How to hand grind 55 degree cutter for 32TPI?

I've always ground screwcutting tips off-hand but I need to cut internal and external 32TPI (not 48 as title) threads in ali and not having a tool and cutter grinder, would like to know what others do?

With the aid of a magnifying glass, in the past I have been able to check the progress of grinding with a sample of the thread against the light. As my sample thread this time is 60mm diameter, none of the magnifiers I have are usable because the closest the lens can be is 30mm away from the point I want to examine.

Ian P

Could you use a 55 degree thread gauge or a thread angle gauge to use as a template? If you have these things!

Ian beat me to it, but here's a link anyway

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.

Its quite a good quality gauge but at high magnification the edge looks quite rough.

Ian P

My gauge does not have the clearance slit like the one in the link. Out of interest, how wide is that slot likely to be?

Ian P

Edited for spelling

Edited By Ian P on 02/02/2020 21:52:57

Also a thread gauge might be hard to use on a boring bar (unless the tip was highre than the rest of the tool.

Ian P

32tpi would be so close to a point that thinking of a tip radius of 0.0037" produced by hand is mind boggling.

I would get er11 carbide tooling for that job, especially as the diameter is so nice and big.

While I have the wherewithal to accurately grind a HSS toolbit (Clarkson T&C grinder and swivel vice) I'm idle and just buy threading inserts.

Andrew

I'd grind it as close as I could to a 55 degree point set in a Vernier protractor or one of those digital angle gauges from Lidl, then stone the tip rad by hand with a medium/fine India oilstone. If it's 32 TPI on a 60mm diameter it's not gonna matter much if the tip rad is too small and the root's a thou or two deeper than it would be if the tip rad's crack -on size, unless the thread's on an extremely thin-walled tube. So my stoned rad would do little more than break the sharp edge.

I have just had a quick look for an insert but small sizes of 55 degree ones seem rather thin on the ground and I'm unclear about the what the tip radius is (or how fine a pitch it could cut).

I have internal and external holders and it looks like I could get away with just with an internal tip.

Ian P

Slight correction to make as I have actually have two different parts to make and I got mixed up. One is a female 1"x32TPI (C-Mount lens thread) and the other is a 60mm OD extension tube with 58x0.75mm male and female threads at the ends. My sample is made out of 1mm wall tube (although I will use 2mm wall).

Ian P

Fine pitch Whitworth thread form inserts are hard to find, but Cutwell do them down to 48tpi. They are not cheap and the op would likely be better off going the hand finished way.

I've just read your last post, are you sure they are 55 degrees and not 60 degrees?  

I would also be inclined to make a bored sleeve to go over the thin wall to stiffen it before threading. 

SRB photographic specialise in these types of adaptor and a look at their website is recommended.

Edited By old mart on 02/02/2020 22:37:01

Edited By old mart on 02/02/2020 22:39:59

Edited By old mart on 02/02/2020 22:42:48

After roughing out the tool use double sided tape to attach a wooden angle template to the tool that you can align with the side of the grinding wheel, Then do the other side so you have a true angle and a dead sharp point.
Next attach a toolmaker's clamp to the tool using paper for grip just to establish a reference position. measure the distance from this reference to the absolute tip with a height gauge or equivalent. Calculate the amount to be removed from the tip to bring it to the flat top or peak of the modified tool. Remove a little from the tip with a slip stone and measure until the calculated height is achieved. Then for rounded thread types round off by guessing how much is needed from the effort required to take off that tip in the previous stage.

The 60mm threads are 60 degrees, but the few details I can find on the C-Mount lens thread seem to say it is 55 degrees.

I will get a better magnifying glass and have a go at making a HSS tool.

