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.

I'd go for "blacksmiths" reduced shank drill bits held in the chuck if its a 7×16 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.

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

Jim

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

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

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

You can still buy those bars, Tracy Have them for starters

None of my books gives numbers for the power handling capacity of a taper, either in practice or theory, The grip rises as the taper is forced home and the grip is good enough on a press-drill to see tapers smaller than MT2 happily coping with 1000W. Mills need drawbar help because sideways forces tend to loosen tapers rather than tighten them. It's highly unlikely that a pressed MT taper would slip on a mini-lathe.

Motors limit the maximum diameter of a twist drill because large diameters call for slower turning speeds. Many motors stall easily when run slow because they lack torque, "turning power". Lathes with back-gear do better at low speed than lathes fitted with electronic speed control, because gears and belts multiply torque as speed decreases.

A hand-crank does well at slow-speed because muscles develop high torque at low speed, and a fit man can develop up to about 1.5kW in short bursts. But only in short bursts! If a large amount of metal is to be removed, it's faster to use a motor within to the best of it's ability because motors don't get tired!

Twist drills are good for making not particularly accurate holes quickly. My mini-lathe, from memory 600W max out, drilled well between 3 and 10mm, but it was too slow to drill well with small diameters, and too fast to drill well with big ones. In practice, 13mm was its outer limit. No problem drilling free-cutting mild-steel,  ordinary mild-steel was noticeably resistant.

The same twist drill happily goes into ordinary mild steel on my cheap bench drill, which has a 500W motor. The difference is the belt driven pillar drill has a shade more torque at slow speed. Though better, it's close to it's limit as well – struggling above 13mm.

Boring has much lower torque requirements, and will produce big accurate holes all day long! But boring is much slower than drilling.

Big rigid powerful machines with plenty of torque are needed to remove metal quickly. Mini-lathes bless `em, aren't designed to remove metal quickly.

Dave

Edited By SillyOldDuffer on 07/01/2023 15:11:51