and most of their lathe offerings start at the $6000 mark and go up. How do you balance the fine art of machine cost vs travel/shipping costs vs rehab costs?
 
Check out machine dealers like HGR Surplus. I bought through Mohawk Machine in Cinci, and chartered transport for it, but I was after something a little larger than the usual shop lathe. Mohawk sells stuff that is resaleable to industry, HGR sells stuff to industry and hobbyists. Aggregator sites like Machinio are a good starting point for browsing, then follow the links to the actual seller's site (typically way down on a google search, hence Machinio exists) for better pics and prices. Prices may or may not be flexible, but you should always at least ask. My nice way of asking for a better price is to ask if that is the cash price if you aren't financing. Salesmen know what that means without taking offense.

Freight cost can vary. I've paid for several freight shipments in the last couple of years. Sometimes, you can get freight cheaper than the actual fuel cost, it happens, don't ask me how it works. Most machine dealers have freight contractors they work with, and can get good rates. Picking up yourself is easier to figure costs, just rent a drop trailer, fill up the tank, and budget the time.

You can always post pics of candidates you find on this site for feedback... you'll get all of the answers, and most of them will be correct one way or the other. You might appreciate the perspectives. I favor cold, hard quality and am willing to pay for it when it's high on my list. My equipment is a little on the heavy side. Many folks like to run useful, more compact, or more affordable equipment, and their perspectives may align with yours better than mine. So read up, and ask away.
 
Orphanbrg said:
So that brings me to another question, which is off the original topic but since it's my post, I guess that's OK. As a newcomer, it's sometimes difficult to reconcile when I veteran hobby machinist says that you can pick up a quality lathe for a few grand when looking around the machines you are finding are much higher in price. A couple of years ago, I contacted an ebay seller about 2 hours away to do an inspection on a Clausing 6300 he had posted for $2000. Being the first lathe I had inspected, I educated myself on the Clasings operation and what to look for when buying a used lathe and wound up knowing more than the owner about how it worked. While the lathe looked good and ran well, he had been running it without oil in the sight glass, not even knowing it had sight glasses or that he had to put oil in it. Since he was firm on his price, I took a pass. To the novice's eye, given that the Cincinnati area produced several brands of machine tools, the area is rather dry of findings. CW Wood is the only local machine dealer that I've run across and most of their lathe offerings start at the $6000 mark and go up. How do you balance the fine art of machine cost vs travel/shipping costs vs rehab costs?
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First off, it sounds like you are already on the right track.

When it comes to balancing these, I can see it in terms of balancing the associated risk. For example, if you have the ability to work on these machines in terms of experience, tools and rigging, then you may be willing to take a little more risk in terms of condition. This ability can reduce the risk for used machines that you can’t evaluate in person.

On the other hand, I may be willing to pay more for a machine that I can evaluate in person. Additionally, shipping costs and distance all eat into the amount I’m willing to spend on a machine.

Knowing what your goals are for the machine in terms of use and condition can help you find the right balance. In the end, there is no substitute for researching and patience.
 
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