Lathe: when to change speed?
Discussion
Got myself a little hobby lathe. I've never changed the rotational speed since receiving it but when the belts broke (perished) I bought new ones and installed them to ensure the same speed as before. Got me thinking... I assume a larger diameter item dictates a slower speed but what material-themselves need faster/slower speeds?
Nice machine, lathe speeds as with twist drills the smaller the drill the faster the speed.
The formula for working out cutting speeds for a lathe is cutting speed (N) = 12 X S divided by Pi X Diameter
Don't panic it's simple really.....
Mild steel has a cutting speed of 100 ft minute. (S) and say the material is an 1" diameter.
12 X 100 = 1200
3.147 X 1 = 3 (round up Pi to 3)
1200 / 3 = 400
400 RPM is the recommended cutting speed for machining 1" dia mild steel
800RPM for 1/2"
1600 for 1/4"
works the other way too
200 for 2"
100 for 1" and so on...
Ally IIRC has a cutting speed of 1000 ft min (S) using the a formula
1" ally = 4000 RPM
1/2" = 8000 RPM and so on.
You don't need to keep changing the speeds after every cut just as the diameter gets smaller you need to increase the RPM.
If you intend to use the formula for metric change the 12 to 1000 and use metric for the diameter.
HTH.
ETA
Just found this
http://its.fvtc.edu/machshop2/Speeds/RPMcalc.htm
The formula for working out cutting speeds for a lathe is cutting speed (N) = 12 X S divided by Pi X Diameter
Don't panic it's simple really.....
Mild steel has a cutting speed of 100 ft minute. (S) and say the material is an 1" diameter.
12 X 100 = 1200
3.147 X 1 = 3 (round up Pi to 3)
1200 / 3 = 400
400 RPM is the recommended cutting speed for machining 1" dia mild steel
800RPM for 1/2"
1600 for 1/4"
works the other way too
200 for 2"
100 for 1" and so on...
Ally IIRC has a cutting speed of 1000 ft min (S) using the a formula
1" ally = 4000 RPM
1/2" = 8000 RPM and so on.
You don't need to keep changing the speeds after every cut just as the diameter gets smaller you need to increase the RPM.
If you intend to use the formula for metric change the 12 to 1000 and use metric for the diameter.
HTH.
ETA
Just found this
http://its.fvtc.edu/machshop2/Speeds/RPMcalc.htm
Edited by Big Al. on Thursday 28th April 22:36
forgot to add those cutting speed are primarily used for HSS (High Speed Steel) cutting tools
Why do you need to change speed?
Simply because you wear your tool up if you go too fast and waste a lot of time if you cut too slow.
In general with steel using HSS if the swarf is discoloured it's too fast.
Roughly in order of most dificult to cut it's high tensile steel(bolts etc.), stainless, cast iron,
mild steel, free cutting steel, bronze, brass, aluminium, plastics, wood.
A drop of cutting oil helps impove surface finish.
Eye protection prevents blindness.
Sharp swarf cuts skin with ease.
If you are using tungsten carbide speeds can be trebled and blue swarf is no problem(except it's
very hot).
Never leave the chuck key in after use. It will hit you in the face when you inadvertantly start the machine.
Simply because you wear your tool up if you go too fast and waste a lot of time if you cut too slow.
In general with steel using HSS if the swarf is discoloured it's too fast.
Roughly in order of most dificult to cut it's high tensile steel(bolts etc.), stainless, cast iron,
mild steel, free cutting steel, bronze, brass, aluminium, plastics, wood.
A drop of cutting oil helps impove surface finish.
Eye protection prevents blindness.
Sharp swarf cuts skin with ease.
If you are using tungsten carbide speeds can be trebled and blue swarf is no problem(except it's
very hot).
Never leave the chuck key in after use. It will hit you in the face when you inadvertantly start the machine.
mr.man said:
Why do you need to change speed?
Simply because you wear your tool up if you go too fast and waste a lot of time if you cut too slow.
In general with steel using HSS if the swarf is discoloured it's too fast.
Roughly in order of most dificult to cut it's high tensile steel(bolts etc.), stainless, cast iron,
mild steel, free cutting steel, bronze, brass, aluminium, plastics, wood.
A drop of cutting oil helps impove surface finish.
Eye protection prevents blindness.
Sharp swarf cuts skin with ease.
