Chassis Drilling

[GregS]

I have noticed with the previous builds that I've done that the chassis holes I drilled drifted off centre by at least 1/16" from their intended location. However, all the holes drifted in the same direction by the same amount so were still well aligned. For the previous builds I did this wasn't a big issue. Now I'll be using pre-drilled panels over the chassis so it is important that the holes I drill in the chassis align properly with the panels.

I don't believe the drill is wandering at start up. I first make an indentation at the desired centre location with a sharp pointed tool struck with a hammer. This guides the 1/16" bit that I use first. I drill a second hole with a 1/8" bit before using my stepped drill bit to complete the hole. The other reason I think it's not a case of the drill wandering is that the drift is always in the same direction (towards me) rather than random.

I'm wondering if I'm exerting a slight unintentional pull towards me as I'm drilling or that I don't have the drill exactly perpendicular to the surface I'm drilling. Also the drill has a slight wobble but I wouldn't think that that would be doing it.

Anyway, what I want to know is:

Is there something I'm obviously doing wrong with the hand drill that can be easily corrected or is it necessary to get a drill press when it is important to be very accurate? I can't afford to be wasting chassis.

Thanks,

Greg

[gsr]

You might try checking how you hold the drill with a square. I sounds like you may be inadvertently tilting the drill, always in the same direction, which would cause the hole to be off opposite the tilt. A helper holding the square, or better yet two, should help keep you plumb. (Some drills have levels built in, but that requires the chassis to be level first.) Hope this helps.

[olddawg]

Stepped drills will always drift. There is always some play (wobble) transitioning to the next step. Is te material to thick to use a chassis punch? You can pick one up from Harbor Freight for under $50.

[GregS]

Thanks for the replies.

I do have another drill that has a level indicator so maybe I'll try that out on a scrap piece of metal first before getting a drill press.

Greg

[Raybob]

I have noticed with the previous builds that I've done that the chassis holes I drilled drifted off centre by at least 1/16" from their intended location. However, all the holes drifted in the same direction by the same amount so were still well aligned. ...

Looks simple enough to me. Since it's consistantly off in the same direction by the same amount each time, just mark where you want the holes 1/16" off of where it's intended to go.

Raybob

[dchang0]

If you look carefully at the bit tips, you'll notice that they don't come to a perfect point. They are designed to actually come to two points. The distance between them is usually the amount of drift introduced.

I'd recommend getting a drill press. You can get a tabletop one for as low as $25 on sale at Harbor Freight.

[Chuck H]

I make a custom one off order now and then and often drill by hand. With the awl dent as you are already doing you will get less drift with high speed and light pressure. There will always be some drift by hand but 1/16th is a lot. I always order the face plates with the intended hole locations and sometimes I do need to file a little on a hole to mate the chassis up. But I'm never off by much and any switch or pot mounting hardware always seems to cover the gap. I do use step drills but not with a pre hole. I always use the bit for the intended hole size. Punch a dent, use high speed and low pressure with the bit for the intended hole size.

Maybe someday I'll get a drill press. Or better yet a proper punch set. Making IEC shapes with a Dremel is a PITA.

[guitarmike2107]

If you have to get the holes to match a faceplate, get a bit of 9mm MDF wood cut to the same size as your chassis front.
Mark and drill the wood to match the faceplate. Drill with a small 3mm (1/8th) drill first then drill with the proper size for the components.

If you muck it up just make a new one, once your done you have a template which you can clamp to your chassis and drill
much more accurately with little drift. And you can use it again for the next amp if you designed your spacings to be as flexible as possible.

Chuck H: I can highly recommend not drilling your hand

A drill press is very usefull, I picked up a second hand “antique” one for £25.00 ($37.5) because of its 30 something age and made in the UK when that counted for something, its built much better and is more serviceable than the modern ones in the shops now

[Steve Conner]

I've never heard of all of the holes being "off" in the same amount and direction. Even if the drill does wander, and more so in one direction, the amount should be random. Are you sure you centre-punched in the right place to start with? 1/16" error is a little wild, but it's hard to get much below 1/32" with a hand drill.

