Different ways of boring a hole

Could you please help me to understand the relative merits of these 3 different ways of boring a hole ? Happy to read a book or a web page if you know of anything relevant.

1. Milling Machine : Mount workpiece either in vice or on table, open up with a drill bit then use boring bars in boring head to achieve final radius.

2. Lathe method A : Mount workpiece on faceplate or chuck and then same as method 1 above, this time the workpiece is spinning instead of the cutting bit in method 1.

3. Lathe method B : Mount workpiece on cross slide, drill out pilot hole, then mount a bar between headstock and tailstock, this bar holds a cutting bit which enlarges the hole as the workpiece is moved along the lathe bed on the cross slide.

I have never used method 3 - am guessing the cutting tool is less prone to flexing due to tailstock support ?

Any substantial benefits in spinning the workpiece rather than the cutting tool ?

Any substantial benefits in moving the workpiece along the lathe bed as opposed to holding the workpiece stationary and moving the cutting tool ?

My apologies if this is all blindingly obvious to many of you - at the moment its not obvious to me.

I guess the relative sizes/solidity of the relevant lathe and mill would be important. In my case I have a round column Rong Fu 30 mill and a Hercus 9" lathe (Australian equivalent to South Bend 9" and Myford 7"

Bill

Edited By BW on 14/07/2019 10:07:06

Rather depends on the shape and size of the workpiece and how deep the hole needs to be.

Pointless to use 3) if only a thin item?

Pointless if item is so out of balance for 2).

1) is an option if one actually has a mill.

I do it the simplest and easiest way dependent on the job.

Edited By not done it yet on 14/07/2019 10:11:43

Providing it's in balance, there's a slight advantage in rotating the workpiece in that the swarf is less likely to be be forcefully flung about the place when drilling.

The only reason for doing 3) would be a long precision hole where the boring bar in the chuck alone could flex.

If it's a thin item, see NDIY's reply, or trepan it if the bore's big enough to make a tool for it. It's obviously more difficult to set up a trepan op on a faceplate than a chuck. You might use a combination of trepan and parting if you're cutting a thin ring from a bigger billet.

Thanks for the response. Let me ask the question in a slightly different way.

i.e. ............ You have a workpiece that is of an appropriate size and shape such that any one of the three methods would be viable, and you do have a mill, and assuming you are capable of holding the workpiece equally well in all three cases, is any of the 3 methods substantially better than the other two ?

Edit : Written at the same time as Mick's response. Thanks Mick.

Bill

Edited By BW on 14/07/2019 10:28:33

• If you do have a mill then the size of object you want to bore is virtually limitless
• V bed lathes can be limited for method 2 compared to a gap bed lathe
• T slotted boring tables are not always provided on smaller lathes making method 3 difficult, even though it would be most useful on these machines (but see Neil's article on mini lathe abuse!!)

But there is usually a way as this image from an old Drummond catalogue shows!! (Method 3 taken to an extreme)

Edited By Nick Clarke 3 on 14/07/2019 10:37:31

I avoid boring with the mill as the feed (assuming vertical mill) has to be applied by hand. In the lathe the feed rate is taken care of by the machine making the process controlled.

The tool stationary and the work spinning (as on the lathe) could lead to tool flex and a tapered hole. Also any out of balance or spindle bearing issues will affect the hole geometry and finish.

If you keep the job stationary and spin the tool (as on a boring head or between centres bar) then the only path the cutting point can follow is a true circle and you are more likely to get an accurate and parallel hole, this assumes the spindle bearings are good.

Just think about how often you see jobs in mainstream industry bored on a lathe faceplate compared to on a jig borer or horizontal borer, however as model engineers, we have to make do with what machines we have available.

But as a rule of thumb spin the tool not the job wherever it is practical.

Phil

Good question. I suppose the answer is 'It all depends...' Major determinants will be the configuration and size of the job and the relative accuracy and suitability of the machines to hand.

One consideration. If you want to make something like a machine spindle housing, or quill, with a bearing housing at each end, a between-centres boring bar in the lathe can bore the central hole, be adjusted to bearing housing dia, then bore each housing, by swapping the bar end-for-end, with guaranteed concentricity of bores, all without having to change the set-up of the workpiece. I can't think of another way to do this so simply (and am happy to be corrected...).

Edited By Kiwi Bloke on 14/07/2019 11:18:53

A lathe and rotating work may allow you to face the end truly square better that a mill unless you have a boring and facing head. Also with larger bores you have the potential for a huge boring bar while a boring head will be limited.
The saddle mounted setup was much more common in the days when milling machines were rarer.
An horizontal mill adds a set of variations on all three methods.

As the others have said its dependant on the job.

Mill will do odd shapes that can't be rotated on the lathe and if you have several holes they can all be done at one setting ensuring they are all parallel to each other and if fitted with a DRO for positional accuracy you can place them better than marking out. Most won't have power feed on a hobby machine so that can be a disadvantage. Big holes can make the boring head out of balance so speeds will have to be adjusted accordingly.

I don't see a problem with rotating a part on a faceplate that may be out of balance as you can just bolt on anything to hand to bring it back to an acceptable level of balance for the speed required. Not quite so easy if using a 3-jaw or small 4-jaw, big 4-jaws have tee slots for adding weights. Power feed available for an even finish. This is also the simplest method if you want a tapered bore either using the topslide or taper turning attachment.

