How to hand grind 55 degree cutter for 32TPI?

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Just to show solidarity with Hopper ... Permit me to amend that statement, for clarity

”But producing an accurately formed tool for 32tpi is likely to require an optical comparator to check your progress.”

Would probably better read:

”But producing an accurately formed tool for 32tpi would be likely to require an optical comparator to check your progress,; and, for a C-mount thread, such detail is probably unnecessary.” [*]

Incidentally, however: Whitworth’s own test gauges are a joy to behold !

MichaelG.

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[*] and would be irrelevant, if the information in my update is confirmed.

Morning all

Hopper mentions Martin Cleve above: in his book ' screwcutting in the lathe' he describes a simple jig to aid producing a screwcutting toolbit on an off-hand grinder. See pages 123 et seq. l made one and am very pleased with it. It is worth getting a copy as it is a mine of useful information.

HTH

Dave

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I suppose it might be wise to ask whether the proposed mount is for an exotic camera/lens, before deciding that this is ‘general work’. The cost of recovering jammed or worn components could be high.

MichaelG.

Just draw a couple of lines on your offhand grinders rest to line up the shank of the boring bar to so that each edge is presented to the wheel at the required angle and grind away, clamp something against the lines if you want to have something to slide the tool against. Same with the external one but easier to do.

If it is that large a thread then use a holder for a HSS bit then you don't have the difficulty of doing a solid boring bar with cranked end.

I also don't see the issue of grinding the angles for 32tpi, they are the same for any 55deg thread so grind something large and then just stone the end if you want the radius.

If you want to use your usual method of cmparing the angle with a screw the just put a piercing saw cut into the bottom of a thread to remove the root radius and then use that just like a thread gauge with it's clearance slot.

Edited By JasonB on 03/02/2020 07:48:50

Edited By JasonB on 03/02/2020 07:49:26

Would a partial profile insert do ?

55 degree PP inserts are available that can cut a range of threads starting at 48tpi ( like eBay item number: 293401899996 as an example of an external type).

The tip radius will be a bit smaller than 32tpi, but this usually compensated for by going a bit deeper. At work we use PP inserts most of the time, with the (metric) inserts being able to cut a range of threads from 0.5 to 3mm on the same insert. We typically use Korloy PP inserts from Cutwel, but a quick look on their website now only shows Vardex inserts of this type & the online catalogue doesn't state the tpi range for these.

Nigel B.

Never mind idleness, inserts are ideal for this kind of job. Whilst it's certainly possible to hand grind a suitable HSS cutter the answers show the DIY approach involves a good deal of fannying about. Not straightforward to grind, or to confirm that the finished tool is correct.

This being a fine thread on a large diameter object hints at optics, where failure to match threads accurately could cause expensive bother. As it's not difficult to cross-thread professionally made components of this type, I'd want any thread I made to be as good as possible. The coarse threads I usually cut allow more room for error in cutter shape and my iffy craftsmanship; fine high-quality threads need more care.

Easiest way of getting an accurate fine 55° thread cutter is to buy one. Let a professional like Cutwel make it! They have all the gear and expertise...

Dave

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I think you will find that Ian was concerned about the [Whitworth] specified radii

MichaelG.

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More detail about that thread-form than any of us could probably work-to, available here: **LINK**

https://amesweb.info/Screws/AsmeUnifiedInchScrewThread.aspx

Just select 1-32 and click the button !

MichaelG.

Staple jobs in our local section workshop at RARDE / DERA / DRA was producing metalwork components for optical and IR research equipment including (lots of) one off and special purpose lenses. Normal practice was to use appropriately modified versions of the flat bottomed thread form whether Whitworth, UN or Metric depending on what existing equipment the lens in question was to mate with or what was appropriate to the rest of the design.

I'd be unsurprised to discover that, even when I left in 2004, most of our in house components were still basically designed to BS 1618 : 1949 and its successor standards.

