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Left Hand Milling Cutter

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norm norton09/09/2020 14:53:15
186 forum posts
9 photos

I often have small work items (e.g. valve blanking caps) that I want to hold on the rotary table in order to machine a square or hex head on the top. These often have a thread on the other end and so easily screw into a threaded mandrel that I can grip in a collet chuck on the rotary table. They are short and offer little other surface for gripping.

valve cap.jpg

The problem is that if the mandrel is held vertically, and you touch the work with a conventional (clockwise) rotating tool, the work item will unscrew from the mandrel. Ahh, I thought, I need a left hand spiral milling cutter to be running anticlockwise. But there aren't any, why not? There are LH taps and drills. The only LH spiral milling cutters are designed to turn clockwise and the spiral forces the swarf and hence work item down on the table.

So far I have got round this by setting the rotary table vertically, the mandrel thus horizontally, and now a clockwise cutter drives the work into the mandrel. But the bulk of the rotary table gets in the way of the milling head.

The thread on the work is cut first on the lathe, then the item is parted off. I cannot see that there is another work sequence to let me cut the hex first on a longer piece of stock.

Any thoughts people? and why are there no LH cutters I can find? I shall have to make a simple one.

JasonB09/09/2020 15:01:21
22744 forum posts
2653 photos
1 articles

you can get LH cutters such as these EDIT OPPS, that's only the flutesblush

The alternative is to hold the work horizontally while still part of the parent bar so there is enough sticking out of your R/T, indexer or block and mull six flats behind the flange then part off.


Edited By JasonB on 09/09/2020 15:18:22

Howard Lewis09/09/2020 15:02:14
6104 forum posts
14 photos

IF you have a Rotary Table, Dividing Head or Stevenson Hexagon Collet block, you could mill the flats, in the appropriate place on the raw material, with a small diameter cutter, before transferring to the lathe to screwcut, face and part off.

In this way you can use normal RH cutters without unscrewing the yet to be cut thread.


Tony Pratt 109/09/2020 15:06:17
1961 forum posts
12 photos

When I had a paid job last year I couldn't find any cutters which rotated in the opposite direction to 'conventional' cutters not sure if they still exist unless custom made, Jason B's link goes to LH helix cutters but they rotate in a conventional direction.


Howard Lewis09/09/2020 15:10:26
6104 forum posts
14 photos

If the R T fouls the cutter / holder, is there enough raw material to centre drill, and extend clear of the RT to be supported by a Tailstock?

That is my method for cutting small gears (If you call 12 - 20T 20 DP gears small )


Tony Pratt 109/09/2020 15:23:59
1961 forum posts
12 photos do proper LH cutters. USA supplier though


Tony Pratt 109/09/2020 15:29:32
1961 forum posts
12 photos
Posted by Tony Pratt 1 on 09/09/2020 15:23:59: do proper LH cutters. USA supplier though


LH router bits on Ebay.


DC31k09/09/2020 15:31:18
686 forum posts
2 photos

Would it be possible to do something to the mandrel to increase its holding force?

There are lots of solutions to stop Myford chucks unscrewing when the lathe is used in reverse.

Schaublin W-series spindles use a split clamp to secure the threaded-on chuck.

Look at how the track rod ends are clamped on your car.

Martin Kyte09/09/2020 15:57:25
2751 forum posts
48 photos

1. Mount the rotary table vertically and flycut with a cutter of 90 deg included angle. More clearance.

2. Knock up a mandrel with the appropriate thread and loctite them in. Warm up again to remove.

3. Split threaded collet in a chuck mounted on the rotary table.

regards Martin

not done it yet09/09/2020 16:05:49
6808 forum posts
20 photos

Horizontal milling cutters can be turned to cut the other way. So can slitting saws.

Neil Wyatt09/09/2020 16:36:48
19032 forum posts
734 photos
80 articles

Try making your own milling cutters.

If you are like me a proportion will come out left handed at random...

But seriously, as you are working in brass a simple 'slot cutter' style made by grinding a silver steel bar down to 2- 3mm thick and relieving behind the cutting edges will do the job (after hardening of course).

Here's one - to an LBSC design!


slotting cutter.jpg

John Haine09/09/2020 16:38:57
4671 forum posts
273 photos

Make your threaded mandrel long enough so you can have a smaller screw entering a threaded hole from the back. Actually you could simply have your threaded hole right through. Then have another screw that enters the mandrel from behind that you can tighten against the item you want to mill, so it is acting as a "locknut".

Or, make the mandrel from hex bar so you can hold it horizontally in the machine vice, indexing round by lust loosening the vice after cutting each face.

John Haine09/09/2020 17:09:05
4671 forum posts
273 photos

Oops! That should read "just loosening"....

Dave Halford09/09/2020 17:37:04
2047 forum posts
23 photos
Posted by not done it yet on 09/09/2020 16:05:49:

Horizontal milling cutters can be turned to cut the other way. So can slitting saws.

Thats when you need a key otherwise the nut comes undone

Martin Connelly09/09/2020 18:01:54
2137 forum posts
222 photos

Fly cutter running backwards with a suitably ground HSS tool in it.

Martin C

Paul Kemp09/09/2020 18:44:40
712 forum posts
27 photos

Gang mill with two horizontal cutters appropriately spaced. Added bonus you can do the job in half the time


roy entwistle09/09/2020 19:15:12
1525 forum posts

If you have the cap screwed into a hole in a bar bolted to the table and go round it clockwise with a normal cutter it shouldn't unscrew, it should tighten up

norm norton09/09/2020 21:43:52
186 forum posts
9 photos

Pleased I have caused a little bit of enjoyable thought Some interesting feedback, thank you.

Responses in reverse order.

-touch a clockwise rotating cutter on the edge of a round object and that object will try and rotate counter-clockwise, i.e unscrew upward.

- Fly cutters by definition have a moderate radius and this means a thumping wallop each rotation.

- Putting a small piece of work in a Stevenson hex ER block, and rotating that in a chuck manually, produces errors of at least 0.010". Been there, tried it, a mess. I am looking for 0.0001" in the vertical otherwise that small hex I show in my photo has all manner of irregularities in the horizontal land around it.

-The above is why working with the rotary table horizontally is better by keeping the cutting tool at a fixed depth of cut as it runs in the X or Y past each of the six faces in turn. Hence a perfect face around the hex sides.

- A small, 6mm, three flute carbide cutter is the ideal cutting tool for the brass of this size. I just wish the Chinese made left hand cutting ones!

- I like Neil's suggestion and it is what I thought I would do. Make a 6mm dia cutter from silver steel, mill cut a pair of teeth with rake and relief, harden to a straw temper, then diamond hone the tips

roy entwistle09/09/2020 22:03:08
1525 forum posts

Norm forget my post above. It could be done but I shouldn't really recommend it.


norm norton09/09/2020 22:06:59
186 forum posts
9 photos

They do exist, found these from America, bit too expensive to import, so might keep looking. 2AAAOSwNhpa55Zd">link



Edited By norm norton on 09/09/2020 22:08:06

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