What is the best stepping bit for drilling a chassis

[Slobrain]

Hey Folks,

I'm about to start working on a amp build using a old Marshall chassis from a 5010 combo. I need to drill 2 power tube sockets and 3 preamp tube sockets. I made the mistake of giving away my Greenlee stepping bit and now I need to buy a new one and see many on Fleabay and I see titanium, cobalt and the regular Klein stuff and wanted to ask which type stepping bit is best to use to drill these holes?

Thanks

Slo

[flyingdutchman]

The old Marshall chassis are steel and really tough to drill. A stepping bit might work for a 3/4" preamp socket hole, but I doubt it will work well for the larger holes. A special cup saw might be a better bet. A hole punch would be better still.

If you're going to use a stepping bit or a hole saw, you're also going to need a proper drill press.

[big_teee]

I've rough cut them out with a sabre reciprocating saw.
Then smooth them up with a dremel or air hand grinder.
I wouldn't want to do lots of them that way though.
Here's a cheap knockout punch set.
Knockout Punch Kit - Knockout Punches at Harbor Freight Tools
I've used Greenlee bolt sets like this.
I always greased everything good before using.
Cutting edge and all.
T

[wyatt]

If you are patient and keep an eye out can buy just about any old Greenlee radio chassis punch on eBay for as little as $10. Cleaner than a step-bit and more accurate than the conduit punches still sold today.

[Slobrain]

Hey guys,

Yes, the chassis is kind of thick, I have done two of the same chassis before with the stepping bit 9 years back but had to take my time and use oil too. The knockout set sounds good but not sure to say if this chassis may be somewhat too thick to use those on. The old Greenlee stepping bit worked ok but dulled it out after using on those two chassis. I see the new cobalt and titanium series stuff but not sure what would work best or go with one of those hi speed Klein bits. I cant believe I gave that old Greenlee bit away, I think I paid up around $30.00 buck or more for it...

The surprisingly cool thing about the old Marshall 5010 combo is that the cabinet is made out of birch ply and the chassis is really heavy duty so it makes for one heck of a cool project to build a tube amp into. the mid 80s SS amps were built really heavy duty. Now that I see the Marshalls soaring in price this makes it cheaper to build into.

Cheers

[big_teee]

Hey guys,

Yes, the chassis is kind of thick, I have done two of the same chassis before with the stepping bit 9 years back but had to take my time and use oil too. The knockout set sounds good but not sure to say if this chassis may be somewhat too thick to use those on. The old Greenlee stepping bit worked ok but dulled it out after using on those two chassis. I see the new cobalt and titanium series stuff but not sure what would work best or go with one of those hi speed Klein bits. I cant believe I gave that old Greenlee bit away, I think I paid up around $30.00 buck or more for it...

The surprisingly cool thing about the old Marshall 5010 combo is that the cabinet is made out of birch ply and the chassis is really heavy duty so it makes for one heck of a cool project to build a tube amp into. the mid 80s SS amps were built really heavy duty. Now that I see the Marshalls soaring in price this makes it cheaper to build into.

Cheers

I have the Kleins bit your talking about.
Don't know how it would work on thicker steel.
Harbor has some of those also.
Search results for: 'step bits'
I think the punch set would work best.
T

[olddawg]

Used to have a tool in the shop called a nibbler. You could drill a 1/2 inch hole with a standard bit, then nibble it out roughly and then finish it off with a half round file. It was not incredibly time consuming and was very clean and accurate if you weren't clumsy about it. Didn't have to use oil, it didn't wander, didn't over heat or warp. You want to do it on an empty chassis though. Bits of metal get everywhere.

[Ken Moon]

To me, a Unibit stepper makes the cleanest holes. The only problem is that the large size one costs a fortune ($50-$75 ea)

I've tried brand new chassis punches and haven't been able to get a perfect hole with no warping of the metal chassis.

I use a big 1/2" chuck hand drill that has a very low rpm and lots of torque - I think the max rpm is 500.

I've tried using a drill press, but I get a better feel for when the stepper is about to "break though" to the next step when I do it by hand.

I haven't had nearly as good results with any other stepper but the Irwin Unibit.

These aren't the best closeups, but all of these holes were made with a Unibit:

[defaced]

I use the ones from Harbor Freight on steel all the time. Use lube and the appropriate feed/speed and they give good cuts.

[eschertron]

I've used step-bits in the past. They were nice for what I wanted, BUT for the holes I needed on amp projects, I found a decent price (comparable to step drills, really) on two greenlee 'radio' punches, (noval and octal socket-sized). I trust punches to hold their own against wear and tear better than drill bits. But then, I'm no machinist.

[flyingdutchman]

Unibit stepping bits are excellent tools, I have a metric 4-12 mm Unibit that's 20 years old by now and it still cuts well. I also had a 6 - 20 mm that has mysteriously disappeared , bought them as a set and paid good money for them but they're well worth the price.

However, I avoid using them on steel.

One particular thing about the type of mild steel used in most amp chassis is that it doesn't cut well: it is relatively soft but very tough. The reason is that in series production, sheet metal is punched rather than drilled, and the properties of this type of mild steel are optimized for this.

[Mick Bailey]

If you want to use a step bit for mild steel, you need to use a HSS bit run slow with a cutting lubricant. I like the sulphur-based ones such as Trefolex. The alternative is a HSS hole saw, again run slowly with lubricant. The rough hole left by sawing needs to be cut slightly undersize and final-finished with a taper reamer or 1/2 round file.

Everyone seems to have abandoned the old approach to this job - chain drilling and filing. I still resort to this for really tough jobs on 'mystery stainless' offcuts from the scrap yard, or for a hole size where I don't have a punch or bit.