Repowering a Lathe

The cautions probably have to do with motor cooling.
If the motor has a fan, using it at full HP and a lower than name plate RPM the motor might over heat.

Last week I picked up a 1920's Dalton bench lathe which has a flat belt drive, with a rather large drive system. I've checked it out well, and gone over it, and decided it's worthy of an upgrade, So I'm considering modifying it.

I'm thinking a 1/2 hp 3phase motor-VFD combination. I'll modify a pulley to go over the existing center flat belt surface on the head stock. With the VFD and lathe's back gear, I think I'll have an excellent range of speeds, and it will be a more compact package.

From what I can see, there are only upsides to the variable frequency drives.
 
cuseguy said:
His
I guess my question is why? Why would you opt for a static converter over a VFD? Price isn't much different for either, from what I know. A 2hp Teco VFD can be bought for $145-175 depending on model. I think static converters are in the same range? Aside from the drawback of losing 30% of the power, you can't run the motor for lengths of time due to overheating concerns and multiple restarts within a short period of time are also an issue. These factors make me question the amount of stress the converter is placing on the motor. You're not saving money if it ruins the motor in a couple of years. Unless something has changed that I don't know about...I would go either vfd or spend the money and do a rotary converter. This will work for you.
http://dealerselectric.com/item.asp?PID=4554&cID=0&scID=165
I own two of the higher level model of the same drive #JNEV 202H1and they work awesome.
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The Static convertor that I was looking at was about $50.00, and the VFD was over $200.00, so that is why I was thinking that route. After checking out the FM50, I think that unit will be fine for my purposes. (Placed the order this morning) Thanks for the help!
Jack
 
jmhoying said:
The Static convertor that I was looking at was about $50.00, and the VFD was over $200.00, so that is why I was thinking that route. After checking out the FM50, I think that unit will be fine for my purposes. (Placed the order this morning) Thanks for the help!
Jack
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Wise choice! I think you'll be much happier.

Sent from my Samsung Galaxy Note II using Tapatalk.
 
Hello Bill:
About the constant horsepower warning.
Its the manufactuer's way of reminding you that you can't get rated motor horsepower at speeds below the rated speed.
Recall that HP is the rate of doing work. James Watt picked 33,000 ft-lb/min as the rate that an average horse can pull a load. This has been the basic definition ever since.
For a rotating shaft (or motor or lathe spindle) HP = 2*PI*RPM*Torque/33000 (were Torque is in foot-lbs)
This is often reduced to HP = RPM * Torque / 5252
So for a constant HP situation, the Torque goes up at the same rate that the RPM goes down.
Some machining processes, like drilling, tend to follow this rule. With single speed AC motors, we use multiple pulleys or gearing to get around it.
Remember that with a DC motor, the available HP goes down linearly with RPM. So at 1200 rpm motor speed your 1 HP, 2400 rpm base speed motor would have 1/2 HP available. Also recall that when torque flows through the machine from motor to spindle, it gets modified up or down by the gear ratios. HP on the other hand, does not change as it passes through the gear ratios.

For applications where there is no gearing installed, the DC motor is often oversized by a factor of 2 or 3 to get good performance at low RPM.
So at first look, your idea of putting a 1 HP DC motor to replace your 1.5 HP AC motor might not make you happy in the end.

But I did a little calculating to see how it might go on your lathe. (haven't done these calcs for many years - it was a good exercise to go through them again).
I won't go into all the details here (see Machinery's Handbook chapter on machining power), but I assumed a specific HP of 0.88 HP/cu in/min which is typical of mild steel with feeds of .005" per rev. I figured you would belt your lathe at the existing 2:1 ratio so that the 2400 rpm DC motor would give you 1200 rpm on the spindle at rated motor speed (2400 rpm).

If you then adjust the motor speed to hold a surface speed of 200 - 300 FPM (typical of HSS cutters) for various diameters, it looks like the 1HP motor can handle cut depths of 0.015 (per side) up to about 3.5" diameter work. Above 3.5" dia, you would have to reduce the cut depth so as not to exceed 1 HP.

For "free machining" steels (like 12L14) the specific HP is around 0.52 and you could probably get up to 6" diameter before you run out of low rpm HP.

