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Amp chassis hole cutting

The Captain

The Captain

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Hi, Im designing a valve amp around a basic chassis- a few Qs:

1. what the best way to do holes? I used hss bits before up to 8mm but to enlargen to say a 10mm vol pot hole becomes a pain with hss bits.. is there a drill bit thingy Im not aware of for eg?
2. which is easier to cut (I dont have specialist tools and need three 1.25" valve holes) aluminium or steel?
3. of these which generally has the best finish?

thanks in advance, Captain.
 
E-mail me at fluffvogel at hotmail dot co dot uk I have full work shop access and our engineer can do the buisness.
 
You need a step drill to make the smaller holes for the pots etc- you can buy them from Farnell or RS.
For the big holes for the valves you need a hole punch, which usually consists of two parts that screw together through a pilot hole in the middle. Again RS or Farnell should have them.
Aluminium is softer and a lot easier to cut.
Good luck!
Sam
 
Lots of ways to cut holes in sheet metal, including;

If you are going to use a step drill ( or Christmas tree as we call them in the Aircraft Structures biz) make sure that you clamp the work piece down securely and ideally use a vertical pillar drilll to cut the hole, feeding the drill in slowly as they can sometimes have a tendancy to wander a little.

Mark the exact centre of your hole and then centre pop it, then use a set of dividers to clearly mark the circumference of your hole. If your hole drill, or hole saw doesn't give a good enough finish you can cut it smaller and finish off the hole with a small half round hand file.

Another method is to chain drill using small drill holes ( e.g. 2.5 - 3.5mm ) to drill a series of holes inside the circumference of your marked hole, then cut the connecting webs out using snips and finish off with a file.

Another method is to use an 'abrafile' blade in a normal 12" hacksaw frame. The abrafile is a round saw blade that cuts in all directions , (a bit like a ceramic tile saw). All you need to do is cut a single small hole to pass the abrafile through and then cut the hole out, finishing off with a small round/half round file again.

A hole punch usually gives a very cleanly cut hole, although they can ofen be expensive to buy, are size specific and you might not get the exact size you want.

One of the reasons people often struggle to drill holes, especially small ones is that domestic DIY drills go quite slowly, often at about 500 rpm. At work when drilling into Metal Aircraft structures made from Aluminium alloys i always use small windy drills that revolve at about 2000- 2800rpm.

A sheet metal punch will give the quickest , cleanest, neatest hole but they normally cost a bit of money.

I when i need to cut a hole ( many, many thousands over the years) I use the methods I first mentioned and have only ever used a hole punch a handfull of times.

Aluminium is easier to cut than steel sheet, however I've sometimes found that a sheet of hard, heat treated aluminium alloy ( such as 2024 or 7075 alloys used in A/C structures ) can be more difficult to cut than a sheet of soft alloy steel sometimes.
 
Cheers for this info- I think chaindrilling and snip (hadn't thought of snipping- good idea that) for the 2cm valve holes & with zener's help of a gismo for the power valve holes those should be ok. The pesky ones are my 2x ~16mm and 7x 11mm holes which will be on the facia- those xmas tree bits are not cheap so Ill plough on with hss bits and handfile methinks.. much swearing involved Im sure. Thanks Capt
 
I think a drill press is a good idea, though I've done all mine with a hand electric drill - and as S&C says, they have a tendency to wander. I recently bought one of these which works really well, and doesn't take up too much room in the workshop.
 
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That looks pretty damn good- I should really get one. In meantime Ill attack this one I think with a handrill, patience and profanities..

is a trick to cover the facia with masking tape 1st, plotting holes on this and drill- would the tape stay on ok and not skew when drilling if Im right?
 
The Captain said:
That looks pretty damn good- I should really get one. In meantime Ill attack this one I think with a handrill, patience and profanities..

is a trick to cover the facia with masking tape 1st, plotting holes on this and drill- would the tape stay on ok and not skew when drilling if Im right?
Click to expand...

What are you drilling and how thick is it ?

yes masking tape is a good thing to protect the surface from a 'skating' drill bit.

I recommended a drill press for completness, i never use one and always do it by hand, I've drilled lots of holes though and can usually save it if it starts to go bad. (I have often been embarrassed though :)

Drilling through thin sheet with a xmas tree is easier than going though thicker plate , especially by hand, if the drill isn't exactly straight( perpendicular) you can end up with a slanting hole, not a problem in thin sheet.

Drill a smaller 'pilot' hole first just to get the xmas tree point started (e.g. 2.0 - 3.5mm).

A hand drill will work just fine it will just take longer, if it is thick plate keep checking your hole regularly. Possibly put it vertically into a vice and get somebody to align the drill from the side for you ( Up & down ) , whilst you look down on it from above to ensure the correct side to side angle. ( A small spirit level could help in this respect to if you stick it on your drill )

Find a gash bit of similar metal to practice on first, this isn't rocket science, it's just drilling a hole , you will do it just fine :)

Doing it yourself is far more rewarding than getting somebody else to do it for you. Probably why i **** up so often, i hate getting other folk to do stuff for me even if i get it wrong first time around.

Asking lots of questions is a good thing, get the kowledge then put it into practice.
 
Just bought myself ( yesterday) a valve amp kit off World Designs. The KEL84 kit. Pre drilled holes and soldering by numbers for beginners, It is supposed to be a great sounding little amp. Looking forward to it.

A scratch build like you are doing is just a little too advanced for me at the moment.
 
I like to jump in at the deep end scratchbuilding/ pillaging other designs, the rewards being still a novice are great, besides a blank chassis (from Bluebell- thickness not shown but I suppose they aren't heavy duty for that £- anyone tried one by the way?) being only ~£12 scratch building works out a good dose cheaper. As the panel will be at the rear top of amp aesthetics aren't as important, just a good alu surface under the 9 odd knobs n bobs without any labelling at all will suffice.

Thanks for the input on this folks.. much more prepared now.
 
Rory - I've not forgotten about the drill bit for you - just need to find the bloody thing in the bombsite that is my desk.
 
Rodent activity still curtailed by my lack of time - it's sat on my desk in front of me though, to remind me to get going!
 
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