On the topslide with a between ctrs bar will give a parallel bore as the tool won't flex and is ideal for long holes where excess tool stick out will cause vibration and a taper. Setting depth of cut is often the hardest part but like above you have power feed for a good finish

There is also the 4th option of a boring head mounted in the lathe spindle for those that don't have a mill but want to do a job that the boring head suits with work held on the cross slide

If you don't have a mill and can't swing a large item then 3 or 4 is still a valid method.

Edited By JasonB on 14/07/2019 13:17:28

Worst choice is number 3, I'd only do that if there was no alternative. It works but is most fuss to set-up and it's not particularly rigid.

Usually mounting on a milling table is quicker and easier than mounting the same job sideways on a face-plate. But - and this may be wrong - I believe spinning the work is more accurate than spinning the tool because the heavy rigid work centres on the axis rather than the the lighter bendy boring tool. Milling machine easy, lathe more accurate. Not noticed any difference in practice though.

So, if a job fits easily into a chuck, I prefer to bore on the lathe. If it's awkward to hold, needing a vice, clamps, or a fixture, I bore on the mill.

Dave

When using a boring tool, ie a L shaped tool, it do not see any difference in either rotating the tool or work, if all is rigid and true. Any flexing in the tool will create the same sort of errors in either method. If however the lathe spindle is not aligned with the bed ways it will cut a taper. If a Mill spindle is not perpendicular it will cut a slanted hole.

However if I had a choice I would use the mill as it is the easiest method.

If using a D bit or trepanning tool that has contact on both sides of the hole, I belive it is better to rotate the work. This is because the tool will self centre to the axis of rotation and bore true.

Adrian

I'd try to do it in the lathe if possible, a Myford 7 rather than use the Sieg 2, especially if the difference between the pilot hole I could drill and the finished bore is quite big and the depth of bore is quite short as this means stopping the mill every few seconds to slacken the boring bar off, adjust it and tighten, it's much quicker in the lathe,

Thanks for all of the tips above.

I remember in one of the Harold Hall books where he needs to bore a hole for a shaft in a block (dividing jig) he emphasised that its very difficult to get the hole absolutely straight and same diameter all the way through.

He recommended boring the hole as best as you could and then enlarging the hole at each end and inserting a snug fitting collar at each end to grip the shaft.

Bill

I still think option 3 is going to give the best results, if you want to be quick but not fussy about the end result you can bore out a long hole on the lathe, but the tool will be springing and flexing all over the place and the method relies on your lathe being in perfect alignment and being able to turn absolutely true over a long length.

With a between centres boring bar, granted it is fiddly to set up, but you can be certain that the cutting tip will always follow a true circle and it will continue to do so for the full length of the job. Usually the boring bar can be nearly as big as the hole you are boring so long as there is room for chip clearance, hence it will be much stiffer than most lathe boring tools as it is supported at both ends. The fact that it is between centres adds to the accuracy achieved as well.

Phil

Good tip Phil, I hadn't thought of that. Just shows, learn a few tricks and I think I've seen it all. Nope! Very educational this forum.

Dave

In the, I hope, not too distant future, I want to make a device with its bore on the centerline of the lathe. The bore is planned to be about 70mm long.

The proposed method, unless anyone can suggest a better way, is:

Bolt to Cross Slide (which is where the finished item will sit, when in use )

Centre drill.

Pilot drill, and progressively open up to just below finished size.

Bring to finished size with Boring Head. This may need more than one "spring cut" to allow for tool deflection.

The finished size will be too big for any reamer that I have, otherwise that would have been the preferred method of finishing the bore.

How say you all?

Howard

Wouldn't the spring or deflection in the boring bar stay the same for the full length of cut assuming the depth of cut and feed rate stay the same?

I get why you would think that, in theory I suppose you are right. However in practice what normally happens is that the tool flexes and gets pushed inwards as the cut progresses and you end up with a tapered hole.

If you turn up a test bar with a normal tool on the outside diameter, lets say nominally 1" diameter and 4" long, then measure its actual diameter at each end, unless your lathe is very well set up you could easily find a thou or two difference in diameter across the length. Well this will also happen when you are boring a hole, as the conditions are just the same, but if you spin a cutting tool it does not matter that the saddle and spindle are not 100% in line, the tool will follow a circular path and the saddle will just be moving the job along past it. You only have to ensure the job is set up square on the cross slide.

A lot depends on the depth of hole in relation to its diameter, I read somewhere that anything over four diameters deep and you will encounter the problem of boring tool flexing.

Every job has to be taken on its own merits, there is no hard and fast rule to follow

For a relatively short depth hole, more often than not I will drill a hole somewhat under size, which could potentially have wandered off course due to the drill not being correctly sharpened or flexing off centre. I will then go down the hole with a boring tool to bring it back concentric again, and finally finish it with a reamer.

But for something like an engine cylinder bore, the between centres bar is the way to go, and even after boring it to a few tenths under size I will make an expanding hone/lap to finally finish it to size (I DO NOT mean those three legged glaze buster contraptions).

Howard mentioned he was making some tooling that is going to be used on the cross slide, in that case it would be a no brainer for me to machine it with a between centres boring bar on the cross slide where it will be used, it has to be bang on centre height and truly round if done that way.

Hope this helps, its a case of horses for courses.

Phil