As you can see, the problem is that when you slow down the motor to hold the surface speed on large diameters, you move the motor to lower HP.

Aluminum, of course is much easier to do than any of the steels.

So, if you do most of your work at 3" dia and less, and you are not put off by feeds of .005/rev and cut depths of .015 per side, the 1HP DC motor might get you there.
Plus, if you are willing to make new pulleys to get you a 4:1 and a 2:1 belt ratio, then you would get to go to larger diameters or to larger cuts.

(I checked my work a couple of times, but if you decide to take this to the bank - I sure hope I didn't screw something up)

Terry S.
 
Terry, thank you very much.

My lathe is belt drive and the 1200 rpm is max, using the slow speed pulley arrangement from motor to jackshaft and high speed from jackshaft to spindle. Using the slowest pulley arrangement would give a top speed of 648 rpm. I'll just select the final drive that gives the max motor speed for desired spindle speed. Currently, I seem to use 500 (or less) rpm most of the time. I'm retired and in no hurry to produce blue chips that fly all over the place.

Oh yes, about 95% of my work is stuff under 3". I think I have machined one item that was in the 6" range, although I can't, at the moment, remember what it was. So that may be a damn lie.

Bill
 
I still believe 1hp DC is not going to be very satisfying to use on a 12x36 lathe with a 38 lb chuck! Heck the little 7x import lathes with 3" chucks that weigh 7-8 lbs, now come with 1hp DC. But just my honest opinion. If it works for you, great.

Sent from my Samsung Galaxy Note II using Tapatalk.
 
First, I'm wondering if the "original" motor is really original, because 1200 RPM is kinda weird. Most motors of this sort are at or slightly under 1800 RPM. -Just a thought.

When I installed a 2HP, 3 Ph motor in my lathe, I accidentally left it wired for 480 but supplied 240. -No harm done, just ran at half power and as such, couldn't get up to top speed without really straining and there would be no way it could cut a tough material like stainless steel.

Another thought, is that you can "over-horsepower" the lathe with no problem provided it's the proper RPM. The only downside is A) you're wasting electricity and B) you will be able to cut tougher materials at the cost of putting more strain on the gear-train. Other than that, you could put a 10HP motor in there and if you're cutting a piece of aluminum, the machine will never know it's being powered by a monster.

Anyhow, there aren't too many AC motors that run at 1200 RPMs -that I know of anyhow. There's a bunch in the 14 and 1500 range and going over by a couple hundred RPM is not going to part seas or cause earthquakes.

I know a few folks who've put DC motors in their equipment. It works but it seems they're always messing around with the controller for one reason or another -and there's the added complication of making an adapter bracket to make it fit.

If it were my machine, I'd just go with a modern 2HP motor (as those are most common and cheaper than 1.5) and call it a day. Since I like 3 Phase and use the daylights out of VFD control on all my machines, I'd consider that assuming the rest of the lathe is worth the extra money over a single-phase replacement.

Ray
 
Yes, the 1200 rpm motor is original, I bought the lathe new and believe me, it has never been changed. The 2hp Baldor would be a very good candidate, what would you recommend for a mount. Maybe a couple of pieces of angle iron welded to a steel plate with straps holding the motor in place?

I'm thinking I'll the DC. If it is inadequate I could use it on the bandsaw or drillpress. They seem be to be happy with 3/4 hp.

Sometimes a fellow just has to go ahead and see what happens.

I really appreciate all the input. Sometimes a guy will respond with nothin that is new, but placed in a different context. Gets the brain in gear. Really helpful.

Bill
 
OrangeAlpine said:
Yes, the 1200 rpm motor is original, I bought the lathe new and believe me, it has never been changed. The 2hp Baldor would be a very good candidate, what would you recommend for a mount. Maybe a couple of pieces of angle iron welded to a steel plate with straps holding the motor in place?

you can do it a bunch of ways. if you were so inclined you could gently remove the frame footing from a donor motor and attach it to the motor case, or an adapter could be fabricated to hold the motor as you said.
if you have access to torches or a plasma cutter you can quickly fab a solution, if not you can still do it the old fashioned way with saw, drill and welder.
there may even be commercially available mounts or brackets for the 56C face.
Grainger or McMaster-Carr may be a good place to start
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