indexible internal thread cutting tool

I have a query about an indexable internal threading tool which uses either a 55 degree or 60 degree insert as required.

The tool is one of these

The query is this, when mounted in the tool post the tool and insert sits at an angle of tip down giving a very negative rake angle. I could just live with this if I got a good result using it, but I don't.

I recently cut a 1.25 metric pitch thread using a new 60 degree insert, material was a steel hydraulic tube which already had one end threaded with the thread I wished to cut. The finish on the one I cut was pretty awful feeling very rough and having burrs on every crest. Spindle speed was 240 rpm and results was the same with or without suds. I checked that the tool was at 90 degrees to the work piece and tried various speeds, all giving the same result, am I right to suspect the tool?

I've cut a fair number of internal threads in steel, cast iron and brass with a similar tool, 3/8" BSP in this case:

I've not had a problem with finish. I'd make the following observations:

• The insert holder came from a professional tool stockist - Cutwel I think?
• Likewise the inserts came from Cutwel
• Is the insert full or partial form? If full form it shouldn't be leaving burrs on the crests at full thread depth
• I recall the rake angle being zero or slightly positive
• Is the insert clearing the work? If the holder is at a peculiar angle the insert could be rubbing.
• Have you tried cutting with the same set up in a more forgiving material?

Andrew

Edited By Andrew Johnston on 19/05/2019 21:48:59

You probably need to play with the tool height a bit to find the heigh for the best finish.

The reason for the tip down angle is to improve the clearance on smaller thread diameters.

Thanks for the quick response Andrew and Mark. The tool and insert was from RDG, in the photograph of the tool used on the suppliers web site it looks the same as yours Andrew, however when mounted it is angled (rotated) so that the head of the tool is about 30 degrees lower than a horizontal position (difficult to describe). The tool is in a fair sized bore and does not apear to be rubbing or fouling anywhere. A good suggestion to try a different material, I will give that go. I will also check if it is full form (I think so).

Mark I thought clearance might be the reason for it but I also thought the insert would have been formed so that the tip geometry had positive (conventional rake).

Indexable inserts were not around when I learned to do this type of thing so I wouldn't be surprised if the poor result is down to poor technique with the tool, I will try small adjustments to tool height to see if that helps.

Once again many thanks to both of you.

Edited By Ian Skeldon 2 on 19/05/2019 22:08:27

Are you using the manufacturer's recommended speeds for the tool, or if not provided those from a similar tool from a big player?

Looking at other inserts on the site I didn't see any part or full form tips, no doubt the insert you are using is a std 60 deg angle without crest forming, hence the sharp crests to the thread cut.

Emgee

Edited By Emgee on 19/05/2019 22:20:39

Hi Nick,

I am afraid that the information is not available for that insert from that supplier. I suspect that the optimal cutting speed for the insert would make internal threading very interesting, although I will see what other suppliers are stating as the best range of cutting speeds for such a tool and give it a go

I am going to see if I have a piece of HSS tool lying around, just in case

I bought an insert-tip threading tool to cut an internal thread on the lathe, and used it for thread milling.

It was the usual form with a cylindrical shank, with a flat on top and bottom. After careful measurement I found that it seemed to be designed to work best with the shank axis on centre-line. When I fitted it in a collet to match the shank in the CNC mill, it cut very easily leaving a good finish.

If your tool is the same it suggests that the best way to mount it is with the bottom flat on the base of a tool holder, to orient the tip, and the height set so that the shank axis is at lathe centre height - leave the tool tip to take care of itself.

Hi Emgee,

I think your right, the crest does look and feel sharp. I might have to look at the same supplier that Andrew uses. What is that saying about buying cheap

Hi John,

I will have a look at that, currently I have set it the conventional way so that the tip is at the centre of the tube which could well be part of the problem. Sadly the tool holder and insert came with no technical data.

Ian, be aware that you need the designated thread size for each insert as the crest form varies from the small to larger sizes, best to just buy the sizes as needed and you soon build up a number of common sizes, same info goes for different TPI inserts.

Emgee

I use exactly the same tool from RDG. Generally I get good results, usually winding the spindle by hand. Tougher materials like silver steel do tend to have a burred up finish which needs a bit of work afterwards to clear up the crests with a file. I suspect it is your hydraulic tube that is causing the problem. As has been suggested, you may need a rather more frightening speed to get a good finish straight from the tool. Perhaps the in to out reverse method as advocated by Jason may be the answer.

Rod

Thanks Emgee, I will bare that in mind, it does look a bit on the small side for the job I am doing.

Hi Rod,

Yeah I think the tube is made of some well treated steel, the cutter does cut it ok, just that the finish is awful.

I will give it a go over the next few evenings and let you all know how I get on.

Did you get the tip with the tool? There is two types of insert. One for internal work and one for external work. They look identical at a glance, it's very possible you have the wrong type.