Help with making a rotary table more rigid

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RSG
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Help with making a rotary table more rigid

Post by RSG »

Hello All!

I have been using a 6" rotab for many years (pictured below) and have found it to be good but as you can see with the four jaw and then three jaw soft jaw chuck on top it sits so high that it lacks rigidity. I've tried to beef it up with a heavy steel base and a compression bolt locking the four jaw chuck down through the center hole but during milling there is more deflection then I care for. With the height of this contraption it loses rigidity.

Image

You may ask,"Why use the soft jaw chuck" , it helps support delicate aluminum parts and is quite quick when doing multiple parts.

I was thinking of trying an 8" rotab which is heavier and either trying to make a low profile four jaw system or adapt the whole thing above to the 8" rotab. I'd appreciate your thoughts on the best way to achieve condensing the whole unit. How difficult would it be to eliminate the four jaw chuck and just dial indicate the soft jaw chuck directly on to the 8" rotab every time?

Any thoughts on this would be appreciated.
Vision is not seeing things as they are, but as they will be.
SteveM
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Re: Help with making a rotary table more rigid

Post by SteveM »

If you mount the soft jaw chuck directly on the rotab, you can mill the jaws with an endmill to make them concentric with the centerline of the rotab. Of course, if you want to keep using the same set of soft jaws, this would negate that, as your jaws will have more and more material removed and eventually need to be replaced.

Bolt the three-jaw to a plate so that the plate-to-rotab bolts are exposed and you can tap the chuck into concentricity and then snug it down.

A bigger rotab would go a long way to solving the problem, but even with a larger one, that height is going to be an issue. Nice thing with the bigger one is that you will have the bolts out at the perimeter where they will be easier to get to.

Steve
Inspector
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Re: Help with making a rotary table more rigid

Post by Inspector »

Does the three jaw chuck have a threaded back? Are you doing some of the work on the lathe and then moving to the mill? If so then it would make sense to mount a threaded stub to the rotab and not have the 4 jaw at all. If you want to keep the 4 jaw for other operations then a second rotab would be in order so both setups can be changed quickly. If considering an 8" rotab make sure it will fit your mill as it is much bigger and heavier than the 6" you have.
RSG
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Re: Help with making a rotary table more rigid

Post by RSG »

Thanks for the input guys.

Steve, I like the idea of bolting the softjaw chuck directly to the rotab. I hadn't thought of that until I posted this question and your reply has solidified that idea. I'm no stranger to sacrificing soft jaws as I routinely wear through them on the lathe so that doesn't bother me. As for "tapping the chuck into position on a bigger rotab I didn't know how hard that would be, I need to get it fairly accurate so if it takes too much time then it's probably not a good option.

Inspector, that's how I do it now, I use the softjaw chuck on the lathe then swap it over to the mill to finish the work. I have plain back chucks with no threaded end so that idea won't work for me unfortunately. Good advice on making sure the 8" rotab will fit my mill. I checked and it will. The table is 8" x 29"

I'll keep you informed of my suiccess. I just bought another 5" chuck with removable jaws and an 8" rotab. I'll make a backplate for it and see how it goes. I appreviate the input you guys have given me.

RSG
Vision is not seeing things as they are, but as they will be.
John Hasler
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Re: Help with making a rotary table more rigid

Post by John Hasler »

Can you put locating pins on the rotab?
pete
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Re: Help with making a rotary table more rigid

Post by pete »

Depends on your accuracy requirements of course, what about cutting the backplate on the 3 jaw a bit undersize then drill / tap the rear of the chuck body in 4 places and make a set-tru type chuck? Once it's dialed in with the soft jaws then for multiple parts it should repeat just fine and no need for the 4 jaw. My R/T is a 6" as well and I sure wish I'd spent the extra and bought the 8" instead. I think my R/T has used larger sub plates on it more than I've used the actual table surface. My D/H has a 10" swing and while it's heavy to lift on and off the table I sure don't think it's too large even on a 3/4 sized BP clone.
RSG
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Re: Help with making a rotary table more rigid

Post by RSG »

John, I suppose I could add locating pins, not sure what I'd be locating though, can you elaborate? Do you mean to locate the chuck to the rotab?


Pete, good ideas. As far as accuracy goes obviously I want as accurate as I can get but realistically I'd be happy keeping things around .00005" I bought another 5" chuck and an 8" rotab but have yet to make an adapter plate. Could you describe the "set-tru" style plate for me? I can't visualize it in my head.

Thanks
Ron
Vision is not seeing things as they are, but as they will be.
John Hasler
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Re: Help with making a rotary table more rigid

Post by John Hasler »

RSG wrote: Tue Mar 20, 2018 10:33 pm John, I suppose I could add locating pins, not sure what I'd be locating though, can you elaborate? Do you mean to locate the chuck to the rotab?




