Allen key size for older HP knobs? [Answered]

[Cerebus]

Does anybody know what size Allen key is used for HP instrument knobs of 1970-80s vintage? I'm presuming these are imperial sized. They're smaller than ¹/₁₆", which is the smallest imperial Allen key I have.

I'm taking about the grey fluted knobs that fix with one or two hex headed grub screws. I've got a photo, but for some reason the forum is failing if I try to upload it so you'll just have to do with a description instead.

I'll probably get a kit of 'precision' keys but it'd be nice to know exactly what size I require in advance just to make sure that I get a kit that includes the right size.

[Sbampato12]

Some equipments (old ones) I recent played with knob removal were between 0.9mm - 1.3mm - 1.5mm. The smallest one I could find here was 0.7mm, but never used for this.
I didn't used imperial for these cases yet.

[edavid]

most likely 0.05", but they occasionally used 0.028"

[tautech]

Of course they'll be imperial and with the cheap Fuller set I have, the sub 1/16" sizes are 0.050, 0.035 and 0.028".
They were able to fit any grub-screw in a HP1740 that I fixed a while back.

[Cerebus]

That sounds fairly conclusive. I'll order up a set of Allen keys and I'll report back the exact sizes I ended up using once they're here.

[Sbampato12]

Of course they'll be imperial and with the cheap Fuller set I have, the sub 1/16" sizes are 0.050, 0.035 and 0.028".
They were able to fit any grub-screw in a HP1740 that I fixed a while back.

Sure, almos certain it is imperial. The equipments I've played was national made (on the 80's problably), soo they were metric (but on many cases, even those are imperial on 'some pieces').
But that to give an ideia to what this could be. These sizes on metric is 1.27mm (1.3mm), 0.889mm (0.9mm), prety damn close . And I must say, I don't remember the last time I've used a 0.7mm or 0.028" (0.711mm).

The pain, is that we use to have both kits of metric and imperial tools... due the globalisation I think....

[nctnico]

The one and only correct size is 0.05" (if that doesn't fit it is likely to be imperial). Be sure to get a good quality set though because you may need some force which could break a cheap allen key or worse: ruin the screw.

[tautech]

Of course they'll be imperial and with the cheap Fuller set I have, the sub 1/16" sizes are 0.050, 0.035 and 0.028".
They were able to fit any grub-screw in a HP1740 that I fixed a while back.

Sure, almos certain it is imperial. The equipments I've played was national made (on the 80's problably), soo they were metric (but on many cases, even those are imperial on 'some pieces').
But that to give an ideia to what this could be. These sizes on metric is 1.27mm (1.3mm), 0.889mm (0.9mm), prety damn close . And I must say, I don't remember the last time I've used a 0.7mm or 0.028" (0.711mm).

The pain, is that we use to have both kits of metric and imperial tools... due the globalisation I think....

The Fuller set I have #130-8030 has 0.07 - 10mm and 0.028 - 3/8" (15 sizes for each) and has never left me short of any size I've needed, bloody good for a cheap set and in fact I bought 3 sets at the same time, 1 for dad and his 2 sons.    Never broken any or twisted them. While they aren't Fuller Pro (lifetime warranty) I would recommend them.

[tggzzz]

Some can be 0.050", others are larger.

[bills]

I believe what you are looking for is called Bristol Wrench.

https://en.wikipedia.org/wiki/List_of_screw_drives

https://www.amazon.com/PIECE-BRISTOL-WRENCH-COLLINS-TRANSCEIVERS/dp/B01L7BCCQU/ref=sr_1_2?ie=UTF8

[CatalinaWOW]

The one and only correct size is 0.05" (if that doesn't fit it is likely to be imperial). Be sure to get a good quality set though because you may need some force which could break a cheap allen key or worse: ruin the screw.

That is worth repeating.  On larger sizes the quality of the steel and the preciseness of fit are not terribly critical.  But the smaller size screws are worked much closer to their limits and you can easily be frustrated as your tool rounds off, or infuriated as the tool rounds out the inside of the screw without removing it. 

Easy outs for these sizes don't work either so when you round them out you are either done or drilling them out and starting over.

[Cerebus]

I believe what you are looking for is called Bristol Wrench.

https://en.wikipedia.org/wiki/List_of_screw_drives

https://www.amazon.com/PIECE-BRISTOL-WRENCH-COLLINS-TRANSCEIVERS/dp/B01L7BCCQU/ref=sr_1_2?ie=UTF8

No, what we're looking for is a specific size of Allen key. The type is not in question, it's definitely a standard hexagonal socket, colloquially known as an Allen key.

[Cerebus]

The one and only correct size is 0.05" (if that doesn't fit it is likely to be imperial). Be sure to get a good quality set though because you may need some force which could break a cheap allen key or worse: ruin the screw.

That is worth repeating.  On larger sizes the quality of the steel and the preciseness of fit are not terrible critical.  But the smaller size screws are worked much closer to their limits and you can easily be frustrated as your tool rounds off, or infuriated as the tool rounds out the inside of the screw without removing it. 

