How can I tell if a carbide tip is worn?

I'm in the process of boring out a piece of 25mm x 100mm diameter steel (en3b) to an internal diameter of about 60mms part of making a fixed steady.

I've a tipped boring bar which has manfully chewed through the inside 30/40 iameter or so without any dramas, but it seems to be struggling a bit.

I've been running the lathe at 400 (inside diameter) down to 300 rpm (current speed - 30 mm) and taking off a chunky .5 mm each time. Feed is about .5 mm / sec.

The chips are coming out hot (ow - need to get an overall with sleeves!, but the tool is staying fairly cool. I'm using paraffin as lubricant / coolant.

At the current diameter, I feel there's something not quite right. Is the tip getting blunt? Should I increase the speed, decrease the speed? Start taking a slower/finer cut?

Also - it's rather tedious doing this by hand, but I'm not sure what gearing I should put in to get a power feed - the lathe (SPG0618) only has threading gearing on it. Any ideas on that?

Iain

Use a watchmaker/jeweller's loupe or a microscope to examine the tip. That will soon tell you if it is blunt. Chipping or ridges along the tip will indicate wear, and a build up of material behind the cutting edge will also produce a rotten finish and a poor cutting action. Normal HSS cutting speed would be about 30M/s, so a carbide tip will work best at 2 to 3 times that speed. Cut depth will vary according to the design of the tip, but 0.2 is fine, and 0.5mm might be ok, but might also be a bit on the deep side. If you took twice as many cuts at 0.25mm depth you might find the tip liked it better, lasted longer, and gave you a better finish. You need to experiment a bit, I think. There is a sweet spot where the combination of speed, feed and depth of cut wqork well (a small range, actually).

Carbide tips don't last forever, though, so I would examine it closely first.

Marcus

Hi Marcus,
Do you really mean 30 metres per second. I think 30 meters per minute is about right when turning mild steel using an HSS tool. I would not expect a carbide tipped tool to need sharpening (Or tip replaced if it is a replaceable tip.) after so little work. If the tool is one from a set supplied with the lathe it may be very poor quality.

Les.

If you are sure that the cutter is set correctly especially not bending to under centre when boring the best way of telling is that the finish wont be so good, You might see wear on any coating with a loupe when this happens. I'm assuming you can see chips etc.

The problem with boring and the tip going under centre is that clearance is reduced and the tool can rub rather than just cut. If it does that because it bends things get worse - deeper cut so more bend and deeper cut etc.

Can't help on gearing for power feed. The manual for the lathe or a similar one is probably your best bet.

It doesn't surprise me that the chips are that hot at 400 rpm with a bore of that size. Up to an inch or so they should be relatively cool. 2" probably rather hot.

John

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Les, you are of course correct. Mea culpa. 30M/min is more realistic, and what I thought I was typing....

So; for a 60mm diameter and an HSS tool the speed should be around 160rpm, and for carbide perhaps 300 to 500rpm. And hot chips.

Marcus

EN3B is 'orrid stuff and cuts like toffee. With carbide inserts you need to run fast to get a good finish. See the first post in this thread:

**LINK**

Assuming that we're talking insert carbide tooling they can be quite worn and still cut. The most noticable effect is decreasing quality of finish. It will be fairly obvious if the tip is worn, as it will have a different geometry to an unused edge, and may be shiny where the coating has worn away. I'll try taking some pictures later.

Depending upon the material, carbide inserts need a minimum depth of cut to obtain a decent finish. A good rule of thumb is a DOC at least equal to the nose radius. Unfortunately low carbon steels are among the materials that are most finicky when it comes to DOC versus surface finish.

Depth of cut has the least effect on tool life, if you think about it logically. Say you double the depth of cut so you are removing double the material, but you are not wearing the tool at double the rate, as you are using double the length of cutting edge. So to a first appoximation the wear rate will be the same. If the lathe doesn't have enough power then the feedrate and/or spindle speed can be reduced.

Taking lots of shallow cuts simply wears the same small part of the tool. Increase the depth of cut and decrease spindle speed and feedrate if required to maximise material removal versus tool life.

Andrew

A mini lathe isn't as powerful as a Harrison Andrew and EN3B is a pretty variable metal as this links suggests

**LINK**

The ability of a lathe to take very light cuts and give a good finish is mostly down to rigidity, lack of vibration and the state and type of head stock bearing arrangement.

Interesting tests though and it does pay to take cuts as deeply as possible to minimise wear and hit sizes bang on with decent sized cuts as most lathes have some play somewhere and the cut will remove that at some depth leaving a decent finish.

With a metal like that this OP might get better results finish wise with a good sharp hss tool with 15 degrees all round, side clearance and back rake. Maybe more rather than less. I've turned loads of the stuff but some is terrible. At smaller diameters the machines were incapable of reach the cutting speed you suggest.

