How much difference can tooling make on a small, low power lathe?

[Evan]

It can make a great deal of difference. Here is an example.

It is frequently said that negative rake carbide inserts will produce the best finish. In the same breath it is also said that they require a lot more power and rigidity to use. Both statements are more true than false but not entirely accurate. Negative rake is not the cure-all for finish problems and negative rake cutters do not necessarily require a large, rigid and powerful machine.

I was turning some schedule 40 welded seam pipe of dubious metallic content which was imported from India, that bastion of quality control and unmatched consistency of product.

I normally use hand sharpened solid carbide cutters for this, especially the first pass to remove scale. I was not seeing a good enough finish although it was passable for the purpose it wasn't quite good enough for my eye.

Recently I traded an item to Rockrat in exchange for some old stock cermet (CERamic-METallic) coated carbide inserts. They are as plain as they come, simple radiused squares with no chip breaker, no rake and symmetrical top and bottom.



They are intended to run at a negative angle in respect of the stock surface which provides the requires clearance under the cutting edge.

I made up a tool holder from a square bar of cast iron by milling a 45 degree angled step on one end to bring the centre height on line. It is also canted to provide the correct rake angle and exposes two of the corners for use.



I applied this to the job running the lathe at 800 rpm and taking light cuts of no more than .020" and less for the finish pass. The insert cut a near mirror finish with none of the usual tearing, skipping and other usual grief seen when turning mild steel. No lubricant or coolant was used.






As can be seen, even if you have a light import machine there is much you can do to improve the quality of your work by simply spending the money to buy quality tooling. These inserts have 8 usuable corners and the one I used turned at least three passes of nearly a foot by 1.5" diameter. If used for only the finishing pass on 1018 steel that equates to turning at the very least a total of 24 feet of pipe with a realistic expectation of much longer life than that.

[gfphoto]

Evan,

Very informative post for me as I've just found a bunch of inserts of that shape and had been wondering how to use them and If I could make a holder. I think mine are plain steel or maybe carbide and without holes.

I want to make sure I understand "negative rake": is that when the cutting corner of the insert is lower than the opposite end?

Is the tool height set so that the insert contacts the work at the centerline?

Also, in the fourth photo, what it the device opposite the tool holder? Is that a kind of follower rest? And what are the Vise-Grips for?

Thanks,

Gary

[Evan]

The vice grips are clamping the insert to a carbide tool as a test to see if I should bother to make a tool holder.

The large cone shaped object in the tailstock is a live pipe center for large hollow work.

Rake angles:



The insert is set to cut at the centre line.

[dp]

The vise grips look like part of version 1.0 of the cutter holder The finished tool shown later isn't in that picture.

[gnm109]

Very helpful information. I have only just begun to use carbide inserts and as shown in your pictures they can give a very nice finish indeed when used to advantage.

I note also that you are running at 800 rpm. Apparently carbide comes into its own at higher speeds.

Thanks!

[rockrat]

Its good to see your finding the inserts useful. Nice post Evan and a good point about understanding tooling.

I had ran back through my machining book from college some time ago and re-read many different things. Its amazing at what one will forget if it is not used on a regular basis.

Its also easy to just make it through something because one has the horsepower to do it. Combining all of the above requires the operator make good judgements on everything from tooling to speeds/feeds/hp to support of the work and fixturing.

Nice finish.
Cheers!
rock~

[Evan]

I forgot to mention the common name for the pipe center. It is frequently called a bullnose center.

[lakeside53]

Great post Evan. I have accumulated a lot of quality negative rake tooling and inserts over the past year, and had just left them to one side for a future "bigger lathe". Just the negative rake boring bars - at least 12....

Last weekend I was having all sorts of problems boring some 4340 (swath and chips catching in the tooling, tool rubbing the bore... ). I changed to the only tool that would fit well- a negative rake kennematal bar, and hey... it worked.

I have a 1hp Emco V10P. Not particularly "rigid", but it did work well - I ended with a nice bore finish and no taper.

I thought I was just lucky.. but with your post...I'll give some of my other tooling and some high quality iron pipe a try!

[Bill in Ky]

Very informative post, thanks Evan.

[Boucher]

Evan
1. Why did you use cast iron?
2. If you were starting from scratch and had to buy material to build tool holders. What would be your preferences?
3. From the photo it looks like the corner of the insert is unsupported for approximately one thickness. Are there any rules of thumb re insert support? For light cuts?

That really is impressive surface finish.

This is a very good and useful post that helps us wantabe HSM. Thank you for shareing.

[Evan]

I used cast iron because of it's damping qualities and because I had a piece exactly the right size on hand. It has much less tendency to ring than steel.

Perhaps the insert could be better supported but in this application it won't see the large depth of cut that may be used on a larger machine.

The best material for a tool holder is solid carbide because it is about three times stiffer than steel. For this application steel will work just fine. However, I also designed the holder to use as a boring bar and in that use the cast iron may work a bit better than steel.

[lazlo]

Rake angles:

Right, but that picture is for hand-ground lathe tools. Negative rake inserts, including the Cermet inserts you're using, don't slope off like that -- they have 90° sides.

The reason they're negative rake inserts is because the pocket on the toolholder is cut to tip the edge down, to form the negative rake angle.
You mention that in your original post, but that picture is misleading

They are intended to run at a negative angle in respect of the stock surface which provides the requires clearance under the cutting edge.

To further confuse matters, positive rake inserts like TPG's are shaped like the first picture, which is entitled "No Rake."

[gfphoto]

The vice grips are clamping the insert to a carbide tool as a test to see if I should bother to make a tool holder.

The large cone shaped object in the tailstock is a live pipe center for large hollow work.

Rake angles:



The insert is set to cut at the centre line.

Thanks Evan - very clear. Also interesting about the material for the tool holder.

Sorry if it wasn't clear, this is the device I was asking about:


Thanks again,
Gary

[Evan]

The rake refers to the top surface of the tool only and the angle it has relative to the work (when correctly applied).

To further confuse matters, positive rake inserts like TPG's are shaped like the first picture, which is entitled "No Rake."

They may be but they aren't a true positive rake insert. A TPG insert is "T" for triangular, "P"= has an 11 degree relief angle below the cutting edge, "G"= has a chip breaker and hole for mounting. If the insert is really positive rake the third or fourth letter will be either K, P, Q, S, Y or Z. The third letter may be left out as it is a tolerance spec code and replaced by the fourth which specifies the rake and chipbreaker style.

http://www.pgstools.com/servlet/the-...ion/Page#Cross

[Evan]

Gary,

That is my electric power crossfeed drive. I have variable speed reversible power drives on both axes.