Boring bar with inserts shape choice??

Hi,

I will in all probability have to machine up a spindle adapter for a DC motor. The spindle is 10mm (yet to arrive). Having never done any boring I have some advice from Jason in as much drill a pilot hole say 8mm and mild steel will be OK.

For my clock work to date I am persuaded to use quality carbide cutters but for general machining such as this it may be time to look into other cutters. The sherline tool post is as standard 1/4 inch but I also have the 3/8 and 5/16 inch variants. For arguments sake lets say I wish to bore a 10mm hole to a depth of 5/4 inch.

So I put the stock into the 3 jaw and centre drill then pilot drill to 8mm. I then align the boring arm cutter to centre and oil up. OK I get this but the choice of inserts gets me curious surely a hole is a hole so why do boring arm indexable cutters come as CCMT or TCMT (and I expect others). What difference will the triangular cutter make over the rhombic.

As I intend using mild steel are there any makes and particular indexable cutters anyone can recommend. 6mm shanks seem to be most useable in the Sherline and also a 6mm shank with a flat as I know of no 6mm Sherline 6mm round tool post.

Chris

The insert shape used is determined to some extent by the reach and whether the hole is blind or thru. It's a moot point anyway; I doubt you'll find a common boring bar and insert combination that will bore 8mm out to 10mm. Of course there are specialised microbore insert tools available, but you'll need to sit down before looking at prices. A small HSS boring bar will be just fine.

Andrew

OK I can add now the hole will be blind.

Chris

On trick is to use a slot drill of smaller diameter than the hole as a boring bar. Make a holder for the cutter by popping a block in the toolpost and drilling from the headstock to make sure the hole is on-centre, and provide a grub screw to clamp the cutter in place. Align the cutter so the front corner is at centre height. You could for example use a 6mm cutter in a 9 mm diameter pilot hole.

I have this one from Glanze that will fit without problem it will actually bore an 8mm finished size hole if needed, it uses a small form of the CCMT shape

Though you could make life easier and drill to 9mm, say 6mm first then open up to 9mm. This will allow you to use a common CCMT 0604** insert in combination with a 6mm shank tool such as this one. You can with care use this to bore starting with an 8mm hole but not for a first timer.

Both will work fine on through and blind holes, to help give clearance the tips are held at an angle to set the correct height you want to pack them so that the corner doing the cutting is on ctr height

Edited By JasonB on 12/11/2019 16:37:54

+1 for John Haine's suggestion. TBH if the slot drill (or end mill) is mounted close to centre height it can be used to open the pilot hole (say 5mm for a 6mm cutter) to a tad over 6mm. The tool can then be used to finish bore the hole from a size only slightly more than the cutter diameter to a size many times more diameter.

Whilst a milling cutter will never be a rigid as a boring bar, for light cuts it will produce quite deep holes especially long series type. Bear in mind that the cutting end of an end mill will not produce a dead flat bottom face, also the end mill needs to be very slightly askew relative to the lathe axis so that there is clearance for the flutes relative to the corner that does the cutting.

Ian P

Thanks a lot for posts so far. I will spend a while thinking over the suggestions made.

Chris

OK I can add now the hole will be blind.

One can often through-bore and plug.

Chris is good at asking simple questions that provoke complicated answers!

Inserts aren't made in the huge variety of shapes and sizes they are to suit hobby users. Rather they are made in a wide choice of configurations to allow production to be optimised. Getting tooling right is much more critical when manufacturing thousands to a price, and considerable effort is often put into setting up to maximise profit. Typical factors leading to an optimum choice of insert material, size, geometry, angle and rake include:

• The material being cut, for example Aluminium cuts better with a sharp insert, mild-steel prefers a blunt edge.
• The characteristics of the machine available - no need for hefty inserts on a weedy machines, no sense in pussy-footing with a big one.
• Chip control - in production it's better for cutters to consistently produce small chips of metal rather than long spirals of sharp swarf because managing swarf is a serious problem when a machine makes a ton or two of it per day.
• Power consumption. Generally the more brutal the cut, the less power is needed remove a given amount of metal. But the insert has to be sufficiently strong and heat resistant to cope with the stress. Industrial cutting with carbide considers 850C to be "moderate heat", and it's not unusual to go as high as 1500C. This effects insert choice, because reducing a £1M electricity bill by 5% is well worth doing.
• Coolant. Some inserts crack or otherwise react badly to particular cooling methods, and it's often preferable to machine dry.
• Type of cut. Facing, lengthwise, chamfering, profiling, and whether or not a shoulder or other tight operation is needed all influence insert choice. Inserts shaped for best finish tend not to last as long as inserts designed for roughing out, and the latter will also remove metal faster for less power. Round nosed cutters are good for extended tool-life and deep cuts, but are more likely to chatter than small nosed cutters, which are also capable of better finish. Positive rake tools reduce cutting forces (useful on slender jobs) and provide more clearance, which is useful when boring because the sides of the job curve in towards the tool. Negative rake inserts are stronger and last longer, but have less clearance and put lots of force on the job.
• Cost! It may be cheaper to change cheaper inserts than it is to use the best available. The economics  depend on the job. Not sure myself it's a good idea for hobbyists to spend big money on long-lasting inserts when dropping them is a hazard.

Most of this complexity is irrelevant in the average home workshop! In practice, I mostly go for small noses, (low cutting force and good finish), but for most work the choice is non critical. My favourite boring bar carries a triangular insert, handy because it's easy to tell when a blind bottom is being turned flat. My smallest boring bar, which will nearly gets into a 10mm hole has a rhombic insert: it cuts perfectly well but it's easier to use on through holes, unless a small trench at the bottom doesn't matter. With care the rhombic and triangular cutters both do much the same job. I also have a large boring bar fitted with a square insert. It bores large through holes quickly, but is useless for blind ones.

