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Tony
02-06-2012, 10:55 AM
I've never seen a drive center on a metal working lathe until running
across Frank Ford's build on his excellent website.

I've seen plenty on woodworking lathes and perhaps have never
made the connection.

I'm wondering what the drawbacks to a drive center would be? Would
solve a lot of problems for me if I could plant one into the end of any
old (steel) shaft.

here's a link from Frank's site so we're all on the same page:
http://www.frets.com/HomeShopTech/Projects/DriveCenter/drivecenter.html

sure seems to be a lot less hassle (and better balanced!) that putting dogs
in place.

-Tony

ps.. Frank: thanks!

gvasale
02-06-2012, 11:46 AM
they are a common item for certain kinds of cnc lathe work.

Mcgyver
02-06-2012, 12:10 PM
sure seems to be a lot less hassle (and better balanced!) that putting dogs
in place.


The obvious advantage is you can take a light finish cut over the entire length of work. If I had a single surface cylinder to turn it could be a useful way to hold the work.....but I'm trying to recall when I've last had to do so such that a dog would be in the way

There are reasons why its less appealing that a traditional set up.

- Relies on tail stock pressure for drive - tougher on on precision rotating centres
- marks the end of the work (might not matter in many cases)
- I doubt it would have the torque carrying abilities of a dog, ie only good for lighter cuts
- clearance between body and spring centre means:
1) its less accurate than a solid live centre
2) conceivably the cutting force could push the work eccentric by pushing the centre in

its a good idea, but I see some disadvantages that might make it a specialty technique than replacement.

Jaakko Fagerlund
02-06-2012, 01:42 PM
McGyver, it has a good ability to deliver torque, as I've done between centers work many times without a dog or a driving center, just having the center transfer all the torque and works fine for finishing (yes it has some dangers).

And I would not believe that it would be able to push the work sideways so that the center would slip in, as that would mean that the hardened drive pins would have to either shear off or dig in heavily in to the end of the work, and personally haven't never seen that.

Of course one could make a driving center that has a locking screw or similar for the sliding center part.

About the precision: One doesn't need a sliding fit for the centre part, just put a stiff spring inside and use the tailstock pressure to move it. If the center and its hole is properly done, you can get better than 0.01 mm TIR.

Oldbrock
02-06-2012, 01:55 PM
I frequently use a large center machined in the chuck to finish the outside of bushings, yesterday I machined and threaded ten steel sleeves with nothing more than a dead center I machined in the three jaw and a large live tailstock center. use enough pressure and no problem. Bored and beveled the sheeves first so that the od and id were concentric. Expecting a small centerdrilled hole in the end of a shaft, however for heavy cuts is not going to work, good only for very light finishing cuts. Peter

Mcgyver
02-06-2012, 02:55 PM
Jaakko, its a matter of degrees, not that the technique has no merit. If the centre moves within a bore, there is clearance and hence runout vs a solid turned in situ centre (who's run out would be that of the headstock bearings).


And I would not believe that it would be able to push the work sideways so that the center would slip in, as that would mean that the hardened drive pins would have to either shear off or dig in heavily in to the end of the work, and personally haven't never seen that.

It does press in, in that the pins deform the end of work. My point wasn't that it could slip in so far that the piece flies out of the lathe, but if it slips in a thou (because pins are deforming the metal) then then there's play and it's not holding as exactly as a solid centre - in other words that its sprung reduces its ability to maintain concentricity against a radial load

I'd add to the list that often we're working with spindly pieces between centres, ie feedscrews. long shafts etc. If you put anything other an minimal pressure on with the tailstock you can bow the work which makes for a miserable finished product

I'm not saying its unusable as a technique, just that there are a bunch of reasons, some perhaps too small to worry in some applications, some perhaps not, that collectively suggest, to me at least, its not a good replacement for solid centres and a dog ....unless say you are doing light work across a short(ish) single surface cylinder and can really benefit from the one thing does better: turning a piece end to end.

gvasale
02-06-2012, 05:31 PM
Here is some history. In the mid '90s i was working for a handgun manufacturer. One job I did was barrel profiling. Turning down a 1 1/2" (approximate) diameter to .500 for a 9mm pistol. This was done on a cnc lathe.

