Centre drilling long length, small diameter bar

Hello All

I’ve got a 10 mm diameter threaded steel bar that’s 800 mm long, and I want to turn the last 40 mm of it down to 8 mm diameter.

I’m aware that just mounting it in the chuck with most of it sticking out of the end of the lathe is really dangerous ,so I thought maybe I can turn it between centres . But, turning between centres comes back to the same problem because I need to centre drill one end – and to do that I end up with a couple of feet sticking out the other end of the spindle again.

Is there a way to do this this safely , or maybe it just can’t be done ?

Thanks

David

Run the spindle slowly and you'll be fine.

Andrew

Thanks Andrew - what do you think the safe limit is ?

David

You could rig up a piece of plywood (or similar) with a "V" cut in it as a steady and position it to stop the long end whipping about while you work.

Edited By Bizibilder on 06/08/2017 21:14:29

Quite a common issue with rifle barrel work, check out YouTube for 'spiders'.

Here's just one example:

**LINK**

Capstan lathes have a tube that long lengths are fed through to prevent any problems with the long length whipping round. Anything can be used to control the overhanging length, a few bits of wood will do just lashed in with a hole for the rod.

Mike

Hi David,

For doing the sort of thing that you want to do, I use a length of plastic water pipe over the end hanging out. Two G clamps, a length of wood and a stool.

Edited By John Baron on 06/08/2017 21:20:30 "Spelling"

Edited By John Baron on 06/08/2017 21:21:52

I would set up a bit of wood with a 15mm hole clamped to anything suitably solid to keep the far end from whipping.

Neil

Thanks everyone. My lathe is next to my mill, so I can probably take advantage of that fact by rigging something up on the milling table with the block of wood idea – i.e. sorting out the below:

(If that’s a really stupid idea please shout before I start - it won’t be for a couple of weeks!)

David

Probably below 100rpm. Given that the mill is conveniently placed a wooden steady would be better.

A friend wanted me to turn some features on a 3mm, 350mm long aluminium rod. Drilled a 3.5mm hole in an offcut bar and made them hold it.

When I do it I use a cheap bearing with an internal diameter which fits on the bar

Then I put the bearing into a decent fixed steady or a block of wood

simples

Whatever you use, block of wood, metal or whatever, make sure the thin rod passes through a hole in it, not an open-topped V notch or similar. You want the end of the rod to be captive so it can not possibly bend and whip around, turning into a meat shredder.

I saw this happen once when an apprentice put a four foot length of 5/16 rod in a 9" Hercus lathe (same as Boxford or South Bend) without the correct stand in place. (This was a heavy tripod with a piece of 1' pipe welded at rightangles on top of it for the thin bar to pass through.) Another Einstein apprentice walked past as the unsupported bar was spinning around at 1,000 rpm and seeing the end was gyrating around in a small circle, say 4" diameter, gave it a whack with his hand to try to set it on centre.

Instead, the bar bent at 90 degrees at the point where it came out of the lathe spindle and we had a three-foot long propellor blade whizzing around at 1,000 rpm. The end of that bar would have been travelling at about 20,000 feet per minute. Over 220 miles an hour. Luckily none of the half dozen apprentices working within a few steps' distance, including Einstein, was injured. But that was purely good luck. Could have been a fatal or a terrible maiming for life.

If you have a milling table right there, I would drill a 3/4" hole in a stout piece of 4" x 2" wood and clamp it to the milling table, set at the lathe centre height of course. Set this up well in from the end of the bar so it can't flex and escape under rotation.

Edited By Hopper on 07/08/2017 04:41:36

With 600mm between ctrs on your lathe you could just use the fixed steady, have about 500mm sticking out the chuck and 300mm inside the spindle, support end of rod with steady.

Could also clamp the fixed steady to the mill if you wanted which will be a bit more rigid setup as the turning will be near the chuck.

Edited By JasonB on 07/08/2017 08:05:11

Of course, if you manage to mount it safely in the chuck with a small length projecting to centre drill, you won't have to! I'd guess that 40mm would be rigid enough to turn without support provided you take light cuts with a sharp tool, at least for finishing. I guess that the 8mm is to fit a bearing? I have turned 12mm threaded rod down to (I think) 8mm to fit a bearing without T/S support.

One trick is to wrap thin steel wire around the rod so it fills the thread and provides something to grip with the 4 jaw without marking the thread. Wire needs to be slightly smaller diameter than the thread pitch.Wind the wire into a tight coil a bit smaller that the thread dia first then screw the coil onto the bar. Garden wire is OK, but if it's galvanised you may need to burn the zinc off with a torch first as it can be a bit "blobby" (do this outside!). To centre the rod I rested a small flat bit of steel between a boring tool shank and the top of the rod and took a DTI reading from that.

I don't like the setup in the video, it's asking for the bar to roll off the top of the block and start something nasty.

Minimal effort to use a block with a hole in it and MUCH safer.

<edit> or you could fit a g-clamp to the top of the block to surround the bar.

Neil

Edited By Neil Wyatt on 07/08/2017 10:51:47

Neil - I think I must have been a bit slap dash with my explanation and you've got hold of the wrong end of the stick (bar!). The plan to date is to somehow clamp a wooden block to the milling table, drill a hole through it (the block that is), then pass the bar through the block. The video is just to show the layout of stuff - I wasn't planning on running it like that

If doing this job I would grip the studding in the chuck (preferably collet) and support the tail end in a bush. JAR described a way of doing this in his book. This photo shows the tool in use:

Please ignore the nut on the studding which was there as part of a temporary drawbar.

Just set up a stand to take a length of plastic pipe and pass the rod through it,no big deal.

The important thing is that the overhang must not be allowed to thrash around free,good way to get a free haircut,never mind the style.