Bison 6 1/4 chuck installation questions

You may be expecting far too much from your chuck. The internal portion of the scroll most likely isn't dead concentric with the centerline of the helical thread, nor does the helix run perfectly, likely due to minor distortion in heat treat. The scroll will also have a little clearance on it's pivot diameter, so it is free to wander, albeit a very small amount. Further, each socket you use when tightening the chuck will shift the scroll ever so slightly such that you'll get a different reading each time you close the chuck, plus every time you change material size you should expect that the chuck won't tighten the same. Frankly, in my opinion, if your chuck holds within .003", it's not bad. It's even better if you can find a given socket that runs closer for a given diameter. As far as I'm concerned, if your jaws hold material parallel to the centerline (it appears it does), you have a pretty good condition. If you'd like higher precision, you should either buy an adjust true type chuck, or use soft jaws (properly). Personally, I don't recommend the adjust true type, but endorse soft jaws heartily.

Harold

I don't have any experience. I just got my first lathe and the three jaw chuck was dirty and filled with bits of swarf. So I pulled it from the backing plate and cleaned the majority without disassembling the chuck itself. Probably not necessary, but a new owner of a used machine wants to at least make it appear somewhat virgin.

This chuck had radial set screws to adjust true so I spent some time adjusting the body of the chuck to run very true. Then I chucked some drill rod and found it off by several thousandths. More time spent truing the rod rather than the chuck itself.

So this kind of alignment seemed ok to me once I understood what to adjust for. Sounds like you don't like it for some reason. I'm just curious why? Being uninformed, I'm not even sure how the soft jaws are used exactly.

Thanks for any enlightenment or opinions you can give to this new lathe owner.

I am using the socket marked with a 0. Basically, the runout of the chuck body gets transferred exactly to the runout of my test bar. So the jaws are accurately holding my test bar, this does not seem to be the problem. I am measuring runout on the chuck body and this is what now reads .003". So it seems as if the chuck body is not seating properly on the backing plate. The mating surface on the backing plate was flat to around .0002" (measured farthest from the center). So I imagine if the chuck were seated properly and the surfaces where perfectly clean, which I took extra care to make sure, there should be very little runout on the chuck body.

Also, I have not turned down the outside diameter of the backing plate yet, so there is about a 1/4" lip sticking out past the chuck. When I put my test indicator on this surface, which is the same surface the chuck is mated too, it still measured around .0002" so I know the backing plate is mounted properly to the spindle.

I still think the chuck may not be seating properly. I am not unhappy with the set-up, but just not sure if this is typical from a Bison chuck. As David mentioned, he was able to get his Bison 160mm (I believe it is the same chuck I have) to .0005" TIR. So I am just wondering if there were any other tips or techniques to getting the chuck mated to the backing plate.

Thanks.

Right at the step, when you machine the shoulder, it's a good idea to undercut the shoulder just slightly to make sure that no radius is left to throw the chuck off.

Other than that, it sounds like you did fine. .002" runout sounds reasonable for a 3-jaw to me.

Are you comfortable using a 4-jaw chuck? Lots of guys go to amazing lengths to avoid learning how to dial in a part in a 4-jaw chuck. It's a skill every machinist should master.

GWE

What Grant said. The man knows what he's talking about.

Jim

Yes, I did undercut the shoulder. I do have an 8" 4 jaw chuck that I use ocassionally on irregular shapes. This Bison chuck is replacing my old 4" crafstman 3 jaw threaded chuck which was still very accurate and with some tweaking of my work piece, I could get it down to less than .001" runout. I tend to spend quite a bit of time with a dial indicator making sure whatever I put into my chuck runs true. I was hoping this new Bison with a properly machined backing plate would give me some better results with less tweaking of the work piece.

I may pull it apart from the backing plate one more time and re-seat it. If I can not get it any better than it is, I will just use it and it will most likely be perfect for everything I do.

No one has mentioned if I should counterbore for the bolt heads on the back of the backing plate. Is this typically done so the heads of the bolts don't stick out? This would also give me more threads into the chuck.

Thanks.

Terry,

Things I would suspect as being trouble points are the +.001 on the shoulder of the backing plate. If it's a "very tight press fit" as you say, you may be introducing enough stress into the chuck body to warp it slightly. Can you skin off another half a thou? As Grant said, undercutting that shoulder is important too, for the reasons he mentioned. Are all of your mounting bolts torqued down approximately the same amount? If the chuck body is larger in diameter than the backing plate you could mount the chuck on the lathe and indicate the back of the chuck body itself to see if it is seated squarely on the backing plate. One way or the other you ought to be able to track down the error and resolve it. As others have said, scroll chucks are not the most accurate work holding devices, but their inaccuracy has to do with the scroll and the jaws, not necessarily the chuck body. If I were you I'd try undercutting that shoulder first and go from there.

