# Inverse drill bit?



## GrahamRounce (9 May 2018)

Hello - I want to reduce some (threaded) steel rod from 6mm to 3 or 4mm, for about 2cm of its length. Filing on the lathe takes a while and it's tricky to get it accurate.
I wonder if there is a reverse, "outside", drill bit, something like a plug cutter, that I can use on the lathe?
Thanks v much,
Graham


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## novocaine (9 May 2018)

No. 
normally you'd use a cutting tool. you could do it with a hand graver (i'm assuming you are using a wood lathe). HSS will cut all thread, if it's a hardened bolt then you are in for a lot of work. or you could do it with a grinder and a jig. both of these are bodges.


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## GrahamRounce (9 May 2018)

Thanks, no, it's not hardened. Just 6mm studding from the builders'.
Yes, it's a woodworking lathe.

What sort of cutting tool? My chisels only just do aluminium, very thin sections at a time. 

I'll google a 'graver'!


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## novocaine (9 May 2018)

your chisels are sharpened for wood, different relieve needed for steel and you'll need to look at feeds and speeds. also go for a smaller tool tip to reduce the forces further. a graver is just a small tipped chisel for engraving steel.


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## flh801978 (9 May 2018)

If you have an angle grinder then spin your steel in your lathe and use the grinder in your right hand on the top of the shaft at 12 o clock and it will easily reduce your size down.
Opposing directions work best.
Ian


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## sunnybob (9 May 2018)

Spin it in the lathe and use a coarse file from side to side to remove the thread, then retap.


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## CHJ (9 May 2018)

Drill 3 or 4mm hole in the end of your 6mm studding and insert a short section of 3 or 4mm studding .


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## CHJ (9 May 2018)

Longer to photograph and load image than do it.


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## ED65 (9 May 2018)

Nicely done Chas! 

Graham, you could totally grind this, and without needing to go all the way up the food chain to an angle grinder (sledge to crack an egg!), but this is a job that is completely within the realm of file work. To have it not take too long though you do need the right coarseness of file to start with and of course it should be sharp. 

It's not a quick job, but it's doable and with some accuracy _because_ it's slow.


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## novocaine (10 May 2018)

erm, m6x1 root diameter is ~5mm (technically it's 4.9 something something but 5mm is close enough), not much meat left to drill a 4mm hole in it. 
also assuming here (I know I know) that the OP doesn't want a threaded section but a plain section 4mm. 

personally, if i didn't have a lathe I'd chuck it in a drill and spin it against the bench grinder or stick it in the pillar drill and run it down with a file, same as you'd do on the wood lathe. if im felling brave I'd use a small tipped hand graver and a slow speed, but that's because there is something decidely enjoyable about hand turning steel, in fact I do it on occasion when I want a domed head or end on something.


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## Phil Pascoe (10 May 2018)

My thoughts exactly - you wouldn't have much room to go off centre, which without machinery you would - and with machinery the question is redundant.


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## CHJ (10 May 2018)

phil.p":1jhuksgv said:


> My thoughts exactly - you wouldn't have much room to go off centre, which without machinery you would - and with machinery the question is redundant.


The OP has a lathe, drilling a 3mm hole should not be a problem if he has a suitable drill, if plain shank required then a suitable nail or one very near size is a better starting point. 3-4mm is not exactly a tight tolerance on requirement.
If mechanical integrity is important soft solder or silver solder if available* for fixing.

*not something many DIY workshops have.


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## Phil Pascoe (10 May 2018)

I understood it was a wood lathe.


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## Blockplane (10 May 2018)

Actually , there is a tool for this ( two if you count the tangential cutter boxes used on capstan lathes)
It is called a "running down cutter", and doesn't look too different to the plug cutters we know from our woodworking.
Some of the older metal turning textbooks describe them, and at least one gives the 'words and music' for making them.
I made myself a set for making BA bolts of a particular pattern back in the long ago when I was repairing brass and wodwind for a living.
Basically - cut a length of suitable sized silver steel, face the ends, drill and ream a hole of the desired diameter, reverse and drill out most of said hole to give clearance. then file cutting teeth on the business end. Harden and temper, then stonethe cutting faces sharp. 
I think you would have to be making a fair few components to make it worthwhile.


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## GrahamRounce (12 May 2018)

Hi, thanks, that's all very helpful! 
I've actually done it now, by filing it in the lathe, but it's difficult (ie impossible : ) to get it straight and parallel sided and exactly the right size. I should have thought of drilling a hole in the end, etc! My lathe has a swivel head, so it's possible to line it up quite accurately.

I did think of trying to make a sort of "running down cutter", thanks, say from a 10mm rod with a 4mm countersunk hole down it, but was daunted by the "file sharp teeth' bit! I've never been able to do that successfully. I wonder if a pair of hacksaw blade pieces, held rigidly 4mm apart, might do it?
Next time!
Thanks again,


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