I’m planning out a project where I’ll be using a new and unfamiliar tap size, which means I’ll need to do a figurative supply run.
After sorting out the taps I’ll need – OSG spiral point plug and spiral flute bottoming taps, it was time to sort out my drill bit needs.
I’ll be reusing some older 80/20 t-slot framing extrusions – 15-series (1.5″ x 1.5″) in either standard or light and smooth profiles. Both have 0.262″ through-holes, which can be tapped straight away for 5/16-18 thread.
Advertisement
But, I need the end holes tapped to M10 x 1.5 threads for use with swivel casters. This means I’ll need to enlarge the holes a bit, a task that is made a little easier with the light and smooth profiles.
The easiest option would be to find some 5/16-18 threaded-stem casters, but I’m very much set in my brand preference when it comes to indoor casters.
Looking at online thread engagement tables – here’s one by OSG – we can see that 0.262″ would be 70% engagement for 5/16-18.
I have seen general recommendations of 50% for steel and 75% for aluminum. 80/20 sizes their 15-series extrusions for 70% engagement without additional drilling needed before tapping, and so it seems safe to stick with that.
Some engineering references recommend 60-70% engagement in general. Sometimes it takes some trial and error to sort out which is best for an application. I have found that optimal thread engagement can depend on different factors.
For a moment, let’s say we want to go with 75% engagement.
Advertisement
Here’s a Little Machine Shop chart for metric and inch tap and clearance drill sizes.
To prep a through-hole for an M10 x 1.5 tap, that chart says I’ll need an 8.50 mm drill size for 75% thread engagement in non-ferrous materials. That’s around 0.3346″.
The closest imperial drill bit size is R, which is 0.339″ or ~8.61mm. (I have also seen at least one chart list Q as the closes imperial drill bit size.)
So, I can go with 8.50mm or R. There’s no fractional equivalent close enough. I have a letter drill bit set, but if not I’d have to buy either size.
If I wanted to tap ferrous materials such as steel, 50% thread engagement would mean a 9.00mm drill bit size, or T as the closest imperial letter gauge drill bit size, which is 0.358″. This is according to the Little Machine Shop chart, or others’.
(When looking at reference charts online, it’s a good idea to double check with other references.)
Okay – so 75% engagement for M10 would mean an 8.50mm or R (~8.61mm) drill bit size, and 50% engagement for M10 would mean 9.00mm or T (~9.09mm).
That was pretty easy.
The Close Enough Method
What if I want to drill a hole for a tap to cut M10 threads with 70% engagement?
There are not many options that are slightly larger than 8.50mm or R. There’s 8.80mm and S, which I’d assume would be around 62.5% engagement. That seems too loose for aluminum.
It seems best to stick with 8.50mm or R. An R drill bit is approximately ~8.61mm, which would provide slightly less engagement than 75% – which follows closer to 80/20’s example.
Alright, so R it is – that’s close enough. It would provide slightly less engagement than 75%, and should be easy to find at industrial suppliers.
A More Accurate Approach
For the most accurate sizing, you’ll need to consult an engineering reference (such as the Machinery’s Handbook) for the proper equations to use and do some math by hand. I won’t cover that here.
Kennametal has an online calculator that only needs to know the major diameter and pitch. Little Machine shop lists major and minor diameters, but only for imperial tap and drill sizes.
Let’s use nominal dimensions, which would mean 10mm for the major diameter and 1.5mm for the metric thread pitch (length between threads). Checking first for 75% engagement, the calculator says the cutting tap hole size should be 8.5386mm, which rounds down to 8.5mm. This agrees with what many online and reference charts say, leading me to trust the calculator.
For 70%, it recommends an 8.636mm hole size.
Looking online, there are 8.6mm drill bits. I can find them for close to the same price as 8.50mm or R letter-sized drill bits.
But the R-size drill bit is pretty close to the recommended hole size too, somewhat validating the “close enough” approach.
If 8.6mm provided too much engagement, and 8.8mm too little, there are also 8.7mm drill bits.
For my application, the casters will be threaded at least an inch into the aluminum extrusions, which means that I’m not too concerned as to whether the thread engagement is 68% or 71% instead of exactly 70%.
There’s also a bit of trial and error involved. This will be my first time tapping 80/20 materials for M10 casters. I have previously only tapped 15-series extrusions for 5/16-18 mounting bolts, where the built-in through-hole is already sized for 70% engagement.
