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I don't believe that is correct, the cutting speed would be dependent on the circumference of the part being turned.

Circumference is calculated as??Π谤² (Pi times the radius squared).

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To calculate the actual cutting speed? at a specific RPM the formula would be:

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?Π谤²*RPM

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?For an object 3" in diameter at 1000 RPM the calculation would be:

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3" diameter divided by 2 = 1.5" radius

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1.5"*1.5"*Π*1000 = 7068.6 Inches Per Minute

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If you want to calculate the RPM needed to meet a specific cutting speed the calculation would be:

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1/(Π谤²*/Cutting Speed) = RPM

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The RPM required for an object 3" in diameter with a cutting speed of 7068.6 inches per minute would be:

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1/(1.5"*1.5"*Π/7068.6)? = 1000

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Cutting speed nomenclature is dependent on what linear measurement and rotational speed you use.

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If you do the calculation using the diameter/radius measured in inches you get inches out.

If you do the calculation using the diameter/radius measured in millimeters you millimeters out.

If you do the calculation using the diameter/radius measured in furlongs you get furlongs out.

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If you do the calculation using revolutions per minute you get distance per minute in out.

If you do the calculation using revolutions per second you get distance per second in? out.

If you do the calculation using revolutions per fortnight you get distance per fortnight in out.

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?Be aware; as the diameter is reduced the cutting speed is reduced, as you reduce the diameter of an object you may need to increase the RPM to maintain the desired cutting speed.

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Richard

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On Tue, Sep 3, 2024 at 06:55 PM, Peter Brooks wrote:

I think the spindle would have to be inclined slightly to cut the thread at the correct angle?? These angles would change from item to item depending on the pitch.

I think that if the cutting tool is small diameter compared to the thread being cut there will not be a problem. I can't get my brain around the multiple angles and helices and so on, but there will be a cut-off point where the cutter spoils the thread form - but by how much, and does it matter - is another question. A quick calculation shows the helix angle of an M3 thread is 3°. The helix angle of metric thread becomes less as the size goes up, so M3 is a good starting point. Your large diameter threads will be much less - I don't think there will be a problem

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How on earth would you make a cutter? ?I guess you would start with the (for instance) 55 degree triangle profile at the end, then mill alongways to create individual teeth? ?How many 'teeth'? ?Apart from the U3 lathe I have the milling attachment (with fine feed), and the indexing / dividing attachment, so I guess there is no excuse!

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Here is a Woodruff cutter :

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Now imagine the teeth formed with a 55° (or whatever) angle on them. The width of the teeth would determine the coarsest pitch it could cut - your 12 tpi is only 2.12 mm pitch. If you reduce the number of teeth it is possible to cut them out with a mill.

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Here's one I drew earlier :

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This is for 12 tpi. Overall diameter is 15 mm, shaft is 7 mm (to fit an ER11 collet). Tooth width is 2.5 mm, cutting angle is 55°. The notches are cut with zero rake, for cutting brass, but by taking them in a bit further you can get 7° for steel. If you want to go to 20° for aluminium you will only get five notches. Harden and temper it, then a very light touch with a fine hone to sharpen up the edges, maybe giving slight clearances behind the cutting edge. I can draw up similar cutters for other pitches if you like. I think once you've made a couple you'll get the idea. You might need opposite handed ones for internal and external cutting, but probably not.

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And I've realised I have a couple of those smaller fully cased motors that started this enquiry. The bodies are just about 2" diameter. I'm trying to get around to setting one up as a grinder with? 50 mm diamond wheel. I've already satisfied myself that this will be a good alternative to a normal bench grinder, giving very sharp toolbits, with a nearly polished finish, but at a moderate speed so that you can control how much metal it takes off. Also not much noise, no sparks, no smell of burning and no grit spread all over the place. Ideal for working on the kitchen table.