Enlarging an existing hole in a steel chassis.

[Nanozeugma]

Hi.
I have discovered to my irritation that one must assume nothing, lest one commits a faux-pas!
To explain...
I have purchased some new B9A ceramic valve bases to replace old, corroded ones, and (silly me) assumed the size of the drop through ceramic body would be the same diameter as the old one - wrong!
The chassis hole reads 18.85mm and the new valve base ceramic body 21.95mm. I reckon I need to enlarge the hole by 3.5mm.

I was wondering what would be the most appropriate tool to enlarge the hole neatly? I could, of course, use a half round file, but that would be a bit crude and possibly end up off centre to the existing holes for the fixing bolts.

Your thoughts much appreciated

[emeritus]

Draw a circle of the correct size and position with a Sharpie (other permanent markers are available!) and use the file. I do have a set of circle templates that I use for this sort of thing but I also use anything circular of the right size (pipe offcuts, bottle tops) , or use compasses to draw a circle on a piece of card (corn flakes packets are of suitable weight) and cut out the hole. I have used a Swann-Morton No. 11 scalpel blade clamped in the drawing ink pen attachment of the compass of my geometry set for this.

[vinrads]

I did come across this problem , for future ref you can buy the correct base , to enlarge the hole I use a stepped drill bit, apply a squirt of wd when drilling ,I had eight to do on this Rock Ola amp all good fun , Mick.

[chriswood1900]

I would use a taper reamer like this obtainable from most of the usual suspects. ebay, Amazon etc. https://precisehandtools.com/tr-04-t...mm-30mm-holes/
Other options might include a step drill but that will usually cut in 2mm intervals so 22mm is nearest.
Good luck

[Ambientnoise]

A “Conecut” or look alike will do it. Like a step drill without steps. Cheapo versions come up at Lidl/Aldi quite regularly. Only problem is that they obviously cut tapered holes but not an issue on thin chassis metal.

[snowman_al]

Or buy the correct sized sockets?
If this is for the MA-12, I used to ones that look like this https://www.ebay.co.uk/itm/Ceramic-T...oAAOSwbsBXliqB

I have seen them from UK suppliers too, but a quick search has not got me a result so far.

Alan

[JonSnell]

I use my trusty SnapOn Tools reamer and a T bar or low revving power drill. Makes a neat job.
Pop into ToolStation or order from RS Components if SnapOn is financially prohibitive.

[David G4EBT]

Originally Posted by Nanozeugma

.

I was wondering what would be the most appropriate tool to enlarge the hole neatly? I could, of course, use a half round file, but that would be a bit crude and possibly end up off centre to the existing holes for the fixing bolts.

Your thoughts much appreciated

It wouldn't be that much of a chore with a half-round file - to enlarge by 3mm only requires 1.5mm to be removed around the perimeter. That said, if you have a pillar drill, a 'step drill' (often incorrectly terms a 'cone drill', which is tapered -not stepped), is by far the best solution. They've been mentioned before, but are worth repeating. A step drill would do the job neatly and in a jiffy. A set of three isn't expensive, and they're excellent for cutting holes in metal, ABS project boxes, and especially acrylic sheet without snatching, and even cut their own pilot hole. EG:

https://www.amazon.co.uk/Malayas-Tit...74251327&psc=1

Hope that might help.

Good luck with it.

[JonSnell]

Originally Posted by snowman_al

Or buy the correct sized sockets?
If this is for the MA-12, I used to ones that look like this https://www.ebay.co.uk/itm/Ceramic-T...oAAOSwbsBXliqB

I have seen them from UK suppliers too, but a quick search has not got me a result so far.

Alan

https://uk.rs-online.com/web/p/vacuum-tubes/6784094/
Better quality and probably cheaper than fleabay. And you will receive what you want, not what they send!

Cone cuts have the advantage of cutting any size albeit with a slightly tapered hole.

[snowman_al]

Unfortunately Jon those on RS are the ones that are too large for the hole. They are also from China, but as you say...

ps. Colin, 'Mullard Magic' are advertising the original type socket at the moment, a little scabby but you never know.