I may take another pass on the shoulder because it is a very tight fit. I did undercut the shoulder. The backing plate is larger than the chuck, so I can not indicate the back of the chuck. But I can indicate the front of the chuck, and I did this very carefully by closing the jaws all the way, and lifting the indicator point over each of the three gaps. This showed it was about +.003" on one side of the chuck indicating that it was not square to the backing plate which measured .0002" on the part extending past the chuck.

I will pull of the backing plate, counterbore the bolt holes and take another .0005" from the shoulder and try it again.

Thanks

Suggest you find out if the front of the chuck body is parallel to the back. This would be easy if you had a surface plate and surface gage onto which you could catch a dial test indicator, but there are many ways to do it.

Easy does it taking off that .0005"! I've been known to use a (shudder) file under such circumstances ..

GWE

Yes, I would suggest a very fine toothed file, low RPMs, and frequent test fitting. Break the top edge of that shoulder while you're at it.

If the chuck is axially true to the spindle (the only really important thing) you have the best precision the chuck can produce. Three jaw chucks are not intrinsically accurate radially. My three jaw (older than the local hills) is axially true. I have machined the 'spigot' undersized and the bolt holes oversized. Thus it is a trivial matter to slack the bolts and 'bump' the chuck until the workpiece runs true. Otherwise, IMHO, chasing a 'few thou' on a three jaw is an exercise in insanity.

Regards. Ken.

Taking off that small amount will be interesting. Just hope I don't go to much and scrap my backing plate. If I keep my gibs tight I think I can do it ok.

Adjust type chucks: They're not a bad instrument, it's just that you can work yourself into a frenzy getting them to run well for a given size, but you haven't really solved any of the other problems the way soft jaws do. The purchase of a chuck with master jaws (two piece jaws) is a far better investment, and usually less expensive.

It would be easier for me to point you to a post I made long ago, which explains in fine detail the benefits of, and methods to apply soft jaws. Please check this link for some very helpful information, along with some pictures to help you understand soft jaw usage.

Ok, I might get chastised for this by the more experienced machinists in this group, but here's something I've done when I need to squeak off a tiny little extra amount of metal in my lathe: I take a piece of fine emery cloth and back it with something rigid, like a piece of flat metal stock and then apply it evenly to the workpiece. Basically, it's like combining the simplicity of a file, with the finer and smoother cut of toolpost grinding. And by selecting the correct width of emery (and backing piece), you can hit the entire area at one time so you're less likely to get steps, gouges, or helical patterns. The key is keeping your backplate pressed lightly and evenly across the surface of the work. If your shoulder is critical (as it is on your lathe chuck, you want to make sure the emery does not extend past the side edge of the backing piece and that your backing has a smoothly polished edge so it won't cut the shoulder.

Often, all that's needed is to remove any high-ridges that were created by the single-point cutting tool and the fine emery does an exceptional job at producing an almost mirror finish. But like filing, don't get carried away and take off too much.

- Michael

Terry - please forgive a bit of an idle question: what is the logic behind relieving the outside section of the backing plate surface? Also, is the runout you mention a variation of the radius of a round bar held in the chuck?

Hul

Terry wrote:

Chuckle!

You won't get chastised by me, but I sure as hell have plenty to say to those that use files for such operations. That's nothing short of insanity, and is very poor practice.

Your points about leveling the highs with abrasive cloth or paper are right on, as is your method of applying them. It would be very difficult to lose center, or create steps by your method, due in part to the relatively slow cutting process, although you do run a slight risk of rounding edges. Beyond that, it's very best way to go, short of grinding, or *turning*, my preferred method, considering the part is already in the lathe.

I would assume that in this instance, the material in question is cast gray iron. Unlike mild steel, it will gladly allow a cut of a few tenths, with excellent results. That would be my choice of methods to reduce a diameter by a small amount. You risk nothing by this method, unlike all others. Putting a file to a surface that should be considered precision isn't a great way to go.

Harold

I think you are talking about the cut I put in at the bottom of the shoulder. This is to make sure there is nothing interfering with the chuck against its mating surface.

It also worked well becuase when I cut the shoulder, I left it about .015 oversize, cut the relief, then I machined the mating surface perfectly flat, When I came back and turned the shoulder to the right size, I stuck my bit into the relief making sure not to touch the mating surface, and I was able to machine a nice surface without any lips for the chuck to catch on.

I am sure someone else can explain it much better. This was the first time I have done this. I found an article that explained it very well. This is what I used to make mine.

I did use one of my micrometer standards in the chuck, and it showed the same runout as the chuck body did.