Do I need metric drill bits?

[n8n]

Good afternoon folks,

First of all I apologize as this is going to be long, but I don't know how this is going to make sense unless I explain why I'm looking at these now.

So I have a little project that I want to do. As I have been playing with audio gear a lot lately, I occasionally need to mess with a turntable. Now typically a cartridge will be mounted to a headshell with either M2.5 or 3-48 (I think? It's a number 3 for sure, obviously will confirm before going at it.) Usually M2.5 is used but some headshells like an old AR XA (which I have) use a threaded brass insert in a blind hole for each screw so not only MUST I use a #3 screw, but I need a pretty specific length, at least within 1/8" or so.

So here's my thought. Once upon a time, I collected a pile of vintage pressed brass switch and receptacle plates for an old house, and when I went to install them, I found I needed several different lengths of screw. So I bought a box of solid brass oval head screws from McMaster-Carr that was the longest length I needed, and when that was too long, I'd cut them down using a cheap crimp tool. I was thinking of doing the same with stainless filister head screws. I've already determined that the washers that come with cartridge hardware kits are most likely Mil-Spec NAS620 #3 washers (M2.5 and 3-48 being close enough they will work on both.)

So my first thought was that someone ought to make a screw cutting tool for these because this shouldn't be an odd request. Well, there is one for metric that ought to work, but it's $44

https://www.amazon.com/gp/product/B000YTAAUW/?tag=atomicindus08-20

Every similar tool I've found, for metric, will cut M2.6 and I cannot find a fractional one that has a smaller size than 4-40. My next thought, then, would be to buy one of those tools where the pivot is a bolt so it can be disassembled, and just drill two new holes in it to make cutters for the sizes that I want.

So I look it up and the thread pitch for M2.5 is not 0.5 as you'd expect but .045 (????) for a theoretical tap drill of 2.05mm. which is 0.0807".

I have letter, number, and fractional drill bits but no metric. I've never even heard of a 2.05mm drill bit although now that I look I see that they in fact exist, presumably for just this purpose. It certainly isn't part of any set I've seen as they usually step by 0.5mm, or 0.1mm if you buy the big fancy set.

I do have both my grandfather's old Ace Super Set (fractional) and a newer Irwin Super Set (both fractional and metric) but only have letter, number, and fractional drill bits.

To tap a M2.5 hole in what I assume is at least semi-hardened steel, what do I do?

a) use a #46 (.0810") drill bit and let it be a little sloppy
b) use a #47 (.0785") drill bit and hope the tap doesn't break
c) stop being such a cheap illegitimate child and buy the damn 2.05mm bit, and leave it in the box with my tap and die set as that's literally the only time it'd be used.
d) oh, come on, just pay the $10 for a hardware kit every time you get a new headshell, why are you doing things the hard way? (that doesn't solve the AR XA problem though, and that's the simplest - read most reliable - turntable I have and yet at the same time it sounds great, and I'm not the only person who thinks so.) In case you aren't familiar, the hardware kit usually has a bunch of different M2.5 screws with a pair of each length, stepping up by 2mm or so increments so you can pick the right ones for the cartridge/headshell combo that you have.

I'm leaning toward a) as being just fine as the difference is only .0003"

sidebar question: being a completist I now am thinking having a metric drill bit set might be good anyway if I ever get back into doing serious work, as this is the Way of the Future (tm). My existing drill bits are a mix of old stuff from an employer 20 years ago, whatever they have in the pick a bins at Fastenal to replace missing or broken ones, and a number set that I bought from McMaster-Carr as I didn't have any of those at all (I think maybe Chicago-Latrobe?)

What brand would you recommend? Thinking just bog standard black oxide, 118 degree, jobber length. 0.5mm steps good enough for having to drill and tap a hole maybe once a year and most likely on automotive stuff? Or pop for a 0.1mm set?

Thanks for reading my long winded post and for any opinions.

 

[American Locomotive]

Use a #46. I doubt normal consumer grade taps (or drills) are even held to a 0.0003" tolerance anyways.

 

[4xdog]

I brought a set of metric drill bits home from a trip to Europe a few years ago. I've found them useful.

A set almost like this, from the same company. The one I have has wood, metal, and masonry bits too, just a few more of them.

 

[2ndGearRubber]

Use a #46. I doubt normal consumer grade taps (or drills) are even held to a 0.0003" tolerance anyways.

This. 3 ten-thousanths? The machine is likely adding that much wobble.

 

[MikeF2316]

I'm about as metric as a person (in North America) can be, I try to do everything I can in metric. When I'm using Ikea's catalog, I use the metric dimensions. But that's one thing I don't do in metric, I don't have a single metric drill bit. I'd use a #46 as suggested.

 

[Kodiak]

I seriously doubt I will ever own a metric drill bit unless it is given to me. I agree with the others - not needed.

 

[ecotec]

I have metric drill bits. I collect NOS USA drill bit sets, drill bit stands, drill bit indexes, tap and die charts, ephemera...

That said, I do not really think that you need metric drill bits.