I’m not concerned about how things turn out here – I’m sure the R drill will be fine for my application – but I thought it would be interesting and potentially helpful for me to share the steps and resources I used to land on a drill bit size.
There’s another challenge – building a jig or otherwise using an R-sized drill bit to accurately enlarge the 0.262″ hole. This isn’t exactly something I will want to do free-hand with a handheld drill.
MattW
Tap drill size for approximately 75% 60 deg threads = Major diameter – pitch. Thread strength doesn’t change much with minor changes to the minor diameter, so isn’t worth sweating absent a critical need/requirement.
MM
Wow that’s a lot of text when tap/drill charts are so easily found and when tolerances matter so little for this application!
You can easily drill the holes freehand if you track down a counterbore with the appropriately sized pilot instead of a drill bit, or have a piloted drill custom made. You can probably make modify a drill bit yourself if you have some kind of stationary belt sander, bench grinder, etc. You’re only drilling in aluminum and this application does not require high precision. It would also be very easy to make a jig using a drill bushing and some scrap wood.
Stuart
Maybe half the charts posted online lack context and proper guidance. You have to know what you’re looking for, how to read the charts, and know when to take what they say as a suggestion.
I have a lot of inch tap drill sizes committed to memory, but not metric. My first few searched turned up garbage references that would have steered me wrong.
I’ve used piloted counterbores before, and own a bunch in standard inch socket cap screw sizes plus one or two special sizes. I can find ones in close-enough sizes, but it would add up to $60 to the tooling cost and still require the use of another drill bit to get the hole to final size.
I can build a reusable jig for less than half that, and much less if I only build it to last for this project.
MM
I’ve got 3 tap/drill charts within eyesight: A Starrett, a Morse, and some no-name one I printed off online and taped to the wall years ago. There’s also the chart glued in the lid of my metric tap/drill set (no name, made in Japan). They all give 8.5mm as standard size for M10x1.50, and as you explained R is a very close size if someone doesn’t have or want to buy and 8.5mm drill
For some critical or precision applications % thread engagement is very important but for such a basic task as putting stem casters on the bottom of something it’s completely overthinking things.
But yeah, I don’t see the need for some serious solution here. Build yourself a one-off jig or sacrifice a single drill bit to the grinder to make yourself a piloted one, it only has to work for what, 4 holes, and surface finish doesn’t matter one bit since you’ll be running a tap down the hole.
MM
If Stuart had access to a machine shop this would be done in 5 minutes. Stick the extrusion in a lathe with a 4-jaw chuck or put it in a horizontal mill and drill it out in one step. Depending on the length of the extrusions and size of the equipment available a big drill press might be an option too.
MM
Whoops, that was meant to reply to fred below.
Stuart
Yes, but I’ve used extrusions as legs up to maybe 72” before.
Fixturing 30” and longer extrusions to a drill press isn’t easy. Can it be done? Yes, but building a reusable portable jig seems a lot easier.
It cannot be done in a vertical mill, and I’m not sure if or when I’ll have a lathe capable of handling even a 30” extrusion let alone longer.
Kent
YUP!
JoeM
I suggest a flat block to make a jig. Mark the measured angle on the side, then follow it with the expected Drill Bit in either a Drill Press or a Drill Guide, like the Milescraft or Rockler ones. The Rockler one, you can see on AKlein’s YouTube channel, comes with a built in clamp for the base, so that you can lock the material in place.
Once you’ve made the jig with the guides and marks you’re read to use it freehand, just as if it was a Kreg. Especially if you make it out of something harder, like cast iron, so repeated use doesn’t wear one side or another to knock you off target. As long as it’s thick enough to hold you straight, but not enough to stop you from getting to the intended Aluminum, you can make them out of nearly anything, as long as it’s not softer than the Aluminum you’re using.
But, you probably already know this stuff. If I’m stating the obvious, I apologize. I just don’t see much use in trying to buy anything more than a chunk of really solid material to make a jig. You can make single-use jigs easier than you can find factory-produced ones by the top companies, like Kreg. If it were something everybody does regularly in a field, Kreg likely has what you need, and I’d suggest them first. But this sounds more like you have a very unique setup you want, therefore, their jigs won’t necessarily cover it. So… Make your own seems reasonable to me.
I’m going to stop at just making the Jig. I’m a Metric user, and I know it’s really difficult to get SAE/Imperial users to simply jump over and use whole Metric sizes instead, for ease of calculation. Plus, you mentioned you only have the Imperial/SAE drill sizes right now, and I’m not about to recommend you buy hundreds, upon hundreds, of dollars’ worth of new bits, when you have just one size in mind.
