# Tapping size hole for 1 1/8 whitworth



## firebird (Oct 20, 2011)

Hi

I have just bought a 9 inch face plate for my Myford lathe. Its a casting that needs to be machined. Can anybody tell me what size I need to bore it to so that I can screw cut the internal 1 1/8 x 12 thread.

Cheers

Rich


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## mklotz (Oct 20, 2011)

My DRILL program says that the tap drill for a 1.125-12 tap would be:

1.0438 in = 26.513 mm

assuming a 75% depth of thread.

Checking with the formula, we have:

TD = MD - 0.013*75/12 = 1.125 - 0.08125 = 1.04375


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## firebird (Oct 20, 2011)

Hi

Thanks Marv 1.0438 it is then. Thm: 

Cheers

Rich


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## Tin Falcon (Oct 20, 2011)

well according to Machineries Handbook a 1 -1/8 -12 UNF classes 1A, 2A 1.0240 class 3a 1.0258 reference. 

the whitworth chart shows 7 tpi for 1 1/8 course whitworth and 9 tpi for fine hmm. the minor diameters .9420 and .9828 respectively.
Tin


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## MachineTom (Oct 20, 2011)

A Myford is a 55° vee angle, not 60. For that tap drill= major dia - (1.1328* pitch) so that would result in 1.125-.09439= 1.030 tap drill. It may vary as a BST has a rounded root and crest, if the cutter is ground to spec. or ground as a flat root and crest.


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## mklotz (Oct 20, 2011)

My apologies. My mind didn't register the "Whitworth". Go with Tom's numbers.


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## BobsModels (Oct 20, 2011)

Rich

Having had a similar problem here is some additional information for your consideration.

First is a thread I started on the HSM site

http://bbs.homeshopmachinist.net/showthread.php?t=41954&highlight=myford

The second is a site that sells a tap specifically for the Myford spindle. This gentleman was a seller of Myford Lathes a number of years ago.

http://www.lautard.com/

Go to the link for the tap.


Lastly I have a collection of old machinist handbooks. The newer ones do not have details of some threads. I am not at home but at the Model engineer show in Ohio so I am not sure which version. It had all the details as described by Rich Carlstedt.

However as I mentioned in my followup post the variation was noticeable. I also had measured the internal threads on several chucks, back plates, and face plates.  At home I have the chart with the variation. I do not recall what size tap hole drill I decided on but it was close to what Rich indicated.

Not sure this will help but will make you cautious. The key is to have a spindle thread to test the cut on as you progress. I had the Myford nose for the tailstock, actually two of them so I figured when they both fit nice I would stop.

Good Luck

Bob


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## tel (Oct 21, 2011)

RDG do a set of three taps for 20 quid

http://www.rdgtools.co.uk/acatalog/info%5f8675%2ehtml


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## Blogwitch (Oct 21, 2011)

Tel,

I already have a set of those taps, and the die to go with them.

If I thought they could have cut the thread, they would be on to the way to Rich right now, he would only need to ask.

Maybe into brass, but nothing any harder, and even then only about 50% to 60%. They would be OK for cleaning threads up, which is what I use them for, but for nothing else.

Maybe the one from Arc Euro would do it as they are HSS, the ones from RDG are carbon steel.

http://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/Taps/Taps---Other


John


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## kvom (Oct 21, 2011)

One way that might work well is to bore the hole to 1.03 as per Tom, then single point in small stages until close to the major diameter followed by use of Bogs' taps to finish.


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## Blogwitch (Oct 21, 2011)

Rich,

I would suggest you have a look at my post on here, just the first bit about cutting a myford internal thread.

http://madmodder.net/index.php?topic=2249.0

The nearest thread size that I could find on my large tapping chart that came anywhere near close was 1 1/8" X 12 ADM. Don't ask what the ADM stands for, I haven't a clue. But here are the figures that I used for cutting my thread.

Thread angle 55 degrees.

Core dimension - 1.018"

Depth of thread - 0.053"

So core plus 2 times depth = 1.018 + 0.106 = 1.124"

The overall figures were round down slightly, but to me, they are close enough for what I want, and you start to come down in thous towards the end, so just cut to depth until the thread fits the male thread perfectly.

Hope this helps


John


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## tel (Oct 21, 2011)

Bogstandard  said:
			
		

> Tel,
> 
> I already have a set of those taps, and the die to go with them.
> 
> ...



Thanks Boggy, I was seriously considering a set too.


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## firebird (Oct 21, 2011)

Hi

Thanks for all the input gentlemen. 

John, I was also considering the taps but having had some success cutting external threads for the nose on my high speed headstock I thought I would try screw cutting the internal thread myself. I have just read your links, very useful, thanks. Looking at my chuck back plates and other fittings I think as long as I machine the register accurately that is the main thing then cut the thread last.

cheers

Rich


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## bob ward (Oct 22, 2011)

If the tap is readily available, and because its an important thread, I would be single pointing that to 90/95% and finishing with the tap.


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## Tin Falcon (Oct 22, 2011)

What I have done is copy the spindle nose as close as possible. then use that as a plug gage to check internal threads. I did it for my south bend and 2 Machinex 5s.
tin


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## Blogwitch (Oct 22, 2011)

Rich,

I made my register holes somewhere between 0.001" and 0.002"larger than my spindle adapter, you should use the spindle measurement off your lathe, any tighter than those figures, you risk things sticking together.

I measured up some ready manufactured fittings and they were way out on register diameter, if I remember correctly, up to 0.005" larger.

It is the back face and the register that really matters, the thread could be almost anything as long as it tightened up.

If you need the taps I have, you only have to ask, but IMHO they are not worth bothering with.

BTW, I found the Arc Euro ready threaded backplates just a little easier to machine than the RDG ones.


John


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## firebird (Oct 23, 2011)

Hi

Thanks for the kind offer John but I had a bit of luck today. I went to a big car restoration show at Stoneleigh today. There were many stalls selling all sorts of new and used stuff and at one selling engineering goods I picked up a tap 1 1/8 x 12 for £4.00 ;D ;D ;D.

I will take the route of copying from my existing fittings to within a few thou then finish with the tap and as you suggest I will take care with the back face and the register.

I have had a couple of chuck backplates from Arc and had no trouble machining them.

Cheers

Rich


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