Member postings for Andrew Johnston

If it's not worth setting up a tapping head then I use two methods. One, just put the tap in the drill chuck and make a few turns by hand to start the tap. As a first choice I tend to use spiral flute taps so a few turns gives a couple of turns of a full thread. After that finish the thread by hand away from the machine. Two, just tap the whole thread by hand away from the machine. For free hand tapping I tend to use normal taper taps as they are easier to start nearer the vertical than spiral flute taps. After a turn or so I'll check that the tap is perpendicular to the surface from two directions and then continue turning. To some extent the tap is self-aligning when tapping by hand.

Andrew

Yes, they are. I've just double checked 25mm/1" to 50mm cutters and they're all 20 tpi Whitworth.

Andrew

The threads on cutters intended for Clarkson style holders are Whitworth form and 20 tpi, irrespective of whether the cutter is imperial or metric. I was under the impression that the shanks of such cutters were of a limited range of sizes. My smallest holders are 1/4" and 6mm and have 1/4" and 6mm by 20 tpi threads respectively.

I'm not aware of special taps for any of these threads (other than 1/4" BSW) and 5.65mm seems non-standard. Not much help I'm afraid.

Andrew

Yes, but only for spotfacing or counterboring, never for milling. I also use milling cutters in the lathe tailstock for creating none critical flat bottom holes or rough drilling brass and bronze where I don't have a suitable slow helix drill.

Andrew

I do; there are a pair of slip-on industrial steel toecap shoes by the workshop door that I put on when machining. They keep my tootsies warmer than slippers too.

Andrew

They're made by Carmex, who are based in Israel. I bought online from the UK distributor ProTools, based in Kent.

ProTools

Total cost was a few pence over £50 including next day delivery by APC and a 10% new website discount. So not cheap cheap, but less than a proper thread mill.

Andrew

Don't know about the scam, but it would confirm that the email address is valid and ties it to a person and 'phone number. That's more valuable than the scam as it can be sold on. Best to tell him that you can't receive emails because the internet has been compromised.

New toy day:

Tiny 55° partial profile carbide threading inserts. I've bought them for short internal 7/16" x 32 and 1/4" x 40 ME threads. I declined to buy the holders at about £100 each. Note the small holes as the tool shank goes to full diameter. These are for through coolant, which is probably why the holders are so expensive. I'll be machining brass so won't be using coolant, through or otherwise.

That's blown the discretionary budget for this month.

Andrew

Surely the important parameter is how accurately the centre is located when the needle movement is nulled. I don't see that has anything to do with the calibration of the indicator itself? Although of course one would need to make an independent measure of the centre to get the numbers.

Andrew

I used to do that but the toolpost/cross slide always seemed to be in the way. So I bought a tailstock die holder for the few times that I use dies.

Andrew

My Haimer Centro claims a centring accuracy of ±0.003mm. A quick look online and some of the cheaper units don't seem to quote any accuracy. That's strange, and rather worrying, for a device that is supposed to offer precision by its very nature. I expect that for co-axial indicators you really do get what you pay for.

Andrew

The Haimer Centro shown in my previous post claims ±0.003mm centring accuracy for shafts and holes. The Haimer Zero Master claims ±0.01mm accuracy for axial and radial probing. I've got a wiggler set somewhere but have never used it. I don't have a mechanical edge finder. In the past I used fag papers and either the tool in use or a piece of silver steel. Accuracy was probably ±0.04mm at worst. I still use fag papers on a regular basis and it's my go to method for centring on a horizontal shaft for cross drilling.

I've learnt two things today by reading the Haimer manuals. The Centro can be used to check perpendicularity. I'll use that next time I tram the Bridgeport. The Zero Master manual gives a procedure for centring on a shaft or hole, which is a truncated form of Osborne's Maneuver.

Andrew

It's interesting to note that the die holders for both the Britan and Arc systems use four screws as described. But in both cases the dies are a snug fit, no more than a barely detectable shake. So I can't see the fourth screw being much use for aligning an off centre die.

I'm not a fan of split dies. Where possible I prefer to screwcut or use Coventry dieheads.

Andrew

There's virtually no "float" in the Britan system. So they must rely on the thread in the die being concentric?

Andrew

That triggered a vague memory. In a copy of Machinery's Handbook from August 1943 all the threads listed in the OP are in a table headed "French Standard for Metric Threads". What we now consider to be the standard pitchs for M3, M4 and M5 are not listed. The notes say that the French system followed the ISO system between 6mm and 90mm, but below 6mm they did their own thing. Even more bizarrely for threads less than 3mm the thread angle may be 60°, or 55°! There's also a table for a German Metric Thread that starts at 12mm, but with different thread depths and clearances from ISO, so not standard.

So much for standardisation.

Andrew

The second one - it's called Osborne's Maneuver. It's different from the method described above in that it is an iterative process and ends up with the X and Y axes aligned with the centre of the work. Of course it was developed before DROs were commonplace. I have used it in the past, but now I just use a co-axial indicator:

Quicker than faffing about with edge finders.

Andrew

According to the chart for my lathe (Harrison M300, imperial with a 4 tpi leadscrew) any number from 1 to 8. But I've never tried it as I replaced my thread dial indicator with a high speed threading unit which automatically looks after all that.

Andrew

Indeed, but split dies also work fine under power. This is a sample holder and reversing dog clutch that I use on my repetition lathe:

I've just used it to cut some 2-56 UNC threads in brass at 500rpm, as I didn't have suitable chasers for the Coventry diehead.

Andrew

PM sent - Andrew

By nominal I mean the thread size. So in the case of your 3/16" x 40 thread I'd turn to 3/16", without worrying about the 5 tenths, say 0.187".

I don't think having the blank at nominal thread diameter is the problem. I've recently made a batch of wick feed oilers for my traction engines:

The threads on the spigot underneath the body are 1/4" x 40. They were screwcut using a full form insert; the OD measures 0.25" give or take the odd few tenths, some over some under. Likewise the lids bottom left have a 7/16" x 32 screwcut thread. They measure 0.4375" on the OD.

The interesting part is the 5/32" x 40 threads on the copper tubes. The tube OD measures 0.1575". A 2.5mm drill slips neatly inside. That gives a wall thickness of 0.0295". A 40 tpi Whitworth thread has a depth of about 0.016" so that leaves 0.0135" minimum wall thickness after threading. When I made the first batch of these oilers I screwcut the 5/32" x 40 threads. But it was a pain as slightly too much depth of cut and the remaining thin wall gave way. So this time I resorted to a die in a tailstock dieholder. It worked fine and the OD of the resultant thread is 0.1575", so slightly over nominal. But they fit nicely in the mating tapped holes. The threads are properly formed for the full length and without breaking the remaining thin wall. That's why I don't think making the stock undersize is the solution. It's perfectly possible to cut nice threads starting at nominal diameter with a die. Reducing the diameter may appear to help, but it's covering up the problem not solving it. The die I used was part of a present (Triangle brand) some 50 odd years ago and the tailstock dieholder came from Arc, so neither are top notch professional (*), but they did the job fine. A few questions:

Where did the die and tailstock holder come from?

Have the die and/or holder been used satisfactorily in the past?

Was the die opened out to its maximum to check the fit before adjusting?

Have you tried the same setup on something more forgiving like brass?

Andrew

(*) Apologies to Ketan - I'm more than pleased with the tailstock dieholder set from Arc and it does what I expect without fuss, with one minor annoyance.