First attempt at trepanning.

[Robin GrahamParticipant @robingraham42208]

I have a job which involves making 100mm holes in 150mm x 6mm discs of black plate:

As I have a few to do, and the cutouts would be more useful than a bucket of swarf, I thought I would have ago at making a trepanning tool.

As you can see, it worked – but there is room for improvement.

I made the tool from a piece of 4mm square HSS:

I locked top and cross slides and fed manually using the saddle handwheel. Spindle speed was 65rpm giving ~20 m/min cutting speed.

What I found was that sometimes the tool chattered with a low grumbling sound, sometimes it squealed, and sometimes it went like a hot knife through butter. The discs were bought off eBay so I don't know how consistent the metal is, but my gut feeling is that the variation in behaviour was more likely to do with my inability to maintain a steady feed – especially as I've never done this before, so was perhaps overly hesitant. Something I've learned about parting off is that hesitancy is not good!

If so, I suppose power feed might be the way – but I have no idea what feed would be appropriate.

Or maybe the tool geometry is wrong. I used 4mm square because I had it – but perhaps that's not rigid enough and I should be using rectangular section, more like parting tool geometry.

Sorry that this is a bit rambling and unfocused – any education about this technique would be most welcome.

Robin.

[Robin GrahamParticipant @robingraham42208]

[HopperParticipant @hopper]

Yes 4mm square could be a bit small. I have done similar jobs and used a piece of 10mm square ground more like a parting tool but with the extra clrearance on one side for the curve of the resulting groove.

Looks like you have the toolbit sticking out too far for such a small cross section too. Might work better if you can tuck it back in by 50 per cent or more.

Also, grinding that notch on the top surface can cause problems if the back of the notch enters the groove because the parent material is wider than the tip of the tool because the tip is further down the tapered cross section. Hard to explain without pics, but I always run the top of parting tools dead flat for this reason. Grinding notches in the top leads to grief.

[old martParticipant @oldmart]

That is something I have not tried, but if the finish is not great, just cut undersize and finish by boring normally.

[larry phelan 1Participant @larryphelan1]

This is something I also have not tried yet, but I,m sure it will come along one day.

I seem to remember that L,C Mason mentioned that when talking about cutting out blanks for making gears, using 3/8" plate. His book "Using the small lathe" is a mine of information, well worth a read.

As I recall, he managed to get three blanks from the one piece, with little or no waste.

Might be worth looking up.

[Anonymous]

Posted by larry phelan 1 on 07/05/2021 12:51:22:

As I recall, he managed to get three blanks from the one piece………..

Correct, and he shows the tool shape he used. We can summarise, relative to the picture in the OP, as half the width and twice the height.

Andrew

[Robin GrahamParticipant @robingraham42208]

Thanks for replies. I confess that I didn't think this through too well – I just ground a 4mm square bit with enough clearance on the outside edge. As I (sort of) got away with it the first time I thought, pending arrival of a more suitable piece of HSS, I'd try another disc and experiment with speeds and feeds. The tool broke.

I haven't got Mason's book, but Andrew's comment that the tool should be half the width and twice the height (ie 2mm wide by 8mm high) made me think – surely that would foul on the outside edge? So I calculated:

 

It seems that for my 100mm hole a 2mm thick by 14mm high tool in that geometry would (just!) clear if I've got it right. Obviously that might not be the optimal tool geometry, it was just to give me a starting point. If any kind soul has a pic of Mason's tool shape to share, that would be interesting.

Hopper – I had to read your point about the notch about six times, but I think the penny has now dropped. Possibly!

Robin.

 

 

 

 

 

 

Edited By Robin Graham on 09/05/2021 00:10:16

Edited By Robin Graham on 09/05/2021 00:11:46

[JasonBModerator @jasonb]

I grind the outer edge with a bit of a curve, the more metal left in the tool the better, something like this

You can see how the commercial face grooving tools take it one stage further and curve both sides to give the insert plenty of support

Edited By JasonB on 09/05/2021 07:33:54

[Nigel McBurney 1Participant @nigelmcburney1]

I have trepanned a lot of black sheet steel,I have found that a parting tool ground with adequate clearance and some top rake and a flat cutting edge towards the work dos not work very well and tends to chatter.An easier way is to grind a full rad on the front of a parting tool start off by feeding in by hand until it starts protesting,then move the tool by the cross slide by just under the width of the tool,and feed in again,then move it back until the tool is central to the two groves then feed in until the tool protests,then repeat the process until the centre is removed,Now the centre piece could go anywhere and jam possibly damaging the lathe bed so its best to drill a hole in the centre and mount a rod in the tailstock chuck,feed the rod via the tailstock thro the hole in the plate and into the bore of the chuck this will act as a catcher,keep your hands out of the way until the lathe is stopped.finish off the bore in the outer plate with a boring tool.I would not use power feed on this type of work.

[Robin GrahamParticipant @robingraham42208]

Thank you Jason and Nigel for further info. Some more ideas to try – as I have several to do it is worth my time experimenting to get this technique under my belt. Apart from the pleasure of learning something new!

In my first (successful) attempt I went through until I saw a bit of a bulge emerging on the back of the work, then turned it over and took off the bulge with the trepanning tool. I then knocked out the centre piece and finished by boring. I can see that trying to go all the through without a centre support as Nigel suggests would be a surefire recipe for disaster.

Thanks again for all the suggestions , Robin.

[JasonBModerator @jasonb]

If you don't want a hole in the middle bit then I go in from both sides stopping just short of half depth so there is just a couple of thou left. You can then knock the middle out off of the lathe as the thin material is much like the top of a pull ring tin can and just peels apart.

[Mick B1Participant @mickb1]

+1 for the rad on the leading edge of the tool. Doesn't IMO need to be a full rad, and even a highly-obtuse vee will do – anything to divert the swarf towards the centreline of the tool to stop it jamming down the sides. Also as above, if you do put some front rake on it, make sure it doesn't run out back to the top surface of the tool within the thickness of the workpiece. Compound slides often offer better mechanical advantage and feel than saddle handwheels for feed, but that varies between machines – and users …

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