Lathe Spindle RPM?

B

bpimm

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I'm going to be changing to a treadmill motor on my lathe and have been reading about cutting speeds. Using a SFM to RPM calculator and the numbers I got from a SFM chart I saw SFM numbers for different materials I might want to work ranging from 26 for roughing steel to 3300 for finishing Aluminum. I ran the numbers for 1/4" diameter and 5" diameter (that's what I can swing over the carriage) using both the minimum and the maximum listed above and came up with a range of 19 RPM to 50,400 RPM to cover the listed SFM's and sizes. Obviously this is well beyond reality so I'm wondering what the max RPM you guys would recommend for an old (110 year old) split brass bearing lathe would be?

The way it was set up when I got it it's 3 speeds are 380 RPM, 640 RPM and 1060 RPM plus back gears, I didn't measure it with the back gears in.

I would also be interested in seeing pictures of your motor and drive setups on old lathes if you are willing to share.

Thanks
Brian
 
Hey Brian

I am unclear what you mean by 54,000 RPM. is that the motor RPM?

You are calculating the motor RPM, multiply times the motor pulley circumference.

Then divide by the spindle pulley circumference.


Another cheap way is to print a fluorescent fixture RPM "gage" for the back of a chuck or faceplate. It uses the "strobe" effect from the 60 cycle electricity (in the United States) like an ignition timing strobe.
Come to think of it, I believe you should be able to scale up or down any one you find. Sherline has a free printable one:

http://www.sherline.com/rpmgage.pdf






Bernie
 
... Then you can set up your pulleys etc, dial your machine to the appropriate RPM and mark the motor speed control dial.

If you know the motor RPM is 54,000, and you are SURE want to use the WHOLE MOTOR RPM Range, then you have to just do the math to arrange pulleys.

If the max RPM you want is 1060 RPM, you really need to gear it down with a few pulleys.

A 1 inch pulley on the motor- belt to an 8 inch pulley, next to a 2 inch pulley, belt to another 8 inch pulley gets you to about 1,700 RPM.

THen you just put a smaller pulley next to that 8inch pulley, and choose which of your spindle pulleys to go to.


Bernie
 
I guess I didn't explain that very well, the 54,000 rpm is the theoretical spindle RPM to maintain the 3300 SFM on a 1/4" part if the calculator I used was right. Obviously this old lathe would explode long before it got there.... What I'm after is the max spindle RPM that would be safe for this old lathe? 1000 RPM? 2000 RPM?

It runs 1060 RPM now according to my HF contact tachometer. I can calculate the needed pulley sizes once I have the max spindle number. Most of the treadmill motors are good to 6000+ RPM.

Brian
 
I had a 100+ year old lathe once it was a Le Blond 8x30 . Babbitt bearings . 3 place belt pulleys and clutch . I don't remember the RPM's , but it did have back gear. Calculating SFM I think should be known by a Machinist . But really is it that important for us as hobby Machinists? I think, to a
point for sure . But most of us are not programing a CNC lathe with a 30 inch swing , Where you live and die by SFM .I think, with older and antique
machines calculating SFM is a rewarding part of hobby machining but not super important . In prospective a guide line ,but it's also about how much the machine can take .
And really as hobbyists, I don't think we need super fast speeds and feeds . Man I wish I still had that old Le Blond !
 
Crossthread, your last statement is kind of a yes and no thing. You are spot on about the surface speed on a CNC in a commercial setting where time is money. People change tooling to gain cutting speed and material removal rates. As a hobbyist, those are not the main factors to consider. BUT, you still have a legitimate need to know enough about the proper speeds and feeds to get the best finish when you need it, and the proper speed to avoid burning up and prematurely dulling bits and inserts (if you use them). So it's not a trivial thing even for a hobbyist, but if you're trying to squeeze every second out of an operation, then by all means, yes it is absolutely necessary to be able to push the tooling and machine to their limits, without exceeding any limits. Hobbyists need to know for other reasons.

