Some (daft?) boring bar questions plus related one on drilling.

As my machining interests seem to demand boring large holes, decided it was time to get a better grip on boring on the lathe

My home ground hss boring tools work, 3/16" & 5/16" blanks but seem painfully slow.

I need something a little larger.

First the drilling question. What size hole would people consider a sensible maximum to drill on a mini lathe? Ok ,that;s material dependant~ let's settle on "mild steel" for arguements sake.

I'm quite prepared to use the spindle hand crank if the motor can't cut the mustard but there must be a point @ which the MT2 taper 'lets go'?

Now to the boring bar questions.

First, if i make say a 1" dia boring bar but reduce the shank to fit tool holder to say 5/8" will this be any more rigid than a straight 5/8" boring bar?

Second, I believe lead filling a boring bar is a fafoured way of reducing vibration~ how exactly would one go about filling a 1/2" dia hole with molten lead? they never tell you that part~ sounds like a recipe for an unholy mess on the kitchen table or a very unhappilyl burnt cat.

Thirdly & finally has anyone experimented with tapered boring bars?

In another life i remember a top notch turntable tone-arm being made tapered (using some mathematical curve) to reduce resonances.

Makes sense to me, though not sure how practical a tapered boring bar would be.

Thanks in anticipation...

The question of maximum diameter hole you can produce before starting with a boring bar depends on how much material each drill is removing if you start small and step up to final size. Trying to drill a Ø25 hole may be possible if you can go up to it in many steps whereas trying to cut it with just a Ø3 pilot hole will probably not work. There is also a chance, if the material is not too deep, of cutting a large hole with a hole saw or broach type cutter (my preferred option, it produces less waste and a useable solid plug). Also blunt drills will not get you to the maximum size you can cut with something sharp.

Martin C

Thanks Martin, I like that idea of a 'useable solid plug'~ waste not, want not.

MT2 drill bits usually go up to around 20mm, you can probably find a few up to 23mm but I'd probably look for a MT3 above 20mm.

I have a feeling the motor on your mini lathe will be the deciding factor in the largest hole you're able to drill.

I also can't think why you'd need a 1" diameter boring bar on a mini lathe, I've got couple of nice Sandvik 1/2" boring bars and I doubt a mini lathe would have the power to give them any problems.

The taper letting go will not be a factor. I have successfully drilled 1" diameter holes in steel in my old Drummond Flagelator lathe with a No1 morse taper in the tailstock. Those tapers hold better than you might think, if clean and tapped home with a brass hammer etc. No idea though on how big the rest of your mini lathe can take.

How big are these holes you want to bore? It seems tothe like in a mini lathe you should not have to go any larger than the toolbits you are already using. For most work I use a boring bar with a shank about 3/8 or 7/16" diameter taking a little 1/8" square HSS tool bit in a square hole at the end. It will take a fairly decent cut.

It's all about how you sharpen the toolbit. They need a lot of extra clearance on the side next to the curvature of the hole being bored. And a good bit of top rake to provide a sharp cutting edge. Also don't put a big radius on the cutting tip, it increases chatter. I grind mine to a sharp corner than just round it off on the bench oil stone a tiny bit.

The other alternative, I bought a set of those cheap as chips insert boring bars that are everywhere on the net. Four boring bars ranging from about 6mm shank to 13mm shank and they work a treat. Very good for shifting metal quickly. But a well sharpened HSS will give a smoother finish on fine finishing cuts and better control over final sizing.

Pics show 1" drill in MT2 on the ML7, and 13mm shank insert boring bar doing a 2" or more hole in steel.

Most, if not all, drills, lathes and mills have a specification - including maximum drilling capability. Doubtless this is reasonably conservative and can be safely exceeded by resourcefulness or care by competent operators. I suggest you look at the specification of yours.

It is likely that only you know your level of experience/competence. It can be somewhat easy, to cause problems with machines powered by DC motors. Those problems are often expensive in comparison to the initial machine cost. So beware, if you intend yo exceed those specifications.

I'd go for "blacksmiths" reduced shank drill bits held in the chuck if its a 7x16 as if the drill is going to slip I'd rather have it slipping in the chuck than the tailstock taper but you may be limited on length.

