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Looking for very slender live center

Schjell

Aluminum
Joined
Jan 16, 2020
Hi,
I've come to realize that I need a live center (MT4) that has a point that will allow work on OD's as small as approx 12mm.
The static holders I use are std. 25 mm and I have failed to find live centers that have a sufficiently reduced OD point to allow access for typically threading.
Considering buying a live center with replaceable inserts and making my own in case I can't find anything.
I generally work with light parts with low tolerance requirements, so this live center "tip" does not need to take up great loads.
I have looked into Royal (US) and Rohm (GER), but I suspect there probably is an out of the box product for me somewhere. Cost not a big factor, I only need to buy this once:-)

03-SKP_01.jpg

Thanks in advance for all tips!
 
Hi,
Thanks for Riten tip!
I haven't found all dimensions on the skinny CNC live centers that I've found, such as the Rohms picture above.
Threading is the most limiting one I think - it looks like when my holder is to start threading at typ. Z1.0(mm)then it will require about 26mm of space towards the tailstock/live center. If the live center tip was a bit longer before it tapered out to full OD then it would work (for sure).
The threading holder that I use Vargus) has got an internal cooling channel so I have not yet tried grinding an angle on the holder to give me more space. I don't use the internal coolant for threading, but considering connecting it up. I'm also a bit concerned that I need the rigidity in the holder, but I'm guessing it's overkill as it is.
Still learning every day, maybe it's everyday practice to take a holder to the grinder to get the job done.

Below is a picture of current situation, which clearly would collide. Maybe a set with replaceable inserts is the best way to go.

received_390164606084031.jpg

EDIT NOTE:
Just had a thought.
If I drill and tap a hole in the stock, put a custom "bolt" in it, then I could offset the point where the live center engages. Just a thought. Might work if I keep loading very low. Disastrous if it fails as the hydraulic tailstock would slide forward:)
 
The common kind of live centre with a point unit projecting from a relatively large diameter bearing carrier doesn't play well with small shafts. Especially in large taper mount sizes where the bearing and its carrier is sized for the hefty jobs normally done on such machines. You end up with a silly long point unit to get tool access between the job and bearing unit with corresponding lack of rigidity and degradation of accuracy.

Although the general range of interchangeable point kit types do appear to have sufficiently slender centre points quality seems to be something of a crap-shoot. As usual with such semi-professional (at best) the price / market demand curve won't accept high, guaranteed, fully inspected quality.

Best design for this sort of thing is the inverted type where the centre itself is tubular with the bearings inside carried on a plain stub on the end of the taper mount. Load carrying capacity is less than the conventional type due to the smaller bearings but they can obviously be made very slender.

No idea if they can still be bought commercially.

Believe the one I had dealings with was intended for small electric motor rebuild duties but it was certainly adequately accurate for light machining. That one had a simple ball in the centre at the front end of the spigot to take thrust loads and a basic magneto style bearing at the back to hold it all in-line. On an MT2 taper and around the same OD as the lathe tailstock barrel.

If they are still made, and intended for machining work, I imagine the bearing arrangements would be more solid.

I've several times contemplated making one on the bi-annual or so occasions when it would make a "why did I agree to do this?" type job easier.

Decent quality needle roller with inner and outer races sitting on a plain end MT taper to carry the side loads with the same sort of single ball up front for thrust. Use a radius centre drill rather than the normal variety to form the bearing seats. Hardening the ball seats isn't really practical but if reasonably obdurate steel is chosen and lubrication ample it shouldn't wear out on my watch. For practical assembly reasons the bore in the rotating centre "tube" body for the needle bearing outer race needs to be a bit over-long. Hence it will self adjust slide a little to take up minor wear.

For best accuracy it would seem wise to arrange some form of zero side thrust locking arrangement so that the unit can be mounted in the lathe headstock so the actual centre point can be formed in situ. My provisional plan was to loctite an alloy sleeve on to join the centre to the body. The sleeve could easily be turned off, after making the centre point true, before removing the unit from the lathe headstock.

Clive
 
An easy alternative is to switch to sandvik for threading.

Their insert location system allows them to make a holder that has no back edge. Use in combination with a standard extended/double angle point centre.

202419697_d50_0_0~tl02_04.jpg
 
Yeah, I clearly see what you mean!
As for my current situation, I don't really have the luxury of replacing the chuck with a collet type. I make three different parts in a day typically, typically between 1 and 15 in a series. So I went with the straight collet shank solution that I grip with the soft jaws. It's ER32, so I can only take up to 22mm stock.

I also get the point of doing stuff close to the chuck to avoid deflection and trouble. I was hoping to have a do-it-all-in-one-go with parting at the end of the cycle, but I see that this is not really feasible without a skinny live center. As it happens, I ended up doing what you suggest. Dividing it in to two ops where I move the set point (G54) after threading to avoid req. for tailstock support.

