19brendan81 7th December 2011, 03:55 PM I have never used a dead centre before and was wondering how its done and what they are used for.
Is there any advantage in using a dead centre over a live centre in the tailstock when machining between centres?
Also when using a dead centre how fast can you spin the piece RPM wise without burning something out? And how frequently do you need to lubricate it?
Brendan
pipeclay 7th December 2011, 04:07 PM Dead centres are used the same as the live centre.
For locating into a centre drilled centre or a machined centre.
These days most use would probably be for a half centre if threading small diameter material,it allows the threading tool to get to the end of the job.
The speed you turn your job would probably depend on the type of dead centre whether steel
or Carbide.If you turn a job at a high RPM lube would need to be monitored as it would not take long for a steel centre to burn away and then the job would more than likely fly out of the lathe.
The dead centre is normally used like a live centre in the tailstock of the lathe.
Some times the dead centre might have more of a point on it than a live centre.
Dave J 7th December 2011, 05:10 PM Hi Brendan
I would say keep it under 800 rpm and as far as lube it depends what you use, but just keep your eye on it.
A dead centre is more accurate than a live centre because a live centre usually has run out from machining and the bearings.
So if you places say a elec motor shaft in between centers, with a dead centre you would be guaranteed it would revolve around the centre line, where a live centre will have run out making it slightly orbit around the centre line.
For us home shop guys we would probably never tell the difference, but for high precision stuff it's something to consider.
You will see that tool and cutter grinders use dead centres and I would say that is the reason why, but they only turn at low speeds.
Dave
Michael G 7th December 2011, 05:34 PM Centres offer repeatability- that is, a job on centres will go back on the same axis if you remove it, something that can't be guaranteed with a chuck. As Dave says live centres will still have an opportunity for some error, a dead centre less so.
A live centre can be used in a tail stock but a dead centre can be used both in the tail stock and the headstock (with a dog to drive it).
Whenever I've used a dead centre in the tail stock it has been lubricated with grease - preferably a thick EP type. The big danger is that the centre will pressure weld itself to the work.
Michael
Steamwhisperer 7th December 2011, 05:55 PM Hi Brendan,
At one time all work was used a dead centre. When live centers became more reliable they went out of vogue and the skill was lost. When using a dead centre, there are a few things to remember. First make sure the tailstock screw handle is on the side where the weight of the handle will tend to tighten rather than loosen the spindle to the work. Lock the spindle in position. Use grease in the centre hole never put grease on the centre. When greasing, less is more. Adjusting end float is the critical part. The job must rotate without binding, this should be checked frequently as the heat generated by the cutting action of the tool can take this out of tolerance. Did I mention lock the spindle. Re-grease each time you check your job. Cleanliness is next to good machiningness. One last thing, lock the spindle. Attention to these points and speed of rotation is at your whim. Just be extra careful at high speeds. I don't have a live centre at work, it is all done with a dead centre.
Phil
.RC. 7th December 2011, 06:12 PM I was getting chronic chatter one day when turning a long slender part with a live centre....
Nothing would make it go away.... Then I switched from the live centre to the dead centre and the chatter disappeared...
nadroj 7th December 2011, 06:33 PM For between centres work, best accuracy is with a soft centre trued in the headstock. Put it into the spindle (or spindle adaptor) and do a truing cut with the compound slide set to a 60 degree included angle.
As it doesn't move relative to the workpiece, it won't tend to wear. But once removed from the spindle, it can be trued again for the next job.
Jordan
Stustoys 7th December 2011, 06:58 PM I rememeber a titbit from somewhere was to rest your thumb on the dead center. The story went you'd know things where heating up before any damage was done.
Stuart
19brendan81 8th December 2011, 09:56 AM Sweet. Thanks for the tips guys. Im going to have a go at machining between centres and i'll be sure to keep all of this in mind.
Whilst on the topic of machining between centres, does a lathe dog mar the surface of the piece you are driving? From what I gather you would have to crank it up pretty tight on a smooth shaft in order to get it to drive properly.
pipeclay 8th December 2011, 10:47 AM Are you planing on using a standard cast dog or something you have made yourself?
