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Thread: Safe speeds for large chucks
02-28-2010, 09:28 PM #1
Safe speeds for large chucks
What is a good guideline for determining safe maximum spindle speeds for lathe chucks? I have a 12" buck for example, and feel quite safe running it up to about 1600 rpm, but I also have a very old 16" 4 jaw, unknown manufacture, that scares me at 800. I have had a bad injury to my face in the past, 4 months of too weak to do anything, and still 4 teeth missing... and the thought of 60 lbs of cast iron coming at me at 80 mph makes me sick, and I just want to know what I'm doing is relatively safe and withing normal operating standards. Maybe chucks don't blow apart, I haven't heard of it, but...
02-28-2010, 09:40 PM #2
I would not run a 12 inch chuck at 1600 unless it is a forged one. I have seen the end results of chuck explosions, real ugly. I have an 8 inch buck forged chuck that I run to 1000 regularly no trouble but a 12 is getting pretty big for that type of speed, just my opinion though, someone should know the surface speed of that puppy it has got to be hauling.
02-28-2010, 09:46 PM #3
Modern chucks usually have a max speed stamped on them. If not I would be very cautious . Hmmmm there must be some info on this somewhere.
02-28-2010, 10:21 PM #4
Bolony. Complex highly stressed cast iron components are run at thousands of RPM every day. Look at standard transmission flywheels and pressure plates for example. Consider autoengnes, turbine rotors, and other high speed apparatus.
Just because somethng looks a little scary doesn't mean it actually is. Don't get me wrong, out of balance workpieces, defective chucks etc pose their hazards if run at high RPM. And workpieces dislodged from the chuck from wrecks or high risk operation have caused their share of injuries too. You have to be sensible about thngs like this. Work a little engineering math, consider structural safety, clamping forces being self relieved from centrifugal force ect.
The mechanical strength of chuck body materials can be ranked in descending order for assesment. Heat heated forged alloy steel counter-centrifugal production chucks naturally are at the top of the list followed by cast alloy steel, semi-steel, solid body cast iron, cored cast iron and old worn-out chucks. Three jaw chucks resist the effect of centrifugal force better than four jaws.
I've seen 10 ft vertical boring mill tables run at 30 RPM (truely frightening) and a 26 ft vertical boring mill table run at 4 RPM with the table extension outrggers deployed (brave men run at that sight.) It aint really scary unless you plan to dive in it.
Some machine are chuck speed limited because of the power loss from jaw windage or wear and tear on the clutch from accellerating a large mass to high RPM. It aint that the chuck's gonna blow up; they're worried the clutch might fry.
Friable frangible vitrious bonded grinding wheels run safely at 6000 ft per minute and may burst at 14,000. Even a miserable quality import cast iron chuck can hold together at 6000 feet per min with the jaws firmly gripping a balanced work piece. So let that be your limitation. 6000 feet per min on a 12" chuck is 1910 RPM. Problem is windage. The jaws are like a squirrel cage fan. They can eat up 3 or 4 HP blowing air around running that fast. If your lathe has only 5 HP there's not much left over for cuttin'.
02-28-2010, 10:39 PM #5
I don't consider Bison/Toolmex to be a "miserable quality import".. but their semi-steel 12.5 inch 3 jaw scroll is listed as 1500rpm. The 12.5 inch forged steel "precision series" are around 2200rpm.
Other data (smaller chucks).
My 6 inch Rohm cast iron 4 jaw scroll is listed as 2000rpm, my 8 inch "Bison" is 2500rpm and my 8 inch 4 jaw is 1800.
03-01-2010, 07:35 AM #6
First of all, 12" is not a large chuck.
We've had 32" chucks running at close to 1,000 RPM. That's with an import chuck. Those cheap SMW ones. The Cushmans that size only run about 500 RPM, but they're on manual machines.
Come out and watch our 9' Gray run at 150!
03-01-2010, 07:53 AM #7
Lakeside: I referred to "miserable quality import cast iron chuck[s]". Bison/Toolmex are definitely quality import. Don't read a sneer when none was intended.
JR: A 9 ft vertical running 150 RPM? Holy smoke!! THAT'S scary. Wish I could see it. And it's about 4200 feet per minute.
03-01-2010, 09:16 AM #8
Manufacturer's safety factor has surely increased over the years, likely resulting in decreasing speed ratings.
Here's another data point.
Rohm standard steel 12" is listed at 2300 rpm.