Wouldn’t mind a link, or article, on Tap Drill purchases/recommendations though. That’s something I would like to get eventually, so I can make easily-disassembled furniture for myself.
Stuart
End-drilling t-slot extrusion to enlarge their holes for larger tap sizes is a challenge because of their size. You also need the holes to be well-centered. That the lighter profile holes have relief cut-outs presumably for weight reduction might complicate things.
I have in the past used larger feet with 1.5″ x 3″ profiles, which means tapping the end holes for 5/16″ bolts and then installed the feet to a mounted block with 1/2″-13 or similar central hole.
I’m starting to use more M10 threaded stud casters and 3/8″-16 leveling feet in new projects, and also re-cutting older profiles for new quick builds.
As an aside, 3/8″ and M10 end taps are now $3.45 via 80/20, and the price per cut is now $2.79 each. So if I want say 4 legs each 30″ long with an end tap, ordering 2x 60″ extrusions vs 4x 30″ and doing all the taps myself would save 2 cuts and 4 taps, or $19.38. If I have 4x 32″ extrusions in my spares pile, I can lop off 2 inches and save $24.96 on machining costs alone.
Anyway, I have built 8020 jigs in the past, and have found it easier to attach a jig to the profile compared to trying to lock down the profile and steady a jig over it.
My 10-series (1-inch) jig was 1″ wide and 1/2″ deep. I’m thinking a 15-series drilling jig can be 1/2″ wide and 3/4″ deep. Down pins can be used for alignment. I’ll try an L-shaped jig with drill bushing and locking alignment bar stock attached to it. I’m thinking one or two dowel pins with a 5/16″ threaded stud and locking handle. This should allow for easily going from one bar to another. Locking it down with a hex key would save a bit money but hamper longer projects that might require drilling a lot of holes. The same jig can potentially be used to drill access holes for end fasteners.
Here’s what 8020’s access hole jig looks like: https://www.amazon.com/80-20-Inc-Access-Drill/dp/B006YW09TU/?tag=toolguyd-20 . It’s easy enough to replicate if you have the patience.
Alternatively, you can 3D print a C-shaped block and add a bushing for drilling access holes. A lot more can go wrong with the greater forces involved in enlarging an end hole. Maybe a 3D printed jig can be built for this too, but for me it’d be easier to make out of wood, machinable plastic, or aluminum.
Sam S.
Could you simply cut off a 1″ piece of 80/20 and drill it with your drill press to make a guide?
Stuart
Potentially, yes, and it could be clamped with flat plates.
If there plates on adjacent sides it can then be used on any 15 series extrusion ends. I’m not sure how good of a drill bushing the relieved hole will be though.
fred
We had skilled machinists on the payroll that we relied on for this sort of thing. Being more of a plumber and woodworker the creation of precision dimension holes was mostly not in my wheelhouse. But I did learn over the years that precision might involve the multiple steps of initial drilling – followed by boring/milling to enlarge the drilled hole and then reaming to a precise diameter. I seem to recall that many of the items that we fabricated from aluminum had specs that required the use of threaded inserts for threaded fasteners – when rivets were not being used.
AC
Once I found combination tap and drill bits, I never looked back. They’re so easy it feels wrong to use them, but I’ve yet to have a problem.
Stuart
How do you use a combination tap-drill to make deep holes in solid materials? Or a blind hole?
All of the tap drills I’m familiar with were designed for use in sheet metal or very thin plates.
fred
Combination tap drills seem to be popular with electricians. I would think that typical use would be for drilling/tapping holes for screws to hold down cover plates or for adding a ground screw. Such tasks, as you suggest, would probably be making through holes in not-too-thick metal.
Peter Fox
Picking a tap drill for metric is honestly easier than it is for inch.
Subtracting the thread pitch from the nominal major diameter gets you a size that is suitable for most applications. As an example M4x.07 4mm – 0.7mm = 3.3mm this works off of the geometry of a 60 degree equilateral triangle so it scales equally from small to large threads. Unless you are working with difficult materials or something critical this will work just fine. If you are doing more demanding work that you really do need to spend some time and dig into machinery hand book or other more substantial reference. Tap drill size is only one of the relevant variables, the H or D limits of the tap can also matter.
The only difficulty is that we use such an antiquated system for measuring and specifying drill sizes that unless you have proper metric drill bit you need to look up the nearest size in a chart.
you can use the same method to determine tap drill size for inch threads as well you just need to convert TPI (threads per inch) into the pitch of an individual thread (1 / TPI).