This also brings us to depth of hole I'd not want to be using a small 5/16" boring bar on a hole of any depth say something like a Stuart engine cylinder which could be 65mm long average bar is good for about 5 x dia protrusion

It sounds like you want to make a boring bar so I would say start with some 1/2" or 12mm material cross drill the end for a 3/16" or 5mm dia piece of HSS and drill and tap a hole at 90deg to that for a grub screw to hold it in place. Ream a hole in some 5/8" square bar and then saw a slot along its length. Bar goes into block and then toolpost screws close the block onto the bar.

Alternative would be start with 3/4" ort 20mm bar and mill flats onto it so it fits tool post but that can limit the ability to adjust tool overhang. But if going that large it's probably better to clamp the bar direct than use a toolpost particularly if it is a quick change one.

A 20mm hole should allow the 12mm bar and it's bit to enter.

Other alternative is buy a 16mm bar to take inserts, I tend to use these in various sizes these days only reaching for my ones that take HSS bits when doing large cylinder bores that can be 6" or so long

Can't see much point in a tapered bar as you will end up with a tool that will be restricted on what size hole it can enter the longer the depth of the hole is.

The deflection and vibration characteristics of the bar will depend on its projection from the holder (how much it cantilevers).

Thus, the very best thing you can do to improve things is to reduce the projection to the bare minimum for the particular hole you are boring.

Having a variable projection dependent on the work in hand makes any tapering or vibration tuning very difficult. The tone arm on the gramophone is a fixed length and has only 3 modes of operation (33 1/3, 45 and 78) so can be closely optimised as its entire working life is known in advance of its manufacture.

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Please consider an economic analysis of the annular cutter idea. You need to buy the cutter and arbor to hold it. What is the cost of these compared to the plug of metal you will throw into the offcuts bucket when it releases and that your executors will throw in the skip when you die?

I've had very good results with elastic bands (wrapped around) and plasticine (just squeezed around) on boing bars to help damp vibration.

Jim

Well first off don’t use any more overhang than you need on your boring bar and ensure your topside is wound right in and clamped.

Interesting that Martin as I tend to have my topslide mid way which gives me the most dovetail engaagement when clamped.

Also depending on he size of work and chuck jaws etc if I wind my topslide right back I would need to increase bare stick out if I am to avoid the topslide/carriage hitting a revolving part.

Edited By JasonB on 07/01/2023 10:00:04

I don't know if they are still available but I have a 23/32 "coring drill" on an MT2 taper. These need ~1/2" hole to start as they don't have a point but they do have 3 edges/flutes. Excellent for getting out lots of material quickly when you are boring larger holes. I think I got mine from Tracy Tools.

I seem to get horribly tapered bores unless I clamp the cross slide. I am surprised that locks are not fitted. Or it just me?

That’s what I meant, you have expressed it better. I’m on a Myford so I work with the upper and lower ends of the dovetails on the chuck side flush. I also have installed George Thomas’s top slide locks.

regards Martin

I'm surprised and very disappointed that nobody seems to be selling these things any more:-

A 12mm or 1/2" bar with 45 and 90 degree square holes for 3/16" square HSS toolbits.

Wassamatter with 'em? Are they too cheap, too simple and too enormously versatile for anybody to stock 'em any more?

You can set your overhang to suit, adjust rake angle and centre height, grind the toolbit to suit threadforms or whatever profile you like, you can take 'em out the holder and use like a flycutter for flats or bore a concave radius with a carefully set standoff. The carbide insert bars on the net look like they couldn't do half those tasks.

I don't remember how much I paid for mine however many years ago, but it was cheap enough not to tempt me to make one.

You can see how much use and abuse mine has had, but now I'm bothered I might have to make one to replace it! ARC seems to sell hoders, but not the bars.

If the OP can get hold of one of these, do so.

Edited By Mick B1 on 07/01/2023 10:25:48

How long is a piece of string ? It's a mini lathe, gear box, belts or varible speed ? High rates of metal removal need power or high torque. Rota broaches work very well but are only made in 2 depths, 25 and 50mm, in diameters from 12mm up to sizes far to big for a mini lathe so would fill the bill and at about £20 for a 25mm a good starting point. A holder can be made from scratch or using a blank 2MT arbor drilled(3/4" ?) to suit with a clamp screw. Can your lathe run at 300rpm with reasonable torque - with out burning out the motor ?

