Acme Taps for cross slide nuts

Some of you asked about Acme Taps for the 7/16" LH and 3/8" RH screws. I bought some from e-taps in FLA. When I got them I put them in a drawer and hadn't really looked at them. A few days ago I took them out and for the first time I measured them and the two finish ones seemed way too large. I got out the Machinery's Hand Book and looked up Acme Taps. Unfortunately, the information was not clear to me. The .4375 tap was .477 in diameter, that's almost .040 over 7/16". From what I see, South Bend cut the internal acme threads a few tho over? I have a 1/2" LH Acme Tap I used for tail stock rams and it .514 which is .014 over .500. I call ed E-Taps and spoke with a gentleman there and he sent me the below chart:



Does any one of you know if this is right? I don't want to use the taps to see how they fit, for now I'll keep them new. I suppose I might be stuck with these expensive taps, oh well, live and learn. May be e-taps will take them back in trade if I order others but at this point I'm not sure what I need. If I find out anything, I'll post it on this thread. Also, I'm not complaining about E-Tap, they have been helpful so far, I'm over my head here.

Thank you, Tex

These screws live in a dirty environment

Where would chips and such go if only a few thou of clearance?

If threads get jammed up with a small chip, think how much bother it will be to take compound or cross slide apart to fix it......

Lube can allow junk to work out the generous amount of clearance.

The critical thing is that the screw fits the nut with minimum backlash. OD of ACME tap has little to do with that.....

The calculations are correct for a 2G-class fit, what you are seeing (measuring?) I believe (someone please correct me if I'm wrong here) is the difference between the min major diameter of the nut (which the tap will cut) and the nominal (basic) major diameter or in other words the max major diameter of the screw. This does not mean that the screw will have that measured clearance on all diameters. The pitch diameter (pcd) allowances,
tolerances and limiting dimensions are calculated to ratios that corresponds to the classes of fit. I think you will find that a new south bend nut is not a 2G. A screw turned or rolled to a 3G-class will still be slightly on the min loose side, but will be within the south bend tolerance for the cross feed backlash. I would suspect a 4G would be a better class of fit, but have not had the opportunity to try this. When you hand fit a single pointed nut on a new screw (or turn a screw to fit a new SB nut) you are in all probability making a 4G or 5G fit. I do know that E-taps can produce a 3G-tap set; I priced one last year, but never got around to ordering one. Now I know I need to go for the 4G set, but they are quite expensive to have made. You can still use the 2G taps, although I would expect you would have turn the screw (or insert blank) to arrive at the backlash tolerance.
Hope this helps Tex, I am by no means an expert here, I can do the calculations and have the formulas, but what it comes down to is the class of fit dimensions for the desired backlash.

Steve

acme taps

When ordering acme taps be sure to order them cleared for the material they'll be used in. For tapping in brass or bronze they need a lot more clearance than for steel or cast iron.
lwbates

Acme Taps for cross slide screws

I looked over my shoulder at one of my book cases and saw an "American Machinist's Handbook" by Colvin & Stanley 1940 issue.
This is their entry on Acme Threads (Modified Acme Thread).

For Screws:

D=(0.375 X P) + 0.010"
F=0.40302 X P
C=(0.40302 X P)-0.0052"

For Taps:

D=(0.375 X P) + 0.020"
F=(0.40302 X P) -0.0052"
C=(0.40302 X P) - 0.0052"


P=_______1_______
No. Thds. Per Inch


D= Depth of thread
F= Width of small end (tip) of thread
C= Width at bottom of thread
P= distance from leading edge of one thread to leading edge of next thread measured at tip.
Angle of threads are 29 degrees

I hope this helps.

Jesse

7/16 - 10 acme taps

Hi all

I went through the very same thing a couple of years ago. I ordered a three tap set of left hand taps to make nuts for SB or any lathe using a 7/16 10 LH thread. I ordered them to be used on Phosphor bronze. when I got them the finish tap was right at .040 oversize. I did cut a thread and as I had a brand new SB lead screw I tried it. Lose would be a incorrect term. Sloppy would be better. I contacted Western tool the company we got them from and complained bitterly. These things cost right at $400 and I wonted correct ones. I was put in contact with the company that made the taps and they asked me what I was using them for. This info had been sent with the original order but somehow it got lost so what I got was a set of taps for threaded fasteners. Bolts in other words. The company made me a new set of taps and if I remember right the finish tap is about .005 over .4375.

