Thread cutting with carbide inserts

[Anonymous]

In general the standard CCMT inserts need to be run fast and with sensible DOC and feedrate to get a good finish. However, that is by no means a hard and fast rule. In some materials, for instance 303 stainless steel, you can get a good finish at low speeds.

Threading inserts are precision moulded and the edges are sharp and well defined compared to the CCMT inserts. As a result it is perfectly possible to run them at lowish speeds and still get a good finish.

Out of idle curiosity I had a go at screwcutting a thread this evening on a length of EN1A. I chose a full form insert for 16tpi Whitworth on a 1" diameter bar, screwcutting at 260rpm. For 16tpi the thread depth is 40 thou, so 80 thou on the diameter, as my cross slide is graduated in diameter rather than DOC. I did a quick run at 10 thou off to check I'd got the leadscrew selectors correct. And then cuts at 30, 50 and 70 thou followed by a clean up pass at 70 thou. Followed by a cut at 75 and 80 thou, and another clean up pass at 80 thou. Finally I did a cut at 82 thou. Previous experience has shown that I need to cut Whitworth threads a thou or two deeper than the theory in order to get a nice fit. So nine passes in total. One could reduce that, and I'd do so if I was cutting a lot of threads. Feed was straight in, can't see the point in setting the top slide at an angle with threading inserts. They cut fine on both edges.

Andrew

[Howard LewisParticipant @howardlewis46836]

Nige,

If you mount the tool on the back, presumably on a back toolpost, needless to say, it needs to be "right way up" for running in Reverse, but inverted to cut in Forward.

Cutting away from a shoulder avoids broken tools. DON'T ask!

Obviously, based on Andrew's DOC, (even as total Diameter changes) I have been MUCH too cautious!

But my Martin Cleeve retractable toolholder is fairly flexible.

Howard

[NigeParticipant @nige81730]

With the SC4 I had noticed that the lathe comes to a stop quite quickly when the stop button is pushed. I assume that the control gear for the brushless motor incorporates some sort of braking effect. This evening I have been experimenting with the SC4 with the half nuts engaged, as it would be when thread cutting, to see how far the saddle travels when the stop button is pressed.

I mounted a round bar in the 3 jaw with about 4 inches out of the chuck and put a thin black line near the end. I ran the lathe at 400 RPM and as the lathe tool came to the line I hit the stop switch, NOT the Emergency Stop switch, and measured how far past the line the tool travelled. I tried this more than 20 times and the tool overran the line no more than 3mm, usually barely 2mm. I would expect that if the tool was actually cutting then the overrun would be less because of the 'drag' of the cut. As you might expect the overrun was greater at higher speeds

[John CParticipant @johnc47954]

Posted by Neil Lickfold on 29/07/2018 12:38:55:

Posted by John C on 29/07/2018 10:00:56:

Nick L – please could you give details of your microswitch/ VFD set up? It sounds interesting.

John

So I have an album here, https://www.model-engineer.co.uk/albums/member_album.asp?a=48822

I have the microswitch on S1 one connection. So when the switch (normal closed) is opened, it stops only the forward direction of the motor. Reverse direction still works . I set the VFD to stop in about 0.3 seconds I think, maybe 0.1 seconds. When the switch opens, the motor stops, then I switch to reverse, wind back.and can take another cut. The microswitch is set on a movable stop to set it in the Z distance.

Will add more to the album when I thread cut next.

Neil

Edited By Neil Lickfold on 29/07/2018 12:47:03

Thanks Nick – will keep watching! John

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