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| When is half of 60 not half of 60 ? |
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| DavidA:
Not wanting to stir up any hornet's nest, but here goes. I was reading through my copy of Sparey's 'The Amateur's Lathe' and , where he covers screw cutting, he mentions the 'half angle' method of cutting threads. In his example he uses Whitworth 55 degree thread and says something along the lines of ' Set the topslide over to 27.5 degrees'. Which is half of the 55 degrees. no surprises there. BUT. I have noticed that our most excellent Doubleboost, when setting his topslide to cut 60 degree threads by this method, makes a point of setting to 29 degrees. Not the 30 degrees I would expect. No doubt this is done for a reason as it seems to produce excellent work. But why ? Dave. :scratch: |
| spuddevans:
I may be wrong ( it has happened once before :lol: ) but I think it is so that you are definitely cutting only on one side of the cutting tool. Then for the very last cut you advance the cross-slide instead of the top-slide to give the final true thread shape. I believe the idea is to reduce the cutting forces on the tool for the majority of the cut until the final cleanup cut. Tim |
| DavidA:
Tim, I see where you are coming from there. But won't that leave you with a slightly skewed thread form ? 29 degrees on one face and 32 on the other. Dave. |
| doubleboost:
The last cut is put on with the cross slide This cuts both sides of the thread at once Giving the thread the same angle as the tool (well that is how I do it) John |
| philf:
--- Quote from: DavidA on June 29, 2013, 02:59:52 PM ---Not wanting to stir up any hornet's nest, but here goes. I was reading through my copy of Sparey's 'The Amateur's Lathe' and , where he covers screw cutting, he mentions the 'half angle' method of cutting threads. In his example he uses Whitworth 55 degree thread and says something along the lines of ' Set the topslide over to 27.5 degrees'. Which is half of the 55 degrees. no surprises there. BUT. I have noticed that our most excellent Doubleboost, when setting his topslide to cut 60 degree threads by this method, makes a point of setting to 29 degrees. Not the 30 degrees I would expect. No doubt this is done for a reason as it seems to produce excellent work. But why ? Dave. :scratch: --- End quote --- Dave, It's always safer to be a degree less than the half angle. If you set it a degree more rather than less you can't generate the correct thread form. I did a couple of sketches some time ago to show what would happen if someone set the topslide at 60 degrees instead of 30: The first shows the topslide set to 29 degrees (61 to the lathe axis) and shows that most of the cutting would be on one edge of the tool with just a scraping on the other side. The second shows the topslide at 60 degrees and you can see that one flank of the thread will be cut correctly whilst the other would be stepped. This would apply to a lesser degree to any angle even very slightly larger than half the thread angle. As the topslide graduations aren't usually very precise it's safer to set the angle a degree less. I don't bother - I keep the topslide parallel to the lathe axis and for every 0.1mm I increase the depth of cut with the cross slide I advance the topslide 0.04mm. For the last delicate cuts I just put a cut on with the topslide. Hope this helps. Phil. |
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