Broaching Keyway

This topic has 11 replies, 10 voices, and was last updated 6 May 2016 at 14:22 by Nigel McBurney 1.

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11 February 2011 at 09:34 #63919
Alan Worland 1
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@alanworland1
I have rough machined a cast iron flywheel and need to put a keyway through the bore.

Bore is 7/16 which needs a 1/8 keyway about 0.050 deep, the flywheel boss is just over an inch long.

I am thinking along the lines of using a piece of brass rod (to not damage bore) drilled and tapped to take a ground toolbit which can be ‘unscrewed’ and gradually cut the keyway.

I have a small flypress which I was going to use to drive my cutter through – but it all seems very small, is this the best way of going about this?

Alan
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11 February 2011 at 09:34 #5399
Alan Worland 1
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@alanworland1
11 February 2011 at 11:42 #63924
Gordon W
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@gordonw
I would “plane” it. Put a square ended cutting tool, like a parting tool on its side, in the biggest boring bar that will fit in the hole. Set the center of the tool at center hight. Lock the mandrel and cut by moving the cross-slide, putting on a small cut each time. Doesn’t take long for a one-off. Might have to adjust the key thckness to suit.
11 February 2011 at 11:46 #63925
Speedy Builder5
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@speedybuilder5
Seems a good way of doing it, but I would use steel for the shaft, the bearing area would be large enough not to cause damage to the flywheel, just make sure there are no sharp edges on the ‘mandrel’. You may find brass is too soft to hold the tool bit rigidly enough.
11 February 2011 at 14:00 #63927
Michael Cox 1
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@michaelcox1
I made a small hole keyway cutter that was essentially an adjustable broach. I think this would work quite well for what you want to do if it were scaled up in size. Full details are here:

http://mikesworkshop.weebly.com/small-hole-slotting-tool.html

Mike
11 February 2011 at 15:47 #63929
Francois Meunier
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@francoismeunier96697
Hi

a crucial point in making a broaching tool is to allow sufficient room for chips, otherwise the tool bind and get jammed with disastrous results, your bore may be ruined. the best way is IMBO, as GordonW suggest, by planing in the lathe, a safe, reasonable and a 10 minutes job, including sharpening and setting the planing tool.

Zephyrin
11 February 2011 at 16:51 #63938
JasonB
Moderator
@jasonb
Its fairly easy going planing the keyway into CI and not that much harder into steel, just take a thou or two each cut and let the spring run out of the tool every so often.

This is one I did earlier, four 3/16″ keyways in steel about 1″ long, don’t think I would want to use this method on much over 3/16.

I did recently find a site in the US that had a very reasonably priced set of small broaches and bushes 1/16, 3/32 and 1/8 for about $80 and just about ordered them until I saw the postage at $170

J
12 February 2011 at 16:17 #63975
WALLACE
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@wallace
I know it’s a cop out – but a local engineering company might be persuaded to do it for not too much. I needed a few keyways in some stainless pulleys I made up – I coldn’t see myself doing that many of them in the future so it was £20 well spent . .

W.
12 February 2011 at 19:58 #63987
Alan Worland 1
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@alanworland1
I know I could ‘farm it out’ but that sort of defeats the object.

Tha methods described above sound good – thanks for your advice!
12 February 2011 at 23:53 #64001
John Olsen
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@johnolsen79199
Some of the above strikes me as using a sledgehammer to crack a walnut. All you really need is a scrap of cast iron turned to fit the bore, a drill the size of the key you want, and a small square file. Turn the cast iron to a nice fit in the bore and loctite it in place. Face it flush with the flywheel on one side. Make a centrepop mark exactly on the joint line and drill through with a 1/8 “drill. This should be a good drill that you can trust not to wander. This is also why the plug you fitted should be the same sort of material as the flywheel.

Once you have drilled through, you have removed 78.54% of the material to make the keyway. Knock out the plug and carefully file out the rest. The half hole makes an excellent guide to keep you lined up. Use a piece of the key stock as a gauge, it is not too hard to get a very nice fit. I have done several for Stuart engines like this.

A few things to bear in mind…make the keyway in the flywheel first, it is much easier to make the one in the shaft to match if you end up a little wider than you meant to. Same goes for the key, make that last to fit both the keyways. Don’t trouble yourself too much if the fit ends up a little looser than you meant, a little loctite will cure things, and for a key it does not need to be the super retainer grade, the ordinary threadlock will suffice to keep things in place.

regards

John
6 May 2016 at 07:40 #237702
guri alley
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@gurialley39278
I agree with you, Alan. Methods explained are very interesting….
6 May 2016 at 14:22 #237760
Nigel McBurney 1
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@nigelmcburney1
Alan I can see your point,if you do not try yourself one does not learn ,so have a go, I make my boring tools from silver steel just as I did 55years ago during apprenticeship,in those days the holes in the bar were square,no doubt laboriously filed from drilled holes,since those days I cheated a bit ,Just drilled a hole int the bar and used a round HSS toolkit..now should the tool bit be the same diameter as the required keyway just grind a short length of the tool bit,down to the radius so you produce a flat of the diameter in width, for small keys side clearance is not required,just some clearance ground on the end, this make a nice broaching tool. One snag is the size of your lathe,and the size of the saddle hand wheel,as it takes some effort to plane out a long keyway. also be aware that the side load on the boring bar tool holder will have a tendency to rotate the tool post so you may be merrily adding small cuts and not get very far as the tool post slips.This method of using round toolbits is ok up to about three eighths of an inch,above this they are too weak and break,Care must be taken with set up to ensure the keyway is central in the hub,a wonky keyway which is off to one side and nor truly radial looks awful.One thing myself and fellow apprentices found was that the lathe spindle did not tend to rotate when cutting ,just put the lathe in low speed and back gear ,the lathes used were a large Willson and a Boxford neither had a spindle lock.
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