Strange steel

[Roger B]

I have a bit of workshop time and thought I would have another go at a ER 25 collet chuck for the Hobbymat. I have a piece of scrap bin steel of a suitable size, but my previous attempts at getting a good surface finish were not successful.
As a first foray into replaceable tip tooling I bought a set of Proxxon tools (the thought, possibly misguided, being that they were designed for a small lathe not an industrial size machine).  This actually worked, but only at, for me, high speeds. The piece of steel is 60 mm diameter and at 500rpm 0.2 mm depth of cut I got a mirror finish. When I tried to face it the finish was okay on the outside but went rough in the middle. When it was cutting properly, without any lubricant the chips were coming off blue.

I finished the middle piece and bored a short 26 mm dia hole at 1000 rpm.

This piece of steel looks like stainless, shows no rust marks, and is magnetic, my DTI base will hold it firmly.

Any ideas how to work with this when I have to cut the thread for the collet closing nut? I cannot thread towards a blank end at 1000 rpm, I'm not sure if my HSS threading tool will cope and give an acceptable finish. My first thoughts were run the lathe backwards with the tool upside down so I cut towards an open end. My tips have a 55° angle so I can start the thread with them and then finish with the 60° HSS tool.

As a final question, the boring bar has flats on it, I assume to set the angle, but the tip slopes down. When I set the tool tip to centre height it appears to cut nicely but looks strange. Is this normal?

[Stuart]

Yes

It's normal it's to give you some clearance .what you are not seeing it the tip geometry with the chip breaker it is infact slightly positive rake



Stuart

[Roger B]

Yes

It's normal it's to give you some clearance .what you are not seeing it the tip geometry with the chip breaker it is infact slightly positive rake



Stuart

That's what I was wondering. I have often had problems with conventional round boring bars set in a clamp. If I set the top face at what appears to be a positive rake nothing happens or it simply gouges the job away. This is the first one I have had where the angle is pre defined.

[PStechPaul]

I've never seen chips or swarf quite so blue as that, but from what I learned in a brief search, blue chips are indicative of a lot of heat being transferred to the chips and not so much the tool. They are desirable for carbide tooling and indicate the cutting speed is high enough to make full use of the high temperature abilities of the carbide. Here is something I found:
http://www.mmsonline.com/articles/read-your-chips

Apparently stainless steel will usually have a straw brown color.

I think your facing operation may be better with a greater angle on the cutter, with the leading edge about perpendicular to the face, and the trailing edge just a few degrees off the face. Perhaps something like this:



or this:



Or this:



Not that these are ideal, but they worked for me. And the last two are cast iron.

[fumopuc]

.....

As a final question, the boring bar has flats on it, I assume to set the angle, but the tip slopes down. When I set the tool tip to centre height it appears to cut nicely but looks strange. Is this normal?

Hi Roger, yes, that is the correct position for this kind of tools.
If you like to improve your cutting results, I would re comment industrial tips. Since I have changed all my tips to Seco  (SECO SWITZERLAND could be your first contact)  I am able to cut SS or CI like butter. Also the ALX type for aluminum and brass is cutting like a sharp knife and not pressing. There is a big difference to all stuff offered bye all  the Hobby Shop suppliers.
If you like to understand  the code, than you will find in this catalog at the first pages a good explanation. You can download by a click in the catalog picture. The pictures are self ex planing.
http://www.xact-gmbh.de/product_info.php/hauptkatalog-hauptkatalog-p-699
Until now I have never used the tips of xact, only the milling cutters, but I will do it in the future.

[Ian S C]

Roger,  I wonder if you bit of steel might be a bit of car half shaft/ back axle, your description of the cut sounds like a bit of nitride steel, hard on the outside, and leaves a mirror finish, face the end and the finish is rough, except for the outer few mm.  I use it to make holders for carbide tips, its ridged and wears well.      Ian S C

[Roger B]

Thank you for the comments. This is a new bar end from a scrap bin. There is definitely a threshold when cutting with the tipped tools. As I got in to the 35mm diameter range I had to step the speed up to 1000 rpm again.

It seems to drill reasonably with low speeds and plenty of lubricant, the complete opposite to turning. I now have to decide how to cut the thread.

[dieselpilot]

Try to follow the inserts specifications for feed and speed to get the best results. As you found out this can be a challenge keeping the feeds and speed up. I wouldn't bother with the 55° insert for roughing the thread. Do try all of what you have for lubricant on the thread. Some type of oils will work better than others. I had some pretty hard steel than came with a toolbox purchase, that seems to cut just like you describe. My guess is some kind of prehardened chrome steel. I made a chuck backplate from a piece and had no trouble cutting the internal threads with an HSS bit.

Greg

[Roger B]

I had a look at some similar off cuts from the same source and one had M238 written on it in felt pen. A search suggests that this is a 'mould' steel similar to 1.2738 or P20. I couldn't, however, find any information on machining it.

[dieselpilot]

That's a nice piece of steel. I wonder if you have the softer ~300HB (30RC) or the harder ~360HB(50RC). You were at the lower end of the suggested surface speed and the chips were dark blue.

http://www.bohler-edelstahl.com/english/files/M238DE.pdf

Greg

[Roger B]

Greg, thank you for the link. If I have understood it correctly I would need to use my HSS tool at around 1/10 the speed I have been using the tipped tool. This would be around 100 rpm which is a sensible speed for screwcutting.

[Ramon Wilson]

A search suggests that this is a 'mould' steel similar to 1.2738 or P20.

Roger - that's the first reference I think I've see to 'P20' since I machined it in the early eighties 

I clearly remember screw cutting it to a fine tolerance on a regular basis - it is very tough indeed and we would struggle on the last cuts as it really does have a propensity to work harden. I machined quite a few tough basic steels over the time - none of the esoteric materials though - but apart from one other (I made the slide block on my fly-press from it!- that was really tough) I don't remember anything as tough as P20.

You will need very sharp tools for the final cuts when screwcutting - we used HSS - and the surface speed should be around 40 ft per min if not less.

Good luck with it

Ramon

[Roger B]

Good luck with it

Ramon

I must admit I am somewhat apprehensive about the screw cutting but will probably have a go this weekend. it is the Swiss national day on 1st Aug. so it is a long weekend 

[tangler]

For what it's worth, I always cut fine threads like this by hand using a handle in the lathe mandrel.  It makes the reversing for the next cut easier as well if you can't release the clasp nut.  I use a normal replaceable tip carbide threading tool and put on a cut of 5 thou (0.1mm) per traverse.  As I get close to the final depth I take finer cuts as I try to make the thread fit the nut.  Running a slip of hardwood down the thread improves the look of the thing.  It worked on the ER25 holder I made for my Worden grinder which was, I think, EN8 - it certainly wasn't free cutting.

HTH

Rod

[Roger B]

Thank you for the thoughts. I have a fixed leadscrew so I have to reverse. I don't have a handle for the mandrel yet, but I think that the slow speed attachment will give me enough control. I also think I should set the compound slide at 30° so I am only cutting on one face. I currently only have an HSS 60° threading tool.