A maths problem I can't solve
Gary Wooding  27/03/2019 10:52:25 
729 forum posts 192 photos  The diagrams show a Vgroove in a solid block. The angle of the groove is V. There is a right circular cone, included angle C, resting in the groove. The point of the cone is at the bottom of the groove, and the cone just lies there touching both sides of the groove. The axis of the cone now lies at some angle X relative to the bottom of the groove. The problem is simple: what is angle X? 
Gary Wooding  27/03/2019 11:09:10 
729 forum posts 192 photos  Hmm, I've somehow managed add this posting twice. Could a moderator remove one of them please? 
Trevorh  27/03/2019 11:14:17 
302 forum posts 87 photos  Isn't it just a right angle triangle  the right angle being the base of the V block, the hypotenuse being the centre line X extended to the edge of the V block gives you the Right angle called a scalene right angled triangle with no equal sides so the inclusive angle will be 35 degree at the point and 55 degrees at the vertical end
regards Trevor Edited By Trevorh on 27/03/2019 11:16:52 
JasonB  27/03/2019 11:42:21 
Moderator 18310 forum posts 2024 photos 1 articles  Trevor how do you get those figures when none of the angles are known? or did you just measure off the drawing. I would have thought Gary was after a formula so that if V and C change you can work out X 
Gary Wooding  27/03/2019 11:49:16 
729 forum posts 192 photos  Jason is right, I'm looking for a formula that calculates X for various values of C and V. 
Paul Lousick  27/03/2019 12:09:16 
1451 forum posts 555 photos  Easy enough to work out X in a 3D model but need value of C & V

JasonB  27/03/2019 12:35:41 
Moderator 18310 forum posts 2024 photos 1 articles  For those that don't have cad and want to see if they can come up with a formula then these are some random CAD values. V = 80deg C = 40deg included angle X = 32.15deg 
Andrew Johnston  27/03/2019 13:04:29 
5555 forum posts 650 photos  I don't have time at the moment to do the analysis. But by inspection we can say that the following inequality is a necessary condition: C ≤ V In the limit when C = V then X = 90°. Andrew 
Frances IoM  27/03/2019 13:12:50 
780 forum posts 26 photos  I'm sure I've seen this solved  look up analysis of conic sections  was in all advanced geometry books many years ago (eg late Victorian text books)  had a quick look at my bookshelves but can't find the book I was thinking of 
JasonB  27/03/2019 14:02:21 
Moderator 18310 forum posts 2024 photos 1 articles  This looks like it may do, going to try it with those figures
Edited By JasonB on 27/03/2019 14:03:57 
JasonB  27/03/2019 14:41:17 
Moderator 18310 forum posts 2024 photos 1 articles  yes that works Sin X = Sin M x Cosec N Where M is half the value of C and N is half the Value of V from the original question. So taking those CAD figures I posted earlier Sin X = Sin 20 x Cosec 40 Sin X = 0.342 x 1.558 Sin X = 0.533 X = 32.21 which allowing for me rounding to 3 decimal places is right.
I did have to go and look up what Cosec was as I only went as far as O level maths Edited By JasonB on 27/03/2019 14:42:53 
Gary Wooding  27/03/2019 14:46:01 
729 forum posts 192 photos  Posted by JasonB on 27/03/2019 14:02:21:
This looks like it may do, going to try it with those figures
Edited By JasonB on 27/03/2019 14:03:57 Thanks Jason, I can verify that formula does work. How did you do it in CAD? I could only do it by trial and error. 
John Haine  27/03/2019 14:53:47 
3174 forum posts 171 photos  Or sin(x) = sin(M/2)/sin(V/2) since csc(x) = 1/sin(x) Now to prove it! I'm never quite sure why sec and cosec were ever invented since they are just reciprocals of cos and sin respectively. 
JasonB  27/03/2019 14:54:10 
Moderator 18310 forum posts 2024 photos 1 articles  I drew out the vee block and cone to those angles which is easy enough. I then assembled the two using a tangental mate to first mate the slope of the cone to one side of the vee and then a second similar mate to get it to sit against that. Then just a case of measuring the angle between base and the central axis is the cone. I've made the block semi transparent so you can see things better
Edited By JasonB on 27/03/2019 14:54:52 
SillyOldDuffer  27/03/2019 16:02:24 
5925 forum posts 1281 photos  Posted by John Haine on 27/03/2019 14:53:47: ... I'm never quite sure why sec and cosec were ever invented since they are just reciprocals of cos and sin respectively. Me neither, except reading an old book about navigation suggests an answer: it does sums using haversines, covercosines and other weirdness. The reason seems to be simplification and error reduction of calculations at a time when they were done manually in midAtlantic by seasick navigators in rough weather. It's quicker and safer to look up cosec x in a table than it is to crunch 1 / sin x, or to look up sin x and then it's reciprocal in two tables. The formula involved are simplified too, for example the Haversine formula is hot at calculating great circle distances. Likely the functions also helped on land, perhaps laying out curves on railway lines. Other examples of 'labour saving' trig functions include Versine, Coversine, Vercosine, Secant, Cosecant, Exsecant, Excosecant, Cotangent and probably others. Once you have a computer, calculator or even a slide rule, the need for them largely disappears, I guess. I've never come across them in real life! Dave 
Gary Wooding  27/03/2019 16:13:09 
729 forum posts 192 photos  Thanks Jason, I tried, and failed to do it in Fusion, which is the reason I wanted a formula. Fusion doesn't use mates  it uses something called joints instead, which I couldn't make work. 
John Reese  27/03/2019 18:43:59 
842 forum posts  Posted by John Haine on 27/03/2019 14:53:47:
I'm never quite sure why sec and cosec were ever invented since they are just reciprocals of cos and sin respectively. In the days of solving math using pencil & paper it was simpler to multiply by the inverse function than to divide by the function. Some of the early mechanical calculators were not capable of division hence the need for the inverse functions. 
Neil Wyatt  27/03/2019 20:50:18 
Moderator 17970 forum posts 709 photos 77 articles  Posted by SillyOldDuffer on 27/03/2019 16:02:24:
Other examples of 'labour saving' trig functions include Versine, Coversine, Vercosine, Secant, Cosecant, Exsecant, Excosecant, Cotangent and probably others. Once you have a computer, calculator or even a slide rule, the need for them largely disappears, I guess. I've never come across them in real life! Dave Most of those sound like decongestants. My brain needs one after reading this thread... Neil 
Chris Trice  27/03/2019 23:24:09 
1362 forum posts 9 photos  The thing that threw me was that the round disk of the wide part of the cone, when angled forward starts making contact with the V block nearer and nearer to the east and west positions. Looking from the front, the disk starts to become elliptical the more it leans. 
Brian Wood  28/03/2019 09:53:21 
2203 forum posts 37 photos  Jason, May I ask which book it was you found that explanation in please. My various reference books, including the full set of three Chapman's on Workshop Technology doesn't have it Regards Brian 
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.