Question for Ragarsed (and others) on Thread Forms

[Guero]

Ok... here's the situation. I have Myford's cast iron mounting blocks which fit under the Myford's "feet". I want to mount my Lathe on a maple slab which is 1 3/4" thick; the bolts which came with the mounting blocks are 5/16 X 24 Whitworth and are 4" in length. When the bolts are screwed into/through the mounting blocks I only have 1" remaining to pass through the maple block... not enough, in other words. The mounting blocks are used not only to mount the lathe to its bench but to perform very precise leveling (and I don't want to lose this feature). My original thought was to "convert" the 5/16 X 24 Whitworth theads in the mounting blocks to a standard V form thread by passing a tap of that size through the hole(s) on the mounting block and then cutting and threading some 5/16" rod to the proper length. Ragarsed Raglan has given me the impression that were I to do this I might be committing an act of stark-raving lunacy and I don't want to do this. I'm open here to any and all advise and will be the first to admit that entire books could be written about what I don't know.... as well, I'll be grateful for any words of wisdom.

[SGW]

One thought: counterbore the underside of the benchtop enough so the bolts are long enough to go through and get nuts on them in the counterbores. I think you could go down to 1/2" remaining benchtop thickess and still be okay, as there is little or no force trying to pull the bolts through the top except the bolt tightness, and you'll have large plates of cast iron pressing down on the top of the bench above the holes.

[Forrest Addy]

5/16-24 Whitworth happens to have the same size designation as 5/16-24 Unified.

Have you tried a Unified thread in the Whitworth hole? If it goes without excessive rattle I'd say use it.

There's nothing sacred about keeping things absolutely standard (or Whitworth) unless your going for a museum restoration.

I knew a Norton motorcycle nut. He swore Whitworth fasteners were superior to Unified and Metric fasteners to the point of unhealthy obsession. He had a complete set of Whitworth wrenches, taps, dies, and everything all purchased at great cost.

I didn't dissuade him for his motorcycles were marvels of restoration even to me having to make him oddball cable fittings and electrical hardware.

Some screwballs are constructive so you have to serve them as best you can.

[Guero]

I'm just realizing here that this Whitworth thread might actually be 5/16 X 22.... not 5/16 X 24. I went to E-Taps webpage and didn't see any such reptile as a 5/16 X 24 listed. (I said entire books could be written about what I don't know.)

[Guero]

I just went and did a careful count on this bolt - this is a 5/16 X 22 BSW (apparently a "standard" Whitworth). SGW, I may go ahead and counterbore as you suggest.... however, at this point I'd still rather get this question of threads resolved. I definitely would be committing an act of outright heresy and general lunacy to try and convert a 5/16 X 22 BSW internal thread to 5/16 X 24 V form (in some countries one is taken out and shot for such things). I'll give E-Taps a call tomorrow and see if they might be able to sell me a 5/16 X 22 BSW thread die. I'd like to screwcut the threads on the 5/16 rod but don't have a lot of confidence in grinding a HHS tool bit to the proper angle with nose radius. (But, this is what makes all this so much fun as well as a challenge.)

[SGW]

It's probably 5/16-22 BSF, which is "fine thread Whitworth," more or less.

I wouldn't worry *too* much about the proper radius on the threading tool...the main thing is to get the 55 degree flank angle correct.

[wdtom]

Just cut the head off the bolts, weld on a "longer" head from another bolt.

[NAMPeters]

The other approach would be to drill out the threads in the block and tap for keenserts (key lock thread inserts) with the thread of choice. Just a suggestion.

------------------
Neil Peters

[Thrud]

Guero:
You can get the proper center tool from B&S, Moore & Wright, or Starrett. I have all three. The best is the M&W as it covers every threading tool form including acme, 47.5*, 55*, 60*

The suggestion of cutting the head off the bolt and welding a extension in is a good alternative. TIG welds would be spiffy.

[Guero]

I've just ordered from Brownells their 55* HSS thread cutting tool - somewhat expensive but on receipt I'll know I'll have a tool ground to the proper angle. As well, I'll call E-Taps today and see if they can provide me with a 5/16 X 22 BSF die... in case I bugger up the threads while trying to screw cut them I might be able to salvage the job using the die. Thrud, Brownells used to sell the center tool but apparently no longer carry it; I'll check my Travers catalog to see if they carry the M&W. Where do Ragarsed Raglan be on this thread?

[Ragarsed Raglan]

Cooeeee! I'm here Guero,

Sorry about being late for the party, it's the time difference between GMT and EST,or CMT or whatever your on.

The rules to remember are that: Between UNC and BSW all sizes are identical up to over 1" diameter as to their respective tpi's ~ with the exception of 1/2" which is 13 tpi for UNC and 12tpi for BSW.

BSW is a 55* flank angle thread ~ UNC is a 60* flank angle thread. This means that, rather dangerously, a BSW bolt will fit a UNC nut (due to the flank clearance). But a UNC bolt will not fit into a BSW nut (flank interference)

As SGW has correctly identified the 5/16 x 22 tpi is indeed a BSF thread (again a 55* flank angle remember!)- but in this case I cannot remember where the two same sizes coincide for tpi (somewhere like 1 1/2" I think?) the UNF equivalent of the 5/16" being 18 tpi.

All I'm saying is that you do not have to 'bodge' the job ~ if you want a handful of BSF bolts let me know size and length and I can supply F.O.C If you tap out to 5/16th UNF you will loose a significant number of threads (20% or there abouts). I like the idea of the counterbore - that's neat, but you may want to consider Helicoiling the 5/16" BSF to 5/16" UNF as an alternative.

Then again, I see you are a true Home Shop engineer by making the 'little buggers' your self!!!

P.S Thrud ~ Thanks for the vote of confidence in our threads that you posted on the other Guero led thread(!) ~ too many threads here!!!

RR


[This message has been edited by Ragarsed Raglan (edited 03-11-2003).]

[Guero]

Thankee Ragarsed. I believe at this point I've got things figured out and your counsel was good counsel. I called E-Taps today; they don't have the size die I need but will order it and alert me when they've got it. I expect to get the 55* threading tool from Brownells early next week and at that point will attempt to screw cut... as long as I take light cuts I'm generally okay. Thanks again... you all will probably hear from me again.

[Spin Doctor]

Since this seems to be working out for the best I'll just throe this in as an aside. The last time I had to cut a Whitworth thread for a Norton or BSA or Triumph buff I cheated. Rather than go to the trouble of grinding a special tool I just took and set up a D-series positive rake insert in the 21.51 size and used that. The inserts have an included angle of 55 degrees. Not ideal but it worked!

[Thrud]

RR
You da man! How could I not compliment one with such a fine last name...

[uute]

Question on British threads:

Where might I find specs or an approx. equivilncy chart between British - UNC/UNF threads.

I often see projects from ME, everything is held together with 2BA screws, didn't find a cross in Machinery's Handbook (#23), its all about British metric!

Thanks
uute