Lead loading on boring bars of mini lathe size - not worth the trouble. Tapered bars, 2MT letting go, it's a mini lathe? NOT a Dean Smith and Grace ! Noel.

It all depends on what is meant by a large hole?

I suspect that on a small lathe there is little to be gained by step drilling. There are not many things more tedious that continually changing drills going up in small increments. A drill is an efficient way or removing metal if the width of cut is large. I normally go up in steps of half or three quarters of an inch on diameter and use feedrates of 20 thou per rev or more, if I can wind the tailstock handle fast enough. But I have a large geared head lathe that will push a 1" drill through cast iron with no pilot hole if needs must. On a small lathe it is probably just as quick to go up in steps with a boring bar.

It's a myth that chatter equals resonance. They are two different things. Chatter is a forced oscillation. It might coincide with the resonant frequency of the boring bar but mostly doesn't. The important parameter for boring bars is stiffness. So bigger diameters are better. But using a large diameter boring bar with reduced shank to fit the toolpost is pointless. Increasing the stiffness can also be achieved by a change of material. Carbide boring bars are much stiffer than tool steel, but are also eye-wateringly expensive, especially in larger sizes. A lot of the old techniques such as adding lead or weights are aimed at damping any vibration that does occur.

Always operate with minimum tool overhang. Like Jason I run with my topslide in the mid position, but with the gib screws locked and the seperate topslide lock done up. I've never needed to use the cross slide lock to get a parallel bore. I find the best way to deal with chatter is to increase the feedrate and depth of cut. That loads the tool more and reduces the effect of the inevitable tool deflection on the cut.

Coring out the work beforehand can be useful, but for me is not worthwhile unless the core is going to be 1.5" or more. It is tedious but in my case I can use power downfeed on the mill quill and a bolt pattern on the DRO:

I've looked at annular cutters, but they don't seem to be available in the sizes and depth of cut that I would need.

Andrew

+1 for Mick's boring bar. Dead simple to make if you can't find them commercially. If you use a round HSS bit then even easier.

Andrew.

Wow! A lot of useful information in this thread

Thanks to all that have chipped in so far.... keep it coming.

Yes i'd like to make my own boring bar for holes in the 1 1/4"- 3" range.

Why would want 1" dia?

I can easily see the 3/16" tool flex.

I'm led to believe using steel boring bar, rigidity limits , DOC to 3D best practice... with 6D being a realistic limit?

Even 6x 5/16" is not very deep.

6X 1" would be enough 'wiggle room'

I have, probably 2 or 4 boring bars of the type shown by Mick. Various makes, not identical but they all do the same job.

Rule 1 for boring is always to use the biggest bar that can be finagled into the hole. Which means very short tool bits. To my mind the one Mick shows is way too long. I recon less than 1/4" stick out at the front, nearer 1/8" for smaller ones, and nowt out the back. Theoretically it wastes tool steel because you only have a few re-grinds before it is too short to hold. But I have a couple of baccy tins full of itty bitty bits. That said I suspect most neophytes grossly over estimate how many HSS tool bits they will use in a lifetime once the iniial experimentation period of learning how to sharpen has passed.

I'm stupidly overstocked, about 4 lifetime worth. Partly through hoovering up cheap bargains and partly though buying machines with a big box of stuff included.Not to mention the "can you use these" offerings which fall into the grab first ask questions afterwards category.

Not over-impressed by the smaller inset bars. The shank is always rather slim and we generally have problems getting the speed and feed right. Andrews increase speed and feed suggestion works well but generally folk using small lathes will run out of speed. These are generally intended for CNC and book figures for the two I have are impressive.

If the hole is relatively short, but large diameter, I will often just sharpen the end of a piece of HSS tool steel and mount it up at a slight angle to get clearnce behind the edge. The guy who showed me that dodge reckoned it was quicker just to clean the end and put an edge on it than sorting through the collection for the right bar.

With smaller machines I feel its worth while removing the topslide and holding a large bar in a simple block. Mount up and drill so you know its on centre. The extra weight helps absorb vibration and eliminating the sliding joints on the topslide improves stiffness. Consider using the tailstock quill to help push it along. The saddle rack is (almost) inevitably set to one side so there is a twisting tendency.

Clive