Lathe Spindle RPM?

[onecut jimmy]

Le Blonds were and I know of 2 of them that still are excellent machines.
IMO You need to know and understand, surface feet per minute, for the same reason that, although you are not a NASCAR driver, you still need to know and understand miles per hour.
It is what governs the quality, of yourworksmanship. It will effect both the finish appearance and the ability to hold sizes.

 

[Tony Wells]

One factor that hasn't been brought up yet is power transfer ability. You say the motor is rated 2.5 hp. You simply cannot transfer unlimited power between two shafts with just any belt and any pulley diameters. It's all about the grip area, and very controlled slippage. It's a given that the sides of the belt must slide (or slip) up the walls of the pulley (if you are using a vee belt anyway) as it exits the pulley. But you must have enough surface contact between the belt drive/driven surfaces, in the case of vee belts the flanks, to transfer the power to and from the pulleys. Imagine, as an exercise, how much power it would take to make a belt slip on a 3 foot diameter pulley versus a 1/2" pulley. Enormous difference in the gripping surface area. Ignoring the flex around the 1/2" pulley, which one would slip first? Of course it would be the smaller of the two.

Now in your case, what kind of belting are you going to use? A flat belt, or a serpentine belt has other limiting factors, but let's just talk about vee belts. I haven't looked it up, but will a typical a section or b section vee belt handle 2.5 hp on a 1 1/8" pulley? I doubt it would be even recommended to bend an a or b belt that tight, so you have another problem with belt selection. Of course, the lathe pulley is already there and let's assume it's fixed and you will not change it. Probably true anyway. Would be in my case. So there is a sizable dilemma in itself. I believe you need additional information on belt drive apparatus before you select the pulley.

The speed ranges you have are about right. I would go for the middle I think, aside from all that above. Even then, the back gears only will give you around 60 RPM for the lowest speed though. If you're OK threading and knurling at those speeds, then it's fine. Lots of lathes turn slower though. Just your own comfort level.

 

[onecut jimmy]

I look forward to seeing these factors if you find them. this lathe has brass bearings, probably about the same. I have trouble with your last quote... I'll go along with it in the shop, but having played with and raced cars all of my adult life I can't completely agree with it.) I've even hot rodded my lawn mower...

I know almost nothing about cars or hot rods. All of my adult life has been spent designing and building machinery. Before that I listened to metal forming and metal finishing every night at the dinner table. My father was a machinist and my Grandfather was a blacksmith.
I was referring to shop machinery and any machine you want to last awhile.
All that said and done. The reason for SFM is it will affect the finish of your parts and the size the tool cuts. Depending on the tool geometry it will remove more or less material depending on the speed.IMO this is more apparent on a lathe than the other machines.

 

[Ray C]

Something worth thinking about is that DC motors (and treadmill motors) produce their torque at low RPM. They have opposite torque curves from AC motors...

As far as SFM, I wouldn't get hung-up on it. RPM translates roughly to 4 x SFM / Diameter. For Aluminum, most manual lathes cannot muster-up the 3000-4000 RPMs needed for 1/4" stock -but it doesn't stop anyone from making beautiful cuts. I think the SFM charts are to obtain both optimal removal and optimal finish. So, when you can't muster-up the RPMs, you take lighter cuts and spray a little WD-40. I cut 1/4" AL all the time -and it looks beautiful.

Another consideration is that most manual lathe chucks are rated only up to 2500-3500 RPMs. In all honesty, I would feel uncomfortable in front of a jawed chuck spinning faster than 2500 RPMs. The vast majority of my daily work is between 500 and 1600 RPMs and I use carbide for most of my work. With HSS, I could get away with much lower RPMs...

If you look at the higher end manual lathes these days, they all go with a 3-5 HP, 3 phase motor and control it with a VFD. That way, you still get the torque needed at lower RPMs.

Just some food for thought.

Also, Tony's thoughts were dead-on. The proper gears are needed to transmit torque properly -and to cloud the waters, the rules that apply with AC motors don't apply to DC motors.


Ray

 

[bpimm]

One factor that hasn't been brought up yet is power transfer ability. You say the motor is rated 2.5 hp. You simply cannot transfer unlimited power between two shafts with just any belt and any pulley diameters. It's all about the grip area, and very controlled slippage. It's a given that the sides of the belt must slide (or slip) up the walls of the pulley (if you are using a vee belt anyway) as it exits the pulley. But you must have enough surface contact between the belt drive/driven surfaces, in the case of vee belts the flanks, to transfer the power to and from the pulleys. Imagine, as an exercise, how much power it would take to make a belt slip on a 3 foot diameter pulley versus a 1/2" pulley. Enormous difference in the gripping surface area. Ignoring the flex around the 1/2" pulley, which one would slip first? Of course it would be the smaller of the two.

Now in your case, what kind of belting are you going to use? A flat belt, or a serpentine belt has other limiting factors, but let's just talk about vee belts. I haven't looked it up, but will a typical a section or b section vee belt handle 2.5 hp on a 1 1/8" pulley? I doubt it would be even recommended to bend an a or b belt that tight, so you have another problem with belt selection. Of course, the lathe pulley is already there and let's assume it's fixed and you will not change it. Probably true anyway. Would be in my case. So there is a sizable dilemma in itself. I believe you need additional information on belt drive apparatus before you select the pulley.

The speed ranges you have are about right. I would go for the middle I think, aside from all that above. Even then, the back gears only will give you around 60 RPM for the lowest speed though. If you're OK threading and knurling at those speeds, then it's fine. Lots of lathes turn slower though. Just your own comfort level.

The treadmill motors are equipped with a serpentine belt pulley so I will stay with that type of pulley as they are far more capable of transferring power on a small diameter pulley. I'm going to try running the serpentine belt strait to the lathe's flat belt pulley and see what happens, it should self center on the crown just like the flat belt does and my thoughts are it will provide as much or more grip than the flat belt does, this remains to be seen. If it doesn't provide enough grip I may cut the groves into the existing flat belt pulley for the serpentine belt or make a new pulley to replace the flat belt pulley. As for HP the DC motor is rated at 2.5 and the AC motor on it is a .5 HP. the need to transfer the full 2.5 is not likely to ever happen, I have never stalled the .5 HP motor and I think that I will use belt slippage as a safety clutch so it can't put max torque to the lathe is something goes wrong. I'm thinking that I will use the 1 1/8" drive pulley to start with and make the motor mount allow for shifting between the 2 largest existing flat belt pulleys, that will give me the option of slowing it down even more and if the motor can drop to 1000 RPM as well that would give me 33 RPM for the slowest back gear setting if needed.

As for comfort level threading and knurling... as a new hobbyist in this field I don't have one yet. I don't have change gears so I haven't been able to try threading, I have a knurling tool but haven't used it yet.

- - - Updated - - -

I listened to metal forming and metal finishing every night at the dinner table. My father was a machinist and my Grandfather was a blacksmith.

I would have loved to listen in on those conversations. metal forming and blacksmithing are very interesting topics and a dying art form.