# How fast is too fast?



## Reddinr (May 13, 2019)

I am working on a VFD for my lathe, a Grizzly G0509G, 16" gear head lathe.  The lathe nameplate rates the RPM at 1800.  I'm trying to get an idea of how much faster I can safely run this lathe with a small chuck or collet chuck for 2" and smaller diameter, short parts.  The spindle bearings are rated for at least 4000 RPM, maybe 5000 RPM (NHK HR32016 is the PN of one of them).   But, there are other bearings and gears in there to worry about too.   I'm thinking 20% faster would easily be safe enough.   I wonder if 50% or even faster would be out of the question for short periods, maybe 30 minutes at a time and not very often.   I realize there is no "right" answer to this but does anyone have experience with this or have recommendations?  (Warranty is long gone so that is not an issue.)


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## T Bredehoft (May 13, 2019)

If you disengaged the gears, )moved the banjo away from the spindle,) you could run the spindle at your 4 or 5000. it's the gears that would (might) fly apart at that kind of speed.


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## Illinoyance (May 13, 2019)

Do a little research on the RPM ratings for cast iron chucks.


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## Reddinr (May 13, 2019)

Great points regarding gearing and chucks.  For some reason my Atlas PB23 hand-wheel collet chuck isn't specified for RPM in the documents that I can find.  My 3-Jaw seems to be 3000 RPM rated.  I'll need to get in touch with the manufacturer for the collet chuck I guess.


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## Bob Korves (May 14, 2019)

Remember that when you go outside what is in the manual, at that point YOU become the test pilot, regardless of whether somebody else says they got away with it.  Understand that concept carefully, and maybe study how professional test pilots go about testing new parts, tools, aircraft, and anything else -- and do not want to hurt themselves while doing so.  They make sure they understand the risks as well as possible and approach the often dangerous testing as carefully and intelligently as possible, trying to mitigate all potential risks.  Most test pilots pass away from old age.  Please consider doing the same...


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## ChrisAttebery (May 15, 2019)

There’s a post on Practical Machinist by Forrest Addy about someone who overspeeded a lathe with fatal consequences. You should find and read it. 


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## Reddinr (May 15, 2019)

Thanks for the safety advice.  I'm fairly conservative so I would likely sneak up on any speed increases slowly and methodically.  I'll read the Forrest Addy art. for sure.  I found the data on my collet chuck too.  Surprisingly 2500 RPM only!

Chris, any suggestions on keywords to search?  Mr. Addy has thousands of posts and searching on "lathe" with his name does not bring up his post about a lathe disaster.


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## ChrisAttebery (May 15, 2019)

It was a reply to a post under “speeding up a lathe” or something like that. It’s been years since I read it. 


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## Reddinr (May 15, 2019)

I did find "lathe horror stories" thread there.  One was a large CNC lathe with a big chuck and large part accidentally set to 5000 RPM instead of 50 RPM.  Chaos occurred.  Lots of other cautionary tales in the thread too.


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## Illinoyance (May 15, 2019)

Just as an example my Nardini 16" has a decal stating: 4 jaw 400rpm max.  That must be with their OEM 4 jaw that is lightly built.  My 4 jaw is a Horton steel body chuck.  I think I can disregard the decal on the machine.


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## Aaron_W (May 15, 2019)

Illinoyance said:


> I think I can disregard the decal on the machine.



Just remember ignoring safety decals are a good way of becoming immortalized in an internet meme. 


Do you really want to be associated with these people?


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## Bob Korves (May 15, 2019)

What could possibly go wrong???

That question can be asked jokingly, carelessly, or seriously.


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## francist (May 15, 2019)

Bob Korves said:


> What could possibly go wrong???



And for whom?

Machines get sold, sometimes by spouses or relatives. Another good reason for the next buyer to do all his math before putting on a chuck and powering the machine.

-frank


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## Mitch Alsup (May 16, 2019)

Safety third ....................


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## Toolmaker51 (May 17, 2019)

When it comes to lathes; the limitations start with what is attached to the spindle. Jawed chucks have cast iron or semi-steel bodies. Cast iron is definitely lower RPM being of lower molecular integrity. They may have balancing performed at the factory but not outside a limited speed range. Another issue for jawed chucks are those with self centering; the scroll subject to inertia beyond a rated RPM, and SFPM for it's size.
Don't overrun tooling. Machines too are restricted by design and weight bearing components; chiefly at the spindle where factory 'X' cannot control what a user attaches. Like why your little Clausing hasn't a D1-6 spindle, or my Pacemaker a D1-8. 
My favorite appraisal of lathe design rigidity is length of headstock, representing distance between main bearings. IMNSHO a good ratio is 3-4 times swing for heavy duty and real nice toolroom, about 1.5-2.0 for an average engine lathe.


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## Reddinr (May 17, 2019)

Thanks for all the valuable input.  I enjoyed the over-the-top use of an excavator on the lift.  Someone actually did that??   

In my case, I'm pretty level headed and not felony stupid (just normal stupid sometimes).  If I was that dumb, I wouldn't have asked the question in the first place and would have just cranked the thing up to max and "give her a go" with predictable results.   I learned a bit about my chucks' ratings so that will be some guidance.  Great point about the potential next owner.  If I do anything, I can set the upper limits of the VFD temporarily and then back to the normal 1800 RPM limit.   

I believe that this lathe was not designed so that at 1801 RPM things start flying apart, especially with a small chuck/part mounted.  The trick is "staying well inside that envelope" and not pushing it.  I think centrifugal force increases with RPM^2 so a 10% RPM increase is about 20% more force.  So, if I do anything at all, after considering it, I'll likely keep it to at most 110% of nameplate.


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## yendor (May 17, 2019)

Just remember that metal fatique is an accumulated long term non linear thing.
You might be just fine at 10% over the rating for 20 years or more.
but move that up to 20 or 25% over and it might only last for 2 years of service before something gives out.


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