Thanks
Ron
Yes.
pete
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Re: Help with making a rotary table more rigid

Post by pete »

Sure Ron. In my opinion real set-tru lathe chucks are way over priced for what they are. A bit hard to make out the finer details in your picture so I've no idea just how well it might work with your chucks. A hardened steel chuck would be tough or almost impossible but I very much doubt that's what you have. The whole thing works much better with what are called plain back chucks, those either bolt the chuck to the back plate through from the chucks face or the back plate get's bolted to the back side of the chuck. Plain back chucks will have an internal female step so the chuck locates on the back plate's male step and that's usually finish machined to closely fit the chucks dimensions on the machine the chuck will be used on. Direct mount chucks with the mounting integral with the chucks back likely couldn't be made into a set-tru type. Since I've never used that type I can't say for sure.

Normaly it's recommended that the back plate be finish machined to fit the spindle mount. It's then mounted on the lathe's spindle and the back plates face is trued and male step turned to .001" or less to properly fit the chucks dimensions. For a home shop set-tru style if I recall the numbers correctly that back plate step is turned about .020" undersize. At the rear of the chuck's O.D. they have 4 set screws that work exactly like a 4 jaw independant does. Except they move the chuck body on the mount. In use you chuck up a round straight rod in the size your planning on using for your parts. Indicate that or even the part itself, if it's outside your accuracy limits you then slacked off the bolts that are used to bolt the back plate to the chuck, then adjust the set screws to dial in your chuck for that diameter. Then retighten the back plate bolts. "Most" but likely not all chuck types can be drilled and tapped for those set screws and they bear against the back plates male step. Only you can properly judge if that's going to be possible with the chuck you have. Non hardened chucks will usually be either cast iron or steel that's easy enough to drill and tap. You've already got the R/T so the dividings easy enough, all you need to do is decide on the location to allow those screws to line up with the back plates step. In use the chucks a little bit more fragile if you have a hard crash since the chuck isn't positively located on the back plate and could move a bit with a crash. But it's how the factory made set-tru chucks work and no one complains about any issues with the chuck maintaining it's concentricity to the spindle under normal operating conditions. To make it all work your adapter plate to marry the chuck to the R/T has to raise the chuck and the back plates rear face above the R/T's table surface just enough so you have room for a wrench to loosen and tighten those mounting bolts after making an adjustment. With a lathe chuck that's also going to be used while milling I might consider using two set screws in each hole. Remove one set, make whatever adjustments you need to then use the second set of set screws to lock the first one's to the adjustment point you made. Milling vibrations might or might not be enough to change that adjustment over a large number of parts but I don't know that for sure.

Chucks even brand new ones will have slight variables in the scroll and the jaw teeth, that's why they show varying amounts of runout over there holding range unless you go extremely expensive and get into very high accuracy chucks. Add any wear with a used chuck and that changes things even more. Grinding the chuck jaws even with them preloaded can only correct the jaw tips and get them concentric to the spindle C/L, it still does nothing to correct or compensate for any internal issues. You can grind the jaws so they hold one part size very concentrically at a single diameter but the wear or other variables will still throw the concentricity out at diameters other than the one it was preloaded and then ground to. That's why the soft jaws work so well since the jaws are machined to the diameter or shape your parts are going to be.

Bison and quite a few others make 3 and 6 jaw set-tru type chucks but there not cheap. As I said most plain back chucks should be able to be drilled and tapped to do exactly the same. Google images should turn up a lot of pictures of where the holes are located on the rear of the chuck body. Luckily plain back chucks are the most common type so that's likely what you already have. The one complication is you do have is to get that chuck concentric with the R/T's rotation and the part concentric to that but your already having to do that with the 4 jaw. A set true type works and does what your already doing, your just removing the 4 jaw and as a side benefit it gives you an adustable set-tru chuck for lathe use as well. Once it's properly adjusted on the R/T and with those soft jaws you shouldn't have to touch it for a long time.
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Harold_V
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Re: Help with making a rotary table more rigid

Post by Harold_V »

RSG wrote: Tue Mar 20, 2018 10:33 pm I'd be happy keeping things around .00005"
A typo, perhaps?

.000050" would be a challenge for a jig borer. Can't imagine working that closely on a mill, no matter how much attention and time you dedicate to the project.

I think you'll discover that working to a half thou, on a mill, is extremely challenging. It can be done, however.


H
Wise people talk because they have something to say. Fools talk because they have to say something.
pete
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Re: Help with making a rotary table more rigid

Post by pete »

I just assumed a half thou Harold, but if it wasn't a typo that's way out of my league. :-)
RSG
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Re: Help with making a rotary table more rigid

Post by RSG »

Harold_V wrote: Wed Mar 21, 2018 3:38 am
RSG wrote: Tue Mar 20, 2018 10:33 pm I'd be happy keeping things around .00005"
A typo, perhaps?

.000050" would be a challenge for a jig borer. Can't imagine working that closely on a mill, no matter how much attention and time you dedicate to the project.

I think you'll discover that working to a half thou, on a mill, is extremely challenging. It can be done, however.


H
LOL ooops! Sorry it was late I came in from the shop and was tired :lol: Yes I meant .0005"
Vision is not seeing things as they are, but as they will be.
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