The kind folk at ArcEuroTrade (and others) sell the type of holders and inserts most useful to hobbyists. Unless something special is being done, I don't see much advantage in plunging into grown-up catalogues and selection criteria. For a long time I only had a 6mm set of right, left, facing and boring holders with rhombic inserts: later I found value in having triangular and square inserts as well, but I managed perfectly well with rhomboids for a couple of years.

Insert holders are another game again. They range from quite expensive to faint on the spot extraordinary. I think the best are engineered for best possible strength and anti-vibration properties. Jolly good in an industrial context, but I'm not sure a chap with a Sherline would get much value from inserts and holders designed for 1500°C cutting and several kilowatts at the spindle.

Dave

Edited By SillyOldDuffer on 12/11/2019 17:43:57

One thought, that I don't think has been mentioned Chris; bear in mind I'm self taught, so don't always know how a job is supposed to be done.
In normal outside turning, with the tool bang on centre height, any distortion in the tool, or workpiece, is likely to reduce the depth of cut and help prevent a dig-in.

By contrast, a boring tool is working in an internal circle, rather than the cutting forces being more or less a tangent to an external one.

Hence, during boring, a slight distortion of the tool will move the cutting edge into the workpiece, and thus a deeper cut, perhaps then distorting more, and leading to an even deeper cut and the eventual dig-in and or chatter.

To that end, for boring and internal thread cutting only, I found that having the tool just above centre height to be advantageous. If it distorts a tad downwards, depth of cut is reduced; this applies to either a boring bar, or using an end mill as described earlier. (or even a woodworkers carbide router cutter, depending on what you have available.)

Keep on asking the questions though, I'm learning from the answers as well as you.

Bill

I bought a set of micro boring bars from Bangood, they work rather well. Much better than the set I got with my boring head.

Micro Boring Bar

For a 10mm blind hole a good boring bar solution is to grind away half the thickness of a 6mm or 1/4" HSS bar, angle the end at about 10-15 degrees, slightly round the corner (in top view only).

Mount in a drilled 8mm block, split along one side so tightening it in the toolpost clamps it. Shim up centre of bar to slightly below centre height and angle it so the tip is dead on centre heigh and the bar is at about 5 degrees to the lathe centre line.

Alternatively, get some 10mm silver steel and grind and harden your own d-bit and use it to finish the hole.

Neil

As (far from old and far from being a duffer) SOD remarked that ChrisTT was good at asking questions that inspire long answers, or something to that effect. It made me re-read the original question and I think the most significant thing is that Chris says he has never done any boring. I would suggest that this adapter he is planning to make should not be his first attempt. He could learn a lot just by getting some bits of offcuts and have a play, experiment and practice just to get a feel. I dont think one can chose which boring bar, type of tool, or many other of the factors involves without first getting some experience.

I'm sure a lot of Chris's questions would answer themselves, or at least be better focussed once he has spent more time cutting metal.

If I were making the said adapter, my first thoughts would be how to keep the external (wheel fitting) section truly concentric with a bore on the opposite end. Machining the bore to the correct diameter is only one small part of the job but to make the part, the whole machining sequence needs to be thought through in advance. (not that I always practice what I preach!)

Ian P

Thanks Neil, I have just come across a boring bar in my box of cutters I never knew I had that came with my Sherline that i never knew I had. As the L/D ratio is within that recommended by steel this HSS would be OK for that. however I am stil researching what cutter to use for what type of hole (End facing, through hole etc., etc ...getting these defined is where I is).

Chris

Boring bars are not really separately defined into blind and through hole types, mostly the same tool can be used for any type of hole unless there is some special requirement.

There is no right (or text book) way to bore a hole, my suggestion would be to get some hands on experience and see what works for you.

Ian P

Going back to using a slot drill, actually you don't need to angle it away as suggested above to clear the flutes. If you think about it, slot drills are designed to plunge, and can in fact be used for ordinary drilling provided the machine is rigid. The flutes are designed to have clearance even in a hole of the same diameter. When boring, so the tool is offset from the centre line, the flutes curve away from the bore more quickly as they have smaller diameter, so will not touch except just behind the cutting point where they are cutting anyway.

I'd agree; "rules" in books, and on forums, are for guidance not blind obeyance.

While boring bars are not generally defined as thru hole or blind they are defined by the features they will machine. Tthere are many that will not bore and face a blind hole. Search for SSKCR/L, SDZCR/L or STWPR/L for instance.

This is what I meant by microboring tools:

Microbore Tools

Andrew

John are you saying that slot drills are wider at the end than near the shank (so slightly tapered)?

That would make my suggestion to slightly angle the cutter superfluous but I had always assumed that end and slot mills were parallel.

Ian P

No, I'm not saying that, they are parallel. But if the flute edges of a slot drill don't rub when plunging vertically in a hole the same side as the drill, they aren't going to rub when boring out a larger hole, especially since the geometry ensures that nearly all the flute is well away from the bored surface.  (Actually the latter is true only whilst the hole depth is less than the pitch of the flute helix.)

Edited By John Haine on 13/11/2019 14:05:22

For a blind hole, end facing is required.

I have a through hole ones (strictly the bought one is both with an angled HSS insert at the through end and a 45-degree one at the blind end).

In simple terms the through hole one has a shorter protrusion so is more rigid = heavier cuts. The cost of this is it can't reach the bottom of a blind hole.

A third type is between centres, the use of which should be obvious from the name, very rigid but obviously requires a through hole it fits through and the work mounted on the saddle or its various slides.

Neil