Cutting a slot in a turned piece

Hi, I restore old motorcycles. I sometimes have to make custom ferrules for control cables. These are simply turned and drilled mild steel bar.

The turning and drilling is easy but what would be the best way to cut the slot?

I have tried hacksaw and small files but its not very neat and a bit of a bodge.

I have a Myford ML-7 with a collet chuck. I also have an Amolco milling attachment that will take the Myfor collet chuck. Also a milling slide.

This is the sort of thing:

Slitting saw. Basically a small circular saw blade for sreel. Mounts on an arbor in your lathe or mill and the job held in a vice etc and fed into the blade.

Might also be worth mentioning to hold them lengthways in a machine vice while slotting, if they were held on the diameter there is a possibility of them either closing up on the cutter or flying out the vice, sorry if I am teaching grandma to suck eggs.

You might also be able to use a very small slot drill in your mill. Such as these sold by RDG Metric Slot Drills

Good point Baz, thanks, im pretty new to anything other than basic turning and drilling operations.

Thanks Graham, thats a good idea.

Couple of points; using a small slot drill, you will need to take very small cuts to avoid expensive pinging noises. Don't expect to do it all in one pass. High speed is good.

A slitting saw would be better, but it needs to run slowly; you're not using a circular saw on wood. (I recall one chap asking what he was doing wrong with a slitting saw on steel as all he got was sparks and broken teeth hurtling about...)

I'd agree with Nigel that a slitting saw's probably preferable, not least because it's difficult to feel how much side stress you're applying to a small slot drill.

You have to be moderate with the feed on the slitting saw, because you'll only be able to hold on just over half the end diameters of the ferrule to allow the cutter entry and exit, and each cutting tooth's going to be trying to hook it out the vice. My instinct would be to cut from small end to large, but that's a guess - there could be risks either way.

If you have to hold on the diameter you can install a close-fitting sacrificial plug to prevent close-up.

Hi Adam,

If you use small dia slot drills or endmills, use high speeds. Slitting saws, start at the slowest speed and change up to next one and so on. Key is to listen to the tool, loud screeches or other noises, its saying I'm in pain! In other words, something wrong like too high a speed, or work not adequately secured against vibration. Even just clamping work will dampen vibrations leading to a better finish. Be ready to experiment a bit with setups and speeds. Above all, use at least good eye protection - NHS dont do spares yet.

Good luck with job

John

Edited By DMB on 27/04/2020 14:14:09

Have sacrificial chucking pieces been consigned to ancient history?

Rather than trying to hold tiny pieces, hold a big piece and do the work on it, then cut off the small piece you want as a final operation.

All very useful advice thank you everyone.

Yes, I did think a 2mm dia milling tool looked pretty "snappy"

I have to confess that it was the accountant in me talking. Slot Drill £7 slitting saw and arbour £21...

I would slot them before parting them off to length. That way you can hold the bar stock in the machine vice with the end for slotting overhanging.

Then you'd have to plunge-form the smaller diameter so as to give drill access to the larger end, unless you think you can drill that after slotting - which seems to me to offer extra risks and difficulties. If you did plunge-form, it'd probably be easier to add the width of the part-off blade to the form length.

Yes Mick that’s exactly what I was thinking.

It can be done neatly by filing but you need a knife file the correct size. Generally used for creating screw slots.

regards Martin

I've not done anything like this myself, but I'm wondering if it would be sensible to start with bar stock in the lathe, drill out the centre, turn the outside, then transfer to the milling machine to cut the slot before returning to the lathe for parting off? That way you have the ridigity and chucking ease of the bar stock while turning, and something to clamp onto while cutting the slot. The only tricky part is parting off an interrupted cut.

Have to say, I agree with you!🙂 Your maths is impeccable. You are correct in that the initial outlay clearly leans the accountant in the direction of a slot mill.

However: Slot drill life span - not long; abor lifespan - a once-off purchase which will not need regular replacement, probably not even in the longer term.

Real on-the-shop-floor costs would be far lower for the slitting saw option - but an account (as you say) wouldn’t have a clue as to the practicality of making an investment for the future. All an accountant sees is the £££ return on investment over the next few days/months.🙂

I expect capital costs and operational replacements are treated differently by accountants, but as a practical costing (non-accountant) the outlay for the arbor would be a long term investment, whereas repeated seven quid a time (for regular slot mill replacement) would soon mount up.🙂

For a one-off operation, slot mills might just be the better option, but definitely not for on-going manufacturing in our very ‘manumatic’ operations environment.

The common disagreement between engineers and accountants, admirably demonstrated in a short post....short term monetary outlay and return versus a far better long term investment with a better practical solution to the problem at hand.

The company I worked for suffered from the same issue choices - do it cheap or do it properly. I won’t trouble you with examples, but the approx half million quid annual budget I had (35 years ago) was soon reduced by ‘circumventing’ the accountants directives.🙂. Accountants had no clue as to the equipment down-time with resultant loss of production (or increased repair costs) on the production line.

Edited By not done it yet on 28/04/2020 05:04:11

I wouldn't even consider an end mill or slot drill for this job. That slot is about 1.6mm (1/16" ) wide on most cables. I am far too good at snapping small mill cutters to even attempt it. Also the Myford will not spin fast enough to properly power such a small diameter cutter.

Correct theoretical speed for a HSS slitting saw cutting mild steel would be around 100rpm for a 4" diameter cutter, 200rpm for a 2" diameter cutter and 400 rpm for a 1" diameter cutter. In practice, start out slower and work up to it if it feels comfortable. And remember, the slower the rpm the slower the feed. If you are making them out of stainless, slow down by half to start with and keep a good steady feed up so the cutter does not rub and work harden the job.

But these days you can buy boxes of cable fittings online that include such ferrules so hardly seems worth buying tooling to make your own?

Edited By Hopper on 28/04/2020 05:13:53