Threading the other way you suggested is an annoyingly good suggestion! I will try this - as mentioned I'm still learning. 1,5 years and going - always something new to ponder upon - never gets boring!
 
NAKANE has some options. I have 3 of the standard LS models as they're the only ones I ever found available in North America.
An MT2 in a MT2-MT4 sleeve might be an option. Small live centers need less pressure to spin too on small parts with a tiny center drill.

NAKANE LIVE CENTER


Other option may be a Long Point spring loaded center, some of them are fairly small but I haven't tried one yet.
Spring-loaded, “Concentric” Live Centers | Riten.com
 
Good that we are talking about quality name brand lives.
When I was an apprentice old-timer told me that a center will take off the TIR from one side and leave it on the other side.

I have always held the premise and have solved many tooling problems with demanding that a live center error should be less than half the part tolerance.

You see unbranded live centers advertised too .005, in my opinion, is good for allowing .010 parts
->.0002 good for a half thow.

I think it is best to buy a small point live center rather than a multi-point one.

Good lives are rated .0001 and 50 millionths, and most often they run better than advertised.
 

Haha! Thanks again - Love the Dorian brand, never thought of checking them for live centers! Will definitively get a quote there! I remember coming across a fancy dampened 16 thousand dollar Dorian Boring bar once! Had to read it twice to believe it. It had an incredible L/D ratio, but that price tag made me chuckle!

The Sanyo link that another guy posted here is also very interesting. I'm spending the better part of a day finding this live center, but it's hopefully going to solve 99% of the jobs I do.

Best part of running a business is buying tools, never get fed up with it, however my two colleagues in the office end of things might disagree:D
 
An easy alternative is to switch to sandvik for threading.

Their insert location system allows them to make a holder that has no back edge. Use in combination with a standard extended/double angle point centre.

202419697_d50_0_0~tl02_04.jpg

Hi Gregor!
Tried to make my Vardex holder look more like the Sandvik one on your picture:D
Not sure if it's strong enough anymore; worst case it's a 300 buck holder down the drain!
Hopefully fine, there's still plenty of steel to support the insert (I think).

IMG_20211018_160542.jpgIMG_20211018_160536.jpg
 
Hi Gregor!
Tried to make my Vardex holder look more like the Sandvik one on your picture:D
Not sure if it's strong enough anymore; worst case it's a 300 buck holder down the drain!
Hopefully fine, there's still plenty of steel to support the insert (I think).

View attachment 332469View attachment 332470

Pretty standard practice TBH, it will get you through as long as you're not cutting big threads, or marginal on helix clearance (which tries to pull the insert off-axis). I have a few older ISO holders that look like that :)

Sandvik's system has locating grooves on the bottom of the insert so it only needs one shoulder for the insert to pull up against, and the groove locks it in position. The holders are more expensive than ISO and so are the inserts, but it's a better system that is worth the extra cost IMO.

As far as centres go, there are plenty of well made live centres available. I mainly use Rohm, but I have a few Korean or Taiwanese ones that are just fine. Bison are fine too at a lower price point. Most of the US brands are a pain to source over here (Riten in particular, I have one of those spring loaded ones and I had to jump through hoops to get it). MSC carry some Royal ones, but just the basic range. No idea about Nakane.
 
MSC are a pain in the butt for a Norwegian - the UK branch refuses to ship!
Biggest "standard threads we turn are M95x5 in 316, so that's going to be interesting, but worth a shot nevertheless.
I sent a request to Dorian for a quote for live centers with "two steps". Rohms one I've seen are probably small enough to do M10 or M12 threading, especially after my fix with the grinder. Thanks for input in this matter, there's always something:)

BTW - I see you're Aberdeen based - I suspect we work in the same industry!
 
Scjell

In your situation, especially given the supply constraints, I'd still seriously consider making an internal bearing one as per my original post based around components that are easily obtainable for you.

They do work well on appropriately lighter work and, being inside, the thrust ball can be gotten very close to the tip of the centre making it easier to keep TIR down.

Heck if I'd needed one right now it would have been made, installed and working well enough using stuff from the "bound to come in handy" overstock box since I read your original post. By no means what I'd design for a proper job as I'd have to use an ordinary ball race further back instead of the high end needle bearing set but "good enough right now" beats the pants off "proper job next month" or "perfect at Christmas".

If I needed one for tomorrow morning and had to start right now I'd use one of my stash of dry bearing bushes and only be 1/2 an hour late for dinner! Bushes would probably be trashed in 100 hour running but if it gets the job out on time.... That said I'm onesie, twosie or "nobody else is mad enough to take it on" guy so probably won't live long enough to put 100 hours on.

