Milling cutters with screwed shanks

[Chris FarbraceParticipant @chrisfarbrace]

Hi,can anyone enlighten me as to what thread is used on screwed shank milling cutters,I have been given several metric sizes and am trying to id the thread.checking a 12mm size with a thread gauge seems to indicate a whitworth thread and shows 20G 1/4  on the matching gauge leaf,is this a special whitworth thread?Many thanks if you can help.regards Chris.

[Chris FarbraceParticipant @chrisfarbrace]

[Tony Pratt 1Participant @tonypratt1]

Hi, from a quick Google search the thread profile is Whitworth and all are 20 tpi, irrespective of O/D.

 

Tony

[Keith LongParticipant @keithlong89920]

Hi

20tpi on all sizes, including metric. Whitworth form but cut 0.003 – 0.006 inch undersize (source – Model Engineers Handbook, Tubal Cain).

As I understand it they should be a fairly loose fit in the threads in the collet as the idea is the thread will cause a jacking effect on the collet and hence tighten the grip if the cutter tries to slip. A tight fitting thread may not allow this.

20tpi 1/4 inch standard BSW – you should be able to use the same tap for both metric and imperial cutters – just be a looser fit on the metric

20tpi 3/8 inch standard BSF – same as above applies to 10mm shank

20tpi 1/2 inch standard UNF – in theory wrong thread form but shouldn’t make much odds of its a loose fit anyway – as above does 12mm as well

20tpi 5/8 (16mm) and upwards – sorry can’t help, but at least the hole is getting to be a reasonable size for screwcutting.

Hope this helps. Keith

[Chris FarbraceParticipant @chrisfarbrace]

Thanks for the info.guys,much appreciated.Chris

[KWILParticipant @kwil]

It should also be said that the Clarkson system also relies on the rear threaded end being centralised by the center inside the Clarkson Autolock holder.

[CirclipParticipant @circlip]

Metric and Imperial collets have DIFFERENT bores, the cutter shank HAS to be a close/sliding fit in the collet, the collets DO NOT collapse to grip the shanks like others.

 

  They are ALL 20TPI as stated, but cutting at 1.25mm pitch is fine cos you’ve only about 6 threads to pick up in the collet.

 

  Regards  Ian.

[mgjParticipant @mgj]

Agreed Circlip – about -.001 works fine.

 

And given that a whole thou is a long way on the modern lathe, that shouldn’t pose any problems for anyone.

[Nigel McBurney 1Participant @nigelmcburney1]

Hi  Agree with above, the thread is 20 tpi for all diameters,The thread on the cutter was part of Clarksons patent and if a cutter was ordered from a say a specialist cutter manufacturer and the Clarkson thread was  specified on the drawing a royalty had to be paid to Clarksons for every cutter which I think was one shilling.( in approx 1960) My employer told me that until the Clarkson holder was invented there were lots of methods of retaining cutters in milling spindles but the main problem was getting them out after use,they usally jammed tight,the Clarkson scheme made the release a lot easier,plus cutters were prevented winding down into the work by the thread. The more recent use of multi slotted collets was brought about by the need to grip plain shank solid carbide cutters.

[babielsParticipant @babiels]

Hi I have an old mill with a spindle that has an integrated (non-removable) Clarkson C Type chuck.

I want to put a face mill on this machine , so am planning to cut some external threads on a 1" shank face mill holder/arbor using a die. It will be used with my 1" Clarkson C Type Collet.

I'm ordering the die from Tracy Tools but there are so many options, hoping one of you might know:

L/H or R/H?

Cycle Pitch, BSC, BSF, BSP, Whitform, UNC, UNF, UNJC, UNJS, UNS ?

Also would Carbon Steel or HSS be better for cutting threads with a hand die. I think the face mill arbor is softer than HSS but would potentially want to thread 1" HSS cutters too.

Thanks a lot!

[JasonBModerator @jasonb]

It's whitworth form so go for that if they have it listed in that size, personally I would thread cut at that diameter to get a nice concentric thread. It's right hand. HSS probably best assuming the shank is not hardened.

Edited By JasonB on 30/01/2022 08:26:02

[Anonymous]

Don't even think about using a die. The thread should be screwcut for the reasons given by Jason. I use full form insert tooling to get the correct profile. The 1" diameter 20tpi Whitworth thread on this embryo worm wheel was screwcut to fit a large size Clarkson chuck:

Andrew

[SillyOldDufferModerator @sillyoldduffer]

Posted by JasonB on 30/01/2022 08:25:02:

… assuming the shank is not hardened.

Easily tested by running a sharp file over the shank. If the file cuts, the die should cut too. However, if the file slides off or is reluctant, then the die will probably make a mess of the job and end up blunt. (scrap) Ask again if the shank is hard.

The advice to lathe-cut a starter thread is good. Apart from making cutting much easier for the die, lathes cut straight whereas hand steered dies tend to wander askew. So quite common threading larger diameters to begin with a half deep thread on the lathe without worrying about the form (thread shape), and to finish off with a die that gets the form and final depth correct without wandering.

HSS dies last longer and are less easily damaged than Carbon Steel. For a one-off I'd probably buy an inexpensive Carbon Steel die, but be aware they're brittle and easier to spoil than HSS.

Don't forget to lubricate. A proper cutting fluid like CT90 is best, but almost any oil is better than nothing. Also, reverse the die a quarter turn after each full turn or so to stop swarf jambing the die.

Dave

[JasonBModerator @jasonb]

Me and Andrew were thinking fully cut . If you are set up on the lathe it's only another pass or two to get a finished thread with an FF insert and it's more likely to handle a harder material.

You will get a lot of change out of the £50 the die costs by the time you have subtracted the cost of an insert.

[Nigel McBurney 1Participant @nigelmcburney1]

Grind up an HSS toolbit and it costs next to nothing,if you already have a hss bit ground up as a cutter,then use the other end.

[Martin ConnellyParticipant @martinconnelly55370]

I have cut a lot of these threads, I use a brazed carbide single point tool that I ground to 55°. Things I have threaded to suit my Vertex Posilock include Little Hoggers, Ø16 insert mill, Ø10 insert mill, ER16 parallel shank collet holder, edge finder. DTI holder.

I set the item up in a collet or 4 jaw (clocked near the jaws) depending on diameter being held and clock the free end to ensure it is running as true as possible then centre drill the end before cutting the thread with the end supported. Some items need a plug in the end first to centre drill. You can check the thread is free running in the target collet and polishing it a bit with Scotchbrite before removing it. As the earlier post by Keith stated slightly undersize gives a better result, you need to be able to screw items in and out with fingers when they are in use.

Martin C

[peak4Participant @peak4]

Posted by babiels on 30/01/2022 07:44:09:

Hi I have an old mill with a spindle that has an integrated (non-removable) Clarkson C Type chuck.

…………………

For your info, Tony's site has a little about a variation your mill, albeit under a different name.
If you have any useful photos, he may well appreciate them.
http://www.lathes.co.uk/carstens/page6.html

Bill

[babielsParticipant @babiels]

Thanks for all the responses!

I should have stated that I don’t have access to a screwcutting lathe – I have a micro/baby lathe. The very best it could (possibly) manage is holding the die concentric to the shank, with the die being held in a Chuck and rotated by hand. But in reality I imagine I’ll be using a regular ol’ manual die holder.

I’ll try my luck with the Whitform die as suggested.

oh and I have sent details to Tony in the past, one day I’ll get round to sending him some good photos!

thanks

[old martParticipant @oldmart]

Just in case it is not generally known, the Vertex Posilock uses the same collets as the Osborn Titanic II in the smaller sizes up to 16mm.

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