Easy way to centre rectangular shape in 4-jaw

As a novice is there an easy way to centre a rectangular shape in the 4-jaw. I have to machine a taper on my tender spring buckles. I have the basic profile laser cut as shown in the first picture, with the taper in the second. Thanks in advance.

Dave

Place in 4 jaw with packing plates to cover the open sides. Set one face of the job to vertical. Wind in the cross slide to touch the job preferably with a DTI, but can be done with the tool tip, and set dial to zero then rotate chuck through 180 degrees and touch the job again. Note the difference and adjust the chuck jaws to move half of the difference. Repeat for other sides. Colin

A great deal can be done by using the grooves machined into the chuck face and visually aligning the jaws with them this will get you close - then use an indicator as colin has described .

An opportunity for me to share yet more blunders from the Duffer Workshop!

The method is as described by Colin, and best done with a DTI.

First time I tried similar I had a bit of bother. Post mortems after three failures suggested:

• The work tilted slightly in the 4-jaw during adjustment. (Faces not quite parallel?)
• Jaw not tight enough, allowing the work to move during cutting.
• Error because chuck wasn't returned perfectly vertical between centring adjustments.

The cure is close attention to detail and double-checking everything. When satisfied all is well and secure, spin the lathe up and check again before cutting. Fortunately I wasn't working with laser cut parts or hard to replace castings...

Dave

Once you have the first one set in position, the subsequent parts can be positioned more quickly if you release only two of the four jaws, remove the part, insert the new part and then tighten up the same two jaws again. Thus the two unmoved jaws act as fixed datum surfaces. Of course this assumes parts are identical in size. If there is some small difference, the necessary small adjustment will need to be made, but still quicker than starting from scratch.

I use the technique colin describes too.

A self centering four jaw chuck is easy - provided one can put up with the small errors of most self-centering chucks. I don’t often use mine, but it is handy for chucking square or octagonal bar. I don’t like to chuck ‘round’ in it unless perfectly round, or close to. Still time for a Christmas prezzie?

I always use two chuck keys on opposite jaws. Makes life a lot easier

Thanks for all the replies. I had worked out that the method was as per Colin's response, but was concerned about the problems that Dave highlighted. Just worked out a possible helping hand though. Use my Wixey angle gauge to set the part parallel/perpendicular to the cross slide before attacking it with the DTI.

As you have a lot of them, set it up on an angle plate on the faceplate. With the angle plate level you can set the height with a pin In the tailstock, same diameter as the thickness of the part. Get the first one then central and you can get the Have a loose clamped stop ready along the side and fix it when all is in place.

The result is only having to set up once, all the rest will be positioned by the stop.

Edit  re 4 jaw self centring chuck

A self centring 4 jaw is useless here it’s a rectangular component ,  but you don’t use it often!  Wow. Mine is on almost all the time. It gives a better hold on the stock than a 3 jaw esp when threading anything bigger than M6 and contrary to popular belief you can hold hexagonal as you grip it on 4 points of the stock.  My 5” is in constant use.  It’s also a highly accurate chuck

Edited By Zan on 21/12/2018 13:30:57

Another option for repeat work like that is a ring that's a good fit around the part to be held, slit and held in the 3-jaw.

Neil

It may be, but I can make any rectangle into a square by adding a couple of pieces, which this apparently needs, to mount on any chuck. Rather depends on how many he might have to do and the size variations between parts. It looks like it needs a 2mm piece either sde.

Yes, it gives a better hold on perfectly round stock, but anything out of round will only be held securely by three jaws. Mine is ‘highly accurate’ but no better than my three jaw variant by the same manufacturer. My four jaw independant gets most use, I would say

If you have two centres and a dial indicator Dave - this works very well and is quick.

Regards,

IanT

A MUCH simpler, quick and foolproof way - I have used it countless times....Thanks Joe...

**LINK**

Joseph beat me to it.

Mark out the centre, centre pop, and then use two centres to centre the round portion in the 4 Jaw, as Ian T shows..

The rectangular faces can then be milled (or turned to the required dimensions relative to the centre of the round section.

Howard

He's starting with rectangular laser cut parts...

Not done it..... shame you have had bad experiences with the sc. 4 jaw. Mine is brilliant, but non round stock? Bdms is quite round I have experienced no problems.. often people are expecting micron precision for the price of a bacon butty .

I can take a piece of work out, put it back ( which I avoid at all costs) and it’s within  1 thou without any special precautions  , I like bacon but my chuck is a basic level TOS chuck and I stand by the accuracy statement

If you use extra pieces to bulk out to a square shape for the 4 jaw s.c chuck, double sided tape is a great aid to hold it all together while setting up. Still think the faceplate is best though

Edit, ...blasted autotype with technical terms on the pad!

Edited By Zan on 22/12/2018 00:44:39