I want to make some gear cutters with 15 1/2 deg pressure angle. I have Ivan Laws book (17) but it only list 'button' sizes for 20 deg and 30 deg. Does anyone know of a source of data or correction factor I can use on these tables. Thank You
This is something I have just been chewing over, so I was interested to see this thread. However, I can't get your spreasheet to reproduce Ivan Law's numbers. This could be me being dumb - but for a 20 degree PA, and 1 DP, and gears between 17 and 20 teeth, Ivan says the diameter of the button cutters should be 7.8 inches, and the spacing
8.7 inches - but your spreadsheet says diameter 5.8 to 6.8 and centres
6.8 to 7.8.
I can see it is recommened that the tool holding the buttons mounts them at an angle of about 5 degrees so as to get front clearance, and then grinding them at about 9 degress to get 4 degrees top rake. Which I suppose makes the cutting edge oval - so maybe the diameter is a bit tricky to define. Maybe this is part of it.
Does it reproduce Ivan Law's number when you use it ?
I can see Duncan's spreadsheet but no a way to use it. There are two ways to use buttons to produce gear cutters. One was devised in the 19th Century by various persons, Grant was one and the method used by Ivan.
Both are correct although they do come up with different figures.
Where they differ is the old version which was also copied by Unwin in ME in the 60's, use a blank of a known width and touch the buttons on the edge of the blank THEN infeed.
In Ivan's method which isn't clear from the small illustration. Although he specifies a width, that part isn't taken into account in the calculations as he touches the button face on to the blank, moves sideways to centralise the buttons on the blank, THEN infeed's.
If you draw both sets of calculations onto a geometrically correct gear they overlap almost exactly.
May I point you to an article I wrote some years ago?
I think I can see one reason why your spreadsheet doesn't match the numbers that are in Ivan Law's book - I think Ivan has optimised things. I see you have used the radius at the pitch point (as defined by Ivan) as the cutter radius. I have set up a spreadsheet to vary diameter and seperation of circular cutters and then use the Excel solver to optimise for a best fit over the whole involute surface. You can get a better match to the involute than using the radius at the pitch point - and it seems to invokve slightly larger diameter cutters. I suspect the values in Ivans tables reflect this. He has done some optimisation he has not told us about.
Although I can see a better fit using his cutter diameters, I can't reproduce matching figures for the centre distances for these cutters. Could be something in my spreadsheet !
Of course none of this might matter in the great scheme of things - I have no feel for how sensitive the smooth running of a gear is to deviations from the involute curve - clearly the use of single cutters to cover a range of gear sizes in commercial practice suggests near enough is good enough.