Is the finished surface over the full length of the job a concern?
If you are roughing the job out it wont matter how tight you do the dog up,if you were taking really aggressive cuts even grind a flat onto the shaft for the bolt to bite onto.
If after roughing you can place a protective shim of suitable material around the end of the job to protect it from the dog.
If you needed to machine the total length in one go,you could drill and tap each end of the job if needed and use a bolt to drive the job if the face was not critical.
If the dog was to slip under heavy cutting you run the risk of damaging the drive centre.
Dave J 8th December 2011, 11:30 AM There are also what they call hydraulic drive centres, which have a centre point and drive teeth to dig into the work to save using a dog and marking the work. Very similar to what you use on a wood lathe, but more expensive. I think someone here has one.
Here are some pretty coloured ones I just found to show you.:ohttp://i01.i.aliimg.com/img/pb/668/399/252/1275729337727_hz-fileserver3_1873486.jpg
If you are after lathe dogs, I bought 4-5 from ezyaccessories off ebay and they where something like $5-$9 each 3/4 to 2 1/2 inch (from memory) where Hare and Forbes wanted $40 odd dollars for a single 38mm one. They also combine postage and I got some 5 inch V blocks ($26 a pair) and several other things in the flat rate box, a huge saving from buying local just with the lathe dogs.
If you don't want to buy them, for the smaller sizes I used to make them up from off cuts of 12mm thick by what ever width suited flat bar, with a hole drilled to suit the and a thread taped in the end for a bolt. For larger ones I used 2 pieces of something like 20 x 8-10 mm flat bar off cuts bolted together with a bolt each side, and leaving one longer for the drive pin to pick up on.
Dave
franco 8th December 2011, 12:51 PM I was getting chronic chatter one day when turning a long slender part with a live centre....
Nothing would make it go away.... Then I switched from the live centre to the dead centre and the chatter disappeared...
I had exactly the same thing happen with the old 9x18 Brackenbury & Austin lathe when cutting a 5MT recently. This lathe is not prone to chattering normally. After checking everything I could think of on the lathe, the problem turned out to be the live center in the tailstock - replacing it with a dead centre completely fixed the problem.
Frank.
steran50 8th December 2011, 01:01 PM HI:),
All the above and You really shouldn't use a Live Centre when Knurling this is due to the Forces involved. The pushing force of the Knurling Tool can damage the Live Centre, but a Dead Centre is fine to use.
Those Centres look pretty Flash in their Colours Dave.
Steamwhisperer 8th December 2011, 03:04 PM HI:),
All the above and You really shouldn't use a Live Centre when Knurling this is due to the Forces involved. The pushing force of the Knurling Tool can damage the Live Centre, but a Dead Centre is fine to use.
Those Centres look pretty Flash in their Colours Dave.
Unless of course you use a clamp type knurling tool. With these you don't need a centre at all.
Phil
19brendan81 8th December 2011, 04:08 PM Thanks Dave and Pipe for the info on dogs. dave those driving dogs are pretty fancy looking, they do indeed remind me of the types used on wood lathes.
Re dead centers and knurling - thats interesting...I havent really cracked knurling yet despite owning scissor, and push type knurls...might give it another go with a dead centre and see what happens.
Dave J 8th December 2011, 04:48 PM Those Centres look pretty Flash in their Colours Dave.
I would like to have just one of those set ups, even if they where that colour.
They where just the first ones that showed the tips the best when I searched for pictures on Google.
The link led to Alibaba.com
Multi-functional Machine tool-Face Driver for lathe,driving center - Detailed info for Multi-functional Machine tool-Face Driver for lathe,driving center,Multi-functional Face Driver,Multi-functional Machine tool-Face Driver for lathe,driving center, (http://geli.en.alibaba.com/product/316911501-210242888/Multi_functional_Machine_tool_Face_Driver_for_lathe_driving_center.html)
Coming from over their in China they are probably not that expensive, but I think the one I saw on a forum was from Germany or somewhere like that, so it would be.
Dave
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