Tailstock pressure

How does one correctly set the pressure of the dead center, into the end of a piece of stock? How much pressure should there be?

What materials are you turning?

Some thoughts come to mind:

• The least preload possible to obtain the desired outcome. Larger Øs and heavier cuts (work deflecting) will require more preload.
• Some type of grease (moly/bearing) to interface contact surfaces.
• Slowest speeds possible (reduce heat/friction).

I usually turn plain mild steel, but it would be great to know what pressures for other materials as well. Also, how deep should i be drilling the hole with a center drill?

i recently burned up a dead center, turning the shank of a grade 8 bolt (super tough). I’m trying to determine if this was caused by too much preload, or some other issue. Was it just tougher material than the dead center? I was using grease as lubricant.

Use a live centre for 95% of the work, finish with a dead centre

edit: you can make your own one, it's an interesting and useful project

Edited By Ady1 on 14/05/2021 06:50:01

Use a fixed centre on my SC3, slide the tailstock to part for light end force, lube with oil. As long as cutting forces on the part don’t bend the part or force it away from the tool then support should be enough. A live centre should be centred with enough force to make it turn with the part. Always check for heat in both the part and the centre while in use, particularly with higher speeds, obviously that all depends on size of part in relation to the machine. I have burnt centres, many years ago, you learn from your mistakes, we hope.

As for how deep to drill with a centre drill. I always shorten my centre drills to prevent the end breaking off, not often I use one smaller than number three (1/4" to suit the work I do. With a shortened centre drill 3mm or 1/8" is enough to still hold a bit of grease and not let the point of the centre bottom out. A drop of oil during use is a good practice to start.

Edited By Chris Evans 6 on 14/05/2021 08:04:58

One thing to watch out for is heavy cutting on long parts heats them up, makes them expand and thus makes the dead centre a tighter fit against its hole, which is how they can overheat and get burnt. Check for heat and smoke during operation. Back off the tailstock pressure if the centre gets tighter as you go. Keep dripping a bit of oil on the centre as you go. And as Ady1 says, the best way to use a dead centre is by replacing it with a revolving centre for most work.

You will note that a centre drill has a pilot. Do you fill this pilot hole with lubricant? If not, that might help.

Another way to avoid ‘burning up’ your dead centre is to use a carbide one.🙂

The centre is there to support the part, not squeeze it between the chuck and tailstock in an attempt to distort the part! It’s not likely doing the lathe spindle bearings any good either. Anneal the bolt? A better, sharper cutter?

With a revolving centre I wind it in until it revolves with the work, plus a tiny bit extra but not so much as to heavily load the lathe bearings. You will be cutting axially, not radially, most of the time, so cutting pressure should be towards the chuck. Radial cuts are best carried out closer to the chuck as there is more support at that end.

Plain bearings cannot work at the same high speeds as ball or roller bearings (unless with pressure-fed lubrication think car crankshafts?) and require regular lubrication (think myford spindles), so work more slowly and lubricate more often.

Items are often joined together by ‘friction welding’ where rotation and pressure are applied simultaneously. No need to try to go there!

Tallow is a useful dead centre lubricant, it is a traditional lube and being solid with a low melting point tends to supply the centre with lube as things warm up. Obviously it will fling off as it melts and will need reapplication. Revolving centres come to suit every pocket but I would be wary of the very cheap ones as their accuracy and rigidity can be poor. Check out the price of Gepy centres and work your way down.

Mike

Another advocate of lubricating with grease, molygrease or tallow, and not to use excess pressure.

As NDIY says, the centre is there to support and locate, so only apply just enough pressure. You are not trying to, launch the Great Eastern, (Which stuck on the ways )!.

If it gets hot, the pressure is too great, back off!

Howard

I have five bob each way and lube with moly grease at the outset and drip a little oil on the centre as I go just to be sure. But it's a rare job where I use a dead centre these days. Only if utmost rigidity and precision are needed or when using a ground away half centre to allow access to a small diameter job etc. But I have a revolving centre with the two-angle tip that is just as good in all but the most extremely small cases.

I'm in the habit of easing then re-tightening the centre at intervals on long turning work, rather than waiting until things start to complain.

Also re-set it after a long interruption in the middle of a lengthy turning task, to compensate for the work-piece having cooled.

Great info from everyone.

Do you recommend i purchase a live center, instead of a dead center, now?

Unless you are working to super precision limits, I would say a decent quality live centre will cover most of your work.

You can use it for the heavy cuts, and then swap to the dead centre for the finishing cuts if you wanted to.

The only big downside of a live centre is that it can be quite bulky and get in the way when turning small diameters, I quite often use a "Half dead centre" for those jobs.

When you get a live centre, just test it with a DTI to make sure it has no run out. You get what you pay for as with most tooling.

Phil

For a beginner with limited mechanical background, a revolving centre has a higher survival rate.

Jeremy,

You will find my my Gepy ‘pin- up’ and some other useful info. here : **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=134291

MichaelG.

I think Sparey covered this point in his book ????