Milling that slot

[martyn nutlandParticipant @martynnutland79495]

Helped with milling a slot on round bar

[martyn nutlandParticipant @martynnutland79495]

A few weeks ago forum correspondents were good enough to advise me on milling a slot in a round bar and I thought it would be an appreciative gesture if I let you know the denouement.

We explored a number of possible causes for difficulty. Unknown specification of the metal – I was using a surplus Austin Seven gearbox shaft; lack of centricity and/or rigidity of the workpiece, suitability of, and working procedures in using the slot drills I was 'hell-bent' on employing, and so on.

 

So, I threw away the bit of A7 gearbox and ordered virgin, free-machining steel from that most excellent supplier, R C Machines in Luxembourg. I took the precaution of buying both 15mm and 20mm diameter lengths as the finished part was barely 14mm in diameter and break out either side when machining was an issue, particularly if I let the tool deviate to one side or t'other.

 

To that end I trammed the vice to within 0.02/0.05 mm parallelism which, in spite of watching numerous 'how to You Tubes' from the great and the good, is about the best I can manage. I also decided to use about half the 300mm long bar although the finished part needed to be no more than 30mm long, the thinking being it would give me plenty of material to 'hold on to'.

 

To do so I set up twin precision ground V-blocks held as far apart as practicable in the vice jaws. I found the jaws would only grip one block. This could be because there is slop in the newish Vertex vice, although none is perceptible, or the identical V blocks (Chester) aren't in reality .

 

I have to confess to not taking the advice on centring, largely because I didn't really understand. I guess I could have touched on one side of the bar with an edge finder, done the math, touched from the other side, calculated again and divided, but given my level of competency and the back-lash on the Warco I stuck with my dangly inverted V gadget, that I do happen to like, in the mill spindle. But I did double check with the old machinists' trick of touching down with a centre drill on a steel rule placed across the circumference and eye-balling it for levelness.

 

I then drilled into the top of the bar at each end to seat the clamp of each V block and I overcame the fact that one was not gripped by the vice by building up with parallels a step block pressed across the end of the recalcitrant block. I also packed with spacers the gap in the vice jaws between the V-blocks.

 

I found I could get away with an 8mm wide (as opposed to 9mm) slot, so I amended the 'spec' accordingly. I felt the rigidity and centricity(?) I now had was the 'answer to a maiden's prayer' so applied the previous technique. Drill holes on the ends of the required length; cut through with an end mill and widen accordingly. M-I-S-T-A-K-E! Worse mess than before.

 

Thus I decided to 'bite the bullet' and experiment with a brand new 8mm slot drill. In a test slot on the material I found the slot drill would plunge 0.75mm static then

happily slot at that depth over a test length (12-ish mm -18/20 required). 'Perfick'!

 

Try it for real. Problem. Slot drill is pushing the bar along the V blocks. This problem was identified in the forum when I was holding with a collet block in the vice. Paper stuck to the ground surfaces was suggested. Because I didn't want to disturb the workpiece I didn't do that but instead pressed step blocks against both ends of it. Slot drill now plunges and advances with no problem and makes a perfect slot on the axis of the bar. I finished to diameter with an interrupted cut on the lathe. I ran the mill at 360 rpm and plunged the slot drill by 0.5mm increments.

 

Delighted with the result so thanks again.

 

Martyn N

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Martyn Nutland
10 avenue de la Porte de Ménilmontant
75020 Paris

[EmgeeParticipant @emgee]

Thanks for reporting back Martyn, you have discovered that when milling the material must be held securely to avoid cutting forces moving the job, that always results in tears.

Emgee

[MeunierParticipant @meunier]

Thanks Martyn for the link to RC-machines. They appear to carry a wide range of stock in grades I can get my head around and hopefully less expensive on shipping to SW France than from UK.
DaveD

[Howard LewisParticipant @howardlewis46836]

Thanks for the update.

Glad to hear that you succeeded. And more experience gained.

"Experience lets you recognise the mistake, the next time that you make it"

So often, we finish up wondering, "Did that fix the problem?" or "Which was the successful method?".

Howard

[simondavies3Participant @simondavies3]

Posted by Meunier on 04/03/2019 20:57:14:

Thanks Martyn for the link to RC-machines. They appear to carry a wide range of stock in grades I can get my head around and hopefully less expensive on shipping to SW France than from UK.
DaveD

I have recently used "acier-detail-decoupe" on several occasions who, despite their name, also stock Al, brass, cast iron, copper bronze and composites – although I have only taken steel sections and silver steel and aluminium from them.

Prices are displayed and the postage has various options – and is weight driven so 2 smaller packs can be cheaper than one large one with a big carrier.

Ships across France I believe, certainly to this corner of the SE (PACA)

Regards and apologies for any accusations of thread highjacking,

Simon

[MeunierParticipant @meunier]

Thanks for that one, Simon
DaveD

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