custom big tap needed

[douglas coatesParticipant @douglascoates40572]

Thanks for all the knowledge and interest – various bits of feedback as far as I know them:
– relief – I didn't make the current tap or guide (I did make the cutter and box) but in these things the cutter depth is adjusted so the screw runs smooth but not sloppy (in the wood nut). I suppoose this has the effect of giving some axial clearance, but it's a bit more crude – pretty much everything is as we are dealing with wood, grain, atmos movement etc.
– the angle is definitely 60 degrees, pic is not very good, but I know from making the cutter
– my sums were rubbish, the current 4tpi uses a 35mm bit for female blank, and a 41mm approx dowel that the screw is cut from. I need to check the tpi, do we talk points or whole teeth?
– optimal tpi – from playing with many different wood screws for vices, I am sure on this one that my 4tpi is a tads steep, 6 is too fine, but best guess would be I'd like about 4.5 – 4.8.
– bad drawing, the thread is cut on axis, the taper creates progressive flats towards the leading end.
– rads and flats – my guess would be make the tap to apex points on the thread, then nick the tops off anyhow. Make the die / thread box so a small flat exists between threads (maybe a mm) – the tip of the screw has no strength and does no useful work.

I promise I will do another drawing soon as I get a minute…!!

[douglas coatesParticipant @douglascoates40572]

New sums, I checked the current tap which is not exactly 4tpi. I've done a re-calculatiojn as I seem to get strange answers to the 2 diameters. If I set pitch at 6mm but with 1mm flat, the triangle is 5mm, so perpendicular is 4.33. According to that the I/D and O/D differ by 8.66 mm?? I can get a 35mm Forstner for the nut blank, and a 45mm for the vice guide which I can grind down to about 43.7 say. Which bits of this are wrong !!?

 

Edited By douglas coates on 05/05/2016 09:16:31

[Martin ConnellyParticipant @martinconnelly55370]

Douglas, your 4.33mm is correct but you will not have a sharp vee at the bottom. It is usual to have a small radius or flat, after all the mating part is going to have flats. If you have a mating thread with a 1mm flat then you would expect the base of the thread to have a flat smaller than this or a radius to give a small clearance between the two parts. A radius of 0.58mm at the bottom of the vee would be one solution and a flat of 0.75mm would be another. these both have the effect of increasing the inside diameter.

If we have a radius of 0.58mm at the bottom of the vee then the flank height is no longer 4.33mm but becomes 3.46mm and the radius after this goes down a further 0.29mm so your 4.33mm is now 3.75mm.

If we have a flat of 0.75mm at the base of the vee then the flank height is now 3.68mm.

See the drawing below for the details.

Martin

[HopperParticipant @hopper]

I think you need to stick with a standard thread form and use the standard dimensions commonly available. For a 60 degree thread, you could use either UNC or Metric forms. Plenty of standard charts on the net and in ME books that will give you the thread depths, diameters etc including or excluding the flats at top and bottoms.
Eg, metric:

[Author]

Posts