Drilling chassis bolt holes in the cabinet

[Ron Worley]

Now that I seem to have gotten my amp into a functional state I'm building the cab.

I bought the "Drawer Cab" that Jem and Dr. Tim came up with and put it together. (I have to say that with 1/4" rounded edges, this thing looks pretty damn sweet... enough that I going to stain it..)

Issue is getting the chassis bolt holes exactly right. Much like the standoff holes for the boards in the chassis, even when you measure it as precisely as possible, it's always a little off.

Any thoughts / ideas here? Perhaps a template?

Ron

[hairyandy]

Curious to hear how people do this as well, I'm in the same boat with a cabinet that needs to be drilled for an Express.

Andy

[CaseyJones]

(double post)

[CaseyJones]

I bought the "Drawer Cab" that Jem and Dr. Tim came up with and put it together. (I have to say that with 1/4" rounded edges, this thing looks pretty damn sweet... enough that I going to stain it..)

Issue is getting the chassis bolt holes exactly right. Much like the standoff holes for the boards in the chassis, even when you measure it as precisely as possible, it's always a little off.

It's a pretty simple job if you haven't assembled the cabinet. Dry fit the cabinet, make where the insides of the sides are, use pencil if you're going to sand it off later, masking tape if you want to save sanding. The trick is to mock up the chassis exactly where it's going to live in the finished cabinet.

I have a couple good ways to mark the holes. I have a set of machinists's centers, they're a metal rod with a point on one end. Find the one that's the correct OD to match the ID of your chassis mounting holes, drop it down the hole with the chassis mocked up on the cabinet bottom, tap your center lightly with a hammer, there you go. No excuse for it not bein' right.

O.k., most of the time I don't use the machinist's centers. I just grab a brad point bit out of my index near the drill press, drop it down the hole, same deal, tap it lightly, there's my mark.

Ultimate cheap-o center marking... a pencil is about the same diameter as your mounting holes. If not hone out your holes or sand down the OD of your pencil to fit. Drop it down the hole. There's your mark.

It sometimes helps if you mark out benchmarks on the cabinet before you round over the edges, that way you can guage your marks from a clean square edge. Make your marks, take notes as to what your marks are supposed to represent, proceed. If you're covering your cabinet you can scribble all over it, you won't see your scribbles once it's covered.

[Ron Worley]

Unfortunately, in my eagerness to have a playable amp I have already assembled and rounded the edges of the cab...

When you say "mock up the chassis" are you making a template??

[leaveitalone84]

Unfortunately, in my eagerness to have a playable amp I have already assembled and rounded the edges of the cab...

When you say "mock up the chassis" are you making a template??

Casey was using the chasis as the template. But make a cardboard template that fits the bottom of the chassis then use that for your holes.

[Ron Worley]

Sounds good... I may make it out of clear plexiglass- that's what I used for a drill template for 12" speakers... since it's clear, it's easier to mark the holes...

[CaseyJones]

When you say "mock up the chassis" are you making a template??

If a template is what it takes make a template. Your template doesn't have to be anything special, I make templates for all sorts of things from cereal boxes. The important thing is not to stack up your tolerances, that means make the template as close to the real thing as you can. "Stacking up tolerances" means that if you have +/- 1/16" to work with and you cut it short the full tolerance... 1/16" short... then you cut the finished piece to the short end of tolerance, say 1/16" short again... you'll be 1/8" off when you're finished. In machinist's terms 1/32" = .032", 1/16" = .063" and 1/8" = .125". 1/16" off is a lot. Stack 1/16" a couple times and you're way off.

Good tools make a difference. A plain old carpenter's framing square is at least square. There's a cheap square available at Home Depot that's 12" on the long side, it will save you a lot of hassle if you make templates with at least one reference edge that's very close to square. Ditto for your cabinet being square, don't assume the dovetails will jig it up square. Square it when you glue it (too late!) 'cuz if it's out of square then it's out of square forever.

A good 18" steel ruler is worth its weight in gold... even now that gold is around $1k per ounce! Staples sells one that a lot of graphics designers use, the old school ones who still check their layout after it prints out from the computer.

"Measure twice, cut once". Measure at least twice. It's much less hassle to eyeball a project for an hour to make sure everything is right than to start over if it isn't.

[Buschman]

Trace the bottom of the chassis on a sheet of paper & poke holes in the paper where the screws go. Then transfer to the bottom of the cabinet. It will not hurt if your holes are a little bit bigger than the screws.

[Ron Worley]

Trace the bottom of the chassis on a sheet of paper & poke holes in the paper where the screws go. Then transfer to the bottom of the cabinet. It will not hurt if your holes are a little bit bigger than the screws.

I have a Allyn Meyers chassis, so you really can't do that- the chassis bolts do not go all the way through the chassis like a Fender... Perhaps I am not understanding what you mean exactly....

Casey- I have a decently equipped shop and have all the tools you mention (and a whole lot more). You make some great points on the cumulative error aspect...

I just figured that one of you Wreck masters would have come up with a neat trick to do this... I am going with a plexi template I think...

Ron

[Structo]

The wreck cabs are a little different than what I am used to since my amp hangs upside down but couldn't you drill the holes on the large side and use finish washers to cover the holes?

[Richie]

Things to remember, before you make a template.
The transformers on a wreck sit back on the chassis about 1/4". So if you put your chassis in the cab,it won't come out flush with the bevel front. It sits back about 1/4" or half of the thickness of the front piece.

If you were to slide in a chassis without transformers ,as to use as a guide,make sure you have it sitting back,and not exactly flush with the beveled front. because if you did make your mounting holes or template with the chassis flush with the front, then when you put the transformers on the chassis, it would hit the back of the front piece,and your holes would be off.

Its not to hard to just measure the bottom, or set your chassis in the cab,measure the space you have from the back of the cab, to the chassis.
Then you have a 1" to the center mounting hole. So add those together,and thats where the back screw holes would sit. And you measure from the side,allow for the thickness of the side of the cab.
Don't quote me on this,but seems like its 1 3/16" for the side to center of the mounting hole.
Once you measure a few times,and when you think you have it, just drill a small pilot hole. And you can put in a small bolt to use for checking.
I think the distance between the mounting hole centers are 6"
You can check that to be sure. All cabs might have some slight differences, so take that into consideration.

[Ron Worley]

The wreck cabs are a little different than what I am used to since my amp hangs upside down but couldn't you drill the holes on the large side and use finish washers to cover the holes?

On a real Wreck, KF used fairly big washers on the chassis bolts, so perhaps this would work.

Richie, thanks for the insight on this!

Ron

[CaseyJones]

Casey- I have a decently equipped shop and have all the tools you mention (and a whole lot more). You make some great points on the cumulative error aspect...

As a footnote... when I post an answer I try to include information that will be useful to members of all skill levels. I'll wander off topic, wander back on, if I make one point that lights a light bulb over someone's head I've done my job.

Just skip though to the parts you find useful. If none of it is useful just skip all of it!

[Ron Worley]

Casey, please don't take my comment the wrong way... I'm amazingly ignorant on lots of things.. and you never know what will help.

I am no master craftsman to be sure... I've made enough stupid measurements to last a life time...

I have done very little metal work, but I think that I would like to learn a whole lot more... Guys like you can teach us all a lot.

Ron