How to remove four jaw chuck screws?

Clive,

Like a twit I thought I could push the yoke / plug out to the point that the screw would come free, and then just knock it back in without worrying about alignment. That's actually the exact point where the plug comes totally adrift, drops on the floor and rolls under the table saw. It's not over stiff in the bore, just very scratchy; I tapped it out with a light pin hammer.

The state of the bore surprised me, it is very roughly machined; a bit of work with 800 emery on a dowel took some of the high spots down a touch and things are much smoother. Still a press fit but civilised and no longer graunchy. On reflection, it might be the bore is galled from fitting the plugs dry, with no lube; that would chime with the other abuse.

On reassembly I plan on using the heat gun on the main casting to make wiggling the yoke around that much easier. Maybe even put the screw in the freezer for a while. I'll do a few dummy runs with the existing parts by way of experiment.

I appreciate the interest.

Eug

Edited By Eugene on 17/06/2018 22:29:15

Hello again Eugene,

Do bear in mind that the thread helix angle will be affected by the screw diameter as well as the TPI of the screw you are cutting.

I have had success in cutting square threads using round tool steel held in a split on one side clamp, drilled to the diameter of the tool steel, with the whole then clamped in a standard tool post slot. Side relief on the cutting tool should be about 3-5 degrees each side and a small degree of rake angle is helpful in getting a nice sharp cutting edge.

I cut a standard shallow Vee form thread to start with to take out some of the 'meat' to ease the work on the slim tool. Don't make it wider than 1/16 inch otherwise you will remove the shoulders of the finished thread.

Don't try to push it hard either, cuts of little more than 3-5 thou infeed work well and use some cutting oil. I would try to make the thread as snug a fit in the chuck jaw as you can, the degree of engagement is not good and being hard it will not wear like the jaw screw and tend to eat that---if already a trifle sloppy to start with it will soon make that worse.

Final tip. If you have a mandrel handle, I would use it, you are really only shaving the material off the workpiece and with a slender tool you will feel a ;problem before it breaks.

I didn't know you had a wobble broach, that will make a better job on the square corners for the key

Again, good luck. Why not make a spare while you are all set up? There could be micro cracking on another screw that you can't yet see.

Regards Brian

Edited By Brian Wood on 18/06/2018 09:48:40

Yeah those fork / plug thingies are lively little devils. Did 11" four jaw in situ on the lathe figuring the working vertical would, on balance, be easier than chasing such a heavy lump round the bench. Chip tray collected 2 out of 4. Real handy!

As yours are a scratchy fit why not polish things out a touch more so they are a smooth "firm thumb push" fit so they can be turned into alignment without excessive force. Retain them with a bit of low strength loctite on the back 1/16" - 2 mm or so. If you go that way don't put loctite on the whole thing. The bond area will be large so even low strength adds up to a lot of holding power.

My preferred method for square recesses is to cut a slot and weld a capping plate over the open end. I make the slot over length so there is something to grip with a clamp. Weld on bit needs to be same width as slot so you weld across an inside corner. Clamp bit is wider of course. For smaller jobs it may be easier to make the cap plate full width and hold in place by welding the over length part. Then machine down each side where the square is to go making an inside corner to weld. Cut to length and turn back to size when done.

I imagine silver soldering or brazing the capping plate on would work fine. Be a lot closer to proper size too so less clean up. Not something I've tried as welding worked fine for the first one so didn't seem worth trying a different way.

Clive.

Brian,

All four jaws are cream crackered, so I'll be doing a full set anyway.

I already have a split clamp for holding small boring bars and also a square bar drilled lengthways on the end for 1/4" HSS  cutters. Using either one will make setting the helix handle easier ... no need to grind it, just twiddle the bit. I have a protractor I can set to the nearest five minutes of angle.

One piece of blue sky thinking ..... I can buy a 1/16" parting off blade ready ground with top rake and side relief for about £6. If I can find some way of holding it at the correct helix angle I'm home. What think?

Clive, I popped the yoke / plug in the freezer for an hour this morning and it's not far off a thumb pressure fit at -17C. I'd already thought of the polish / Loktite gig; I'll play around a bit and see what works best.

Onwards and upwards.

Eug

Edited By Eugene on 18/06/2018 12:30:58

Hello Eugene,

I would try silver brazing a piece to a suitable round section you can then grip in the small boring bar holder you describe, HSS will stand those temperatures

And a quick way of setting the helix angle is to align the tool with the thread in one of the damaged screws; it will save you some time working it out!

Regards

Brian