Machining what am I doing wrong?

[Martin WParticipant @martinw]

Hi Andrew

 

I dunno either !! Just trying to eliminate things as they have been raised, still its all good experience and a learning curve.

 

Cheers

 

Martin

 

PS

 

Thought your results in the Cambridge Turning Trials were informative and nicely presented.

[Steve WithnellParticipant @stevewithnell34426]

Posted by Andrew Johnston on 31/05/2011 23:00:57:

Well I dunno, I find it difficult to believe that the headstock bearings are being affected by a cut on an 8mm diameter workpiece? Unless it’s a very small lathe and, or, the bearings are shot. A question: do you get the same inconsistent results on other materials, say brass or aluminium?

 

The finish in the picture looks slightly dull and torn to me? If that was with the carbide tooling I’d say you were not running fast enough. For my own curiousity I’ll experiment with some EN8 later this week and report the results in my thread on turning trials.

 

Regards,

 

Andrew

Steve > Bit more info. The lathe is a 5 x 22 Sieg C6. There is a revolving centre in the tailstock (not branded). The lathe will have been running at 450rpm with carbide tooling when I machined this.

 

Regards

 

Steve

[Steve WithnellParticipant @stevewithnell34426]

PS: The piece is being turned between centres and I am using a 1 inch dog to drive the workpiece, so I can’t go that fast (The lathe is good upto 2600rpm when everything is in balance)

 

Steve

[Ian S CParticipant @iansc]

Steve, If you are using a live center, what are the bearings in it like, I had some work turn out similar to that just before the bearings collapsed. It was a 2 MT Skoda live center, I could have repaired it, but it was cheaper to buy a new one. I had it about 15 years at that time, and some of the work done was proberbly too big for a 2 MT center.

If its a plain center, is it getting enough lubrication? Ian S C

[Ian S CParticipant @iansc]

Steve, If you are using a live center, what are the bearings in it like, I had some work turn out similar to that just before the bearings collapsed. It was a 2 MT Skoda live center, I could have repaired it, but it was cheaper to buy a new one. I had it about 15 years at that time, and some of the work done was proberbly too big for a 2 MT center.

If its a plain center, is it getting enough lubrication? Ian S C

[Steve WithnellParticipant @stevewithnell34426]

Hmm, the live centre has very few miles on it, but the manufacturer was so proud of his work, he didn’t put his name on it I’ll take another cut with a dead centre in the tool stock and see what happens.

 

 

Then I’ll play with tool profiles and maybe traverse rates, I’d assumed slower the better (currently 0.01mm/rev), but “Tubal Cain” talks about going as high as 1.5mm /rev based on the finishing tool having a flat section twice that eg 3mm as a target for finishing. I’ve 2mm left on the shaft diameter so plenty of room to play and tweak. Once I’ve achieved an optimum and can then see what the impact is cutting away from the chuck, which is necessary because of the shoulder at the right hand end.

 

Top slide and cross slide gibs are fine, that only then leaves the saddle to think about I guess

[mgjParticipant @mgj]

Affected by a cut on an 8mm dia workpiece – the scenario I envisaged was a touch of clearance (maybe the bearings weren’t properly degreased before assembly?) and you have a touch of clearance. In compression the load drives the taper home, in tension it pulls out.

 

I used exactly that check on my 6″ Chinaman when I put the new bearings in, for exactly that reason. A .003 or so cut on a 1″ bar was enough to move the shaft and a socking great 6″ Pratt chuck and all. All it took was a bit of pressure with the tailstock centre and an extra bump with the hand on the C spanner, and that was that sorted. At which point I discovered just why the bearings were so darned expensive, because the precision suddenly became fantastic in both directions, and has remained so.

 

It is effectively a vibrating load, and its going to find any slack toot-sweet as they say.

[Anonymous]

Hmmmm, having initially suggested headstock bearings, I’ve now changed my mind. Unless the bearings are in very bad shape I don’t think they are the cause of the poor finish. Presumably you do not see the same effects on other materials, say aluminium?

 

In the picture I can’t see all that much difference in the surface cut towards the chuck and that cut outwards. I make 8mm diameter and 450rpm about 37fpm surface speed. Therein lies your problem I think. For carbide, much too slow a speed, and much too fine a feedrate. Result, a torn rather than ‘cut’ surface. See my thread on turning trials; for ordinary carbon steels I had very poor results at low speed with carbide. I didn’t try very fine feeds, but my general experience is that they give a poor finish. I never use less than 2thou/rev and normally use 4thou/rev finishing and up to 20thou/rev roughing.

 

Over the coming weekend I will try some turning trials using EN8, and will report the results.

 

Let us know how you get on.

 

Regards,

 

Andrew

[Ian S CParticipant @iansc]

Andrew, I can’t remember the figures, but I think the feed is related to the tip radius, larger the radius faster the feed. I’m afraid I just set the feed levers for the job I’m doing, and go by feel. There was a bit about all of this a few years back in ME. Ian S C

[Anonymous]

The rule of thumb I use is: feedrate is half the tip radius, may be a little less for finishing cuts.

 

Regards,

 

Andrew

[Ian S CParticipant @iansc]

Thanks Andrew, thats what I thought it was. Ian S C

[Martin PeckParticipant @martinpeck15887]

Your problem almost certainly stems from the very inadequate top slide clamp which flexes under load. I know this from first hand experience!! Look at Steve Bedaires web site where there are lots of mods to improve these lathes. The 9 x 20 is a very lightly built machine and needs bolting down securely to eliminate bed flexing as well. Once these improvements are made you will have a different machine believe me!

[Derek DroverParticipant @derekdrover32802]

Whilst reading the threads, my first thought was the clamping of the tool post… as Martin has mentioned above, go to the Bedaire website and read his ideas on improving the 920.

I have one of these.. its pretty good for general work, the clamp was the first thing I did to improve stability. I did it on the lathe using the 4-jaw chuck, so you wont need any special equipment.

I'm now reworking the bearing plate for the compound screw, and making a replacement "imperial" leadscrew for it too as mine is not accurate (metric leadscrew with imperial markings).

Tumbler reverse will be on the list of future improvements.

Rgds

del.

[OuBallieParticipant @ouballie]

Using the BH600G with the VFD has been a revelation for me.

All I do now is twiddle the motor speed knob and select depth of cut to suit rate of traverse.

It was very hit and miss on my V10P, having all too often to resort to manual traverse to get a decent finish.

Even now though, when required, I will wind the saddle backwards manually, either without moving the toolbit or just a division or two inwards on the handwheel, to get a virtual ground finish. Something I learned when using the Emco.

There was an article in one of the magazines about making a special tool to achieve a good finish.

Geoff – No workshop today, just not up to it. Bummer or what!

[Author]

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