Clearances on threads used with optical components tend to be different, usually larger and assymetric, to the official standard tables to reduce the chance of jamming and allow a tiny bit of float for things like lens mounting rings to settle correctly against the curved lens surface. Making things dead nuts to small diameter standards puts impossible symmetry requirements on the optical surfaces if the rings are to seat properly. Odds are such rings will come loose in service.

I suspect SRB used book form threads as their filter carriers were much more prone to jamming than the ones we made in house.

You haven't lived until you've had maybe £150,000 in mid 1980's money worth of one off IR telescope jam up partway down the Wreathall mount on a TICM 2 thermal imager. 16" long, 11" diameter front element, and several germanium lenses inside = heavy, serious heavy. Cuddle lovingly, not hold in hands.

Clive

Sounded like he was having problems with the angle to me

"I do have a thread angle gauge but with very small cutters the sharp tip (before it has a radius) touches the bottom of the gauge 'V' making it hard to use."

Take a look at an article I wrote for MEW called "Really Simple Threading Tools". It was in issue 159, page 44. I can't remember year.

Some way back on here, someone quoted the thread for C mount as 1-32UN-2A

Does that mean that it is Unified ?

If so, the thread form is 60 degrees. In which case a 0.75 mm pitch Metric insert would have a a radius suitable fora 33 tpi thread. Which would probably suffice.

Also, does it HAVE to be a radius? Particularly with Unified threads it is normal, (or was when I was an Apprentice at Rolls Royce ) for Unfified threads to be truncated, with a flat top.

Whether radiused or truncated, the object is to prevent interference at the crest of the thread.

And C mount sounds like the thread for a cine camera lens, so is unlikely to be so highly stressed as to be in danger of overloading and stripping.

Howard

Clive,

Good to hear that someone on the forum can recollect the "good old days" at RARDE etc, although I was in the RF section in Q7 not the optical section from the mid eighties onwards. I remember the workshops we used to have, Q4/Q7, and I remember you quite well,

Best Regards,

Clive Brooker

PS Apologies for being slightly off topic.

A slightly offbeat thought. 32TPI would be 3/16" BSF in a 55º form.

Maybe use a single chaser out of a set from a ¼" Coventry diehead.
I've just checked my spare ones, but the only 32TPI chasers I have are UNF so 60º

Bill

Wow!

An awful lot of interesting and useful information if all those replies, thanks to all.

Although not at the PC (or in the workshop) I've made some theoretical progress. One of the things that I had forgotten I had bought is an internal screwcutting insert holder, the best bit is that an 11NR-A60 tip is already in it! which is perfect for both the threads I need, just waiting for the 60mm tube now.

Pete Rimmer suggested 'For a fine tip like that I'd use carbide to turn some round HSS using the compound at 27.5 degrees then grind the HSS round down to half thickness then stone the radius'

That sounded very tempting but then I realised that the cutter would be very weak near the tip because there would be very little metal.

Jason's idea of relieving the bottom of the 'V' in a gauge is blindingly obvious now someone has told me.

For info, these are camera related threads and I now know the C mount is 60 degree so the same tip will do both.

Thanks

IanP

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Yes ... and I also suggested [prior to finding the reference] why that was likely.

‘Someone’

Or use a tap held in the toolpost as a multi tooth screwcutting tool bit.

Exotic would be the jesus bolt holding the rotor on a helicopter. A lowly stressed, finger tightened camera lense mount is not so critical. Quite general for a competent machinist. Main thing is plenty of clearance so it does not jam up. So go easy on the tip radius. Too much tip radius on the tool and the thread will bind in that area. Better to have a pointier tool to provide clearance in this area. In fact make the OD undersize and ID oversize to ensure good clearance. Remember that something like 65 per cent thread engagement still gives 95 per cent strength according to Tubal Cain's ME Handbook. And with a bit of extra clearance on thread flanks the exact angle is less critical and jamming less likely.

Edited By Hopper on 04/02/2020 04:16:20

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It appears that you must have some inside knowledge regarding what Ian is building, Hopper

MichaelG.