"superfast" speeds in a hobby shop? Nah, unless you are talking about routers and stuff. So your spot on there too. But it's not practical to try to get the calculated optimum speed for a 1/4" piece of aluminum to turn at it's "best" speed. There aren't that many commercial machines that can do that. And people never worry about that. Unless you are using a collet, I wouldn't run any lathe that fast even if it could. The chuck would probably fly apart. Even if it were only 4" diameter. Plus the faster you spin, the less chuck pressure you have on the part. Most people never even think about that. The weight of the jaws and centrifugal forces are a factor to be reckoned with.

As far as max speeds for babbit bearings, look at auto engines. Granted, they have pressurized oil systems, and that is the real key, but the small 4 bangers regularly redline above 6,000 RPM. Maybe more now days. I haven't looked in a long time. For a drip oiler, I think <2,000 RPM would be my choice, as long as it was tight, and I had sufficient and correct lubrication.
 
Last edited:
[I have rebuilt several machines from air compressors to lathes and pipe threading machines with babbit bearings. IN EVERY CASE , THE BEARINGS WERE IN VERY GOOD SHAPE. tHE SHAFTS WERE WORN AND OUT OF ROUND. i POURED NEW BEARINGS AND HAD THE SHAFTS REGROUND AND REASSEMBLED THEM
dAMN THERE ALL CAPS AGAIN, i GOTTA LEARN TO TYPE.

Any how there are standards for these factors. I may have a book with that I'll look and see . If I have it I'll post it.OH YEAH My point is .The bearings don't burn up, the shafts wear out. The best pratice is never run anything faster than you HAVE TO
 
Tony Wells;117991 "superfast" speeds in a hobby shop? Nah said:
The optimum speed for 1/4" was just an exercise for my feeble mind just to answer what the theoretical best would be, not for practical purposes.

One of the goals I have is to drive the lathe without a jack shaft. With the motors we have on the shelf right now one of them looks good, it's a 2.5 hp @ 7000 RPM with a max speed of 7200 RPM motor and figuring 1500 RPM as the bottom end of the usable range with a 1 1/8" drive pulley going strait to the largest face on the flat belt pulley would give me a working RPM range of 312 RPM to 1500 RPM without having to change the belt position. if I switched to the middle pulley I would get 392-1883 RPM. I also have a 1 3/8 drive pulley that would give me 381-1883 RPM on the large pulley and 480-2302 RPM on the smaller pulley.

Of the 3 speed ranges (2 almost match) which one would you want on your machine? Keep in mind that it has 6.1:1 back gears also for the real slow stuff.

Thanks
Brian

- - - Updated - - -

cross thread said:
I had a 100+ year old lathe once it was a Le Blond 8x30 . Babbitt bearings . 3 place belt pulleys and clutch . I don't remember the RPM's , but it did have back gear. Calculating SFM I think should be known by a Machinist . But really is it that important for us as hobby Machinists? I think, to a
point for sure . But most of us are not programing a CNC lathe with a 30 inch swing , Where you live and die by SFM .I think, with older and antique
machines calculating SFM is a rewarding part of hobby machining but not super important . In prospective a guide line ,but it's also about how much the machine can take .
And really as hobbyists, I don't think we need super fast speeds and feeds . Man I wish I still had that old Le Blond !
Click to expand...

Agreed, at this point in time this is a theoretical problem to solve to pick the right hardware for the motor mod, and then live with what it is. The way it's set up now the motor and jack shaft is way up high creating an 40-50 Lb out of balance mass on a stick to shake the whole machine. With nothing in the chuck the machine shakes and you can see it move almost 1/4" from the vibration of the motor and jack shaft running.

IMG_6255.JPG

IMG_6255.JPG
Click to expand...
 
onecut jimmy said:
[I have rebuilt several machines from air compressors to lathes and pipe threading machines with babbit bearings. IN EVERY CASE , THE BEARINGS WERE IN VERY GOOD SHAPE. tHE SHAFTS WERE WORN AND OUT OF ROUND. i POURED NEW BEARINGS AND HAD THE SHAFTS REGROUND AND REASSEMBLED THEM
dAMN THERE ALL CAPS AGAIN, i GOTTA LEARN TO TYPE.

Any how there are standards for these factors. I may have a book with that I'll look and see . If I have it I'll post it.OH YEAH My point is .The bearings don't burn up, the shafts wear out. The best pratice is never run anything faster than you HAVE TO
Click to expand...

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...
 
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