Grinding trepanning tool ?

[Rich2502Participant @rich2502]

What would be a good profile to grind the cutting tool on one of these, for cutting thin metal sheet ?

 

[JasonBModerator @jasonb]

Much like they have it there will work particularly if very thin. Or you can go for a blunter end more like a parting tool but just watch that the outer edge has enough clearance to stop it rubbing on the outer edge of the cut

[DC31kParticipant @dc31k]

As shown, the ‘point’ is on the outside of the tool, so the edge of the hole will be of reasonable finish, with the centre circle discarded.

Should it be the case that you need the circle as a finished item (e.g. you wanted to cut a washer out of your sheet), then having the point on the inside will give a better finish on the OD of the washer.

it will assist if you can sandwich the metal sheet between two sheets of plywood and clamp them well. Clearly you need to cut a clearance hole in the upper sheet first.

[Mick B1Participant @mickb1]

Standard practice I learned was to grind a broad, shallow radius across the front face of a trepanning tool, in order to divert the chip inwards to reduce risk of trapping and binding. The vee point shown can also do that, but will leave a tapered edge to the both workpieces.

Whether or not that matters depends on the purpose.

 

[SillyOldDufferModerator @sillyoldduffer]

I did a lot of ‘chassis banging’ in my youth.  Radio Hams mostly built their own gear, and the electronics were mounted on a metal chassis, often inside an outer metal box for screening.  Lots of holes needed for valve bases, electrolytic capacitors, dials, meters, cables, loudspeakers, cooling and much else.

Stamping and folding in a big press is the best way to make these; once set-up millions can be knocked out for little more than the cost of the sheet metal.   Not so easy at home with hand tools!   In the worst case, all the holes would all be chain-drilled and tidied up with a file, and straight lines cut with a hacksaw.   Twist drills and sheet metal aren’t best friends because the drill tends to catch and gouge as it breaks through.  Hacksaws aren’t ideal because at least 4 teeth should be in contact with the metal at the same time.

As valve bases came in a few standard sizes, it was worth buying a set of Q-Max cutters to punch them.  (Other brands of chassis punch available.)

The larger holes needed to accommodate meters and a loudspeaker, were usually made with a Tank Cutter as illustrated by Rich.  These were invented so that Victorian plumbers could cut holes in the galvanised iron water tanks installed in lofts and ceiling height lavatory tanks.  (Do any still exist?)

The cutter was held in a gut-buster hand drill – no motor – and turned relatively slowly with heavy pressure against the metal, which usually had to be supported at the back.   My experience with them wasn’t good!  Thin steel sheet and the soft Aluminium alloy used to make radio chassis both tend to distort and tear.   Never tried one on a galvanised water tank, but I suspect these are tough enough to resist tearing.   They work OK on plastic water tanks.

No forum advice available on tank-cutting when I needed it in the good old days, and it’s possible I was doing it wrong!   Am I right in thinking tank-cutters are best avoided, or should I try again?  Nowadays I reach for a hole-saw or step drill and my tank cutter has only been used twice this century (on plastic tanks).

 

 

 

 

Dave

 

[Nigel Graham 2Participant @nigelgraham2]

Avoid the tank-cutter if better is available – but that’s true of many tools. It does at least have the advantage that you can set it to the radius you need.

There is or was an alternative form of the cone-drill you illustrate, being a smooth cone instead of stepped, but I have not seen one either in the steel or listed anywhere for a long time. Are they still made?

.

Incidentally the majority of the high, syphon-action lavatory-cisterns were of cast-iron not galvanised sheet, and came complete with the inlet, outlet, overflow and fixing screw holes at no extra cost. No need for anyone to drill holes in them!

[Roderick JenkinsParticipant @roderickjenkins93242]

Plenty of cone drills about. E.g.

https://www.screwfix.com/p/erbauer-cone-drill-bit-set-3-14-8-20-16-30mm-3-piece-set/9031V?kpid=9031V&cm_mmc=Google-_-Datafeed-_-Tools?kpid=KINASEKPID&cm_mmc=Google-_-TOKEN1-_-TOKEN2&gad_source=1&gclid=CjwKCAiAjfyqBhAsEiwA-UdzJMmgttBFKR_EryIfjyvi1Oy54SJDqAIGz0eX1mLuRnUHAQuvzGNI2RoCrEAQAvD_BwE&gclsrc=aw.ds

Rod

[Author]

Posts