Or grind a D-bit from a length of round HSS.

Neil

I'v got a little boring bar made from a bit of 1/4" sq HSS, it will go down a 1/8" hole.

Ian S C

Solid carbide boring bars are available in the kind of size you need. I bought a set of three a few years ago on ebay and the smallest one can bore a 5mm hole.

Doug

A D bit – the type that does look like a D on the end will give a hole only slightly worse than a reamer. Instead of +0.0005 it's something like 0.0015" or better over size. Don't remove any more material than you would with a reamer.

It would probably be easier to use a 6mm slot drill to drill the hole and the same to bore it out then a D bit to finish but you may need to lap the bore anyway.

D bits are pretty easy to make. It was once done with a file but they can be milled. Take the flat very very slightly below centre rather than above. It's important not to have the flat above centre so depending on size we are talking 0.0001's not thou's at this size. When filed a toolmaker would deliberately add a bit of rock to the filing action which ensures that the edges are a touch below centre. Those days are long since gone though so I suspect people will mill them. Make sure it has a decent finish, harden it and allow the temper on the very tip to go to very pale straw. A cheapo plumbers torch could be used up to say 1/2 dia or so. Bigger may need something more powerful. Polish after hardening and the heat well way from the tip and watch the colours run up to the tip. As soon as it shows slight signs of pale straw plunge it into cold water.

Most models can be changed. I'd be wondering if the bore could be increased to allow the use of an 8mm reamer but you can probably buy a 7.5mm one from drill services. Also a 5/16" reamer wouldn't be much oversized. I/64 less even closer.

John

Edited By Ajohnw on 22/11/2016 11:39:13

The best bet is to find out what is causing the taper. If there is signs of the same cut setting cutting again it might just be slightly loose headstock bearings but bars also bend so the diagnosis has to be done carefully. It can be worth running the same cut several times to see if the taper changes as setting headstock bearings isn't that easy to do.

If your reaming the tailstock alignment is more important. If I have doubts about that and it's not being used to correct for bed twist I know what I do but would hesitate to suggest it on small chinese lathes. it's pretty simple really and assumes that the 3 jaw is in good condition and runs pretty true which means it's a decent make and correctly mounted on it's back plate etc. Hold a morse reamer in that, extend the tailstock quill by a couple of inches and then simply push it "gently" and slowly along the bed onto the morse reamer so that it takes a light cut and centres the morse socket. What you don't do is use the tailstock feed or clamp the tailstock. If you rotate the morse reamer or anything else round by hand just as the jaws close slowly on it what ever it is will run as true as the chuck does. The tailstock needs to be heavy enough to "hold itself down" The more reamer will be cutting on one side to start of with. Should be ok on any lathe say a Myford 7 or heavier.

Light lathes ? Maybe press down hard by hand as well.

I am assuming the bed etc is nice and clean and if messed up don't blame me – it just needs some care and thought. The weight of the tailstock is what is helping the reamer cut on one side initially.

This is one of the reasons I don't strain beds.

John

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If you can't remove the taper turning of your lathe before this job, is there any way you could mount the job on the cross slide and bore it?

Dave
The Emerald Isle

In my last bit it should read "if you ream the hole".

Ian S C

40mm! That's a very deep hole to try to bore accurately at 7.5mm. You might get away with using a piece of 6mm round HSS with the tip ground into a boring bar shape but quite possibly will end up with a slightly tapered, chattery hole. If you can get a reamer (or make a D bit type reamer) at 7.5mm, you could take the chuck off the lathe after boring out to just undersize, stand it upright on the drill press table and ream the last few thou out of the hole in the drill press. This might get around your lathe misalignment problem.