myford nose thread

I am cutting a thread for a myford 12tpi and I am asking if anyone can give me the diameter of the thread root as I want to cut a witness for the depth before I start.

Frank

Look here for 1 1/8 Whitworth spec, you need the core diameter

http://www.motalia.com/Html/Charts/bsw_chart.html

**LINK**

Whitworth thread depth is .6403 x pitch.

But lathe spindle nose threads are often machined a tad undersize so that all location is on the registar collar and not on the thread. Best to machine up a threaded ring that is a nice fit on your spindle nose, then machine your spindle nose thread to match that.

Bricky,

1.036 inches will give you a nice run out for the thread.

Regards
Brian

Bricky, my error forgot to say in the tables you need to look at the thread depth for 12 tip and deduct depth x 2 from the O/D thus 1.125 - 0.054= 1.071 make a small allowance for clearance so I would make the undercut 1.060 + 0 - 5

CheersJohn

Thank you all for your assistance in this matter it's only 4degs in my workshop so I will continue in the morning.

Frank

Frank,

On my S7 the nose thread is not a true Whitworth form with rounded crests, the thread form is pretty sharp.

Normally the OD of a BSW thread is rather less than the nominal thread size because the rounding of the thread crests and roots occurs within the theoretical 55 degree triangle of the thread form.

If the Myford nose were a true Whitworth form then the measured OD would be 2 x H/6 less than 1.125" but my nose is exactly 1.125". In light of this you might want to consider cutting your thread to the depth of triangular height H rather than for the depth of thread h. For 12 tpi the relevant dimensions are H= 0.083, h=0.053. (Illustration and figures from Machinery's Handbook 25)

HTH,

Rod

Allow five thou for expansion then!

Rod,

On my S7 the nose thread is not a true Whitworth form with rounded crests, the thread form is pretty sharp.

​​Looking at the picture you posted, do my eyes deceive me or is there some distortion of the thread form, in particular the ones at the MT socket end?
Adjacent to the register, the form there looks rounded.

​This suggests to me that a degree of thread flank wear from frequent chuck fitting which has over the years left the crests pointed with the wear getting progressively more evident as the affected threads get more 'mileage' in terms of engagement, hence the graduated effect from front to back.

Regards
​Brian

The follow on page of Machinery's Handbook gives the OD of Whitworth as being the decimal of the nominal size. 1 1/8" dia is 1.125" before the assembly allowance is applied. The truncation amount x 2 isn't taken off the nominal outside diameter. Likewise the same on Unified and ISO Metric. which start at that nominal OD after truncation from the sharp V, in that case H/8. The effective diameter is the all important thing then so if the effective dia of BSW form is say 0.010" under then we would expect the OD to be a similar undersize to keep the full radius on the crest................... bit we can't get that easily with a single point tool unless a good die is run over it at the finish.

Here it is, courtesy of Tubal Cain. Model Engineer's Handbook

If I am cutting a full form Whitworth thread, I usually use a inserted tip of that form. The nominal dia is the same as the major. Never had any trouble mating parts.

Steve.

Harold Hall has some info on cutting Myford nose fittings here... **LINK**

He advocates using a slightly bigger bore than the usual core diameter and a 1/2" BSW tap to finally chase the female form.

Rod's nose thread is just the same as my ML7's. The first few threads will naturally get worn over the years because unless everything's lined up perfectly the weight of the chuck bears on these first few but as the alignment improves as it goes on there's less and less effect.

Jon

"Here it is, courtesy of Tubal Cain. Model Engineer's Handbook"

Treat this drawing with great caution because it is riddled with errors. Just to point out a couple, both the thread diameter and the register diameter are incorrect. The taper on the bearing is also shown as 2.25 degree, instead of the correct 12.25 degree.

Mike

I have in my hand a BRAND NEW S7 spindle.

Dimensions as follows:-

Register Diameter 1.250"

Thread diameter 1.124" (over crests)

End of Front Cone 1.725"

The Front Cone Bearing taper angle may be incorrect, I have not measured it though.

Edited By KWIL on 01/12/2016 17:27:11

Apologies- l misread which dimension related to which arrowed lines on the thread and register diameters. Bearing taper is definitely a typo however.

Mike

OK, looks like I got my reasoning wrong, so thanks guys for the correction. I still think though that if the OP cuts his thread to the theoretical depth for 12 tpi that there is a pretty fair chance that it will not screw on to the Myford nose - that has been my experience anyway.

I mustn't forget to take the dried frog pills again.

Cheers,

Rod

I turned the register and the witness depth to 1.036 and proceded to cut the thread , half way there the tool jammed in the thread and rotated the work in the chuck .I finished the thread hoping all was well the thread worked fine and a shaving was required off the register,this showed out of truth.I tested with a collet chuck and some ground bar and I had a runout of twenty thou,.Another shi- -tter for the bin.I am changing tack after this and am going to make the mandrel between centres with plenty of allowance for tools and driver,fingers crossed.

Frank

Kwil,

What edition is your copy of Tubal Cain's 'Model Engineers Handbook'? Mine is 3rd Impression, 1985, and I can't find that illustration in it. Please could you give a page number?

Best regards,

Swarf, Mostly!

I have 3rd edition. It is at chapter 3.8