The driving center had a pilot and 4 protrusions similar in effect to what has been described. Hydraulic tailstock and taking off 1 inch of material total isn't really "light" stock removal. I do not remember that the DOC was, sorry, I remember that there were roughing cuts and finishing cuts.

What it really comes down to is whetyher youre lathe is rigid enough, or if you're using it on appropriate material, in other words, how hard the stock is.

These barrels were hammer forged, heat treated, I'm sure as well.

Having said that, there are jobs for which it suitable as well as those which are not.

Davo J
02-07-2012, 12:43 AM
If you type " Hydraulic lathe drive centre " into Google and click images you will see lots of them.


http://i00.i.aliimg.com/img/pb/858/504/263/1282379982872_hz-myalibaba-web5_7025.jpg


These ones are pretty, LOL

http://i01.i.aliimg.com/img/pb/668/399/252/1275729337727_hz-fileserver3_1873486.jpg

Dave

Tony
02-07-2012, 09:01 AM
Why 'hydraulic' ?

any of the "hobby houses" carry these things? Now that I know bit more
about them if I could pick one up on the cheap might be handy having
it around.

I'll put it next to my adjustable angle block. :D

making one doesn't seem like a fun undertaking.. getting all those pins
to bite at the same time that is.

Tony

PS wondering out loud, what if the center wasn't spring loaded and
instead of pins I used say, three pointed set screws I could advance
from the back side one at a time? This way the center stays put
(and under pressure) and I have the set screws acting as dogs. no?

Davo J
02-07-2012, 09:22 AM
Why 'hydraulic' ?

Not sure, but thats the search they come up under for metal lathe use. If you just type in drive centre only ones for wood seem to come up.

With those ones over your idea, I don't they would allow the centre point to unload from too much pressure from the outer pins, but thats only a guess.

Dave

rkepler
02-07-2012, 10:25 AM
Why 'hydraulic' ?

I have a Rohm driving center that is grease/oil filled and all the pins float in their holes backed by the grease. When you drive it into the work the grease balances the driving force between the pins while the spring loaded center centers the work.

You can see it at the top of page 4 in this pdf:

http://www.rohm-products.com/uploads/tx_userproducts/technische_cok_07.pdf

(warning - 8MB pdf)


any of the "hobby houses" carry these things? Now that I know bit more
about them if I could pick one up on the cheap might be handy having
it around.

So far as I know - no. They're specialty items. The Rohm one that I have is carried here:

http://www.nolansupply.com/superth.asp?supercategory=Centers+and+Center+Drive rs+-+Rohm

Another handy item is the force measuring live center for the tailstock - this so you get the appropriate grip in the face driver. Together you can get them for maybe $1200? About what I paid for my first lathe.


I'll put it next to my adjustable angle block. :D

That's close to where I keep mine. It's another one of those tools that does a critical job that nothing else can do, the job that you do once every 10 years.


making one doesn't seem like a fun undertaking.. getting all those pins
to bite at the same time that is.

Thus the "hydraulic" part.

Tony
02-07-2012, 11:04 AM
thanks rkepler -- a traditional dog it is. :D

Jaakko Fagerlund
02-07-2012, 06:55 PM
If the centre moves within a bore, there is clearance and hence runout vs a solid turned in situ centre (who's run out would be that of the headstock bearings).
Sure, just most probably you don't see it with a micrometer or a 0.01 mm dial indicator, if the center is done properly, meaning honed hole and precisely ground center shaft, in which case the "sliding fit" is close to zero clearance.

Make a test if you have the ability, or make a nicely polished test bar on a lathe and try a commercial bearing on it and get little bit off until you get a nicely sliding fit and compare this to a "loose" sliding fit which is most probably just a tad smaller in size.