How much is too much ?(weight)

[steve]

I have a customer that I've been doing turnings for , they need a colum 8"dia with 1"x1/2"deep flutes. It would be a hollow turning , but still a lot of weight. I have the 6" indexer on my PRS Alpha. I was woundering if that would be too heavy.Oh yea the walls would be 1 1/4"thick , about the same volume of wood as a 6x6x72" solid post

[scottcox]

I think you'll be alright if you keep the indexer rotation speed moderate. Compare the circumfrence of the workpiece to the indexer rotation speed (deg/sec) and calculate the true inch/sec. You can even experiment with the ramping speeds for the indexer to minimize the backlash.

[steve]

Thanks Scott , I am now thinking of an indexer speed of 60rpm , that would be about 25" sec. with a stepover of .02" with a 1/2" round nose. May seem slow , but that should leave a good surface. ...and new bits all around.

[khaos]

Can the stepper accurately stop that much weight from turning at that speed?

I don't know the answer to the question but that is the real question.

[steve]

Joe... well now that is a good question. Maybe 30rpm?. It will be in continuos motion while turning to round , the flutes are straight , I think 30 or 60 rpm will be slow enough to position the blank. ...I think

[myxpykalix]

I think it's not just the weight of the blank but the weight of the blank and the resistance of the bit in the cut that puts the strain on the motor. So if you take a reasonabe cut and since you are only using a portion of the bit for the cut at any one time you should be able to take a reasonable (rough)cut to get to your diameter then follow up with a shallow fine cut to get a good surface.

[bill.young]

Hey Stephen,

There's some information on calculating speeds in the ShopBot wiki. It's focused on people using regular lathes with a ShopBot, but the math is pretty much the same.

http://shopbotwiki.com/index.php?title=CalcLatheSpeed

Bill

[steve]

Jack , I plan on taking light cuts , .15-rough , .02 finish. At that slow speed I think i will get a good surface. There are only two posts , they can take all day. I'd rather take it easy on the equipment and take more time.
Thanks Bill , I saved that page .
The job is not due till the end of Feb , a little time to practise making staved posts.

[jim_stadtlander]

I have read in past posts that with loading heavier items in the indexer, you have to be carefull of not letting the weight turn the indexer motor. They claimed it would "fry" the system somehow. I don't know if this is a true fact/myth, but I have loaded many very heavy pieces and always taken extreme caution. It has been written that it's better if the conrol box is shut off while loading, and I would tend to agree.

Jim

[myxpykalix]

Jim,
I would disagree, using this reasoning. While the control box is on, it has control of the indexer and if you've ever tried to turn the indexer manually while box is on, it is almost impossible. For that reason your blank is held solidly when mounting. I have an alpha maybe that makes a difference?

[jim_stadtlander]

Jack,
Here is the post I found about this: It's in shopbot accesories titled "Indexer with a larger than 6" diameter". The following statement was in there made by Gene Marshall---- "A Good Tip...
turn off the control box and unplug the indexer before mounting the blank. "If you happen to turn that head chuck while it is connected, you will be shipping a control board to durham NC before you can say OH $#!T.
Gene"

I wouldn't know if that pertains to both standards and alphas, or like I said, if it's an actual fact/myth.

Stephen--
The post I listed above also talks about some heavier items that are turned.

[steve]

Thanks Jim , those old posts were usefull. I have turned the indexer with the power off ( slowly) with no ill effect so far. Maybe I won't do that any more!
I assume there is a connector inside that square rubber covering about a foot or so along the cable from the indexer motor , maybe i'll cut that open and unplug when I want to turn the chuck by hand. As for the posts I need to do , they are in cherry , about 6' long , 8" dia. I think I will just slow everything down. Say 30rpm , .125 depth per pass , .02 stepover.
Thanks everyone for your help , I'll post pictures when done.

[jim_stadtlander]

Stephen,

Please read that above post carefully!!!!!! It says also to NOT disconnect the cable wire unless the control power is turned OFF!!! There has been several people warn me about this. If you disconnect the wire with the control box on, I believe that it will blow out the "driver" which is costly.

I, too, have slowly turned the indexer while the control box power is off without any mishap yet. I try to do it as little as possible. It seems as if the warning comes with bumping the indexer with a heavy load that makes it turn while the unit is powered up. To me, it would seem like this would be a similar circumstance as the other stepper motors when a "mishap" occurs and it would not be a complete disaster. BUT, I don't want to take that chance to find out!!!!

Jim