 

[Steve_P]

Generally not needed. But a few years ago I needed to enlarge a 10 mm hole to 10.2; I needed that bit as the closest inch size was too large. So i do have a few metric bits, but not a set. The typical metric sets are spaced so widely that 1/64th inch bits are more than fine

 

[n8n]

Generally not needed. But a few years ago I needed to enlarge a 10 mm hole to 10.2; I needed that bit as the closest inch size was too large. So i do have a few metric bits, but not a set. The typical metric sets are spaced so widely that 1/64th inch bits are more than fine

Yeah I looked and a 2.5mm through hole is also pretty close to a standard size so... maybe that'll be a "when I win the lottery and buy a big house in the country and one garage for my cars and one for my project cars and workshop" purchase

 

[matt_i]

imo the "tenths" place as in ten-thousandths place shouldn't even be in a conversation about a twist drill bit.

If you want to cut down a screw either hacksaw it or just go at it on a belt sander for rough length. Then switch to swiss pattern files for chamfering the end. Highby the lead thread.

Single cut files also work well, in this case you want a fine toothed version.

 

[Iron Beaver]

This is Garage Journal.... You need all the drill bits

 

[HenryAZ]

I bought a small set of metric bits in a metal index case, for what reason I don't know. I work a lot with metric, especially tapping, but I've only used one metric bit once for that, and an SAE bit would have sufficed. My Champion 29 piece index in SAE (1/16" --> 1/2" by 64ths) will always provide something close enough. I do like the way metric sizes the tapping drill bit from the thread pitch.

 

[tyyost]

The last job I was doing in metric was drilling and tapping tie rods for an ATV. Since I was buying both left and right hand taps from Mcmaster I bought one or two metric drills in my order.

Since then I have picked up an Astro Pneumatic 7581 - 19-Piece Metric Automotive Drill and Tap Set, but honestly never even checked to see if the bits are metric.

I say forget the metric set and buy what you need as you need it so you have quality tools for the job you are doing. Often on GJ the desire is to have one in the toolbox/shelf waiting for the job, but many of these items are better off bought as needed, at the desired quantity level the job needs.

 

[vavet]

Buy a small set that includes the size you need. This is a balance between what you want to buy (a huge metric set) and what you want to spend (nothing because you’ll use what you have. There’s nothing wrong with spending money on something just because you want it, assuming of course that there’s a roof over your head and food on your table.

 

[Stephenw]

If a person only has a fractional drill set, they might not have a bit as close to the metric size as they might want. Add a number and letter sized bit set and you will be close enough for every need.

 

[Hammer1963]

I use metric drill bits quite often, but everyone's needs is going to be different. Drill bits are kind of like locking pliers (Vise-grips) You can never have enough!

 

[matt_i]

The one nice thing about having a couple metric bits (typically the 2.5, 3.3, 4.2, 5.0, 6.7 and 8.5....gets you from M3 to M10) is that you probably have a low-use sharp one when its needed.

If you know you are going to be doing a lot of holes, I'd just buy a couple of 2.05mm bits. You could bulk up on #48s but my guess is they would get lost. They typically don't mark the shank for sizes like that, and a person has to resort to calipers or mic to sort them out.

 

[RoninB4]

Use the #46 drill. The .0003 larger size will only be making the minor diameter larger and not enough to even be noticeable. You won't be making the major diameter larger (controlled by the tap size) so it won't be "wobbly" or any looser than using the drill called for. A given drill size yields a % of thread, going slightly larger is often done in machine shops on a regular basis. If you want metric drills that's up to you but it won't make a difference in this application.

 

[glenng]

About 8 years ago I purchased USA-made Precision Twist Drill set 1mm to 5.9mm in 0.1mm increments.

A few months after that, I went to see if they had a larger set in 0.1mm increments, and was able to get them in the Dormer brand (COO Brazil) in 6mm to 10mm in 0.1mm increments.

I also have a Dormer A191413 Jobber Drill Set, 1.5 mm - 6.5 mm x 0.5 mm + 3.3 mm, 4.2 mm Size set.

I also have specialty hobby sets from 0.1mm to 2mm in 0.1mm increments that were made overseas. I've never used those though.



@n8n, Norseman makes a 2.05mm drill bit, fyi.

 

[RTM]

I am not a machinist, but this seems like a low stress job for the screw. I don’t think the 0.0003” will matter either, and suspect your technique will add more error than that.

 

[MushCreek]

I bought a small set of metric bits in a metal index case, for what reason I don't know. I work a lot with metric, especially tapping, but I've only used one metric bit once for that, and an SAE bit would have sufficed. My Champion 29 piece index in SAE (1/16" --> 1/2" by 64ths) will always provide something close enough. I do like the way metric sizes the tapping drill bit from the thread pitch.

The same 'trick' works for SAE taps. For example, a 1/4-20 tap. 20 TPI equals .050" per thread (1 divided by 20). 1/4" (.250) minus .050 equals .200". If you don't have a thread chart handy, or if it's an oddball thread, that's how you figure it out.

As for not having the right size drill, there's a lot more latitude than you would think. In soft/weak materials, I'll go with a smaller hole. In tough/strong materials, I'll go bigger. I spent a career machining tool steels, and we almost always went big on the tap drill sizes.