Easy outs for these sizes don't work either so when you round them out you are either done or drilling them out and starting over.

I'd go the other way and say that it's not if you are going to strip the tools or the screw, it is when. It doesn't matter how good the materials or how well the tools are wielded, tiny hex sockets are always going to be working close to the material's shear strength. Anybody who designs something with such tiny hex sockets should be repeatedly kicked in the unmentionables until their ears bleed.

I just did a back of a cigarette packet calculation on this, and it's a web of material a mere 0.0077" thick that's taking the strain on a 0.050" socket. An generous estimate of the socket depth is 1/16, so that's 0.0077 * 1/16 * 6 = 0.0029 square inches of metal taking the load (probably less in practice). That gives a yield point of around 230 lbf for grade 5 bolt steel (81,000 psi yield strength, 105 kPSI UTS), at the end of a lever that must be multiplying force by 100 (2.5 in wrench, 0.025 in radius head), so just 2.3 lbf on the end of the Allen key to permanently deform the socket, 3 lbf to completely round it off. That's assuming that all the force gets applied evenly, in practice I suspect the edges of the Allen key act more like a very blunt cutting tool. Memo to self: stick some penetrating oil on these grubs screws the night before trying to remove them.

[CatalinaWOW]

More confusion on the original question.  I have some HP stuff from that time frame.  US manufactured so I would expect it to be the US version of Imperial units, which should be identical for this item.

On the stuff I have both and 0.050 inch and a 1.27 mm work, but the 1.27 seems to fit slightly better.  I don't know if either of my hex keys meets specification, or if the spec for the two sizes are different so can't interpret what this means. 

[tautech]

The one and only correct size is 0.05" (if that doesn't fit it is likely to be imperial). Be sure to get a good quality set though because you may need some force which could break a cheap allen key or worse: ruin the screw.

That is worth repeating.  On larger sizes the quality of the steel and the preciseness of fit are not terrible critical.  But the smaller size screws are worked much closer to their limits and you can easily be frustrated as your tool rounds off, or infuriated as the tool rounds out the inside of the screw without removing it. 

Easy outs for these sizes don't work either so when you round them out you are either done or drilling them out and starting over.

I'd go the other way and say that it's not if you are going to strip the tools or the screw, it is when. It doesn't matter how good the materials or how well the tools are wielded, tiny hex sockets are always going to be working close to the material's shear strength. Anybody who designs something with such tiny hex sockets should be repeatedly kicked in the unmentionables until their ears bleed.

Nah, you're overthinking it.
I've done a good deal of mechanical stuff in the last 4 decades and if the Allen/Hex screw isn't corroded in place and with selection of a snug fitting and good condition key into a clean hole it's highly unusual they can't be shifted. The only better recessed drive type screw is Torx but I can't remember seeing Torx in a grub-screw.

[Cerebus]

Nah, you're overthinking it.
I've done a good deal of mechanical stuff in the last 4 decades and if the Allen/Hex screw isn't corroded in place and with selection of a snug fitting and good condition key into a clean hole it's highly unusual they can't be shifted. The only better recessed drive type screw is Torx but I can't remember seeing Torx in a grub-screw.

Actually there was little thinking involved, it was more gut reaction and angst. But my practical experience is that the only things I have managed to mangle since I learned how to use the right size tool for the job (that weren't corroded or part of a vehicle exhaust path) have been tiny hex sockets and JIS screw heads, and I've completely sheared the heads off some woodscrews that someone else stuck into hardwoods without big enough pilot holes.

On the subject of Torx, or the horribly named generic equivalent "hexalobular", I've moved to almost exclusively using Torx headed fasteners for anything I make myself and I too haven't seen any Torx grub screws in the wild, but a grub screw is a rarity for me.

[nfmax]

The one and only correct size is 0.05" (if that doesn't fit it is likely to be imperial). Be sure to get a good quality set though because you may need some force which could break a cheap allen key or worse: ruin the screw.

That is worth repeating.  On larger sizes the quality of the steel and the preciseness of fit are not terrible critical.  But the smaller size screws are worked much closer to their limits and you can easily be frustrated as your tool rounds off, or infuriated as the tool rounds out the inside of the screw without removing it. 

Easy outs for these sizes don't work either so when you round them out you are either done or drilling them out and starting over.

I'd go the other way and say that it's not if you are going to strip the tools or the screw, it is when. It doesn't matter how good the materials or how well the tools are wielded, tiny hex sockets are always going to be working close to the material's shear strength. Anybody who designs something with such tiny hex sockets should be repeatedly kicked in the unmentionables until their ears bleed.

Nah, you're overthinking it.
I've done a good deal of mechanical stuff in the last 4 decades and if the Allen/Hex screw isn't corroded in place and with selection of a snug fitting and good condition key into a clean hole it's highly unusual they can't be shifted. The only better recessed drive type screw is Torx but I can't remember seeing Torx in a grub-screw.

