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Martin W	29/05/2011 23:49:22
940 forum posts 30 photos	Hi The formula given by John can be simplified by approximation with the value of pi taken as 3. The formula then is RPM = Cutting Speed (FPM) x 4 Dia in inches or for metric diameters it can be approximated by: RPM = Cutting Speed (FPM) x 100 Dia in mm Admittedly the second formula is a mix of imperial and metric but still can prove useful at times. Cheers martin Edited By Martin W on 29/05/2011 23:50:03
chris stephens	30/05/2011 01:02:21
1049 forum posts 1 photos	Hi Guys, Before any newbie gets too carried away with getting the speed exactly right, according to the charts, they are the speeds that you can work up to for maximum production rates in industry. Most of us don't need to go anywhere near as fast in our home shops environments. Again for newbies, forget the speeds in the charts for drilling especially when drilling in the lathe, slow right down and give the drill time to do its job. Drilling too fast just makes things hot. Remember that, "back in the good old days" drills used to work even when the only power input was you winding a handle. Oh alright, that way might be a little too slow, but the principle still holds. chriStephens
Martin W	30/05/2011 10:35:49
940 forum posts 30 photos	Hi Chris I totally agree with your comments re cutting speeds and getting too involved with the published data. For me the 'Listen & Look' approach works very well. Listen to how the machine sounds when its cutting. Is it loading the motor significantly or is there tool squeal or any other strange noise. Look at the finish on the workpiece for roughness or any other finished surface problems/defects. If you are hearing/seeing problems then change things like the spindle speed, depth of cut or tool feed rate until things settle down. This assumes that sharp tools are being used and they are aligned correctly If you're getting a good finish and it sounds smooth when you're cutting then there is a fair chance that is what suits your set up best or is pretty close to being right. I have, on my small lathe and mill, variable speed control which I find very useful as I can change the spindle speed easily which often overcomes the problem. I use it when parting off and slowly increase the spindle speed as the tool cuts deeper into the workpiece and the cut diameter is reduced. Just a few jumbled thoughts that might help. Cheers Martin
chris stephens	30/05/2011 12:08:32
1049 forum posts 1 photos	Hi Martin, Your suggestions have merit. What I think you are describing is a sense of "feel", and is this not what differentiates skilled workers from rank beginners, no matter how many books they have read. chriStephens
JDEng	30/05/2011 16:51:25
27 forum posts	Posted by chris stephens on 30/05/2011 01:02:21: Hi Guys, Before any newbie gets too carried away with getting the speed exactly right, according to the charts, they are the speeds that you can work up to for maximum production rates in industry. Most of us don't need to go anywhere near as fast in our home shops environments. Again for newbies, forget the speeds in the charts for drilling especially when drilling in the lathe, slow right down and give the drill time to do its job. Drilling too fast just makes things hot. Remember that, "back in the good old days" drills used to work even when the only power input was you winding a handle. Oh alright, that way might be a little too slow, but the principle still holds. chriStephens Hi Chris and Martin, Martin: You are quite right in what you say about the equation I posted and in actual fact the simplified version is the one I use in the workshop "in my head" to work out RPM. I posted the full version because I feel it's important for people to be told the correct way of doing things; you can always take shortcuts later. Chris: I don't entirely agree with what you say. High Speed Steel is a lot more forgiving than Tungsten Carbide and will tolerate a far wider range of cutting speeds and feeds. The recommended cutting speeds for HSS are designed to give the best compromise between production speed and length of time between regrinds; in other words they produce the optimum production/down time ratio. I totally accept that HSS can be run a lot more slowly without adversly affecting its performance although what is gained by this I'm not entirely sure. Drilling in particular is far easier if the correct speeds are used, the drill cuts more readily and less pressure is needed to feed the drill; if there's a problem with heat use a squeezy bottle full of coolant! Tungsten Carbide on the other hand has been designed for use in a high speed production environment. It is far less tolerant of slow speed which is the point I was trying to make in my original post. If you use it below its designed cutting speed and feed the cutting edge will rub instead of cutting, this results in the cutting edge being worn away or failing prematurely which produces an extremely poor finish. As a comparison it would probably be fair to say that HSS should be used at or below the RPM's calculated whereas TCT should be used at or above if a reasonable finish and tool life is to be expected. I accept that most people don't need to work as fast in a home workshop as would be expected in industry however, if you are to achieve results with TCT then you have to work at the speeds it is designed for. Regards, John.
Steve Withnell	30/05/2011 21:23:42
858 forum posts 215 photos	Posted by Andrew Johnston on 29/05/2011 20:48:40: Hi Steve, What type of lathe? Can you describe the differences in finish? For instance is the surface texture different over a small length, indicating that the cutting process is different, or is it inconsistent over larger lengths, indicating that the cutting process is flipping between different regimes? It might be something to do with the headstock bearings being loaded differently? Regards, Andrew Hi Andrew, Here is a picture of the workpiece showing the different finishes. It's 8mm diameter and the material is EN8. The chuck is to the left hand side of the image. When machining towards the chuck, the finish is better and different to when machining away from the chuck. Regards, Steve
mgj	30/05/2011 23:40:16
1017 forum posts 14 photos	I'm sure its something to do with the headstock bearings being differently loaded - in one case they (it is)are in compression and in the other under tension. It might be worth adjusting them up, because , machining away will pull any clearance forwards and allow a touch of movement in the front bearing. What type of centre are you using in the tailstock?
Martin W	31/05/2011 00:12:27
940 forum posts 30 photos	Hi Taking MGJ's comments as a starting point. Center drill the workpiece then using a running center in the tailstock load the headstock bearings by winding the running center in toward the chuck until resistance is felt, tailstock locked down, then lock the tailstock spindle. Repeat the turning process again and if the results ARE different from previous runs then it is, as mgj suggests, probably a headstock problem. If the result is the same as the previous tests then look at tooling or play in saddle, cross slide or top slide etc. Hope this helps but if I am trying to teach my gran how to suck eggs then I apologise. Regards Martin
Andrew Johnston	31/05/2011 23:00:57
7061 forum posts 719 photos	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
Martin W	01/06/2011 00:20:16
940 forum posts 30 photos	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 Withnell	01/06/2011 07:18:52
858 forum posts 215 photos	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 Withnell	01/06/2011 07:57:03
858 forum posts 215 photos	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 C	01/06/2011 11:00:41
7468 forum posts 230 photos	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 C	01/06/2011 11:01:23
7468 forum posts 230 photos	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 Withnell	01/06/2011 17:03:35
858 forum posts 215 photos	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
mgj	01/06/2011 18:01:47
1017 forum posts 14 photos	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.
Andrew Johnston	02/06/2011 21:18:17
7061 forum posts 719 photos	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 C	03/06/2011 10:47:13
7468 forum posts 230 photos	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
Andrew Johnston	03/06/2011 12:16:22
7061 forum posts 719 photos	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 C	03/06/2011 14:23:49
7468 forum posts 230 photos	Thanks Andrew, thats what I thought it was. Ian S C

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