As an example for 1/4-20
1 / 20 TPI = 0.05″
thus
0.250″ – 0.050″ = 0.200″.
0.200″ is only 0.001″ off of the common recommendation of a #7 drill (0.201″)
Shane
Yep, I came here to say this for metric, it’s super easy (as with most things in metric).
M6x1.0 = 6 – 1 = 5mm
M8x1.25 = 8 – 1.25 = 6.75mm
M10x1.5 = 10 – 1.5 = 8.5mm
etc.
Stuart
Ooh – thank you and everyone else, I’m going to keep this in mind!!
MM
Something else just occurred to me here: This is not a blind hole, thus there is probably no point in using the bottoming tap, the spiral point tap should get you 1 inch deep useful threads by itself.
Stuart
For the spiral point plug tap I’m looking at, the thread length is listed in the manufacturer datasheet as 19mm. It has a neck length of 32mm.
I *should* be okay with just a plug tap here, but if I ever want to use M10 leveling feet in this or other projects, I’ll regret not chasing need a bottoming tap.
I also like spiral flutes for clearing out chips so they don’t get left behind in the hole.
I’ve tapped 10 and 15-series extrusions with better results when using two-step tapping vs single.
MM
I find bottoming taps to be a chore to use. Sometimes they are a necessary evil when you need to do blind holes but they are the slowest and most tedious of all tapping processes. The spiral point tap like the top one in your photo is a pleasure to use because the chips go forward. In a non-blind hole (or a blind hole that’s deep enough) in most materials there’s no need to peck with the tap at all. Just go straight to full depth in one pass, then back the tap out. It shoots the chips forward of the tap with no need to reverse to break and clear chips, which also gives them the name “gun point tap”. This process also makes large pieces of swarf which easily come out, there’s not a zillion tiny little pieces here and there. Also since you’re not having to reverse all the time they are ideal for power tapping. If you don’t need the extra depth I can’t imagine why you’d torture yourself with bottoming taps; with a cordless drill and a spiral-point tap you’re done in just a few seconds a hole.
Stuart
I thought the same a couple of times – “I don’t need a bottoming tap here” and regretted cheaping out on the time and effort.
It has made enough of a difference where for this exact type of application I don’t bother wasting time trying to see if I can make do with just a plug tap.
I don’t back out for either spiral point or spiral flute taps. The spiral point pushes chips ahead, the spiral flute pulls them out. The last couple of times I only used spiral flute plug and bottoming taps. The 8020 profiles I’ve used for the past few years have relieved holes that ensure whether by design or circumstance that chips don’t bind up.
Using both doesn’t take much more time, and ensures I get the thread as deep as possible.
Basically, what I’m saying is that I HAVE needed all of the threaded length I could get, and it HAS set me back to wait for more tooling to do so.
If purchased from a DIY/personal context, I’d likely go the cheapest route and pick up a plug tap for half the price of a spiral, and then a bottoming tap only if absolutely needed right now.
Peter Fox
If you want to have extra deep threads in the end of an extrusion like 8020 you might consider getting Extended length spiral point taps with undercut shanks. Then all you need to do is drill deep enough and tap it extra deep in one pass. They look like they are delicate but the long shank general flexes and distributes any bending stresses rather than concentrating them.
I strongly agree that bottoming taps suck and only use them when I need full threads all the way to the bottom of a blind hole. Otherwise its spiral point taps all the way they cut great and are generally hard to break.
Stuart
Thanks!
I have looked at them in the past, but often the price difference was significant and all I really needed was a fraction of an inch longer thread.
I checked quickly now – I didn’t see a reduced-shank tap, but I did see extended length spiral point, and it was about double the price.
I could also potentially just add nuts to the casters rather than seating the threaded studs all the way, but I reuse 8020 enough that I like to tap end holes a certain way.
For M10, where I have nothing, I’ll probably still go with plug and spiral. For 3/8-16, however, an extended length spiral tap might see more use.
I figure that if I’m building a jig for M10 end hole enlarging, I should do the same for 3/8, which I use for leveling feet.
Peter Fox
They are available, but you are correct they don’t give them away.
For M10x1.5 here is one example https://www.mscdirect.com/product/details/59917781
fred
Sometimes what folks list as “pulley taps” will have extended shanks. Sometimes the extended shank will on a “pulley tap” will also have a reduced diameter.
Peter Fox
Pulley taps usually don’t have undercut or reduced diameter shanks.
