Thursday, January 3, 2013

Four Jaw Chuck Keys

The new lathe in the shop came with tooling for several work holding options. Three jaw chuck, four jaw chuck, two face plates and a 5C collet closer. After I got it wired up I mounted the three jaw chuck. The chuck had soft jaws on it that I took off and mounted the hard top master jaws. The main jaws in the chuck body seemed a bit wiggly to me but I just ignored it for the time being. Well it turns out the three jaw chuck is pretty whupped. It runs out a fair amount and the jaws cock a some near the front of the chuck which is a major wear signal. It makes sense in hindsight that there were soft jaws installed on the chuck from the previous owner. This would be the only way to do decent work with this particular chuck. Unfortunately for me its not an adjust through type chuck. The backing plate is integral to the chuck and non adjustable.

So I went ahead and spent some time cleaning up the nice Rohm four jaw chuck that came with the machine. Like most four jaw chucks it looks like it has not seen much use. Most everybody prefers the self centering three and six jaw chucks for speed. I generally agree with this myself. For a large part of the typical lathe work centering within five grand is fine. If you need it closer than that you make a minor adjustment with the adjust through feature and then your dead on.

Like I said I generally prefer three and six jaw self centering chucks. Too bad I don't have one right now. So I sucked it up and mounted up the big four jaw and started doing some jobs with the machine. In the pile of  tooling I got with the machine there was a vacancy where the four jaw chuck key should have been. I figure oh well its easy to get another one, I'll just buy one. So I start looking around to see what is out there. I measure the screw on the chuck and discover its a funny size. Like metric funny size. Still it shouldn't be that big of a deal right?
This particular chuck has male square screws instead of the much more common female type. They are metric to boot. Finding a chuck key was starting to look like a real challenge. The Rohm website even shows the typical female socket adjuster screws. Where the heck did this alien chuck come from?
Twenty years ago on one of my tool buying sprees I bought a set of eight point sockets for a single job. I could have just bought the one socket I needed but no, I bought the whole set. Well funny thing is I have used the heck out of this set over the years. They fit all kinds of things you run into so the expense wasn't wasted this time. One thing I use them for is driving taps where you need a long extension to reach a hole down a hole. Good thing I had them now. Its the only thing I had to drive the square drive screws on the four jaw chuck.

This socket setup is a sub optimal tool for adjusting a four jaw chuck. Its an inch size so it doesn't fit the square very well. So faced with the non availability of a chuck socket I decided to do the next best thing which is make some. If your going to the trouble to make a square socket wrench you might as well make two. There is a good reason for having two which I will tell you about later.
I poked around the metal rack and came up with some heavy wall steel tubing that fit the recess in the chuck. To make the 10mm square I machined a .400 wide slot through the end of the tube and then created two opposing shoulders that were .400 apart. I then inserted two "keys" into the shoulders and welded them in.
This is a great job for a dividing head or a collet block where you can easily flip the part 180. I faced the end of the tube by rotating the dividing head. In case your wondering why I didn't use a 5C collet block the reason is I don't have a 7/8 collet which is what this tube OD was.
The first .400 wide slot is milled slightly past center of the tube. When you rotate the part 180 you get to see how good of a job you did picking up the center line of the tube. Whatever error there is is doubled when you rotate it 180.
The key step milled in one side. I had a small mismatch between the two sides. Not enough for me to get excited about on a weldment.
Here the keys have been welded into their key slots. Its starting to look like something. The weld prep was important here since I am machining off most of the weld reinforcement. I beveled the edges of the key slot so there is some weld filler metal still there after machining.
One really nice thing about four jaw chucks is everything gets indicated. You actually do better work when you use a four jaw chuck. This is a perfect example of why its important. When I machine the keys off I don't want to go below the original tube surface since the weld prep is minimal. If the part runs out a little there will be a witness step when all the key and surface weld is removed. Your only choice is to go deeper which in this case I really want to avoid. You are fighting runout against the removal of all the key and weld. This all goes away when you indicate and zero the tube accurately.
Turning the OD of the keys to the same diameter as the main tube.
Okay so now we have two chuck keys with square sockets in the end. On the shorter one the square closed up a little from the welding. I compensated on the longer one for the weld shrinkage but I still have to fix the shorty. The only really effective way to do this is to broach the ID of the square. No 10 mm square broaches in the tool room so I will have to improvise. The amount to remove was small. Only .005 per side to get the fit the way I intended. For this job I had to pull out my vertical broach. I don't use this tool very often but when you need it you really need it. I couldn't file the ID very well because the square is blind. Filing into a blind corner is pretty tough to do and a sure recipe for a bozo job.
These are what industry calls slotting tools. Bridgeport used to make a slotting attachment that went on the back of the ram for vertical slotting. Nobody does it anymore with the availability of cheap broaches and the easy availability of EDM work. The tool on the right is an actual Bridgeport brand slotting tool. The one on the left is my improvement. The eccentric shank locates the thrust of the tool more on the center line of the spindle. The body is just steel with a high speed tool bit silver brazed into a mating slot. There are side relief angles ground into the edges for back clearance.
First step is to get the part clamped up in a sturdy setup and straight with the machine axes. There is a parallel under the part to take to thrust of the quill.
Next is to mount the tool in the spindle in low gear. The cutting edge is aligned with the machine axes also. It looks like I didn't get any pictures of the actual broaching. Its a simple enough process. I took a couple thousandths DOC per pass, stepping over half the width of the tool much like a planer would. I took equal amounts off each of the offending faces until I got the measurement I was looking for. In total I scraped off  about .004 per side.
One bad thing about big chucks is they don't close down on small diameters very well. I didn't have a 3/8 diameter 5C collet on hand either or I might have used the collet block. As it was I found a better way to do it anyway. Using a V-block in the four jaw allowed me to only need to use one of the jaws to put a new part it the chuck. Once the rod and v-block were centered it was in and out in a flash like a collet closer or three jaw.
These are the cross hole rods for the chuck sockets that are being radiused on their ends. Material is 3/8 diameter cold rolled.
Ready for welding. I radiused the top side because this is where the hand comes into contact with the tool.
I thought this worked out well to weld the rod into the body of the socket through the end hole. No weld cleanup and it really cant pull the rod out of position from the weld shrinkage.
Finished and ready for lots of four jaw work. The two sockets allow you to work the jaws against one another and dial in the indicator. Here is a link to a great to the point video of the concept of two chuck keys.

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