If I make a nut out of bearing bronze and tap them with these taps I get a really great fit on a brand new SB thread. On Steve Wells threaded stock the nut is just a touch loser but still a very good fit. The scary part of using these taps is the force required to drive them through a one inch long nut. They scare the devil out if me each time I use them for the long shanks will flex like crazy. I mean wined up like a spring. I am always scared that one of these times I will break one of my expensive taps. Trust me you do need a three tap set. First rougher a semi finisher and a finish tap. This is how they are marked on the tap itself.

I also have a set of right and left hand 1/2 - 10 taps but these you can purchase from any number of suppliers. I think the set I have came from MSC some years ago when we were building a special machine for a customer. I have used them many times and they have always worked very well and I have always gotten a very good fit. These 1/2 - 10 taps are a two stage tap with the rougher and finisher all on the same tap. They are rather long because of this.

Cutting a 1/2 - 10 acme lead screw is not a bad job but I have cut some 7/16 - 10 threads and a short one is not to bad but cutting a long one say for a heavy ten is an exercise in patents. Take a pass and let the stock cool. then another. The last time I made one it took me most of the afternoon to cut the thread. Reason? As the screw your cutting heats up from cutting friction the thread pitch will elongate. You can get some really funny theads if you let the stock get hot. So the idea is to keep the stock your cutting at one temperature preferably 68 during the thread cutting operation.

I also have a 5/8 -10 Left hand tap that is also a two stage tap. This I used when making up new lead screw nuts for a 13 inch Standard Modern cross slide and tail stock. This tap was also a special order and was quite expensive though not anywhere near as expensive as the 7/16 taps.

Just some thoughts on a bright and sunny Saturday morning waiting to take the wife to breakfast.

Turk

Substitute thread for SB cross slides

Its me again.

Just thought I would share something I did a few years ago. Before I had a Hardinge lathe here at home to cut acme threads with I would repair SB Dalton and other lathes cross slide lead screws a different way. As I had the 1/2 - 10 taps I was purchasing a good grade of 1/2 - 10 acme thread stock from McMaster Carr that I married to the SB or Dalton cross slide lead screw handle shank. Then I would set the nut up in the mill and Aline it with the spindle by putting a piece of the 7/16 lead screw in the spindle collet and the nut in the vice. This way I could get the nut true to the thread. Then I would drill out the nut from the 7/16 size to tap drill size for a 1/2 inch acme. I then would start the 1/2 acme tap through the nut wile still lined up in the mill. Just enough to get a good start then I would take the nut out of the vice and finish it at the bench by holding the nut in the bench vice and use a tap wrench on the tap.

I know it sounds scary but you can put a 1/2 thread in a SB 9 workshop cross slide nut. I have also done the same for a heavy ten. Its not exactly original but it is a very inexpensive way to repair a badly worn nut and lead screw for a very small amount of money. I think the threaded stock I got is good to something like .003 per foot so not the most accurate acme stock in the world but is quite inexpensive and can be ordered by the foot. Again this is a good way of bringing back a good level of accuracy to a worn machine for little money. The rub here is finding someone with a acme tap to do the job with."-)))))

Turk

As Mr. Turk has pointed out cutting an Acme thread by hand with a tap is not for the limp wrist, and skinny armed. A while back someone posted that they successfuly single pointed a cross slide nuts threads. That is not an easy job in such a small hole because you can not see what you are doing and flex is the cutter. I think the easy way to make the threads for a new cross slide nut would be to thread mill them with the lathe. The way to do that is to make or buy an Acme cutter, mount it in a die grinder mounted on the tool post, set the QC gear box on the correct setting, put the lathe in back gear, un plug the lathe,and turn the lathe over by hand as the cutter cuts the new thread. (Remember it is Left Hand!) The acme cutters are available but it would be faster and cheaper to turn one out of water hardening drill rod, cut the teeth with a file and harden and temper it with a propane torch. Gary P. Hansen

if you are buying acme threaded rod, why not order it a little long and make the tap or taps from the rod?

