What's the fastest my spindle can go?

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bulgie
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What's the fastest my spindle can go?

Post by bulgie »

I replaced the motor on my 40 year old Taiwanese lathe, an Enco "92010" 10x24, also sold under a dozen other names such as Jet, Grizzly, Tida, Select et al. I used a 3-phase 1.5 HP, original was 1 HP 1 phase.

In the VFD, I am hoping to set the max Hz somewhere higher than 60, to get some more top speed, for turning small diameters in aluminum, drilling with small drills etc.

My Q is, what's stopping me from going higher? What's the bottleneck? Is it the spindle bearings heating up, or the chuck flying apart, or...?

Top speed as stock is 1000 rpm @ 60 Hz, not measured, just from the user manual. What would you guys think of me running it at 90 Hz? That should get me 1500 rpm, right? Assuming it's linear. I have a cheap photo-tachometer, so I can measure it once I get my spindle back in. (It's currently out for cleaning & belt replacement)

The spindle is a bit over 2" OD at the right-side main bearing. Bearings are tapered rollers, in good shape, seem high quality, made in Japan (NTN).

I don't know if the motor is "inverter rated" or OK with under- or over-speeding, but it was so cheap I don't care if I burn it out. It's a Weg, made in Brazil, purchased "new old stock" on ebay, must be 10 years old based on the amount of dirt on it, but definitely never used. I've already run it up to 90 Hz for a few seconds with no load, so what could go wrong? ;)

Motor is TEFC (fan-cooled), so I assume my biggest worry is overheating it when under-speeding, would you agree? I hope a little under-speeding is OK because my fond hope is to leave the belts in one place and do all my speed adjusting with the pot on the VFD.

I guess my question is, whay did they make a lathe that could only go to 1000 rpm? I guess that was considered fast enough back then?

Thanks for your thoughts,
Mark
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Bill Shields
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Re: What's the fastest my spindle can go?

Post by Bill Shields »

I have one of those lathes..

Big concern is not spindle but bearing heat ,(just from running) and what happens with all of the gears and their bearings everywhere in the drivetrain and QCGB...50% above rated on my equal of your lathe would really sound like ball bearings in a blender...

1K rpm or 1500 rpm at light loads is probably not a concern to the spindle. I would figure out a way to monitor temp on the bearings.

Also consider...is there a max rated speed on the motor? I have seen rotor parts come loose on motors run above their rated speed.
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Re: What's the fastest my spindle can go?

Post by pete »

And your not going to a get a single belt position to do both high/low rpms with just the VFD. It can do some but not all. And yes, motor heat is going to be a problem with slower running at probably below 40-50 hz for even fairly short lengths of time. Lower rpm usually means larger diameter and higher torque required so the motor is working harder without some form of torque increase to keep the motor where it makes it's rated power and cooling. Ok I get that it would be convenient to not change belt positions, but I knew going in that my step pulley mill when I bought my VFD it wouldn't do it either. It's simply a fact that those belt changes are reduced but still inevitable. People prior to the invention of VFD's had zero issues changing the belt position and it's why there built right in. If they really hated that then they bought machines with Reeves or gear head drives etc. If there's enough clearance under the left side gear cover, make that gear cover tool less to open and the same for the belt tension settings by using something like what MSC refers to as adjustable handles. There's multiple variations as both male threaded studs or with a female thread, imperial, metric, handle material and lengths. There basically a multi position handle that can be reset to the best position for there use and location. Probably 1 minute belt changes if you take your time. $50 to maybe pushing it $100 or so bucks for a then lifetime of convenience and much faster belt changes.

My mill came with those already and for that exact purpose. I added them to my little Emco lathe for both the belt cover, tail stock bed locking and it's quill lock instead of the factory cap screws. That was a huge improvement. It's a small lathe and I just didn't have the room to shoe horn one in for the belt tension nut, so I keep a ratchet, extension, and the correct socket handy. And every thing Bill said I'd have to agree with. Motors get balanced and use bearings for there expected rpm. 50% more may or may not create issues. but there's that noise and possible issues within that gear train he mentioned. Most machines with some type of back gear list maximum rpms while it's in use for that reason. So gear trains do have design and speed restrictions. Ideal cutting speed recommendations are just that, it can help, but those limits are mostly for industry where time is money and cutting tool life is factored in against maximum metal removal rates. And most seem to be listed where the expectation is you'll automatically have some type of flood coolant available. Yes with carbide that higher speed can work a whole lot better. But if your machine just can't do so then maybe carbide isn't the best choice. And machine tools are built to a price point with good and poor engineering. That 1k rpm might be all there design allowed without more complex, better parts or a full redesign. It is pretty slow with tapered roller bearings though. Mine will do 2400 rpm with a factory stock single ph motor. In theory it should be possible on your lathe, but that doesn't pay for replacement parts if it doesn't. You'd lose the power feeds, but do you have a set of tumbler gears that can drop the gear train out of engagement? It's not ideal and your not going to thread with that 90 hz rpm anyway, but you could still hand feed and drill at those higher spindle speeds.
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bulgie
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Re: What's the fastest my spindle can go?