However old pattern nut taps usually did along with a linger tapered section at start of the tap. These features allowed easier starting and cutting of threads as well as room for already tapped nuts to accumulate in the shank. These type are mostly obsolete for production work but there are probably still a few companies offering them
James+C
That’s what I was thinking. If it can’t go deep enough for the caster stems then the stems are longer than they need to be. Your choice, buy the extra tap if you think you’ll need it again, or shorten the stems if that’s a problem.
Stuart
Most leveling feet stems are 2-inches, minus an adjustment nut. Getting the additional tap gives me options.
I could also spare the expense and just get general purpose hand taps. McMaster has 4-flute M10-1.5 taps for $11.38, but I stick with what has worked for me in the past.
James+C
Well I don’t mean to talk you of all people out of buying more tools. I’m just getting into more metalworking, including thread tapping, so I’m happy to go along for the ride here. Lots of good info in the post and comments.
Stuart
Even with threaded feet, I don’t cut them down to size. Many of my 80/20 projects are disassembled and rebuilt as needs change. I can usually design around common threaded stud sizes, but it’s a lot harder to work with ones that are too short.
I’m not saying everyone needs to use bottoming taps, but that I do based on my experiences with this type of application.
If you’re just getting started, shop MSC’s sales flyers and learn what you use and don’t use.
Plug taps are most common. Taper taps are useful in trickier situations or more critical applications, bottoming tasks for “you’ll know when you need them” tasks.
Sometimes 3pc sets can be economical.
Spiral taps can be used at higher speeds, but you’ll want to gain experience hand tapping before you even think of putting a tap into a drill or machine.
Don’t forget cutting fluid.
Addicted2Red
Meh, get metric extrusion instead
Zyltech.com has saved me tons on 3030 and 4040 extrusion vs 8020.
Stuart
Looking at that website, their T-slot looks suited for small 3D printers and such. They also don’t have extrusions close in size to what I use.
Their largest extrusion looks to be 3030, with a 6.80mm through-hole that would also need to be enlarged with a 8.5mm drill bit for an M10 tap. They don’t have 4040, which would be roughly comparable in cross section to 15-series imperial.
Besides that, I already have plenty of 80/20 from past projects that I plan to repurpose.
Dave P
Tremendously overthought, but as long as you’re having fun…….
Bruce
Holes drilled with fluted bits are going to end up slightly oversized. You’ll end up just about on the mark using the R. If you want to actually hit a hole size, you’re going to have to use a reamer.
Stuart
Yes.
Some drill bits are better than others as well. For instance, 5/16″ can be used to drill for a 3/8-16 tap, but I don’t use construction brands’ drill bits for this.
I bought one size of reamer to experiment with at some point a long time ago, but have yet to use it in an actual project application. I have yet to run into a situation or design a part where I needed a reamer over a drill bit.
fred
I seem to remember that when we were connecting up for structural steel elements with multiple bolts – the guys would ream through both holes at once then insert the bolt – repeating that process for each bolt and hole That may have had more to do with insuring alignment.
M
….Whatcha buildin? Always love seeing aluminum extrusion projects. If the stuff wasn’t so bloody expensive I’d probably use it a lot more. Right now I just use it with 3D printers and CNC routers.
Stuart
Another cart.
The cost is pretty much why I don’t post more 80/20 projects. It makes sense for business needs but not personal.
This one is going to be for my vacuum sealer so I can bring it to the kitchen easier and have space for accessories and season-rotational storage.
If I didn’t have older bars to cut and repurpose, I’d build it out of 2x construction materials.
JR Ramos
Saw the other post on taps and threading….lots of great commentary here already. I’ll add a couple considerations.
As MM said, don’t fret so much over engagement, especially with aluminum and what are likely not the greatest threads on the caster stems. Maybe get a scrap block of material and drill three or four different holes in it for your tap…get a feel for that and see how the fit changes (using the same bolt). You may notice that it doesn’t feel a whole lot different unless the male threads you have are actually cut threads (not rolled/formed as your caster threads almost surely are) and/or if they are a higher tolerance than the normal range(s).
To that end, give yourself a visual reminder of what twelve thou looks like between caliper jaws, and imagine half of that amount as an extreme shift in thread engagement from the normal %. Twelve is “a lot” but really in most materials/situations, it isn’t. But here, with aluminum and caster threads and play in the caster bearing race(s), you have a lot of room to toy around with.