A bit OT, but I've just been gumption-trapped by a recessed, hidden 2.5mm Allen screw which retains the swivelling spout of my kitchen sink tap, the O-rings of which are no longer sufficiently O to do their job. Limescale from the leak has locked (one could say fossilised) the screw imovably into place: the damn thing is now completely rounded out (as far as I can tell - it isn't possible to see it), I can't drill it out without removing the whole tap from the sink, as there is a solid wall some 10cm behind it. That requires unsoldering the (rigid) tails. And the hot water service valve for the tap doesn't shut off properly either.

I've ordered a new tap. And I will hire a plumber to fix it. The new one has a sensible size, surface-mounted slot-head screw for the same job*. No more expensive Swiss rubbish.

 

* The new tap I mean, not the plumber

[Paul Moir]

On the stuff I have both and 0.050 inch and a 1.27 mm work, but the 1.27 seems to fit slightly better.  I don't know if either of my hex keys meets specification, or if the spec for the two sizes are different so can't interpret what this means.

I think you're just looking at the tolerances.  For one thing, 1.27mm is by definition .050, since an inch is now defined as exactly 25.4mm.  At a .050" the difference between no fit and a good fit is going to be around .001", which isn't much.  And when you design the keys, are you going to push the size a little down to accommodate out-of-tolerance screws for the sake of a little sloppiness in good ones?

The only better recessed drive type screw is Torx but I can't remember seeing Torx in a grub-screw.

As a Canadian I can't let that one go.  You've clearly never used a Robertson. 

[HighVoltage]

On older HP equipment that I have worked on, sometimes people have used a wrong size allen key and the screw hex is damaged.
What I have found working very well is a slightly tapered torx wrench, like the small tools made by WIHA / Germany.

[PartialDischarge]

Just received an HP 3575A manufactured in 1972, suggested by edavid btw, and a H1.3 cheap "dexter" branded hex key fits perfectly

[CatalinaWOW]

On the stuff I have both and 0.050 inch and a 1.27 mm work, but the 1.27 seems to fit slightly better.  I don't know if either of my hex keys meets specification, or if the spec for the two sizes are different so can't interpret what this means.

I think you're just looking at the tolerances.  For one thing, 1.27mm is by definition .050, since an inch is now defined as exactly 25.4mm.  At a .050" the difference between no fit and a good fit is going to be around .001", which isn't much.  And when you design the keys, are you going to push the size a little down to accommodate out-of-tolerance screws for the sake of a little sloppiness in good ones?

OK.  You made me look.  The tolerance for an 0.050 key is 0.049 to 0.050.  The tolerance for a 1.27 key is 1.244 to 1.27.  Translating the latter to Imperial units gives 0.04898 to 0.050.  Theoretically slightly looser.  The 0.050 key that I used to test the socket measures from 0.0494 to 0.0496 across the three flat pairs.  The 1.27 mm key used measures 0.0499 across all three flat pairs.  Both tools are the same brand, and it happens that this was the first use for each of these (I just set up a new work area), so there is no wear involved.  The measurements confirm the observation that he metric key fits a bit better, but it is just luck of the draw, both are in spec.  I am actually stunned that this rather modest quality set was made to such high standards.

[Sbampato12]

Last time I've had to take out a knob (not an HP) it was Torx. And I've lost good minutes of my life trying to figure out wich Allen size it was...
May be it a good time to buy some magnifying glass, or a microscope.....

[Cerebus]

OK, Allen key purchased and reporting back as promised. As everybody was trending towards 0.050" as being the commonest I opted to grab a single 0.050" Allen key as I could get it delivered faster than waiting for a set.

I can confirm that all the knobs on both my HP 6002A (1980 vintage) and HP 6236B (1984 vintage) take a 0.050" Allen key. These are all nominal 1/2" diameter knobs, your mileage may vary on other sized knobs on similar age equipment.

The actual Allen key used measured 1.263 mm, 1.265 mm, and 1.268 mm on the three faces respectively (last digit visually interpolated, mm because I don't have any imperial micrometers) and so well within the tolerances that CatalinaWOW reports above for both 0.050" and 1.27 mm Allen keys. It's a comfortably snug fit in the grub screws with perhaps 10 degrees of play. It's quite impressive that a tool that cost the princely sum of £1.50 GBP should be accurate to within a few microns.

[45Overload]

OK I guess.  Lots of fanfare for a little hex key.  After 40 years the set screws get some corrosion which makes things more difficult but everyone is settled on a .050" key - great.  Penetrating oil on these older screws helps also.  Regardless, high quality equipment of this age and earlier often DID use spline/bristol hardware and one will rarely see a stripped fastener of this type as long as the correct bit has been used.

[Cerebus]

Regardless, high quality equipment of this age and earlier often DID use spline/bristol hardware and one will rarely see a stripped fastener of this type as long as the correct bit has been used.

Nobody is saying otherwise, whoever my original question was: "Does anybody know what size Allen key is used for HP instrument knobs of 1970-80s vintage?" so it's about a specific manufacturer, on a specific part, for a specific period, that I'd already determined by inspection had hex socket grub screws.

If you think this a lot of ho-ha for a simple hex key, well, we're engineers, we like to talk about minutiae - wait until you hit a thread that combines engineering, crowdfunding and politics.