Reaming cast iron question

Gentlefolk,
Working on my Worden... I have to drill and ream an accurate 3" long by 3/4" diameter hole in cast iron - What size should I drill it to before
reaming ?
TIA
--
Bob Unitt (UK)

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How about 47/64"
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In article

15 thou seems rather a lot to me...
--
Bob Unitt (UK)

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Bob Unitt wrote:

Somewhere I have a dormer product guide that has advice on reaming. It mentions that insufficient material left to ream will result in a less than satisfactory hole. 15 thou falls within their recommendation as I recall and have always used as a guide. However did a quick google and: http://www.unionbutterfield.com/tech/reamers/applications.asp
Tom
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Doesn't really matter. As you want the hole straight and true, so you will, of course, be boring it before you ream it. About 5 thou. or a bit more undersize would be OK.
--
Charles Lamont

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I'm confused by this - isn't a drilled and reamed hole 'straight and true' anyway ? - the actual position of the hole isn't an issue (within limits), as I'll be locating all the other machining operations from it anyway.
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Bob Unitt (UK)

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Even boring will only get it in the right position if you set it up in the right position:-)
If you are you saying that you are not much bothered about geometrical accuracy (squareness in particular, and to a lesser degree straightness) then there is less need to bore, but for best results I still think it would be a good idea. Don't you like boring?
--
Charles Lamont

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<SNIP - about boring 3/4" hole 3" long in cast iron>

The major requirement of the hole itself is that it slides smoothly and without shake on 3/4" pgms. Because I know that this is going to be the trickiest job on the whole component, I've planned the sequence of operations so that I make the hole first, and then locate everything else from wherever the hole ends up.

Yes, but I'm not tooled-up to bore a hole that narrow and long, and I don't want to get into the syndrome of making tools to make tools to make tools...
--
Bob Unitt (UK)

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wrote:
<snip>

Why not? <G>
Mark Rand RTFM
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<Lots of good stuff about reaming...>
Thanks Dave, I'll probably give it a try this weekend. If you hear an agonised cry from the direction of Herefordshire, you'll know I cocked it up...
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Bob Unitt (UK)

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> Drills have a nasty habit

That's strange. I've always found the opposite to be true. If the dril is fixed and the work rotates, I can't see how it can drill off centr unless the initial entry point is off centre and the drill wiggles i which case it would be obvious that something is going wrong. A goo centre drill usually starts it off OK and if you do get a bit tha starts cutting off centre slightly, you can often just touch the end o the drill bit with a cutting tool to steady it and stop any wiggle. drill in a drill press or milling machine can wander horrendously bu appear to be cutting concentrically as it veers off at a curve. If th workpiece is something that can be bolted to the lathe saddle, try boring bar held between centres. It's a very easy tool to make with bit of round stock and a reground broken centre drill or millin cutter. Either that or a straight forward boring tool will give you truly concentric hole good enough to follow with a reamer
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I know it's considered Bad Form to follow-up one's own posting, but I never claimed to be perfect...
I made the mistake of going to my local (good) tool-store to enquire about the purchase of a 47/64" drill - only to be quoted an astounding 60 plus VAT !
Does anyone know of a _cheap_ source for this drill, as I'll probably only ever use it once ?
TIA
--
Bob Unitt (UK)

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Bob Unitt wrote:

J & L Industrial have one for 16.97. Its item no. TDI-10734J
Regards
Peter
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That's more like it !
Thanks.
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Bob Unitt (UK)

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writes

I have an old 47 / 64 drill, 2 Morse taper, 270mm overall length, it needs sharpening
If you want to borrow it E-Mail me.
--
Jonathan

Barnes's theorem; for every foolproof device
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<SNIP>

Thank you for your kind offer, but I've just ordered one from J&L.
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Bob Unitt (UK)

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writes

It's extremely easy to make your own adjustable boring bar for a job like this. Just a length of mild steel bar with a 5/16" cross hole drilled through about 1/4" from one end and a thread for a 6mm grub screw tapped in through the bottom of the bar to hold the boring bit which goes in the cross hole. Make the boring bit from an old broken 5/16" centre drill, milling cutter or normal HSS drill bit etc. Grind a flat along one side for the grub screw to engage on and then grind the cutting edge so it's in the correct orientation relative to the flat edge. In fact mount the bit in the bar, lock it in tight with the grub screw and then grind the cutting edge in situ so to speak so you can see the orientation as you hold it against the grinding wheel. At a pinch you can manage without the flat edge if it's too much extra hassle to grind it but it does help the grub screw get a secure hold and it also keeps the orientation of the bit correct at all times.
I have a bunch of these in different sizes and designs which I use for boring out valve throats in cylinder heads, making fixtures etc and all sorts of general big hole drilling where I don't want to be spending a fortune on big drill bits. I generally hold them in a 3/4" R8 collet so they run nice and true but you can hold them in a chuck at a pinch. You'll probably need to make yours out of 1/2" bar if it's to bore a 3/4" hole.
Grind the back end of the boring bit flat and true at 90 degrees relative to the shank so you can mount the tip of a dial gauge against it. Then you can set the bit by taking a trial cut, measuring the bore size, mounting the dial gauge against the back of the boring bit with a magnetic stand and adjusting the bit out to the new desired cutting radius. Or you can just bore a hole out say 20 thou at a time to any size you want by adjusting the bit 10 thou at a time etc etc. It's easier to actually do than describe. You can get to within a couple of thou accuracy on the hole size easily enough with practice and a few trial cuts. Ample accuracy for something that's going to be finish reamed anyway.
For my bigger boring bars I use carbide tipped boring bits. You can get these mounted on a 2" length of 3/8" mild steel bar for a few quid each in K grade (non ferrous) and P grade (ferrous) types from places like Cromwell Tools and cut them down to the desired length. You can really shift material fast with a carbide tipped bit held in a sturdy bar. A few weeks ago I had to bore four 84mm holes through a 3" thick aluminium plate (actually an old cylinder head) to make a clamping fixture for honing out cylinder liners. I hacked the bulk of the metal out fairly roughly with an ordinary milling cutter to give me a through hole to work in and then cleaned the bores up to final size 2mm at a time with one of my carbide boring bars running at 550 rpm. A hell of a lot of metal to shift but quite a satisfying job to do with home made tooling. -- Dave Baker
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<SNIP - Useful write-up on making boring bars>

Thanks for that - but, as I said before, I'm trying to avoid the 'making tools to make tools' loop until I've got a decent tool-grinder to start with...

I'd be a little concerned with the stiffness of the bar - as a matter of interest, has anyone tried carbon-fibre in this application ?
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Bob Unitt (UK)

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wrote:

Since carbon fibre composites only seem to have about half the Young's modulus of steel, they would be rather bendy. Lightness wouldn't be much of an advantage except, maybe, in the International Space Station (Images of Dean, Smith and Grace with beryllium bed :-).
Tungsten is twice as stiff as steel and is used to make tool holders for lathes using ceramic inserts.... Think 75hp going into an 8" diameter super alloy workpiece. Tungsten carbide is nearly 3.5 times as stiff as steel and is used for boring pars.
Hey grandma, do you know how to suck eggs?
Mark Rand RTFM
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<SNIP>
Doh - I forgot that carbon fibre is only stiffer weight-for-weight.

Not my boring bars - mine are mild steel with tungsten carbide tips, and flex like anything - teach me to cheapskate with Chinese stuff !
A set of Arrand bars is on my Christmas list - anyone here got any experience of them ?

I've been trying to stop her doing that for years - makes a terrible mess of her false teeth...
--
Bob Unitt (UK)

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