If fixturing the long pieces is a challenge and you end up having to do this by hand, then that sloppiness and play in the whole affair may work to your advantage as your hole/tap may be off a little (but try to keep it plumb). Ballgame changes when you move to thicker steel, turned threads, higher tolerances and mating parts, etc.
But drill four holes with a wider spread in size, tap ’em all, and see what you learn. It’s a good exercise and will help keep the reference charts in a practical perspective.
Stuart
That’s a good point too – to try a range of drill holes in scrap materials.
My overthinking things isn’t tied to a particular project, but a global understanding, if that makes sense.
80/20’s 10-series extrusion has a 0.205″ through-hole. Why is it that size?
Well, the 15-series hole is sized for a tap drill for 70% thread engagement, and sure enough the same looks to be the case for the 10-series.
The 10-series is a moot point as it looks like it can only be tapped to 1/4-20 or M6.
But if I want to tap 15-series extrusion to say M8 in the future, I just have to consult a calculator for 70% engagement.
80/20 is made from 6105-T5, which I’m not familiar with compared to 6061. So for something like that I don’t like to stray too far from manufacturer recommendations, otherwise I’d lean towards time-consuming trial and error.
For many things, I don’t pay much attention to tap drill size or thread engagement. But there are some applications where I very heavily load 80/20 extrusions. There are some things I prefer to overthink about on occasion rather than learning the hard way.
JR Ramos
My overthinking and wanting to know how the watch works is the same…which is most often a good quality! I don’t know about that hole sizing choice. Could be a passable quick-assembly tolerance for 1/4″-20 tapping, or perhaps 5mm rods/allthread are a popular option and a quick slide fit that can accommodate variation (or bends) is desired, or maybe some particular thing in that niche of server rack infrastructure where I think these got their launch and widest use? Unless there’s something else with the extrusion process but I would imagine they could control that somewhat precisely if they wanted to.
I’ll have to look up the 6105…new to me, too, but generally within the “family” there won’t be huge differences (sometimes it’s just slight qualities that make it better for one purpose in particular…like welding or bending but most properties will remain pretty close to one another…the varieties in brass are a good example). Maybe the composition and/or the particular tempering time/method is helpful for extrusion.
Examining thread failures is interesting. Often it has to do with properties of the metal used more than thread engagement. I can’t recall exactly now but I think 70%-ish is the sweet spot for strength with acceptable necessary force for tapping (and life/integrity of tooling) and is something like 95% strength. Moving up in engagement exponentially increases the force necessary to cut while providing little additional benefit. If more strength is needed then perhaps a different fastening method should be investigated.
As an aside, a couple months ago I wanted to tap a 5mm hole to M10x1.5 in a rather soft aluminum handwheel. I don’t have a mill so there’s plenty of room for error and creativity and getting something like that completed with accuracy. For the application I decided to play with it and see what 95%-ish engagement would be like for tapping and mating. Good hss tap, soft metal, not my first hole. I’ve tapped a lot of 6061 and some 5xxx so I know how it feels normally, and the tap was large enough that I wasn’t real concerned with breakage. It was surprisingly easy to turn (other than being a blind hole and needing a bottom tap)…but comparing that handwheel with another that I had tapped with the standard drill size, the play and engagement didn’t feel all that much different (didn’t measure it). Pull out or stripping won’t be an issue with this but honestly my intuition says that the greater engagement isn’t really don’t anything on this one.
There are parts/materials/applications where it can be important (NASA is pretty particular about it in most instances) but for most things, shoot for by-the-book and adjust when appropriate, but generally don’t sweat it too much. As great as threads truly are, sometimes another approach is best suited for the application, or perhaps switching from aluminum to bronze or cast iron or steel, or thread inserts, etc. (not like you have a lot of choice here in this project…just saying).
Should add that most of this is truth for typical 60° unified thread standards but changes with other thread types or multi-pitch and such.
Travis
Often when I have a task like this (and have access to a decent lathe) I will just turn down the shank of the tap- extending the shank relief further back. It sounds like the material should be too hard, but good old CCMT or VCMT (or a host of other) carbide insert turning tools will do this without too much fuss or drama. I imagine a bit of care and a bench grinder could accomplish the same result. Shouldnt reduce the usability of the tap for normal use (just dont overheat the shank when you do it) and it prevents having to buy another tap.
Bug
Torment yourself by buying a go/no-go gauge.
Set a backup plan with threaded inserts.
Make life easier with impact taps.
Drew M
Don’t google “punch taps”
;^)
Stuart
That’s one of several techniques that require CNC machine control.