Normally a tap will cut a thread that's a bit bigger than the thread a matching die will cut, so there is a small gap between the finished pieces (needed so they can turn freely). | --| |-- die | |---| finished thread
| |-----| tap | -| |- finished hole
| -| |- finished hole | |---| finished thread
If I want to cut a thread in a mould with a die, where the finished piece is the inverse of the mould, I assume I can't use a standard die, it would have to cut a slightly smaller thread.
Wrong:
| --| |-- standard die | |---| mould projection | --| |-- finished item hole
The short answer is no, they don't make taps & dies for this purpose.
There is slightly more to it than you have suggested as well (isn't there always!!). A thread form is usually truncated at the tip and base. Obviously the amount it is truncated by at the tip is more than at the base, giving a clearance fit. If you use a tap to cut a hole, and then mould something in that hole the part you have moulded will not fit into the relevant sized nut as these clearances have been lost. The other thing to consider is shrinkage, almost all materials that are moulded shrink as they solidify. I'm used to plastics, so for example if your 36 x 1 thread was to be moulded in ABS it would need to be a 36.216 x 1.006 thread in the die togive the correct form after moulding.
I find the best way to work out the correct thread form is to draw the male and female thread, with the relevant clearances, truncation, radii, etc (these can be found in the standards, or data books -I can reccomend the "guide to world screw threads" as a relatively innexpensive manual that covers most threads I've ever needed, got my copy from J&L). Then scale the drawing by the shrinkage required (you are drawing this in cad, aren't you?), then you should be able to work out the form you need to cut.
You can usually get away without putting shrinkage on the pitch, as long as the material doesn't have a huge shrinkage.
If you are moulding the male and female parts you can ignore shrinkage, as they will both shrink by the same amount.
If someone is going to check the finished part with a thread gauge be prepared to have several goes at it, if it just has to "fit" whatever it goes onto you should be able to get it to fit fairly easily -check whatever it fits to if it already exists -that thread may not be correct to start with!!
I design plastic injection mould tools for a living, and still hate having to do exact threads that will be checked on a thread gauge.
You can. That's why dies are often "preslitted". Finish the cut they started in the factory and press open the gap with the die holder (that's why they have an extra "useless" 4th screw).
More to the point would you want to cut a thread this big with a die if you could get one? IMO screwcutting in the lathe would be the way to go for this as you can set the major/minors diameters to what you want. Make the part as an insert with a taper location into the bse of your mould and key it to stop it turning.
Alternatively you could thread-mill it, even without CNC. I saw a very clever idea for this on the HSM forum and I liked it so much I kept a copy of the sketch. Have a look here:
formatting link
There are pictures and an explanation in the last post. What material are you intending to mould it in btw?
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.