Threads on milling cutters

Some milling cutters have thread on their shanks which is meant for special holder.

I need to extend such 6mm cutter to make 5 inch long mill suitable for drilling screw socket in awkward place.

Its threaded part of shank has diameter 5.65mm and the thread is 20 TPI,

So it is not a standard thread I am used to.

I want to drill and thread extension bar and then drill again and ream to 6mm, so only bottom of hole is threaded. This should prevent mill rotation during drilling this socket.

Which tap is best to use?

Will 1/4" Whitworth 55*/20TPI tap do or are there some special taps for this purpose?

Specials as far as I know. The older 1/4 x 20 Whit of imperial cutters as carried over to metric but they only kept the pitch so your thread is M6 x 20tpi or M6 x 1.27 in new money

Probably easier to get a 6mm FC-3 cutter and make a simple extension that has a grub screw to bear on the flat.

Or if "screw sockets" are counterbores for cap heads then a piloted counterbore would be the best bet as an extended milling cutter may jump about a bit..

The threads on cutters intended for Clarkson style holders are Whitworth form and 20 tpi, irrespective of whether the cutter is imperial or metric. I was under the impression that the shanks of such cutters were of a limited range of sizes. My smallest holders are 1/4" and 6mm and have 1/4" and 6mm by 20 tpi threads respectively.

I'm not aware of special taps for any of these threads (other than 1/4" BSW) and 5.65mm seems non-standard. Not much help I'm afraid.

Andrew

Would an ER11 or ER16 straight shank collet chuck fit into the available space Martin?

I have ER16 collet shanks (that I hold in an ER32 chuck) that can be used to provide extra reach when required.

Regards,

IanT

The extension bar has to be of 8mm diameter due to particular awkwardness of place to be drilled, so the grub screw is out of question. It will be made out of 8mm h6 bar for linear bearings, hardened to depth about 0.7mm.

Regarding counterbore - have to find if suitable size (4/6mm) is available.

Another idea - just make a reamed hole 6mm @ h6 and glue in shank of the mill with metal retainer.

Apply coolant during milling.

Edited By Martin Dowing on 04/12/2020 13:37:57

Drill extension bar 6mm. Loctite. Let it cure. Use it.

Just drill and if possible ream the extension for a close fit on the shank and then use Loctite to fix it. If you need to get it apart just boil hard in water for a few minutes, of gently heat with a blowtorch.

As Jason implies, good luck trying to drill a counterbore with an unsupported milling cutter.

@DC31k & John Haine,

Thanks,

Just got the same idea while you was writing your posts.

Hence my edit above.

It should work.

I can just start initial 1mm with 6mm drill, then finish job with a mill.

Some of the older textbooks describe grinding a drill bit with a flat end as a "bottoming drill".

Could be worth trying with a long series drill, unless you need carbide for the metal you are working with.

45HRc, so HSS-Co coated with TiCN will do. Normal HSS also works but getting blunt fast.

Don't want to extend carbide mill as some loss of rigidity may result in damaging it.

If you are counterboring, it doesn't have to be flat bottomed, you could just use an ordinary drill?

If worst come to worst I may have to settle with that.

Although the thread pitch on all the threaded cutters is 20tpi, the size of the thread is the cutter shank size. That 6mm cutter will have a 6mm X 20tpi thread. I very much doubt if the taps would be available easily. All of the threaded shank cutters up to 16mm have the 20tpi thread, I cannot say whether the larger ones have the same pitch as I do not have any to check.

I would Loctite the 6mm into an extention to do the milling. Make the extention as large diameter as possible for stiffness and take light cuts.

@old mart,

Mill shank is 6mm but threaded part is 5.65mm @ 20 TPI

Have been measured.

It will be used for counterboring, not for regular milling.

Yes, they are. I've just double checked 25mm/1" to 50mm cutters and they're all 20 tpi Whitworth.

Andrew

So well undersized 6 mm?

Tony

Just measured a 6mm shank Dormer and that's a similar 5.65mm over the crests of the threads.

1/4" shank Dormer is also well under at 0.236" over the threads

I see that the likes of Tracy do M6 x 1.25mm taps which would be close to the1.27mm cutter pitch and if as slack as they measure the difference in angles may not be a problem over a short length of thread

Measuring threads usually gives smaller than nominal results.

It is worth reading the datasheet for the proposed 'glue' before you begin.

To work best, it needs some clearance and surface roughness to give a film thickness and a good bond. Hence a reamed hole might not be the best and a rub of the cutter shank with a coarse diamond stone will not harm anything.

Why not use a long series drill for most of the hole depth, then finish it with a D bit which can easily be made.

Phil