If you are using tungsten carbide speeds can be trebled and blue swarf is no problem(except it's
very hot).[b]
Never leave the chuck key in after use. It will hit you in the face when you inadvertantly start the machine.[/b]
No it didn't, hit me squarely in the chest with enough force to leave me breathless, and still enough momentum to bounce of the ceiling and nearly clout the foreman on the head. He wasn't best pleased with my incompetence. I also know of someone who was in the habit of removing the key as the started the machine, timed it wrong and lost their hand in a mess of crushed meat and bone as it hammered the hand onto the bed.Simply because you wear your tool up if you go too fast and waste a lot of time if you cut too slow.
In general with steel using HSS if the swarf is discoloured it's too fast.
Roughly in order of most dificult to cut it's high tensile steel(bolts etc.), stainless, cast iron,
mild steel, free cutting steel, bronze, brass, aluminium, plastics, wood.
A drop of cutting oil helps impove surface finish.
Eye protection prevents blindness.
Sharp swarf cuts skin with ease.
If you are using tungsten carbide speeds can be trebled and blue swarf is no problem(except it's
very hot).[b]
Never leave the chuck key in after use. It will hit you in the face when you inadvertantly start the machine.[/b]
As for speed, much like shagging a virgin, if it's big or hard, go slow.
When you're turning wood and the chisel digs in it's not much fun either! Lathes need respect.
My father happily let me turn wood and metal on hs Myford ML8 but never the circular saw or planer attachments. Brilliant tool; it was the lathe equivalent of a Swiss Army Knife.
My father happily let me turn wood and metal on hs Myford ML8 but never the circular saw or planer attachments. Brilliant tool; it was the lathe equivalent of a Swiss Army Knife.
Edited by Simpo Two on Friday 29th April 17:59
Wow and Emco Unimate 4!
I have one of those at work as a de-piping machine, great little lathes. Just watch out for the belt tensioning. They actually use big o-rings, had two go on mine from what I reckon was just the rubber breaking down. In the end, used a 'make your own belt' kit where you cut it to length and solder the ends together.
Anyway, for speeds, really it becomes a knack judging speeds with machines and materials. I cut anything on mine, brass, 303, 316, mild steel etc. The thing that more judges the speed is how much the lead screws with take it, you should ensure your saddle is bolted down firm otherwise you'll suffer a lot from chatter.
I have one of those at work as a de-piping machine, great little lathes. Just watch out for the belt tensioning. They actually use big o-rings, had two go on mine from what I reckon was just the rubber breaking down. In the end, used a 'make your own belt' kit where you cut it to length and solder the ends together.
Anyway, for speeds, really it becomes a knack judging speeds with machines and materials. I cut anything on mine, brass, 303, 316, mild steel etc. The thing that more judges the speed is how much the lead screws with take it, you should ensure your saddle is bolted down firm otherwise you'll suffer a lot from chatter.
AndyFoo said:
Wow and Emco Unimate 4!
I have one of those at work as a de-piping machine, great little lathes. Just watch out for the belt tensioning. They actually use big o-rings, had two go on mine from what I reckon was just the rubber breaking down. In the end, used a 'make your own belt' kit where you cut it to length and solder the ends together.
Anyway, for speeds, really it becomes a knack judging speeds with machines and materials. I cut anything on mine, brass, 303, 316, mild steel etc. The thing that more judges the speed is how much the lead screws with take it, you should ensure your saddle is bolted down firm otherwise you'll suffer a lot from chatter.
I bought it with largely perished belts then spent ages trying to find out what vacuum cleaner they come off. Then one day, after one of those "PING" moments hit me, I typed in the model into Ebay and found a guy selling them for a fiver. Yay..!! I have one of those at work as a de-piping machine, great little lathes. Just watch out for the belt tensioning. They actually use big o-rings, had two go on mine from what I reckon was just the rubber breaking down. In the end, used a 'make your own belt' kit where you cut it to length and solder the ends together.
Anyway, for speeds, really it becomes a knack judging speeds with machines and materials. I cut anything on mine, brass, 303, 316, mild steel etc. The thing that more judges the speed is how much the lead screws with take it, you should ensure your saddle is bolted down firm otherwise you'll suffer a lot from chatter.
It is a good little machine. I only use it for cleaning stuff up and little jobs - exactly what I imagine it was intended for. And as much as I'd love a Myford, I don't have the room and would never really use it for more than I use the Unimat for anyway I guess.
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