Post by bulgie »

Thanks to Bill and Pete for the ideas.

I guess lacking any scary stories of exploding lathes, I will just try it and see.

Yeah, me saying "leave the belts in one place and do all my speed adjusting with the pot on the VFD" is crazy, I didn't even really think that would work, should have phrased it different. More like do *some* of the speed adjusting with the VFD. How much to be determined after I do it for a while. No way would I over-speed the motor when I'm in the back gears, which would be pointless since they're for when you want slow. And no need to underspeed in back gears either, since the stock setup goes plenty slow for anything I do. I really only want this for more top end.

Pete, I can't make the belt changes any faster because they're super fast'n'easy already. For the first belt (from the motor to the jackshaft), there's one thumbscrew that takes no tools, maybe 5 seconds to undo. The second belt (from jackshaft to spindle), you just open the lid, no screw at all. And of course flip the lever that puts some slack in the belts, flip it back after making the change. I really can't complain about the belt changing time, it's maybe 30 seconds to a minute.

But there are some times when I think changing speed on-the-fly would be handy, without stopping the motor. Like a big facing cut, needs slow rpms out at the rim, but then that's annoyingly slow when you get closer to the center. I think I would like to twist a knob to dial in more speed midway through that. Or if you're getting chatter, to be able to slow it down without stopping might be handy. Unless I'm dreaming this, I seem to remember some harmonics that actually go away at a *higher* rpm, so slowing down isn't the only thing to try.

Yes I can set the tumbler to not power the gearbox at all, I already do that for drilling, or making a taper with the compound. Definitely would want to do that any time I can when over-speeding.

My way of monitoring the temperature of the spindle bearings is to put my hand on the headstock near the bearing, or the spindle itself after stopping the rotation. Not very scientific, but I know from experience about how much temperature rise to expect. That's how I set the spindle bearing preload — you want to feel a slight rise in the temp, or else you don't have enough preload. Glad to hear contrary opinions, but that's what the manual says. It says as long as the bearing doesn't get uncomfortably warm, from extended use at top speed, then it's OK.

After I wrote my original post in this thread, I went ahead and installed the motor. Still waiting on some parts I ordered to help install the spindle, so that ain't happenin' today, but I ran the motor at 90 Hz powering that first belt and jackshaft, and it's plenty smooth, gave me no worries at all for that much of the power train at least. I used the "high gear" belt position, of the two choices for that first belt. Jackshaft bearings didn't heat up at all, at least to the ability of my hand to feel it. I didn't think they would, they're lightly loaded by this test, but it was worth checking. Plus I just liked running it. I'm getting excited about making chips soon!

Sorry to be such a dork, I know you guys all make chips all the time, but for me it's been 25 years since I had a lathe to play with. After my metal fab career, I had a whole 'nother career in computer networks. I really missed getting my hands dirty. Makers gotta make!
pete
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Re: What's the fastest my spindle can go?

Post by pete »

For now my lathe is single ph but still on the fly variable speed. Even with that I still want to go 3 ph and VFD just for what it can do with presets, ramp up/down, it's much smoother etc. Yep, those larger facing cuts and adjusting to suit the diameter the tool is at are really helpful with that variable speed. And as you say, it also allows adjusting up or down a bit on longitudinal cuts to prevent that chatter or find a speed that produces the best or better surface finish. Most or maybe all VFD's will show only the motor rpm. At a 1-1 direct drive that would be fine, but most lathes aren't. I'd recommend adding even a cheap digital tach for what the spindle is actually turning instead of doing mental gymnastics figuring out the pulley ratio, motor rpm verses spindle speed every time you adjust the speed up or down and swap to a different belt position. My overall guide is mostly chip color though. Starting to go a light tan or brown once they clear the tool with high speed steel and a light blue as there being generated in the cut to turning a darker blue after leaving the tool with carbide. But that tach will get you into a ball park starting range and adjust from there. If the motor can tolerate the VFD and most should, I think you'll notice a large difference between the OEM single ph and the new 3 ph. 30 seconds after the first switch on of my then new mill I was a confirmed VFD addict.

