How would you do this keyway?

[HollowpointParticipant @hollowpoint]

Hi guys. A need to enlarge a keyway has arisen, The brass gear below is for a power feed, it has a 3mm keyway which needs enlarging to 5mm. The keyway is almost 60mm long. Having never done a keyway before I don’t have the proper kit or knowledge. In the absence of a proper broach and press, how would you tackle this?

[Grizzly bearParticipant @grizzlybear]

Hi,

Is the keyway 3 mm deep or 3mm wide?

Good luck………

 

[Nigel Graham 2Participant @nigelgraham2]

It may help if you tell us what equipment you have, and what you are trying to achieve.

Though not good practice, as it puts unfair strains on the machine, a keyway can be cut using a lathe as a makeshift shaper. The tool is held in the tool-post as normal but on its side, so the cross-slide controls the depth of the keyway. The saddle is then racked back and forth to produce the cut.

An alternative, if the purpose is to match the gear to a shaft with a bigger keyway, is one I used where the keyway in a wheel hub is slightly wider than the axle keyway. I made a key stepped across its width, so the narrower half fits the shaft keyway, and the wider half fits the hub.

This may be the better way here than trying to modify a slot 60mm long, accurately. Certainly easier.

[paul rushmerParticipant @paulrushmer83015]

Hello where are you roughly? I have a press and access to a 5mm broach. I am east of  cambridge if this helps

Paul

[HollowpointParticipant @hollowpoint]

Grizzly – The keyway is currently 3mm wide and needs to be 5mm wide. I can’t remember the depth.

Nigel – I have a Boxford lathe and a Warco VMC mill, I’m not a complete amateur but broaching is just something that I’ve never needed to do before. I have been watching some videos on YouTube and using the lathe might be the way to go.

Unfortunately I can’t modify the key because there’s 2 separate parts that slip over the shaft. The key sits between where the two parts join or butt together. The brass gear slips on first so obviously the keyway needs to be bigger. If that makes sense.

Paul – I’m in South Yorkshire.

[JasonBModerator @jasonb]

I’ve dome 3/16″ keyways in steel using the lathe so your 5mm in that nonferrous should be easier. Just make a simple holder and grind up a tool, the existing 3mm keyway will help guide the tool.

Iailing that an imported 5mm broack and homemade guide ban be pressed through if you have a reasonably sized bench drill.

[DC31kParticipant @dc31k]

The keyway in the manufactured item may be 60mm long, because it was easiest and cheapest to make it that way when the gear was produced.

In its future application, how long does the keyway actually need to be to do its job?

Any reduction in its length (plug the hole and drill away the excess) will reduce the amount of work needed to widen the existing one. It may also simplify tooling as a shaper-type tool that needs to reach 60mm will deflect a lot more than one that, for instance, only needs to reach 30mm.

If the length can be reduced, careful filing might do it*. Only touch one side at a time so aim to make it 4mm and offset (use a 4mm key or toolbit to check this stage). Then work on the other side to make it 5mm.

*sacrifice a file and make it to have only one cutting face.

[JasonBModerator @jasonb]

Does raise the question if using a 5mm keyway does it also need to be the correct depth for a 5×5 key or are you just going to widen the 3mm and not go the extra 1mm deep?

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Hollowpoint, if Paul can do it, maybe you could post it to him with a payed return label.

Regards Nick.

[Eric OlthwaiteParticipant @ericolthwaite]

The second method in the video might be best suited. Racking the stationary cutting tool bit up and down by the milling machines vertical feed lever is much easier than racking a lathe carriage back and forth by the handwheel. The latter can get very tedious very quickly.

[Charles LamontParticipant @charleslamont71117]

For slotting in the lathe I made a lever and link. I use the base of the hand tool rest as an anchor. This puts much less stress on the machine than racking with the saddle handwheel. The handle is a piece of an apple branch from the garden.

[Clive FosterParticipant @clivefoster55965]

What a splendid idea Charles.

Effectively answers the too much strain on a small lathe when using the saddle feed issue and simple enough to be worth making for “the one and only keyway I shall cut in my life” job. Of course once you have got it …..

