Cuttings an 8tpi thread in free cutting silver steel

[Ken RatcliffeParticipant @kenratcliffe25229]

Hi All,

I am trying to cut an 8tpi thread in a 5/32" diameter silver steel bar. I am using free cutting silver steel as the instructions suggest. I have ground an ACME tool in HSS and have a live centre and travelling steady for support and turning the lathe by hand……but tool keeps catching (digging in) and warping the bar.

Any thoughts or suggestions, as always welcomed !

Ken

[Ken RatcliffeParticipant @kenratcliffe25229]

[Clive Brown 1Participant @clivebrown1]

How long is the thread? 5/32" dia. steel is very flexible for a cutting width of ~1/16"

Could you try a narrower tool and 2 passes, shifting over after the first pass to get a full width cut?

Lots of lubricant aswell.

[JasonBModerator @jasonb]

8tpi ACME on 5/32" rod will be almost impossible as the depth of the thrad is far too great, either the 8 or the 5/32 is wrong

[David Davies 8Participant @daviddavies8]

8 tpi ACME thread has a depth of 0.062. if you are cutting this in 0.156 stock that leaves a core of 31 or so thou. Something isn't right?

[David Davies 8Participant @daviddavies8]

Too quick Jason!

[Clive Brown 1Participant @clivebrown1]

Doh! should've spotted that

[Ken RatcliffeParticipant @kenratcliffe25229]

Hi All,

It is "only" 3/4" long thread. Just checking the instructions and it is 8tpi and 5/32" stock, it suggests a depth of 0.05" for the cut…..and fully admits it isn't going to be easy !

Maybe I should explain what it is for, it is a worm drive on a clock, so the thread drives a wheel, so, the depth of cut only has to allow wheel to engage, hence 0.50" cut.

The book also suggests a tip on the acme tool of 1/32", again maybe not standard. I think I will give Clive's suggestion a go and lots of shallow cuts from reading around

Ken

[MadMikeParticipant @madmike]

As Jason pointed out the core diameter will present problems in cuttink an 8TPI Acme thread. Even with your depth o.050 the core is still only .056 diameter.

[David Davies 8Participant @daviddavies8]

You could try cutting an 8 tpi 55 or 60 degree thread first then cleaning it up with the ACME tool.

HTH

Dave

[JasonBModerator @jasonb]

It will almost need a solid bush type traveling steady as you see on "running down" cutters. Rather you than me, at least it was easier to draw this is to the 0.05" depth and with 1/32" tool tip

 

Edited By JasonB on 02/11/2020 15:24:57

[Mick B1Participant @mickb1]

There'll be a larger helix angle – I think much larger – requiring a lot more side clearance, too, if the thread's got to advance 1/8" on a 5/32" OD.

[Ken RatcliffeParticipant @kenratcliffe25229]

I think I will try the 650 degree and then clean up……here is a photo from the build book, clearly better lathesmiths than me – but then again I thought making a wheel was impossible the first umpteen times I tried and messed up, I can usually get them first time now

Ken

[John BaronParticipant @johnbaron31275]

Hi Guys,

FWIW the laser head drive thread on a CD drive is very close to that size !

[David NobleParticipant @davidnoble71990]

Hello Ken,

Which clock are you making? It looks very tricky

David

(Edit)

Ah, sorry, just seen your other post. The Merlin Band Clock.

Edited By David Noble on 02/11/2020 16:50:58

[Martin KyteParticipant @martinkyte99762]

A mate of mine (sadly deceased) obtained a drive thread for this clock from a bloke in Italy a couple of years ago. Maybe someone out there will have details. I do remember looking at it at the time and feeling quite thankfull I didn't have to cut one. I'm clearing his workshop at the moment, or was until the lockdown announcement, but I hav'nt come accross it yet. If there is more than one I will let you know.

I'm sure the trick is in the design of the travelling steady.

regards Martin

[Ken RatcliffeParticipant @kenratcliffe25229]

Yep David, it is the trickiest yet, for sure. Putting my lathe skills to the test. That and a traction engine on the go !

[Martin KyteParticipant @martinkyte99762]

Aha found it.

**LINK**

Scroll down the page and you will see it on offer for 49 euros plus 12 euros postage.

regards Martin

[Ken RatcliffeParticipant @kenratcliffe25229]

Thanks Martin, just dropped them a note. Much as I hate not making, this one could be beyond me – and the rest of the clock isn't easy !

[old martParticipant @oldmart]

It sounds like it needs a tiny centre in the free end and a little centre to support it.

[JasonBModerator @jasonb]

The photo seems to show the rear toolpost being used as a traveling steady, possibly a half round cutout in the end of the support, topslide set at 90deg to lathe axis to put on the cut.

As I said another option would be to mount a bush on the cross slide to support the work and again use topslide at 90deg to put on the cut.

[old martParticipant @oldmart]

The bush idea would work well as long as the assembly could be separated after each pass to completely remove all traces of swarf. Grinding the ACME form is not enough, it should be honed for the sharpest possible edge. Also, the cutting tip should be aligned exactly to match the helix angle of the thread.

[Dennis PatakiParticipant @dennispataki22631]

Would milling that thread be a better process than trying to single point cut it?

[speelwerkParticipant @speelwerk]

This is similar, silversteel, diameter 2.8 mm, core 0.9 mm, pitch 1.10 mm and 8 turns if it becomes more slender you make an extra support. Niko

[BazParticipant @baz89810]

Milling would certainly be my chosen method of manufacturing.

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