IanP

I use hand grinding method with my 60 years year old Moore and Wright thread gauge,and is sufficiently accurate for most jobs. Another method not mentioned above is to use high speed steel internal and external machine thread chasers, these are not just for finishing off threads correct to form as the old tech books say , they can be used from start to finish cutting threads, and they produce very accurate thread forms.Back in the 1990s when there were a number of recessions a lot of chasers became available on the second hand market quite cheapl,nowadays their price has risen a lot ,I have a lot and cut a lot of threads with them,mainly BSP for full size steam,The chasers must be the machine type where the shank is flat and parallel,the older shape hand chasers which are shaped to take file handle are not recomended, as supplied they do not have top rake but are ok like this for steel if speed is reduced.The skill is that at the end of the thread a very quick two haded operation is required to disengage the half nuts and at the same time withdraw the tool by a quick wind of the cross slide handle, If you look at a box of used chasers you will find that some have had the row of teeth reduced at the L H side reduced to two this allows the chaser to run into a relieving groove,without the quick action required with a multi tooth chaser. For internal threads the chser shape allows the threading teeth to run through the bore,,blind bores are tricky and not for the novice. I have a good english made set of tool holders and imperial and metric carbide inserts but I rarely use them I much prefer the chasers particularly on commercial jobs.In my early working life the thread angle gauge was used for most jobs,there was just one job where the acme thread cutting tool was checked on the optical projector,not available to the amateur. one tip a tool dealer told me if you buy second hand chasers check that they are for right hand thread,he occasionally came across left hand chasers.

I've recently stocked up on 32tpi and 40tpi Whitworth inserts from Cutwel. Errr, a 32tpi full form insert will cut a 32tpi thread, and the tip radius should be as per the Whitworth thread specification. Here's a short 32tpi Whitworth thread being screwcut in cast iron:

Andrew

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As Andrew has noted : The Whitworth thread form was very clearly specified: **LINK**

https://en.wikipedia.org/wiki/British_Standard_Whitworth#Thread_form

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

MichaelG.

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Edit: That said ... I am inclined to agree that the C-mount thread is likely to be 60°, as I believe it was first specified by Bell & Howell.

Update: ... admittedly not definite proof, but quoted from a respected source:

”The C-mount connection is described in the specification 1-32UN-2A. It is an imperial thread with a diameter of one inch and a pitch of 32 threads per inch.”

Ref. https://www.stemmer-imaging.com/en-gb/knowledge-base/lens-mounts/

Edited By Michael Gilligan on 02/02/2020 23:40:36

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. Relying on a threading gauge can easily get you 5 degrees out and still look ok to the naked eye. Sometimes I make a threading tool by putting a broken 6mm carbide endmill in a jig and grinding it on a diamond cup wheel. Using a jig you can set the angles with a protractor.

I have a loupe with a 360 degree graduated reticle and it easily shows up an error you can't spot using the gauge.

You're overthinking things a bit (a lot) with magnifying glasses and vernier protractors and exact tip radiuses..

Machinists have been cutting fine threads like that for over a century by using a standard, good quality screwcutting gauge (eg M&W), the little sheet metal one with the V notches cut in it.

Yes, for small threads, the very very tip of the tool is in the straight clearance slot in the gauge. But that's no problem at all. If the rest of the side matches the gauge, and the side is straight, the tip must be the same angle.

Secret is to finish the tool off by rubbing it on a bench oil stone, the type used to sharpen kitchen knives etc. Rub both sides of the V on the tool bit on the stone to get it nice and flat and smooth.

And agonising over exact tip radiuses (or radii if you are one of those pedantic types) is the realm of pedantic armchair engineers and what Martin Cleeve describes as "over-zealous amateurs". All you need to do is rub the tip on the bench oil stone a few times with a nice rolling motion, just enough to take the sharp point off. The main reason for doing this is not for thread form -- it really does not matter as the point will cut a handy clearance at the root of the thread -- but to stop the sharp tip from tearing off under cutting load. No need to measure the exact radius. Just feel it with your finger.

This is the way threads much finer than yours (32tpi is run of the mill average really) have been cut countless millions of times in machine shops since the days of Mr Whitworth and James Watt etc through to the late 20th Century when carbide inserts started to take over. Somehow since then, people have come to think there is something exotic about the shape of a thread cutting tool for general work.

There isn't.

Edited By Hopper on 03/02/2020 01:33:52

I made one of these 10 years ago and what a lovely tool for precision sharpening HSS tools HH's Grinding Table

Not quite a Quorn but excellent for a home workshop.