[Restoration73]

All this would be too much hard work for me. Instead

https://www.cromwell.co.uk/shop/hand.../p/QMX0451110L

[BRASSBITS]

Originally Posted by Restoration73

All this would be too much hard work for me. Instead

https://www.cromwell.co.uk/shop/hand.../p/QMX0451110L

how does that work when theres already a 19mm hole

[The Philpott]

I think i would be confident enough to do this job with a dremel abrasive stone, going gently and rotating it around the hole at an even speed and even pressure. Certainly easier to get an even round hole than filing.
Dave

[McMurdo]

if there's plenty of room then a cone cutter, (I attack it from both sides gently, to avoid stressing the chassis) or if space is very tight (might catch under chassis wiring etc) then I've reamed the hole out before now with the sanding cylinder thingie on a dremel (it wrecks the abrasive, so used as a last resort).

Watford Valves do a decent selection of valve bases if you'd rather just get the right one!
https://www.watfordvalves.com/products.asp?ID=6&man=83

[Radio Wrangler]

I keep a set of good quality tapered reamers, the long stepless ones. Operating them by hand is fine, food ones are sharp and will cut gently without grabbing. This way I can monitor the centring as I go and a little tip of the tool will steer it's progress so I can control the cutting so it is even around the perimeter.

Brilliant tools. You can smoothly ream a quarter inch starter hole out to an inch or more without feeling like you've run a marathon. The ability to cut is one thing, the ability to control the cut is even more important.

David

[Nanozeugma]

Thanks to everyone for their input and thoughts.

In the first instance, it's my fault that I made the assumption that all drop through B9A sockets are created equal, diameter size anyway.

The originals are ceramic, and I wanted to replace with ceramic but ideally with gold plated pins for better resistance to oxidation.

Therefore, I ordered, and have to hand, sufficient sockets of the type I wanted to fit - except that the ceramic bodies are too large for the chassis holes.

I can quite see the logic of tearing up that idea and obtaining sockets that will fit without enlarging the chassis holes, but they would not be what I wanted.

So, having obtained the sockets, I am, so to speak, committed to using them. All that is required being to enlarge the chassis holes by about 3mm, the bolt mounting centres are, fortunately identical.

Of the possibilities suggested, I rather like the idea of a suitable cone drill.
The chassis is mild steel, approx 1.3mm thick and there is adequate room to use a cone drill.

The problem I have is selecting a suitable specimen.
Reading users experiences of cheap ones tends to suggest that they blunt at anything mildly resistant. Prices range widely.

It would be ideal if someone could point me to **one that experience has proven is successful** pitted against thin(ish) mild steel.
I obviously don't want to spend an arm and a leg on something that I may, quite possibly, never have to use again.

I am, however, grateful to all those who have taken the time to volunteer ideas and experiences (in case you thought otherwise!)

[ex 2 Base]

I got mine from Aldi several years ago, I have used them a lot and they are still in good order. The trouble with Aldi and Lidl is once they have gone, they are gone, and you don't know when they will be available/reappear once again. Forgot to mention the 3 warranty. Ted

[bionicmerlin]

I have got mine through the trade . G&J conecutter. I’m sure they are big money but they do last. Use some cutting compound to make them last.
One word of warning. They can bite and almost instantly the hole is too big.
Andy

[GrimJosef]

Originally Posted by BRASSBITS

how does that work when theres already a 19mm hole

You have to put masking tape on the work surface and make a few pencil marks around a circle centred on the hole you want to expand and with the diameter of the Q-Max die. Place the die, using the pencil marks, so it's also centered on the hole, drop the screw through, use the screw to pull the punch up to the other side of the metal, one last check that everything's still centred and then start tightening the screw in earnest. 1.5mm might be close to the limit for cutting a thin ring, but with a relatively small diameter punch I'd certainly give it a try. Perhaps the biggest worry is the thickness of the steel chassis, which is also close to the Q-Max limit.

Cheers,

GJ