Clive
 
Hi,
Thanks for Riten tip!
I haven't found all dimensions on the skinny CNC live centers that I've found, such as the Rohms picture above.
Threading is the most limiting one I think - it looks like when my holder is to start threading at typ. Z1.0(mm)then it will require about 26mm of space towards the tailstock/live center. If the live center tip was a bit longer before it tapered out to full OD then it would work (for sure).
The threading holder that I use Vargus) has got an internal cooling channel so I have not yet tried grinding an angle on the holder to give me more space. I don't use the internal coolant for threading, but considering connecting it up. I'm also a bit concerned that I need the rigidity in the holder, but I'm guessing it's overkill as it is.
Still learning every day, maybe it's everyday practice to take a holder to the grinder to get the job done.

Below is a picture of current situation, which clearly would collide. Maybe a set with replaceable inserts is the best way to go.

View attachment 332449

EDIT NOTE:
Just had a thought.
If I drill and tap a hole in the stock, put a custom "bolt" in it, then I could offset the point where the live center engages. Just a thought. Might work if I keep loading very low. Disastrous if it fails as the hydraulic tailstock would slide forward:)

Why not hold the material as close to the chuck as possible to thread it and then pull it for anything after the fact? If you hold it close enough the center would not be needed. I thread 12x1.5 with no support right at the chuck with lots of success. Once the threads are done you can use your existing live center for the remaining work.
 
Ouch! As Gregor said, it ain't the same. Insert register, not just support. So it very likely will pay the price in the fullness of time.

Thot' you said you only had a handful of this part to make?

Might be a surprise to a Carbide addict, but Tantung-G - nor even a Rex 95 - don't exactly wear-out on the first pass at threading 1/2" wotevers.

One-off / few-off tasking should leave a skosh of room for one-off/few-off tooling?

Yeah, it's risky to grind it as much as I did, but this has been annoying me for quite some time, so felt good to plug in the grinder. I had a look at it and I think there should be enough support for the insert unless I've completely underestimates the forces involved. I go light though, typ between 15 and 20 passes for a thread. I've got a bunch of Vardex (or Vargus?) Inserts I need to get through before I likely start using the Sandvik suggestion for OD threading. Another reason that I do not like that particular holder is that swarf tends to get stuck under it/around it and balls up when turning 316 and Duplex. Watched a few videos and suspect that an angled threading holder like the Sandvik is more likely to throw off the swarf for each pass, but this is purely speculation on my behalf.

Got a quote from Dorian an hour ago. Think I'll go for the 500+dollar solution, will also allow me to do face turning to smaller ODs where I can drill with smaller drills in center afterwards.

Appreciate all your input here guys and apologies for not answering to your very detailed suggestions Clive, it's just that I am always jumping from design work to machining to assembly work and even some marketing in between. When we're a four man operation we tend to buy ourselves out of trouble because our time is often better spent elsewhere in terms of profit for our company. I completely understand the point you tried to bring across though. Would not need to be so expensive or complicated.
 
I second the recommendation for a "value" live center with interchangeable tips. In a pinch, you can make your own custom tip.

If you are willing to settle for a dead tailstock center for occasional jobs, buy or make a half-center. Take a regular 60 degree center and grind away slightly less than 50% of the cone for a distance from the tip which will allow your tools access. If you make, start with a HSS center rather than a carbide-tipped one.
 
I second the recommendation for a "value" live center with interchangeable tips. In a pinch, you can make your own custom tip.

If you are willing to settle for a dead tailstock center for occasional jobs, buy or make a half-center. Take a regular 60 degree center and grind away slightly less than 50% of the cone for a distance from the tip which will allow your tools access. If you make, start with a HSS center rather than a carbide-tipped one.

Hi, we machine a lot of plastic so will need to be live center, but good advice in any case!
This is the one I settled on - relatively high end live center set where I can make my own tip if required like you mentioned.
Thanks for all advice!

dorian.JPG
 
Another thing about lives is that poorly ones not only have run out but also have wobble. That wobble is run out end from end that differs. That is why poor ones tend to wear out the point more quickly....because the live center point is wobbling in the part...and so also wrecking the parts center

likely poor ones are manufactured in a chucker that has .0001, .0002 or .005 error with holding the stub in a collet or chuck and the point end turned and then ground, with having that same error.

Good lives must be made between centers, assembled with precision bearings with acentric marks in line..and then after assembly, the point is ground with the live center's spindle turning in its own bearings.

I know this because I used to regrind machine live and dead centers for the shop and for tool makers who would bring me their own ones.. I would put a load on the toolmaker's lives and indicate the run-out at an unused place to show them how good or bad they could be at best.

One can find new live centers on eBay from .0002 to .005 run out.
IMHO .0002 is way too much error for doing any precision work.

Regrinding the point as assembled can sometimes save/ make Ok a poorly made live center.
 








 
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