quasi,
It would bind on the flanks. If you lapped it in I guess you could create a 5g fit as there is not a pitch diameter allowance for that class, it is a "fit up as required" between parts.
This might lead to embedding of the lapping material in the parts also. In the basic thread profile for pitches of 10tpi and above the diametral clearance at the major diameter is created by cutting the minimum major diameter of the internal thread .020 thous above the basic major diameter. This is why Tex's measurements shows the tap larger than the basic major. Now to create the correct class of fit you have to calculate the crest or root using the minor diameter allowances and the pitch diameter allowances.
The pitch diameter allowances for the 7/16 external thread would be the sqrt of the nominal diameter x .008 for 2G, x.006 for 3G and x .004 for 4G. Which would be .0049, .0037 and .0024.
Denny's taps are spot on for the south bend thread as are Brian Millers, we have tested my roll threaded 3G material and it does fall in the correct backlash spec for south bend, Brian has the print and oddly it does not give the size. As stated it is slightly below the unknown class of fit they used, but very useable. It's my personal opinion that a class 4G would be the correct fit. Brian's taps were made by the same company that made the south bend taps.

Steve

amce thread question

Steve
This question is only tangentially acquainted with this thread i bought a "lot" of machinist tools on fleabay. and it it i got what looks like a brand new acme thread tap it was marked on the box as "Reagal cutting tools 25/32-8 NA2GDBL LEADACME4FL X 9 0/AN2758 MOD201502-01" ON THE OUTER CARTON.AND "REGAL USA 210502N! 25/32-8 NA2G DB.LD ACME N2758 MOD" on the tool shank. My question is what size is it? I cant find anything like a 25/32" acme tap on any site that I have checked, and the largest diameter on this tool is .804 The tool is Apx. nine inches long and looks in perfect shape.I would like to get some use out of it but I am a newbie at this and have no idea what this is size wise. I would need to either buy the matching rod or make it on my sb9a but I have to know what size we are talking first

As per your other thread, this is a custom two start (double lead) tap and is no doubt someones special. To answer your question, its useless to you - unless you happen on a job needing such a thing.

Its .804 dia because such taps are built to make the approx. .020 oversize for clearance. 25/32 is .781"

robert102653 said:

Steve
This question is only tangentially acquainted with this thread i bought a "lot" of machinist tools on fleabay. and it it i got what looks like a brand new acme thread tap it was marked on the box as "Reagal cutting tools 25/32-8 NA2GDBL LEADACME4FL X 9 0/AN2758 MOD201502-01" ON THE OUTER CARTON.AND "REGAL USA 210502N! 25/32-8 NA2G DB.LD ACME N2758 MOD" on the tool shank. My question is what size is it? I cant find anything like a 25/32" acme tap on any site that I have checked, and the largest diameter on this tool is .804 The tool is Apx. nine inches long and looks in perfect shape.I would like to get some use out of it but I am a newbie at this and have no idea what this is size wise. I would need to either buy the matching rod or make it on my sb9a but I have to know what size we are talking first

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one more question

could i not cut the same thread onto the correct sized shaft and utilize it that way?

Sure,but to be used for what?

I guess it would be good practice to learn to cut 2 start threads but you got to keep,in mind that's 2 4TPI Acme threads cut into a relatively small diameter...

robert102653 said:

could i not cut the same thread onto the correct sized shaft and utilize it that way?

Click to expand...

Plus one on the FOR WHAT?

Multi start threads are for moving a lot in one turn - sort of the opposite of what's wanted in a machine tool

yup, the only place I have seen such used was for small gate valves...and the shafts on those were cast,not cut.

i am thinking of building a small lift table using that thread
might be good practice for me

that will possibly be a horrible,very unsafe thing.

It will not be a self holding thread.

Chuck up a piece of scrap, turn it to 25/32", and take the first cut of a 4tpi thread on it and you will see what I mean.

These types of thread are used in things like the mentioned gate valves in the water industry, where they are either fully open or fully closed.

If you try to throttle such a valve with that type thread it will walk itself back open. In varying flows and pressures can even go the other way.

Don't design around a thread, use a thread appropriate for the design.

iwananew10K said:

yup, the only place I have seen such used was for small gate valves...and the shafts on those were cast,not cut.

Click to expand...

I was working on a 24" 600# M&J Gate valve recently, and it has a 2-3/4"- 2-1/2 double start thread. Kind of an odd duck, but the 60's were groovy man.

sort of the opposite of what's wanted in a machine tool

Click to expand...

And worse yet for lifting - since the weight lifted makes the screw auto rotate - the very last thing you want to happen.