Top-of-the-line Rohm 12" listed at 3300 rpm, and says:
"The maximum permissible speed has been fixed so that 1/3 of the gripping force is still available as residual gripping force if the maximum gripping is applied and the chuck is fitted with its heaviest jaws. The jaws may not project beyond the outside diameter of the chuck. The chuck must be in perfect condition."
"1/3 of gripping force still available" doesn't exactly give me warm and fuzzy feeling!
03-01-2010, 09:36 AM #9
03-01-2010, 10:00 AM #10
I have a 32" Cushman scroll type that runs @ 800.
And a 24" 4 jaw that will run that high - part allowing of course.
IMO the centrifical forces aint nothin compared to the clamping pressure. Of course you will have to add these two together to git combined force at speed.
Think Snow Eh!
03-01-2010, 10:09 AM #11
At the bottom of this page - Kitagawa-NorthTech , Inc. > bbt_series ( DNN 3.2.2 ) - there's a graph of the reduction of gripping force due to centrifugal force on a 3 jaw cnc lathe chuck. You can see how the gripping force of the larger chucks falls off rapidly. Manual chucks can oly make 1/4 to 1/3 of the gripping force of these hydraulic chucks, so you are starting with a much smaller number with a manual chuck, so it approaches zero much sooner. I imagine a 4 jaw would drop faster still because of the additional jaw weight.
03-01-2010, 11:55 AM #12
When that Gray winds up, it sounds like a jet engine, you can hear it from 200 feet away. Most of the time, we're turning a smaller seat and need the 108" for mounting. Last week, there was a part with most of it off center. Video would not do it justice. You put a big part in that machine and you can feel the wind from it. Sorry, no filming in the factory. Somebody might see how screwed up we are and try to copy us.
I almost forgot to mention that we have a wireless chuck pressure gage. Our equipment is supposed to get an annual chuck pressure reading at static and max RPM. The SMWs need to be torn appart, cleaned, and greased every year.
03-01-2010, 12:31 PM #13
How does the loss in clamping power of a hydraulic chuck compare to a manual one? If you push back against a hydraulic cylinder hard enough, you can move the piston, but with a scroll, which is non reversible, like a fine pitch worm gear, you would have to bend something to release the clamp pressure. I don't think they relate.
03-01-2010, 12:51 PM #14
You have a point. The Kitagawa chucks have wedges to move the jaws, the cylinder moves the wedges. The taper and friction of the wedges makes feeding the pressure back to the cylinder a bit like trying to reverse a worm gear by turning the wheel. I suspect the reduction in gripping force is more from deflection than pressure back to the cylinder, but I'm not the expert. I've seen heavy CNC chucks deflect more than .005 just from increasing the clamping pressure from 250 to 500 PSI.
Is there an expert here who can shed light on this?
03-01-2010, 06:36 PM #15
03-01-2010, 06:45 PM #16
Wow, Thanks guys! I guess I'm not way out of line then. My smallest chuck is the 12", so when the work is under 1" I like to spin it... I guess I really should get a smaller chuck. BTW, 35hp has no trouble with a 12" fan!
03-01-2010, 08:18 PM #17
Assuming that the lathe manuf would design their lathe so that the highest available speed would be under the largest chucks rating fitted to the lathe?
03-01-2010, 08:47 PM #18
My psychology major nephew couldn't find work when he graduated, so he worked for me for several years before moving on. He was getting to be a respectable machinist, but his heart wasn't in it and then he married a girl who hated him working in that environment. Anyway, one day I was pontificating about holding a piece at the size limit of a six jaw Buck. After telling about how you had to make sure the scroll was holding each jaw, I started the lathe and the #6 jaw flew out and put a divot in the wall. I should have kept my mouth shut and concentrated on the task. His comment was "I understood the concept. I didn't need the visual aid."
Edit: The angle should be arctan .5 = 26.57 degrees, not 30. The 1/2 tripped the old response that the opposite side is 1/2 the hypotenuse of a 30 - 60 - right triangle and I didn't think it the rest of the way through. A little harder to move, but the same philosophy.
Last edited by 9100; 03-02-2010 at 09:30 AM. Reason: Error correction
03-01-2010, 08:54 PM #19
I build billet flywheels for pulling tractors, Most of the time, a cast iron flywheel on a diesel engine is around 80 to 110 lbs, 17" dia, and good for 5500 rpms. Above that you need steel. Flywheels are much stronger than chuck castings though.
03-01-2010, 10:03 PM #20
I sure as hell wouldn't trust one made from a billet.