Metal for milling spindle?

Now that the stepper controlled dividing head (qv) is commissioned I need to manufacture a milling spindle to cut the clock wheels and
pinions.
I purchased the cutters 4 years ago (thinking then that I was much further advanced than I even am now!) from Thorntons and both the wheel and the pinion cutter have a 7 mm bore.
Therefore it seems that all I will need is a 7 mm shaft with suitable means to clamp the cutters, mounted in ball races (and I just happen to have 4 races with a 7 mm bore in a box of junk) with a pulley on one end, the whole thing to be mounted on a vertical slide or even the topslide mounted vertically.
My question to the NG is, can I just turn up a shaft from mild steel or from brass, both of which I inherited quite a bit in the junk box I was given, or do I need to use tool steel and then harden the shafts? it seems to me that in the first instance because I only have one clock to make, that a mild steel shaft would be more than sufficient and that if I then get the clock-making bug still further then I can think again about a more sophisticated shaft.
Certainly I could suck it and see, but I have an in-built neurosis not to go several weeks down a path and then to have to start again.
So, is mild steel OK for this, please?
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On Wed, 19 Nov 2008 13:57:42 -0000, "Amateur machinist"

A 7mm shaft sounds a bit on the light side to me for that application - you would probably find that it isn't stiff enough, particularly for pinion cutting. I would go for something more substantial than that and machine the end to take the 7mm bore.
A good option for a "ready made" milling spindle would be to buy one of the Taig headstocks and a matching pulley set; they do an ER16 collet version for their mill which would be just the job.
Regards, Tony
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The issue is one of rigidity and all types of steel have the same modulus of elasticity i.e how much they bend under a given load so mild steel will be as good as anything else. However that's not to say that a 7mm shaft of any type of steel will actually be strong enough. I have no idea what you're making so I can't say.
--
Dave Baker



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I'm making (or intending to make) some clock wheels in brass and steel pinions, both of 1 modulus.
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I'm not talking about the end product you're making, which is irrelevant to the problem, I'm talking about the device you're making to make that end product. How big it is, how big the cutters are, what cutting forces it will encounter etc. Without knowing those things no one can tell you if a 7mm shaft will do the job. Obviously how that shaft is mounted and what overhang the cutters have on it is yet another variable in the problem. A good engineer can just look at a device and know in his gut if it's sturdy enough to perform its intended function. If you can't, or don't know someone who can for you, you could very well make something which won't cut butter and will just chatter like crazy every time you try and remove metal with it.
If in doubt make everything much more sturdy than you ever thought you needed. You can't go wrong making something bigger rather than smaller. If you want to post a pic or a scale drawing of what you intend to do maybe we can advise you better.
Personally a 7mm anything sounds very weedy for a machine tool. If that 7mm was just the last few mm on a 1 inch bar rigidly mounted between good bearings it would be a different story.
Back to you.
--
Dave Baker



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writes

I agree. If the cutter needs (for clearance reasons) to be in the middle of a long bar, you may need to have a 2-part bar with the two parts screwed together with a 7mm spigot between them just thick enough to locate the cutter.
David
--
David Littlewood

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The biggest wheel to be cut for the clock is 4" dia, so that specifies the clearance needed from the shaft.
I'm only making one clock and have only two cutters, one for wheel and one for pinion, but they already come with a 7mm bore which seemes to be the P.R.Thornton standard these days.
I need a spindle only for these two cutters and only for the one clock.
I'm a bit taken aback by all the comments from you and others about rigidity because most published designs for such wheel cutting use a "C" of steel with the cutter revolving on a very narrow shaft with the bearings just being two hardened points.
Such an arrangement seems to feature in most designs for a "wheel-cutting engine".
Nevertheless, I thank you and the other contributors for their comments.
The cutters themselves (despite costing 50 ea!) are about the size of an old Two Shillings / Florin coin.
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Hi.
If you pootle over to the MLHorology group on Yahoo, you'll find a design for a cutter frame there, in the files section as part of the design for the "Isaacs Beginers' Clock".
In short though, the design consists of a 1/2" arbour that ducks down to the 7mm necessary to hold the cutter at the appropriate location. No ball races though - a simple cone bearing at each end using a hardened point on the end of an Allen bolt - don't forget the oil !
I adapted it to make something similar that fits in my milling machine 2MT taper spindle instead of being held in a cutting frame - take a look at:
http://picasaweb.google.com/youra.turceninoff/WildingEggTimerConstruction#5207396943643852034
That's one of the Thorntons cutters - you could easily make the shaft diameter a little larger - as long as you didn't exceed the 'flat' clamping area of course. Most designs I've seen for this seem to use 1/2" as the 'main' diameter for the shaft. As everyone else has said though, stiff is good in these circumstances !
HTH,
Youra

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Sorry, I don't work there; I came down from Nottingham last January with a party of year 10 physics and electronics pupils who were engaged on a project involving mobile phones. Perhaps you remember the blond-haired boy who knocked over the display stand?
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On Wed, 19 Nov 2008 13:57:42 -0000, "Amateur machinist"

As the others have said. Within a few percent, all steels have the same Young's Modulus AKA deflection/length divided by force/area. What changes with hardening (apart from the hardness :) is the force at which the part bends or breaks. Although it's nice to have hardened tools because they don't get scratched and dented so quickly, there is very real risk of bending when hardening. This means that hardening is only good if you are set up to cylindrically grind (or toolpost grind) the arbour after you've hardened it.
As also indicated. Stiffness is your friend and size _does_matter. If possible, make the arbour as short and thick as possible, given the diameter of the cutters. Then clamp the cutter with a closely fitting sleeve over the arbour retained by a nut and a thick washer turned to suit.
An example is here:-
http://www.test-net.com/arbour.jpg
That one is really longer than it should be, but seems to work. It's got a 2MT end and takes 1" bore cutters. The far end is centre drilled so that the tailstock can add additional stiffness. Without that, it would be useless.
The one in the picture has got a keyway and key but, if the clamping is any good, keys offer no advantage for gear cutters and slitting saws.
Mark Rand RTFM
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