John

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The recommended surface speed for carbide inserts with mild steel seems to be in the region of 200+ metres per min. As Andrew points out, you also need a decent feed rate.

Together, my conclusion from this is that you need to use power feed (to get a consistent and defined feed rate) - and with our machinery we generally require spindle speeds close to or at maximum capability. Furthermore, as high speeds and feeds mean things moving fairly rapidly, I almost always use the carriage or cross slide stop to control the range of movement.

The other benefit of higher speeds is the chipbreaker operation. The last thing you want are long razor sharp ribbons flying off the work. Many of the chipbreakers only seem to work properly at higher speeds. The limiting factor in all of this is often the motor power required. I keep an eye on the motor power (on the VFD display) to avoid exceeding the allowable motor phase current and bombing out.

The main exception to this is the use of ground (**GT) inserts for light cuts in steel, where the extremely fine / sharp cutting edge seems to work fine with light cuts.

Murray

Edited By Muzzer on 29/12/2015 20:32:29

Thanks for all the advice.

I went and set up the power feed (20, 80, 20, 80). It seemed slow - and MUCH slower than I was pushing it manually.

I kept the .5mm cut and speed up around 300 - 400 rpm, even at the 60mm diameter. It's worked really well (for an amateur).

I feel I could have gone quite a bit faster, but was being attacked by a swarm of hot chips. For some reason the half nuts would not stay engaged so I had to keep my finger on it - which was brutally attacked.

The other thing I've learned is I need a proper coat - an apron isn't enough to keep the chips at bay!

I'll post a photo tomorrow,

Iain

Not the best pictures (Lord Snowdon needn't worry) but here is an unused edge:

And on t'other side of the insert a worn edge, although it was still cutting:

Another way to detect worn inserts, or cutters, is to look at the power being used, but I don't have that facility. Other than listening for the motor to start protesting!

Andrew

For large , short bores like the one you are doing Iain, I usually attack them with an external cutting tool to take out as much as possible, the bore ends up quite a steep taper, then I go for the boring bar. The carbide tool works OK dry. Paraffin/Kerosene is OK for aluminium, steel use soluble cutting fluid if you must, I find the main use of cutting oil on steel is to make the chips stick to your skin while they are still red hot! My preferred tool is HSS.

Ian S C

Chip breakers are fine but when they cause bits to fly it's only a matter of time before one hits your face. The chips can cut and burn at the same time. Not a pleasant experience I have had it happen.

I also had a lathe that tended to cause all tools to behave like they had a chip breaker on them until it was adjusted and had the head stock bearings replaced - ML7 and no further adjustment on them.

The little bits also get all over the place and are hard to clean up. They cut into and stick to the soles of shoes and people who walk about in stocking feet at times had best watch out. Then there are pets.

Swarf - that's what swarf hooks are for, not hands and fingers.

In my view this one is a bit manic and OTT but

The other aspect is that the tip geometry / angle it's used at will always tend to send swarf in a direction that doesn't cause problems.

John

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Hmm. If I have my sums right I was turning this at about 75m/min. Which is under half the recommended speed. I have yet to rise much above 500rpm in my lathing on the grounds that the slower you muck things up the more chance there is of turning the machine off in time...

This is what I ended up with.

From the links provided, I guess I should be looking at a polished finish if I'd done everything just right (including selecting the correct steel!), but (as a newbie) I'm delighted with the results.

In passing, my first inclination was to cut a deep channel in the face of the bar (both faces, meet in the middle) so I could save the inside steel. I ground something like a parting off tool, but with a steepish chamfer on the outside for clearance. This really didn't work (wouldn't cut in, loads of chatter) and I couldn't find any references to cutting a channel on the end of a bar. Could this have worked and how should I have approached it?

Iain

I have a broach cutter (trepanning cutter) with 55mm diameter. I mount these cutters in an ER32 collet mounted in the tailstock. The standard length is good for 25mm thick material but working from both sides increases this. I would use this to remove the core then finish with a substantial boring bar. For spindle speed I use a drilling feed and speed chart. When using hss tooling the drill speed for a 60mm drill is about the same as the spindle speed for 60mm turning, double for carbide. This is a starting point and can be tweaked if required. I have a 3 phase vfd so setting a suitable spindle speed is a case of nearest gearing then speed up or down to get best results. The feed per rev is also relevant to turning, the larger the diameter the larger the feed per rev should be. This keeps chip thickness more constant and pressure on the cutting tool similar at different diameters.

Martin

Iain: Looks pretty good to me. While EN3B can be a PITA to machine it has some advantages. It is widely available in hot rolled sections, which have the advantage of not distorting when being machined and being cheaper than cold drawn. It also seems that EN3B is less prone to rust than EN1A?

Andrew