Rotating bearings are of course different, if there's any sleeve bearings in your gear train that aren't already running in an oil bath, then that 50% higher speed means your going to need to be more on top of there oiling. As far as the actual gear teeth, I read an explanation in one of my books that made pretty good sense. Even when single point threading, the loads on the gear train are pretty light in comparison to something like an automotive transmission. For that reason we can get away with more frequent but light amounts of oil. Dripping in oil and throwing it everywhere is mostly a waste. Back gears because of there high torque duty are more demanding, maybe a heavier oil or light grease and re-lube before each use. I despise grease on machine tools though, it retains any wear particles, chips and contamination if it can get in there, and the oil works better to help squeeze all the garbage out and away from the working surfaces. Grease gets used on slower rotating equipment. (Farm and earth moving machines) Sleeve bushings and higher speeds need the oil unless the manufacturer says otherwise.

Edited to add, I picked up a bit of information about non inverter rated motors and there long term durability with VFD's. The poster mentioned and it makes logical sense to me that if you go enough above or below 60 hz and run for long periods of time at that setting motor issues can develop. I suspect going by that there's at least some degradation to a non inverter rated motor at both high and low hz settings each time it's done. It seems worth knowing about at least. Since they do build inverter rated motors then there must be something to it.
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Re: What's the fastest my spindle can go?

Post by Patio »

I am not expert, but.... I run my Maxmil mill with the belt in the high speed position and leave it there. The only time I move it, is if I need some extra torque, when milling slow and heavy. I do not use my mill for gain, so it is not run for long periods of time with heavy loads. I do often run down around 20hz, regularly. It is just what I do, and have not had any overheating problems. If I am concern, I feel the motor with my hand. My lathe is sort of the same way. Though with the lathe I can not just reach out and feel the motor, because it is under the lathe, in a cabinet. I may one day put a temperature sensor on the lathe, just so I know what is going on. I can not recommend this practice, but it is what I do.
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bulgie
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Re: What's the fastest my spindle can go?

Post by bulgie »

Patio wrote: Sat Jan 22, 2022 12:31 am <snip> I do often run down around 20hz, regularly. It is just what I do, and have not had any overheating problems. If I am concern, I feel the motor with my hand. My lathe is sort of the same way. Though with the lathe I can not just reach out and feel the motor, because it is under the lathe, in a cabinet. I may one day put a temperature sensor on the lathe, just so I know what is going on.
Since I started this thread I've been using the lathe a fair bit at both extremes, well not down to 20 Hz but a lot at 30 to 40 Hz, and a lot on 90 Hz. I did a run-in test to see if the spindle bearing preload was too high, ran it at 90 Hz (1600 rpm) for probably 20 minutes (I was shooting for 15 minutes and overshot). No problems to report.

I bought a cheap two-probe digital thermocouple, under $30, no idea how accurate it is but at least both probes read the same (as each other), at room temperature as well as in boiling water. Doesn't allow calibration, but it was pretty close as-is. I forget the exact numbers I got on my bearings, but it reassured me I'm in the ballpark. The probe wires are definitely long enough to go into your lathe cabinet to monitor your motor, if you were interested. The temperature readout runs on battery, so you wouldn't leave it on full-time, but whenever you want to do a test it's very easy. I've been taping the probe to the bearing housing with aluminum tape on the theory that it conducts heat, but duct tape would probably work fine too. I assume the inside of the bearing is a little hotter than the outside where my probe is, hopefully not too too much hotter.
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Re: What's the fastest my spindle can go?

Post by pete »

Since the bearing outer races are in pretty close contact with there bores, you'll certainly get some lag of temperature change between what the actual temps the bearings are at verses the castings or bores there sitting in. Plus the higher volume of surrounding metal will dissipate that heat transfer a bit. Better might have been to run that rpm test, record your highest temperature, shut the lathe down and watch for any further temperature rise happening. But I'd say 20 minutes without excess heat should still be a decent test on it's own. If you can easily hold your hand or fingers on the area there mounted into after that 20 minutes, it's a good indicator the heat's not excessive. Since most drive motor bearings have a lot more allowable open tolerances on them than spindle bearings, heat generated by higher rpms should be a lesser issue. Except and there's always one, that also depends on how and what there being lubed with. Sealed & permanently grease lubed for example and depending on the speed range there meant to operate at can or could have grease formulations and sometimes more important, precise amounts of grease injected into them for that general rpm range. Extra grease in a bearing isn't always a good thing. At real low rpms it doesn't really matter. But at those higher than usual rpm levels, the grease itself along with those higher rpms can cause frictional generated heat. If that were to go high enough and within the greases melting range, what happens is it begins to melt and flow out of the bearings. Maybe not all at once, but if it does happen enough, you then start to lose that lubrication. And without it the bearings are going to fail soon after. I've seen quite a few much larger roller bearings than our machine tools usually have with clear evidence of that melted grease in fairly high HP situations. In those cases and since the rpms were always constant and what the equipment was designed to do. That melting grease happened because the bearing itself started to fail, the heat from that beginning then melted the grease and total bearing failure didn't take long after that. I'm not suggesting it will happen depending on what you have, only that it could happen. So it's just something to be aware of.

