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| andyf:
Hi Wong, Speaking personally, I would not be too unhappy with a taper 0.02mm (under 0.001") over 150mm (6"). But I'm easily satisfied! Like Graham says, it could be due to a tiny amount of play in the revolving centre, if you used one. Did you use the 3-jaw chuck which comes as standard with the lathe? Nearly all 3-jaw chucks display some runout, often varying with the diameter of work being held. That would tend to produce a taper with its smaller end near the chuck. If that is the cause, I think most folk would be satisfied with a 3-jaw chuck which had such a small amount of runout. Graham's suggestion of a test cut between centres would avoid any errors introduced by the chuck. Andy |
| wongster:
Hi guys, My replies were lost for whatever reason. The smaller portion is at the headstock end. Further tests were done on steel stock of 25mm diameter with 50mm sticking out of the chuck. I've 0.02mm smaller at headstock. The next piece was 9.5mm of unknown steel again, this time supportrd by revolving centre. I've the same 0.02mm but it widened to 0.06mm as the stock was reduced to less than 8.5mm. Total length of stock was 150mm, of which 100mm was turned. One other thing I notice; at 0.25mm depth of cut, 1400 rpm for the thinner stock and 330 rpm for the 25mm. Cutting noise was terribly loud. Up to 0.2mm was ok. I wanted to try greater depth of to 0.5mm but have no courage to proceed fearing that something bad may happen. Regards, Wong |
| andyf:
Hi Wong, Inaccuracy in the chuck and slight play in the revolving centre will both produce the same effect – the stock will taper down towards the headstock. As to the noise when you increase the depth of cut, your “unknown steel” could be the problem. Also, you haven’t told us whether you are using a carbide or HSS cutter. Looking at the cutting speeds in the chart you showed earlier, they are for HSS. Obviously, a blunt tool will cause problems. So will one which has been ground to the wrong angles. The cutting edge should be on centre height, or 3 or 4 degrees above it, but in that case the angle will increase as you take successive cuts, so adjustment may be needed as you go on. Andy |
| wongster:
Hi Andy, I'm using carbide insert tool. Tried HSS, didn't like the finishes. Maybe I'm doing it wrong. Regards, Wong |
| andyf:
I rarely use carbide, Wong, so can't offer any suggestions, other than saying that speeds recommended for carbide are often four or five times faster than for HSS. Andy |
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