Help Needed Fitting an OT that Doesnt Fit - 5e3

[psb962]

Hi, first amp build for me, a 5e3. I ordered Hammond transformers and a 5e3 chassis from Antique Electronics Supply and have immediately run into a problem that I can't solve due to my total lack of metalworking skills.

The Hammond OT uses a smaller bolt spacing than the chassis. (It seems the chassis spacing is correct, but the OT is smaller than most others sold for this application).

I thought it would be easy to drill a 3/16 hole in chrome plated steel plate so I tried and failed, and then failed again even with a brand new carbide drill bit for stainless. Now I read that this means the potential hole is probably 'work hardened' and no amount of drilling or new bits will drill it.

Looking at the OT in place with the bolts I think I might be able to mount it as is, perhaps with 'some type of washer' to hold the OT hole as its tightened down, or perhaps there is another way? The legs of the OT seem to sit a little above the chassis so there is plenty of room for washers or flanges to go under both legs if need be.

Any suggestions as to how I can get this securely mounted after one (more) visit to the hardware store?

[sluckey]

Drill a smaller pilot hole FIRST. May even want to drill 1/16", then 1/8", and finally 3/16". USE OIL to cool the bit! Clamp the chassis to the bench if using a hand drill. Use slow speed.

[psb962]

I tried that with a variety of bits. Not a scratch. the only impression in the chassis was made by the punch. Nothing else has made one scrap of difference.

[johnnyreece]

I've had good luck with Milwaukee Red Helix Cobalt bits in a drill press with oil and working slow. If you're concerned about the current planned hole being unusable, could you maybe mount your OT to the hole beside that one and drill a hole next to the one you have the bolt in (hope that makes sense...)?

[psb962]

That does. The bit I bought for this was indeed a Red Helix!

I have the option of using the 'other hole' up my sleeve right now but I think I might have solved this. I used a terminal crimp where the ring connector fits the bolt, ripped off the plastic over the crimping connection, and bent it up to make a post of sorts that sits in the OT hole. What's really neat is that when bent that way the crimp leaves a slot between the crimp barrel and the ring connector that grips the OT very nicely. With it all fastened down its solid.

So I think I will go ahead and start the build. If the OT doesn't stay solid I'll revisit drilling, or any other metalwork hacks others might think of. I will do anything to avoid having to try and drill that chassis again...

[TUBEDUDE]

Steel.is hard, chrome is harder. Using a punch to set the mark for your hole may help. The dent will let the drill bit get a good bite in the metal. I use high speed with some pressure initially to "break the surface" then slower with less pressure. Use cutting fluid/oil.

[Roe]

Steel.is hard, chrome is harder. Using a punch to set the mark for your hole may help. The dent will let the drill bit get a good bite in the metal. I use high speed with some pressure initially to "break the surface" then slower with less pressure. Use cutting fluid/oil.

+1. thin drills tend to break with pressure though. Get a sharp drill to start with.

[Stevem]

If one reads up on drill bits you will find this out.

The punch mark you make needs to be as wide as the bit used and idlealy as deep as the bit is wide.
This is because the bit cuts mainly with its outside edges, in fact I bit will not pull itself thru what your drilling until its outside edges are full cutting.
This also means that to make things easy and to keep things center you should start off by drilling a smaller pilot hole then the punch make you made.

On chassis I like to drill until the tip of the bit just starts to pop thru, then move to the other side and drill to complete the hole.

I myself would have used a Dremal tool with a small stone bit to first grind away the Chrome plating.

[Phil_S]

I see what you did and it's clever. If it was mine, I wouldn't do that. Try again to drill in the right place. The problem, already noted by many, is getting the hole started. I suggest a diamond bit like this one https://www.amazon.com/Dremel-7144-Diam ... PDKIKX0DER will fit in your drill and will get you some "purchase" for your smallest regular drill bit. OTOH, if it starts to bite and you keep going, you should end up with a 3/32" hole and from there, stepping through the sizes should easily get you to the size you need for the bolt. IMO, it's important to do this properly. On the final bit, try not to go all the way through, turn over, and finish from the other side. This will give you a hole without burrs. Otherwise, you can probably deburr with a little piece of sandpaper, or manually twisting the next size bit in the hole. You can do this!

[GAStan]

Otherwise, you can probably deburr with a little piece of sandpaper, or manually twisting the next size bit in the hole. You can do this!

I have a set of these (different brand but same. 82° countersink/deburring bit. One can literally deburr an entire chassis in a matter of minutes with one of these in a drill. I've fought using a drill bit to deburr and much prefer these bits. They also work great to use countersunk screws such as under transformers or on the ends to smoothly fit in a cabinet.

Countersink Bits
I have not used this actual set, I am just posting it as an example.

[Phil_S]

Well, the OP says he lacks metal working experience, so that probably means tools, too. The way to deburr is with one of these:
https://www.mscdirect.com/product/details/05751003

[psb962]

Thank you all. If I get a chance to practice drilling the chassis elsewhere (for an extra ground if needed) then I might revisit drilling the hole, but as is the fix is rock solid.