How to Drill Chassis-Mount Holes in Headshell?

[FourT6and2]

I know how to wire an amp, but I'm no woodworker. I have an amp chassis I need to mount in a headshell. But the chassis' flange doesn't go outward, the mounting holes are interior to the chassis walls. So I can't just put the chassis in the headshell and mark the holes to drill in the wood. The holes are "blind" so to speak. What's the best way to mark and drill these holes so the chassis sits flush against the front of the headshell without any gaps. Other than eyeballing where they need to go, what do I do?

 

[Mike Oxhuge]

Sheet of paper big enough to cover chasis, mark the holes on paper and transfer from paper to headshell.

 

[griff10672]

I would drop the chassis down on a piece of cardboard .... and trace the chassis with a box cutter ... or use a pen ... and then cut it out ....

mark the mounting holes on it

with the chassis in the headshell measure the distance from the face of the chassis to the edge of the head ... then using a square duplicate that distance with the template on the bottom of the headshell ... then you just have to do the math to space it evenly left to right ....

you could just do all the math and mark it on the bottom ..... but I find it easier to have a visual template

 

[MadAsAHatter]

Method 1 & 2 require drilling from the inside of the headshell. Method 3 is drilling from the outside

Method 1:
This is the best method I've used and provides the best accuracy. It requires sacrificing 4 chassis bolts, but you'll be able to use them in this method anytime.

Make 4 assembly bolts by cutting the head off each bolt then sharpen the cut ends into a conical point like the pic below. The video below is the best I could find to demonstrate the process. You don't need a bench grinder. Just chick the bolt straight into a drill and use a hand metal file. You can also buy cone point bolts, you just have to make sure it's the same size & thread of the chassis nuts.

Screw the bolt into the chassis with the point facing out. You only need the point to be protruding a little bit. Line the chassis up in the shell. Once you have it where you want push down on the chassis. This will make little indentions where you need to drill the holes. Remove the chassis and drill the holes from the inside with a right angle drill.

Tip: Put a strip of masking tape in the area where the holes would be. When you push on the chassis to make the indentions it will be easier to see on the masking tape, especially if there is some tolex where the hole goes.

Method 2:
Next best method is to mark the centerline of the flange nuts on the side of the chassis. Use masking tape if you don't want to mark on the chassis itself. Get the chassis positioned and transfer that mark to the inside of the headshell. Also mark the line of the outside edge of the chassis. This will give you the front to back position of the holes. then measure the the distance from the chassis edge to the center of the nut. Transfer that to the inside of the headshell Now you have the positions of where to drill the holes. Use a right angle drill to drill the holes from the inside of the chassis.

Method 3:
If you don't have a right angle or drill small enough to fit inside the headshell you'll need to drill from the outside. For that follow @griff10672 advice.

Tip: For drilling a straight hole with a hand drill (right angle or otherwise) tack a couple of pieces of scrap wood into an L shape or notch out a corner of a scrap piece. It doesn't have to be big, but the inside of the L shape needs to be straight. Use the inside corner to keep the bit straight as you drill. Basically this...

 

[griff10672]

Method 1 & 2 require drilling from the inside of the headshell. Method 3 is drilling from the outside

Method 1:
This is the best method I've used and provides the best accuracy. It requires sacrificing 4 chassis bolts, but you'll be able to use them in this method anytime.

Make 4 assembly bolts by cutting the head off each bolt then sharpen the cut ends into a conical point like the pic below. The video below is the best I could find to demonstrate the process. You don't need a bench grinder. Just chick the bolt straight into a drill and use a hand metal file. You can also buy cone point bolts, you just have to make sure it's the same size & thread of the chassis nuts.
View attachment 428137



Screw the bolt into the chassis with the point facing out. You only need the point to be protruding a little bit. Line the chassis up in the shell. Once you have it where you want push down on the chassis. This will make little indentions where you need to drill the holes. Remove the chassis and drill the holes from the inside with a right angle drill.

Tip: Put a strip of masking tape in the area where the holes would be. When you push on the chassis to make the indentions it will be easier to see on the masking tape, especially if there is some tolex where the hole goes.

Method 2:
Next best method is to mark the centerline of the flange nuts on the side of the chassis. Use masking tape if you don't want to mark on the chassis itself. Get the chassis positioned and transfer that mark to the inside of the headshell. Also mark the line of the outside edge of the chassis. This will give you the front to back position of the holes. then measure the the distance from the chassis edge to the center of the nut. Transfer that to the inside of the headshell Now you have the positions of where to drill the holes. Use a right angle drill to drill the holes from the inside of the chassis.

