Lathe speeds - Tested.... and interesting

[kevin.decelles]

The lathe I purchased from Modern (C0636A -- same lathe as @Janger) has a variety of speeds in either a high/low range.

The 'posted' speeds in low range (in order of speed) are: 70/115/190/300
In high range the speeds posted are 460/755/1255/2000

Some time back I had a weak moment while browsing ebay/amazon/kbc/princess-auto flyers and bought one of those laser speed readers. I must say that after I pressed the purchase button, i felt buyers remorse..... until it showed up. I use that device all the time now, on anything from motors, fans, pulleys etc.

Anyhow, I put some reflect tape on the 3-jaw and ran it in all speed ranges to see what the laser says.

In high range, all the speeds 'labelled' on the machine are 'over' stated by 15.5 - 16%. All the speeds in the low range are 'under' stated by 15.5 - 17%.

Example: Highest posted speed: 2000, laser reading 1680. Lowest low speed: 70, laster reading 82.

I found it interesting that there was uniformity to it --- it wasn't just one gear range that was off -- it was consistent. And the inverse proportion between high/low was interesting too.

The tech from Modern had said that all the motors had been replaced/swapped on these machines due to issues in the quality with the batch. If a different sized pulley were used, could that account for the offset? Perhaps the 'increased bore' size over the original design from 1.5" to 2"

Ok, now for the question: Comments/theories on the findings? Is this typical? Has anyone else ever tested their speeds? @Janger -- I'd interested to see what your lathe does as it is the same model.

 

[Janger]

Hmmm is a 50hz motor slower or faster than a 60hz? China uses what 220v and 50hz? The difference is %16.6 which is the delta you are seeing Kevin. But it seems backwards to me I would think a 60hz would be faster.

I want one of those laser gadgets...

 

[Johnwa]

If it’s a regular induction motor it will run faster on 60Hz. I will have to check my lathe. I picked up a laser tach from PA a few years ago. Beats me why I bought it as it hasn’t been used other than just after I got it.

 

[PeterT]

Its hard to know where the machine 'specs' came from or how reliable. Most of these lathes have 2 markets, Imperial & Metric by swapping out the lead screws & default headstock gears. So measuring chuck rpms 'presumes' a certain gear train in between spindle & motor, plus yes sheave size combination. It would probably be more meaningful if you measured motor shaft rpm, but (let me guess) nothing to compare or reference it to. The low/high variation does seem odd though.

I actually don't know if 'quality' can work its way into resultant RPM's of induction motor rpms, meaning better windings or whatever. Maybe commutation is commutation so they deliver the same RPM, just different losses? (efficiency). I've also read that line voltages can vary by +/- 5% which I was surprised by. Maybe your house is at the bottom of the hill where the pressure is largest? LOL
https://electronics.stackexchange.c...variable-is-a-typical-u-s-110v-outlet-current

I have one of those tachs too, Jeez we are all junkies. In my case with RC models, we were maxi-ing out the handheld propeller optical RPM meters with the electric motors. They turned +40K on the bench.

 

[Tom O]

If it’s a regular induction motor it will run faster on 60Hz. I will have to check my lathe. I picked up a laser tach from PA a few years ago. Beats me why I bought it as it hasn’t been used other than just after I got it.

I did the same thing.

 

[Alexander]

The machine being off the speed posted on the headstock seems normal. My old mill that john A now owns was out a bit and my new cnc mill is out a massive 30%. I wouldent give either machine the time of day to repair that. It is just me using my mill so I leave the spindle speed knob at 130% to make up for it. The knob is variable from 10% up to 175%.
Also I wouldent worry about the speeds on a manual lathe too much the gears are far apart and you you can only pick the closest one anyway

 

[Johnwa]

I finished wiring up my lathe vfd so I decided to check the speeds. Vfd was set at 60 Hz.
The motor was running at 1788 rpm, I didn’t make note of what gear I was in.
On chart/ Measured
50/ 56
100/ 110
200/ 221
450/ 474
900/ 920
1800/ 1804
I probably should have checked the motor speed for each of the gear settings as it likely ran slower in the higher gears. In any case it was surprisingly close. The lathe is a Standard Modern with a 1.5 hp 3 phase motor.

 

[Tom Kitta]

I also wouldn't worry too much - just maybe take a mental note of the fact that true speeds on fast gear are a bit slower and true speeds on the low are a bit faster.

At least you don't have a massive 30% more on the fast gear - 2600 vs. 2000 may be somewhat bad for the bearings if they were marginal for the top speed to start with.

 

[Janger]

How accurate are these digital rpm readers? There is the claimed value vs what they do in practice. Johnwa your motor measured at 1788 rpm sounds close but not exactly the typical expected speed. Would that be 1750 rpm?

 

[Tom Kitta]

The 1788 rpm and a very small slippage is due to him measuring it while there was no load on the motor. The speed should go down as he applies load to the motor.

Clearly the digital rpm meter has certain accuracy but 1788 doesn't rise an eyebrow - 1800 and up would.

Also the 60hz in land line is within some accuracy.

 

[Johnwa]

I thought it was a little high too until I checked with google. The rpm on the nameplate is at rated load. I measured it with no load other than the friction and windage of the motor and drivetrain so it should have been running faster.

I suspect the digital techs are quite accurate but you either get a good reading or you get one that is obviously wrong. I had to hold mine about 2 feet away from the lathe chuck or it picked up “extra” reflections and told me it was running at 5000+ rpm.