That said I’m unconvinced by the strain on the carriage argument. Yes a keyway is a wide and relatively heavy cut if you go straight at it. However if, as I do, a starter hole is drilled half in the job and half in a plugged bore just a touch smaller diameter than the key width most of the heavy cutting is already done so the stain is seriously reduced. Only the last bit of the end corners being full width. A little bit of attention to the corners with a file helps there. I routinely drill the starter hole before boring.

If the OP is worried about stress a bit of filing to remove a useful percentage of the metal to be cut isn’t too difficult. Alternatively a narrower tool could be used and one side at a time cut. If working in the lathe use a 3 mm tool and shim up 1 mm to do the top side and down by 1 mm to do teh bottom.

Clive

[Michael GilliganParticipant @michaelgilligan61133]

It may be worth skimming through this for ideas:

https://gisstec.com/top-7-internal-keyway-cutting-methods-how-to-choose-the-right-one/

… I was immediately struck by the resemblance of [5] to some tooling for the Ornamental Lathe.

MichaelG.

.

Edit: __ some videos here:

https://www.youtube.com/@Gisstec/videos

[ChrisLHParticipant @chrislh]

The first keyway I cut I used the racking the method and ended up with a lot of play in my Myford saddle (a very poor guide arrangement for resisting this sort of abuse !). For my second go I removed the topslide leadscrew and rigged up a handle and link similar to Charles Lamont’s above but anchored to the saddle and reciprocating the topslide. Arrangements with adjustable tool height are useful here. It’s still hard work and I wish I had come across Clive’s method above of easing the job at the time.

[Eric OlthwaiteParticipant @ericolthwaite]

Ian Bradley published several designs in ME in the 1940s for lever-acting the topslide for keyway cutting. Theory was it saved wear on the main bed ways.

 

[Dave SParticipant @daves59043]

Why not make a stepped key, 5mm on one side and 3mm on the other.
Only thing modified then is a small piece of square key stock, and it’s a simple job to make it

 

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Good thinking, Dave !

I guess it depends whether or not the ‘machine’ actually needs the strength of 5mm

MichaelG.

[Tony Pratt 1Participant @tonypratt1]

Go for a stepped key.

Tony

[Eric OlthwaiteParticipant @ericolthwaite]

Hasn’t Hollowpoint already said a stepped key won’t work because it has to fit a second component on the same shaft as well?</p>
But could 2 keys be used? One stepped, one not?

Or mill the step halfway along one one long key, letting the curve from the cutter extend past the join line if necessary.

[HollowpointParticipant @hollowpoint]

OK guys, here is the situation. These parts are for a power feed on the VNC knee. The brass gear must slip on the shaft first as shown below. The second, castellated part goes on second. Both parts span a single key.

[HollowpointParticipant @hollowpoint]

I have decided to have a go at doing it on the lathe as suggested above, I spent this afternoon making a tool similar to that shown in “blondiehacks” YouTube video. I tested the finished tool in some scrap brass and as long as I take very small cuts (0.2mm) it seems to work fine. In fact, I am pleasantly surprised at how well it works!

[JasonBModerator @jasonb]

With the smaller one going on first a stepped key won’t work unless the shaft keyway extends to the end of the shaft so it can be pushed in from the end.

But they do not have to “share the same key” they can share the same keyway in the shaft but have separate keys.

Asuming the shaft and nut have 5mm keyways just use a 5×5 square for those. When it comes to the gear make an inverted “T” shaped key with a 5mm bottom to fit the shaft and a 3mm top to fit the gear.

 

EDIT posted between your two, so may as well just enlarge the keyway.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I don’t like stepped keys, and I’ve seen many that get sheared off, or badly deformed, mainly because they are weaker, and the small they are, the more likely they are to fail. Many think they are a passive item, but they are not. They really should only be used as temporary thing, to get a plant running for instance, until the next maintenance period.

Regards Nick.

[HollowpointParticipant @hollowpoint]

Now I’ve made the tool I might as well run with it. The thing that worries me now is getting it all set up perfectly! 😅

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, well providing you set the tool centrally on the lathe’s centre, there shouldn’t be much of a problem. Just lock you chuck by eye to the keyway and start cutting.

Regards Nick.

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