Size Question????

[Christopher Hunter]

Hey guys ,
I am wanting to throw a 21" dia by 1" thick blank of oak on my Grizzly lathe....
The slow speed is 600 Rpm and the motor assembly has to be turned sideways,
would this be a safe thing to do ?
the blank doesn't weigh much but it is a lot bigger in Dia than I have ever turned...
I know that as far as weight is concerned, I won't have a problem, but the outer diameter does seem rather big--
any suggestions or comments are welcome...
thanks
--Chris

[Curt Fuller]

That's about the slowest speed my Oliver will turn also. I've turned a few platters that barely cleared the ways, just a hair under 14". That makes the outside edge traveling pretty fast. So what it does is amplify all of your mistakes and catches. I keep the tool rest as close as possible and take a few light cuts, move the rest closer and a few more light cuts. Also use your tailstock for support until you get to the last finishing cuts.

[Bernie Weishapl]

Christopher one thing that would make it a lot easier to turn is if you have a bandsaw cut it down to a circular form. That will stablize it a lot then like Curt said take light cuts and keep the tool rest close.

[Andy Hoyt]

With the headstock spun cattywampus the tailstock won't be much help.

How about a really big homemade faceplate/backerboard thingamabob coupled with carpet tape or hot melt glue? That'll get one face done, along with the rim and a portion of the opposite side.

Then build one of those doughnut chuck thingies that just grabs the opposite side where you've already turned.

And hang on. That rim will be going fast!

[Ken Fitzgerald]

Chris......That would scare the daylights out of me! I know when I put an out of balance blank on mine 9" at 500 rpm it opens my eyes wide open! If you do it......Try to round it out first with a bandsaw or something else......step to the side after it's mounted and you turn it on!.....Don't stand in the line of fire when you first spin it up......Keep your hand on the OFF button when you first spin it up......Good Luck!

[John Hart]

Also keep in mind that it won't take much of a cut to stop the lathe out there on the outer diameter. Just another reason for light cuts.

[Bill Boehme]

I am wanting to throw a 21" dia by 1" thick blank of oak on my Grizzly lathe....The slow speed is 600 Rpm and the motor assembly has to be turned sideways, would this be a safe thing to do ? ... the blank doesn't weigh much but it is a lot bigger in Dia than I have ever turned... I know that as far as weight is concerned, I won't have a problem, but the outer diameter does seem rather big--any suggestions or comments are welcome...
Allow me to dissuade you from trying this by throwing some numbers your way. Curt mentioned turning things up to 14" diameter at 600 RPM so I will make some comparisons between that and something that is 21" in diameter.

The 14 inch diameter piece of wood would have a rim speed of 36.65 feet per second or 25 miles per hour. The 21 inch diameter piece of wood would have a rim speed of 55 feet per second or 47.3 miles per hour. OK, so just over double the rim speed ..... aww, so who's afraid of a little speed? I'll admit that I am. Here's the really much more significant and scary information:
For the 14 inch diameter piece of wood, the acceleration along the rim is 21 g's. In other words, the wood particles along the outer rim are experiencing a force pulling outwards that is 21 times their weight at rest. Well, that is not too bad -- the wood can handle it until it starts to get thin and then machine vibration, aerodynamic forces, and tool cutting force will cause it to start vibrating.
Now, for the 21 inch diameter piece of wood, the acceleration along the rim is 47.3 g's. That is really significant acceleration and this means that the outward force will be 2.25 times as great as that for the 14 inch diameter piece of wood. All of the things that make the smaller diameter turning chatter will be considerably worse on the larger turning.Keep your shop door open. If it comes off your lathe, your neighbors may think that they have seen a meteorite burning up as it enters the atmosphere and NASA may want to know why you are launching satellites without their permission.:eek:

Bill

[Christopher Hunter]

Thanks for the info guys,
I did try it and it was vibrating so bad that I could not true it up, so I will just stick to the bandsaw and then run it on the router with a template..
this is a table top anyways so I probably shouldn't be trying to turn it ..
next time I will just do it the way it is supposed to be done..
But Thanks anyways --
--Chris

[Christopher Zona]

Allow me to dissuade you from trying this by throwing some numbers your way. Curt mentioned turning things up to 14" diameter at 600 RPM so I will make some comparisons between that and something that is 21" in diameter.


Just to add a little more from another kind of technical way of looking at it:

[Gordon Seto]

[quote=Christopher Hunter]I am wanting to throw a 21" dia by 1" thick blank of oak on my Grizzly lathe....
The slow speed is 600 Rpm and the motor assembly has to be turned sideways,
quote]

I assume "turned sideways" meant you will have to turn with the headstock pivoted and no tailstock can be used.
If you apply the safe speed guide line by Dale Nish:
http://www.woodturnerscatalog.com/pdf/safe_lathe_speeds.pdf
then this is a risk I don't want to take.
Your lathe may walk with too big a blank it can handle.

That is why a EVS lathe with very, very low min. speed is a safety feature to have.

Gordon