Method 3:
If you don't have a right angle or drill small enough to fit inside the headshell you'll need to drill from the outside. For that follow @griff10672 advice.

Tip: For drilling a straight hole with a hand drill (right angle or otherwise) tack a couple of pieces of scrap wood into an L shape or notch out a corner of a scrap piece. It doesn't have to be big, but the inside of the L shape needs to be straight. Use the inside corner to keep the bit straight as you drill. Basically this...

View attachment 428139

I didn't think they made right angle drills ............ I know I didn't have a drill small enough to fit inside ...by all means if you can drill from the inside then that would be the way to go IMO ...

 

[LP Freak]

I didn't think they made right angle drills ............ I know I didn't have a drill small enough to fit inside ...by all means if you can drill from the inside then that would be the way to go IMO ...

Yeah, they make an attachment to go on your regular drill to turn into a right angle

 

[griff10672]

Yeah, they make an attachment to go on your regular drill to turn into a right angle

I had no idea ...... baffles me that I don't have one in my shop already .... the time this bad boy would have saved me on a bunch of projects .....

just ordered this to go with all my other Dewalt stuff

 

[RedPlated]

I use the chassis bolts, but without any need for modifying the bolts. I’ve used it many times and it works great.

For production amps, I have a chassis with extra holes drilled in the top surface. I lay it on the outside of the head shell and can mark holes through the bolt holes, then through the hole in the chassis and on to the head shell. Then just drill straight though from outside in.

My current head shell builder drills all the holes for me which is amazing and saves me a ton of time.

To use the bolt method, simply screw 4 bolts into the chassis and set it in the head shell. Beforehand, cover the approximate hole locations with blue painters tape so you can mark them.

With the chassis in the shell, and the bolt heads resting on the tape, use a pencil and mark around the bolt heads. Make sure the bolts are all relatively perpendicular when doing this. It’s tight, but you can get in there and make a good enough mark to drill the holes accurately.

Remove the chassis, and use a right angle drill attachment to drill the areas you marked. Make the hole a reasonable size, around 3/8” so you have some wiggle room to install the chassis. I’ve seen amps with holes barely bigger than the bolt threads and it makes no sense to do that. Just makes it harder to mount the chassis.

 

[FourT6and2]

Sheet of paper big enough to cover chasis, mark the holes on paper and transfer from paper to headshell.

Yeah but that still doesn't ensure the front of the chassis sits flush against the headshell. I guess I'd have to account for the front panel thickness.

Or you know what... I can use a piece of paper and loosely tape it to the headshell. Then use double sided tape on the paper. Put chassis into headshell where I want it and then lift, leaving the paper behind with the holes already marked?

But that means drilling from the inside, which will be hard without buying a new right angle drill

 

[FourT6and2]

@RedPlated that seems like a great method. Drilling from the inside will be tough without a right angle drill. I'll see what I can find.

 

[RedPlated]

@RedPlated that seems like a great method. Drilling from the inside will be tough without a right angle drill. I'll see what I can find.

Definitely need a right angle. I have an attachment that goes into my cordless drill.

You can get the adapters cheap on Amazon.

 

[Robostyle]

I have one of these- not heavy duty but would be able to do this job imho

https://www.homedepot.com/p/DEWALT-Modular-Right-Angle-Attachment-Set-DWAMRASET/314674938

 

[FourT6and2]

I have one of these- not heavy duty but would be able to do this job imho

https://www.homedepot.com/p/DEWALT-Modular-Right-Angle-Attachment-Set-DWAMRASET/314674938

That'll do the trick, thank you

 

[Robostyle]

That'll do the trick, thank you

NP, glad to help

 

[MadAsAHatter]

I have one of these- not heavy duty but would be able to do this job imho

https://www.homedepot.com/p/DEWALT-Modular-Right-Angle-Attachment-Set-DWAMRASET/314674938

Does this one have a chuck or something to hold a drill bit? I have something similar but it only hold the hex screwdriver bits.

 

[LP Freak]

Does this one have a chuck or something to hold a drill bit? I have something similar but it only hold the hex screwdriver bits.

You can buy drill bits with the hex end that will fit

 

[Mike Oxhuge]

Yeah drilling from inside is the way. Less chance of drill walking ruining location.

 

[Mike Oxhuge]

Also get a screw machine drill bit of correct size. They are shorter and more rigid.

 

[griff10672]

I opted out of the full drill .... and got that 3 pc modular attachment for my impact gun ....... total no brainer ... that thing is bad ass ... and 40 bucks ain't bad IMO