My preferred method nowadays is to make a template in Autosketch, print it out and, after checking dimensional accuracy of the printout (some printers don't print exactly to size) I stick it to the chassis with masking tape. Then I centre-punch through the paper and follow up with a good quality, sharp 3mm drill. I then enlarge the holes to their final sizes using bigger drills, step drills, Greenlee punches or whatever is needed.

I can use the same Autosketch drawing to make a DXF for a laser-cut faceplate that's guaranteed to fit.

I have one of those crap Chinese drill presses, and I can agree that an old UK made one would be better.

[GregS]

Thanks for all the replies. I have also thought about making a template to guide the drill so I might try that. It seems from some of the responses that a drill press would be really helpful here. One concern I had with the drill press is if the chassis could be effectively clamped to the table.

Making IEC shapes with a Dremel is a PITA.

I've been making them with a nibbling tool which is probably more of a PITA. I'm assuming you're drilling a large hole first and then using the Dremel to file back to the edges of the IEC shape?

I've never heard of all of the holes being "off" in the same amount and direction. Even if the drill does wander, and more so in one direction, the amount should be random. Are you sure you centre-punched in the right place to start with? 1/16" error is a little wild, but it's hard to get much below 1/32" with a hand drill.

Yeah, I punched in the places I wanted to. The direction the holes drift in is always towards me which leads me to think it has something to do with how I'm holding the drill. I do like your idea of using a paper template for making the punches. Better than measuring and drawing pencil lines on the chassis like I've been doing.

Thanks again for all the input.

Greg

[dchang0]

There's another option if you don't want to go the drill press route.

Use metal guides or templates.

One way to do this is to get a cheap piece of sheet steel and drill the same size holes that you would drill into the chassis, including starter holes. Then, you clamp the sheet onto the chassis perfectly aligned and use it to force your drill bit to stay true.

You still have to use starter holes, because if that gets off center, then the larger bits will also be off center.

[Jag]

Yeah, I did that using 1/8" plate metal and made a template for the front and back. It's great to have a buddy who owns a machine shop . I'm trying to get him to make the chassis for me on his CNC machines.

[dchang0]

1/8" rocks! Thing is, we don't have to go so far as making a complete template. Seriously, just a single hole the same size as the starter bit is fine. We merely move the steel around to the correct locations, clamp it tight, then drill the starter hole perfectly centered, then move on to the next. Works great for one-off chassis layouts for those of us who don't build enough of the same design to justify the cost of a template.

[Chuck H]

Maybe worth mentioning...

I had to build a couple of identical amps as prototype floor models for a MFG. So the chassis were a custom order. Since I had to have them made anyhow I asked about stamping and it was something like $1.50 for drilled holes and $5.00 for irregular shapes or holes over 1.5". Well this seemed like a damn good deal to me as I was under the gun at the time. So I had the chassis maker cut the tube, pot, switch, fuse, jack and pilot holes as well as the IEC hole. I did all the drilling for screws and such as I built the amps. So...

I know Bud also offers chassis stamping for "runs". My "two only" chassis from a small maker cost only $102.00 each. Custom to spec made from frickin 1/8" aluminum (too thick really but durable as hell). Fully welded corners and all the holes were spot on with the out sourced face and back plates cut for the same dimensions. I still build in off the shelf chassis from Hammond and Bud and cut my own holes. But It did occur to me that I could have any chassis dimensions I want and all the holes cut for me on a seriously HD chassis for an extra $60.00 anytime I want to.

[dchang0]

Wow! I've been looking for a reasonably-priced, short-runs, custom chassis builder. Who do you use? PM me if he/she/they don't want their info posted on the web.

Thanks, Chuck. Oh, and thanks for the tip in the other thread about my reverb pot being wired backwards.