With oil lube such as in a lathes head stock and multiple quarts or gallons in the sump, lube break down is far less likely. But the extra heat within the bearings and because of the close tolerances it's still a real problem. Tighten up those clearances enough due to the heat, and if the bearings or rollers start touch, or the clearances between the races get too tight, the components start to drag, that increases the heat even more. The rest is inevitable. It's also not just your radial clearances on those spindle bearings. A decently designed and built machine tool has some real tight axial clearances as well. Extra heat might affect that enough to again reduce those clearances and cause extra heat. But on the lathes I've owned so far, the front bearings are press fit to the spindle and the rear bearing is a close slip fit on the bearings inner race so it can float a bit due to spindle expansion along it's length. I don't know that all machine tools are designed that way or not, so your on your own there. :-) I do know on very high rpm spindles those bearing clearances are made to be a bit larger than average. They engineer them that way on purpose. The machine is run long enough to come up to it's average operating temperature first. That tightens those clearances back up, those bearing clearances stabilize and there then where there designed to be. Way outside your thread topic, but on the real, real high end machine tools, the bearing lube oil has a refrigeration unit it passes through, so all the parts and the oil itself stays at a very narrow and precisely controlled temperature range.
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Re: What's the fastest my spindle can go?

Post by Richard_W »

On a Jet 14 X 30 gear head lathe with only 2 spindle bearings. After a while of running at the highest speed of 1,400 RPM the spindle would bind up from the heat and would literally slow down. When that happened, I would open up the back cover and loosen the bearing lock nut a little and I would again be able to run at 1,400 RPM.

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bulgie
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Re: What's the fastest my spindle can go?

Post by bulgie »

Richard_W wrote: Sun Feb 06, 2022 12:26 am On a Jet 14 X 30 gear head lathe with only 2 spindle bearings. After a while of running at the highest speed of 1,400 RPM the spindle would bind up from the heat and would literally slow down. When that happened, I would open up the back cover and loosen the bearing lock nut a little and I would again be able to run at 1,400 RPM.

Richard W.
Wow, with no lasting damage to the bearings?

One source I read claimed that since the spindle heats up more and expands more than the headstock casting, that would tend to reduce the bearing preload. That makes sense to me if axial clearance is the main factor, but it's not going to help with radial clearance.

So in your scenario, sounds like this could lead to a cascade, or vicious cycle: friction causes heat, which leads to more friction, and more heat. That sounds like a prescription for disaster if you don't catch it in time.

Not a rapidly cascading vicious cycle in your case I guess. You had time to notice the problem and fix it. Good to know.

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What's the fastest my spindle can go?

Post by Steggy »

“What's the fastest my spindle can go?”

Just to be facetious, your spindle can go as fast as you want it to go. Just try to stay out of the way of flying parts. I hear getting whacked in the head with a chuck can be painful.
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bulgie
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Re: What's the fastest my spindle can go?

Post by bulgie »

BigDumbDinosaur wrote: Mon Feb 07, 2022 2:48 pm “What's the fastest my spindle can go?”

Just to be facetious, your spindle can go as fast as you want it to go. Just try to stay out of the way of flying parts. I hear getting whacked in the head with a chuck can be painful.
Yeah I figger I won't go over 1500 with a chuck. If I program the vfd to let it to go faster, it'll be for use with a collet only, and balanced loads.

Does anyone here think I'm seriously in danger with a "made in Poland" cast iron 3-jaw at 1500? It's been used for 40 years at 1100 so I'd be surprised if I've really crossed some threshold. I still keep my face out of the likely trajectory, should it asplode. My 4-jaw was made in Germany so I have a little more confidence in it, but it's also a bit bigger/heavier so it'll do more damage.

I've heard it said that the centrifugal force on the jaws makes them grip with less force, so a workpiece can come loose. Never experienced that myself, but I'll keep it in mind. Sounds "highly theoretical" (unlikely) at my kinda speeds, agreed?
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