Another what size question, thread this time.

[AndyP]

Buoyed with our success in identifying and sourcing the correct screws for my salt ‘n pepper grinders I’ve decided that my 4” grinders are too small and that it cannot be difficult to make them 6”. The only thing I need to do is make a longer shaft*. Aluminium bar 5mm squared, peened at one end and a thread cut on t’other end. In the picture below the grinder shaft to the left. Square section 5mm. To the right is a M4 bolt. If I understand correctly the grinder shaft cannot have an M5 thread as the pitch would be 0.8mm it seems to have a 1mm pitch. So is this some non metric size? The smallest die I have is M6 which is definitely too big.



*and turn some new bodies of course.

 

[AndyT]

I think it's probably 24 tpi (25tpi would be almost right for M5).

Using Dr Al's handy reference chart on his website, it could be American UNC 10-24 (4.83mm diameter) or even 3/16" Whitworth (4.76mm).

New grinders aren't very expensive, compared to decent taps and dies y'know!

 

[AndyP]

Definitely not M5. I’ve found packet of m5 nuts and they do not fit.
Here is M5 threaded rod with M5 nut alongside the grinder shaft

I have a set of taps and dies so would just need the right size die for the handle.

Yeah a new grinders are about the same price as a die but I thought it would be a challenge and a fun thing to do.

 

[Dr.Al]

Yes, that pitch is a long way off the M5 pitch. From measuring the photo it looks like 1.08 mm pitch or 23.6 TPI so 24 TPI seems likely. As Andy has said that puts it as either 3/16" BSW or UNC 10-24. The main difference is the thread angle but you'd never be able to tell without a thread gauge and lots of magnification. On a positive note, it would probably work regardless of which one you chose and if you bought a split die and the thread ended up a bit tight, you could tighten the die up a bit and loosen it up.

What does the thread screw into and how long is the engagement? Another option is to drill the body out 5 mm and tap M6 but there may be more options depending on what it looks like.

 

[AndyP]

The thread screws into the top cap of the grinder about 1cm

 

[Dr.Al]

The thread screws into the top cap of the grinder about 1cm
View attachment 59130

I reckon you could tap that M6 if that would be easier for you. If you want to send it to me, I'd happily do it for you (I could also turn the thread on a bit of square bar if you've got some). I know you come to the UK quite often so if you can be patient then the postage would probably be cheaper.

 

[AndyP]

Thanks Al that’s kind of you and could easily prevent me from buying a new tool and learning a new skill. I’ve only cut threads in wood up to now; how hard can it be in aluminium?

As well as a set I have just made, I have two more kits ie 4 grinders that are lined up for presents at the end of the year. I can find a 1” die (10x 24 unc ) and enough 5mm x 5mm alu bar to turn all the 4 inch kits into 6 inch with enough left over for the CUF.