View Full Version : plain back chuck
04-30-2003, 07:09 PM
I'm thinking of buying a new plain back 3 jaw chuck for my lathe. But I don't know how to accurately mount it. Please help.
05-01-2003, 03:01 AM
You can buy raw and semi-finished backplates from many sources. You can make one from scratch - cast iron is the preferred material for a backplate as it absorbs vibration better than steel would.
Any of the major tooling distributor like Travers, KBC, et al can sell you them. Or check with thee people: http://www.worldwidechuck.com/
Quality chucks (like Bison) come with a warranty booklet that will also tell you the preferred method of mounting the chuck. barring that required information you need to do the hard work yourself.
The backplate has to be accurately machined to the outside diameter of the chuck. Most chucks have a recess in the back, the backplate should be machined so that it can be pressed into this recess with your hands. Make sure you bevel the outside corners to avoid interferrence with inner corner radii on the chuck back recesses.
If it is a front mount chuck (bolts to the backplate from the front side) you can spot the mounting holes with a tranfer punch. Rear mounted chucks require spotter's or Heillman transfer screws. I also punch witness marks so the backplate and chuck can be re-mated in the same way if ever disassembled. I would also buy qaulity screws (Bison makes nice chucks, but include crappy mounting screws). Use Loctite to assemble.
There is often a void space between the chuck and the backplate that is fairly large. I always turn a tube (or fat washer/spacer) to fill this void out of UHMW PE - this prevents chips from accumulating in the void space (chips in the space can throw the balance off at high speeds).
05-01-2003, 04:49 AM
Just curious - what is the advantage of buying a plain back chuck and what type of mounting system is on your machine?
05-01-2003, 06:54 PM
You're the quality assurance manager you tell me.
Personally, I'd leave out the Loctite, but Thrud has done more of 'em than I have!
I've had one experience with doing a backplate for a Bison chuck. The chuck came with a threaded backplate, but I found that the thread, as supplied, was a "rattling good fit" on my lathe spindle and in addition gave far more overhang than was required. I bored out the existing thread, shortened the "spigot" portion of the backplate about half an inch to reduce the overhang, put in a cast iron plug with Loctite, and re-threaded. Since the o.d. of the backplate was already correct for the chuck, keeping the thread concentric with that was "interesting." If I were doing it again I'd start with a rough backplate. Then I could thread first, then turn the o.d. to fit the chuck, and not have such a setup hassle. By turning the o.d with the backplate threaded on the spindle, the concentricity becomes automatic.
05-01-2003, 10:29 PM
As Stephan has stated, get it threaded first before turning to diameter. I had not stated this, as I presumed that you would get it fitted to your spindle first and foremost - then finsh it to spec for your chuck.
If you feel this is too much for you to handle, chucks can be purchase fitted for Camlock, A and L series spindles, and some threaded noses. They do cost more than a flat back and a good cast iron semi-finished backplate, but if you are cheap and/or desire better fit to your machine - go the separate part route. I prefer this as I can always move my "babies" (chucks) to a new machine just but attaching new backplates for the new machine.
If you do it yourself, it is more fun and you get better accuracy for the extra effort.
[This message has been edited by Thrud (edited 05-01-2003).]
It is easy, and good practice. Also cheap.
Blank backplate is $13 from Victor Machinery.
Machined 1 1/2-8 (and maybe a bad fit) it is about $40. I got a threaded one to save time more recently, and it was a bad fit, but works.
The metal in the last blank one I got from them was soooo nice to machine. Nice and easy to get right.
Put on faceplate (block it up a bit) and bore to minor diameter. Thread to size. Bore the "register" to a good fit, and to your taste (register = controversial item).
Remove, flip over and screw onto the spindle. Turn OD, and machine the spigot to go into the recess in chuck.
Should be perfect or close to it when done. its actually hard to make it wrong if you pay attention and measure twice cut once.
05-13-2003, 07:14 PM
BC21OSH Are you asking a serious question? Or just being sarchastic?
05-13-2003, 09:05 PM
Here is a quick way to rough center the chuck on a backplate.Insert a straight socket with the proper taper into the spindle.Leave bolts a little less than snug and tighten chuck jaws on to socket.Won't get it dead on usually,but as close as the quality level of the socket OD..003 or .004 in my case.Sung bolts a little tighter then remove socket ,reverse and tighten jaws on straight part to indicate in.Use a rubber mallet to move chuck to true up.Reindicate after final tightening to be sure.Robert.
05-13-2003, 10:40 PM
I was asking a legitimate question, or so I thought. I have just acquired a lathe with an A1-6 spindle nose and need a four jaw chuck for it so I was curious as to what advantage buying a flat back chuck had over a preassembled one. The question has since been answered by others responding to your post.
05-15-2003, 05:11 PM
Thrud. It's the spotters transfer screws that give me the dificulty. I don't have any and I'm a cheapskate. Can I make some myself? for a one time use.
Or the old way, paper over the chuck back, rub with pencil..
Transfer paper to plate, and mark out through the to-scale rubbing marks on the paper.
Since the "spigot" on the backer plate should fit close in the chuck recess (they are not really "flat") the screw location isn't critical unless they are off enough to get in a bind
Aluminum foil pressed and rubbed over a hole pattern can be used to transfer it.
Much more accurate than paper.
I think I first read that in the Machinist Bedside Reader.
05-16-2003, 04:43 AM
if you make them you need to provide a way of unscrewing them, and you should case harden them or the points dull in one hit.
The Heinman screws have a small hex with a centerpunch nose on the screw. The transfer screw set is in a machined tube with socket wrench on the end opposite the screw cap - the 6 transfer screws are stored in the handle.
05-16-2003, 06:20 AM
You can make spotters from good quality (eg Unbrako)grubscrews (set screws? to those in the US?) Hold them in 3 jaw and machine a small point on the end, (approx. 1/16" high) then make a hacksaw cut next to the point to enable you to screw them in and out with a screw driver. The hack saw cut will be a little offset, but still works.
I made a set of these years ago, they work well. I can't remember if I heat treated them or not, probably did bring them up to red and quench.
05-18-2003, 03:47 PM
The hacksaw slot is a great idea - I always put a small hex on them for a socket, but the slot is far easier!
05-18-2003, 08:14 PM
I just came across this article "Fitting a Chuck" on Tony's great website;
05-18-2003, 09:05 PM
Interesting to note he refers to "finely finished register". Makes you wonder who is right?
Not that I want to start that all over again.
05-20-2003, 01:54 AM
Despite good arguemnents contrary to what I have stated as being taught, I know what works and what doesn't.
So I won't go there either. http://bbs.homeshopmachinist.net//biggrin.gif