Hard milling of 60+/-2 HRc bar

I need to mill a slot for Woodroff key (no 404 if anyone cares...) in a bar used for linear bearings.

Diameter of bar is 5/8 inch, it is hardened to 60+/-2 HRc ( heat treated C53 steel) to the depth of about 1 mm. I can imagine gushing it a bit with dremmel like hand grinder to remoove most of hardened material but then it have to be milled to size.

How to approach it?

I would not do heat treatment of it and I do not have a surface grinder. Have heard that carbide mills can carry out such tasks albeit they do not last long and of course there are CBN tipped mills, too expensive for me.

Do I stand any chance with solid carbide mill or perhaps it is just destroying nice mill to no avail?

Any other ideas?

Spindle speed is King here. Lots of coated cutters can handle the task if you have the required speed.

Does it have to be a woodruff key as choice of woodruff cutters is quite limited. If you can use a short piece of keysteel then you will have a better range of available milling cutters.

I would go with a straight key as these are stronger and less likely to weaken the bar.

Hi Martin

I would turn a copper disk less spark gap and take it to local toolmakers to spark erode to size. Just a perk of being a just retired toolmaker. I think you will struggle to do a good hub in such a precision hard component.

David

Does it have to be keyed? Could you use something like a Tollok taper lock fitting.

http://www.techdrives.co.uk/mechanical-locking-bushes.htmlt

Drill Service list carbide Woodruff cutters, and claim they will cut up to 62Rc. However, they also say you're likely to break the neck of the cutter. And with the cutters starting at £50 and going up to £80+ that'd be annoying to say the least.

It is possible to mill hardened steel though. If I need a precise shape, or I need to rough out, I use carbide cutters on HSS toolbits. Plain carbide cutters work fine, although I reserve older, well used, cutters for the task as they wear quickly. As Chris says you need a high surface speed with small DOC and reasonable feedrates.The cutting interface needs to be hot!

Andrew

I agree with those recommending changing to a straight key. A carbide endmill should do it nicely. The Woodruff key seat would be nearly impossible with hobby sized milling machines.

Thanks for comments,

@ Chris Evans, What sort of speed we are looking here at? 1500 RPM or 2000RPM for 1/2 inch diameter carbide Woodroff cutter? TiAlN coating OK?

@Jason, Which type of cutters? End mills/slot drills are not really good for keyways due to oversized slots and also sometimes side surfaces of slot are suffering. Disk type of cutters would require quite long slot due to larger disc radius, comparing to Woodroff key. They are also expensive in carbide tipped version.

@David George, If everything else fails or is rejected up front as unlikely to work, then it is a good idea. I was also thinking about dressing small grinding stone.

@Howardt. Has to be keyed. That is what drawing says.

Martin

I don't have too many issues with 1/8" 2 flute cutters and usually have to ease the keysteel or gauge plate when I blue it to fit the slot.

If you are worried about exact width then use a 3mm cutter and open out the width with a finishing cut down either side. 

The other trick is to grind down a 1/2" endmill to make it into a slot cutter but as Andrew says there is a risk of breaking the solid carbide as the neck would be quite thin compared to the head.

Edited By JasonB on 24/09/2017 08:10:24

Martin, I misread the post and assumed it was a straight key not a woodruff. As others have said straight key is the way to go. Use maximum speed of your mill, before I retired we would run small carbide cutters at work up to 18000 RPM that was to slow for some operations.

I'd agree, keyways cut with a slot drill are generally within a few tenths on width, as measured with gauge blocks. Part of that is the tolerance of the cutter.

Andrew

Have seen machines cutting hard steels and it is in another world to model engineering. The tools required are expensive and the machine has to be very rigid with plenty of power and cut dry. Good luck, but I wouldn't attempt it would rather find an alternative way of connecting the pieces.

Have a read of this I found.

**LINK**

Thanks for comments, I will try to make a slot for straight key with slot drill first.

Never have tried hard milling, but I am familiar to a small degree with hard turning.

So yes, I have succeeded with turning of 18mm bar hardened to 1.5mm depth to 62HRc into MK2 taper, and yes, believe it or not but that was done on ML7 but using Sumitomo CBN "600 grade" insert, NU-CNMA120408; BN600", if anyone cares.

Using the same insert i have proceeded further to repair broken off *carbide tip* of MK2 center, which I want sharp as it is used for making punch points on certain jobs.

So yes, I have turned *carbide* using this insert on my ML7. Depth of cut was below 2 thou on carbide and up to 5 thou on 62 HRc bar.

On the top of it insert doesnt appear to suffer damage yet.

The only feature my ML7 has which could assist the task are hand scraped bedways, including these belonging to TTA, what ensures smooth carriage travel. Needless to say, these bedways need to be protected, particularly during carbide turning, which was done with a top slide anyway.

So within 2 weeks or so I will try this straight key slot, later maybe even experimentally a Woodruff one, but then the cutter would be held *not* on a milling machine but on said ML7 held in collet and supported by tailstock to address "neck breaking" risks. For this I will need to make or order new pulley set to get this 1500-1700 rpm at least. for 1/2 inch Woodruff carbide cutter it should be enough.

Anyway, I will report my results, but it will be a while from now.

Slow speed carbide drill each end of slot

cobalt tool on a shaper to make slot

(You'll have to grind the tooling yourself)

Edited By Ady1 on 26/09/2017 09:25:49

Alternatively go in through the skin as above then finish woodruff style with a circular cutter

Edited By Ady1 on 26/09/2017 09:34:08

Why not grind the slot with a small diameter white aluminium oxide wheel?

Neil

Guys,

Don't forget that this is only case hardened. Once through the skin, a normal HSS woodruff cutter will probably be OK.

Just my twopenceworth,

Rod

@Roedrick, This casing is about 1mm thick.

@Neil Wyatt, Grinding will obviously do,

However lacking cylindrical grinder, I would need to use crosslide mounted appropriate hand grinding machine (I have one, Milwaukee brand, it is even adapted by manufacturer for mounting in the lathe). Nevertheless I do not like grinding in the lathe, there would be plenty of wheel dressing too. I see it as one of last resort approaches.