7/16" ACME LH Internal Thread

[Runner]

Hi all,

I attempted to cut a 7/16" ACME LH internal thread. However, I had limited success. The success of such an operation is predicated on the design and construction of the internal thread cutting tool, particularly getting the profile of the cutting bit correct. I only have a bench grinder, so it was necessary to produce the cutting bit first, from a 1/4" square HSS blank. Having cut the thread profile I reduce the square section from a 1/4" to a size that just fitted in a square section hole in the boring bar. The boring bar diameter was 0.210" and the length 1.5" from the tool bit to tool's shoulder. I jammed the bit in from the backside and ground a waist on the remainder of the blank, snapped it off and then ground the backside of the bit flush with the boring bar. To hold the bit securely in the bar I used a screw, that I tightened as securely as possible. I started to cut the thread and for the first few passes everything seemed fine. However, the screw system failed and the cutting bit came out of the back of the boring bar. After several attempts at improving the securing system I resorted to silver soldering the bit in the boring bar. This secured it, well and truly. However after the silver soldering operation the tool didn't seem to be cutting at all well. I assume that the around 650 deg C temp required to silver solder did not affect the material properties of the HSS cutting bit. However the mild steel boring bar began bending and was unable to hold the bit hard enough for cutting to occur. Obviously, the high temperature annealed the mild steel?

I shall remake the internal thread cutting tool, but what changes do I have to make to overcome this problem, bearing in mind that silver soldering is the in my opinion the only viable securing option?


Thanks in advance,

Brian

 

[tel]

What pitch is the thread?

 

[steamer]

I cut a .354-10 LH for my Waltham restoration. The thread is here.

http://www.homemodelenginemachinist.com/index.php?topic=7692.0

I used a much smaller tool "width" wise, and I fed the tool to full depth.

In setting up I put the compound parallel to the work.

Once I was to full depth, using the cross slide feed, I locked the cross slide and put feed on with the compound.

This widened the thread....I slowly widened the thread until it fit.

The bar I used was .1875 diameter with a .062 diameter cutter ground to the correct angle on both sides, but far to narrow to cut to full width.

It was a very laborious cut with lots of passes with small cuts and followed with lots of "dead passes" to take the spring out of the tool before more feed was added with the compound.

Hope that explains it.

Dave

Here's some more from another thread
I would get that as making Acme nuts this small is a challenge. I've done .354-10 LH by single pointing and it really wasn't fun!

The tool was held in with a #4-40 screw...the bar diameter is .1875.

 

[Stan]

Here is another alternative for a boring bar. The one in the bottom right is ground out of a 3/16" HSS tool bit to thread a 10-32 blind nut.
Another suggestion for Acme threads is to cut a partial V thread first.

 

[Runner]

Thanks for the replies.

Tel the thread is 10 TPI.

Dave I reviewed your earlier thread and used it as a basis for the design of my cutting tool. But you didn't indicate how you produced your cutting bit. The only way I could produce mine was as described, I needed to be able to hold the HSS blank to produce the tip. BTW what is "single pointing"?

Stan your approach has merit, I'll keep it in mind.

You may have realised that I am trying to produce a new cross slide nut for my lathe 7/16" x 10TPI LH thread. My tryout to prove the cutting tool was using a steel blank instead of phosphor bronze. Is PB harder or easier to cut than steel?

Brian

 

[AlanHaisley]

Brian,
I'd think that you could use TIX solder, which has a much lower melting point (275F). Even a 63-37 tin lead solder might hold well enough.
Alan

 

[steamer]

Hi Alan,

Is you question how were the angles ground on the tool? or What were the angles ground on the tool?

;D

The how was while held in the bar but extended. I then put the angle and the rake on the tool by using the mitre and tilt table on my import "Baldor" grinder. My grinder is outfitted with an Aluminum Oxide metal backed wheel for grinding HSS lathe tools.

I ground the tool for full depth of thread, and to ACME form using an ACME template tool but narrower than the thread.

The resulting thread is narrower than the finish thread, and to make it wider, I put the feed on using the compound.

Dave

 

[arnoldb]

Hi Brian

Just to answer some of your questions:

Single pointing is a shortened term used for single point threading - where a thread is turned with a toolbit with one point, just like you are wanting to do.

In general PB (Phosphor Bronze) is much easier to cut than steel, but some grades of PB can be a hassle. Just give your toolbit a good hone on an oilstone after grinding to shape, and you should be good to go with a minimum of fuss Thm:

Regards, Arnold

 

[tel]

If you look like doing much in the way of Acme threads it wiould pay you to invest in one of these puppies - not only for measuring pitches, also handy to check the progress of your tool grinding.

http://www.ebay.com.au/itm/300456411737?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

 

[Runner]

Alan, my first attempt after the screw locking failure was to use 60/40 solder, being a long time user of this for many temporary bonding methods. Unfortunately HSS doesn't tin, same results for comsol high melting point solder.

Dave, my problem is I have to hold in my hand what I am grinding so a small tool bit would be impossible to hold. You said that that you held the tool bit in the bar and ground from there, what size HSS blank did you use it appears to be a very small and round?

Arnold if they delineate single point threading does this mean that there are other methods of threading in the lathe?

PS I have completed the tryout on the steel, it appears to be successful. I utilised Stan's approach by grinding the whole tool from a 1/4" Square HSS blank so the previous problems are no longer an issue. I did have difficulty in getting the 29 deg and if I persevered I would have ground the entire tip off. So I made the point about 14 degrees on one side and 0 the other and cut the thread to full depth and then reversed the job in the chuck and cut the other side of the thread. A variation of Dave's approach of a two pass cutting regime.

Brian

 

[arnoldb]

Hi Brian

Yes, there are other methods; though rarely used in home shops.
There's some more information here
Another now-ancient method that I don't see mentioned in the above article is thread chasing - where you'd use a cutting tool with multiple thread points spaced at the correct pitch to cut the thread - sort of like running one of the cutting flutes of a straight-flute tap along the workpiece.

Kind regards, Arnold

 

[steamer]

Brian,

It's a 1/16" diameter HSS round drill blank ( a HSS round shaft about 2" long) cut an ground to the shape

Dave


PS picking that thread back up after you flipped it must have been fun!

Dave

 

[Runner]

Arnold, very interesting thanks.

Dave, using a drill blank is the answer, why didn't I think of that! Picking up the thread after reversing the workpiece in the chuck was a palliative action made necessary by my lack of skill to get the grinding of the toolbit right. I spend more time thinking of work arounds than getting it right first time. Maybe that will change with greater experience. The picking up was done by with the lathe stationary looking at the threaded part that was visible and positioning the toolbit over the thread winding back until it was at the bottom of the thread and then locking the apron to the lead screw and marking the thread dial indicator position. I used this position to engage at the start of each cut. The play in the leadscrew/apron engagement made sure that the toolbit followed the original thread. Well that's my theory anyway. The acid test was that the internal thread mated well with the threaded rod that I had previously produced with little or no play.

 

[steamer]

Runner,

Picking that thread up is a tough thing to accomplish! My (does it belong to me?) method was born of necessity, .354-10 LH is the standard thread size for a BC AMES lathe lead screw. THAT IS what it was It is WAY oddball!

The rough bore is about .254".....not much room to get a tool and bar in there! to keep the cutting forces low and the bar stiffness as high as I could, I went for the narrow tool. IT WAS TEDIOUS! But I got there in the end....I think I just machined it into submission as my friend Dean (Troutsqueezer) would put it!

Now, I see that 7/16-10 is pretty common especially with Southbends...

Dave