Acme Thread 7/8"-7 LH 2 Start External

[screensnot]

I've never cut an Acme thread. So, I picked an odd one to start with (or it picked me).

It's a 7/8"-7 left hand 2 start thread. 80 pieces. 316 SS. The part will be well supported with only 3" from chuck to tailstock.

I am doing this on a Johnford SL-650 (2 axis CNC with Fanuc Oi-TC control).

Some questions I have:

Do I have to use an insert designed to cut a 7 pitch acme thread? I think I do, but I am wondering if I can squeak by with an insert for an 8 pitch? The minor dia has a lot of room in tolerance (I don't have the print in front of me).

Is the insert 'inclination angle' a big factor? I have paid that no attention so far in my career, and have had good success with V threads.

I normally use G76 for threading. Can I time the second start using G76 without moving my Z start position? Since it's a LH thread, I have considered using my RH toolholder and threading away from the chuck. In this case, I would have trouble, because there is a shoulder that is less than 1 pitch from the thread start (if I am threading away from chuck). If I use a LH holder, I have plenty of room to start wherever I want in Z.

How do I measure the pitch dia? I think I will be measuring over two gage pins. I also think I need to buy 2 that are better than the class ZZ minus pins that the shop has.

 

[redlee]

Some info already covered.
http://www.practicalmachinist.com/vb/general/measuring-acme-threads-187704/
Would you not be useing a 14 Insert if you are cutting 7TPI double start?

 

[johnoder]

If 7 TPI pitch and 3.5 TPI Lead (.2857" lead) helix angle will be over 7 degrees (assumes .732" Pitch dia.) and whatever cutting tool is used it must clear that angle.

 

[screensnot]

I think it would be like cutting 3.5TPI x2.

The print calls for pitch = .143, and lead =.286".

 

[redlee]

Some info already covered.
http://www.practicalmachinist.com/vb/general/measuring-acme-threads-187704/
Would you not be useing a 14 Insert if you are cutting 7TPI double start?

Yes,your right- 7 TPI Doublestart-3.5 TPI x 2
Brainfart.

 

[screensnot]

I had a nice long chat with Vardex tech support. They were very helpful, and my tools are on the way.

I think I now own 70% of the world's supply of 7 pitch LH ACME inserts. They made me buy 10.

 

[johnoder]

They still will need to clear that 7 plus degree helix angle

 

[screensnot]

Yes, Keith at Vardex claimed it was a 6.26 'helix angle'. He warned that the seat/anvil that I was getting is both the wrong angle, and the best that he could do (with off the shelf parts). He set me up with a YI3-3P anvil.

I will give it a try. I may hand grind a little more clearance on the leading edge of the insert.

Some things I have going for me is that it is a class 2G thread (not the most discriminating class of Acme).

The things against me include almost everything. But, I'd still estimate my chances of success at 60% or better. Those are good odds in my book.

 

[LexD]

A -3 anvil will only give you 1.5 degrees inclination in a positive toolholder or 4.5 degrees in a negative holder, which is still not enough to clear. You may have to machine the holder on an angle to achieve the extra clearance otherwise the tool will rub below the cutting edge.

 

[PixMan]

Modifying the holder by milling the bottom to a angle would be the best choice, though it means the holder wouldn't be very useful once the job is done. The alternative would be to make a very thin angled shim to put under the holder, though that may cause problems with tool height.

Best of luck!

 

[Forrest Addy]

Careful there. I don't know what the form or gaging requirements of this jab are but you have to remember screw thread flank angles are measured in the axial plane. (OTH worm gear flank angles are measured normal to the helix angle.) If you tilt a formed thread insert, the flank angle it cuts narrows - becomes more acute. If you tilt a 14 1/2 degree flank angle through 6.46 degrees the 14.50 angle projects as 14.47 degrees on the axial plane. That works out to 0.0002" narrow on the thread depth. No big deal.

I'm using this as an example of where to begin sweating the numbers when dealing with more extreme helix angles.

More extreme helix angles rotate the normal flank angle through a greater arc. The normal flank angle and the flank angle in the axial plane follows a cosine curve. 6.5 degrees no biggie, cosine in the 0.993 range. 10 degrees, 0.985 - more effect. 15 degrees, 0.965 - significant effect: 0.0014 change in width over the depth of a 7 TPI Acme. Enough to blow functional gaging if a 3G product thread is cut to the top of the wire size - and there will be localize flank bearing if the mating thread is cut elsewhere with accurate flank angles. I'm just sayin', work the numbers, use the data to dodge the jackpots.

If you grind a tool to compensate for the effect of helix angle - that is, the top has zero rake and is tilted from the axial plane by the helix angle of the pitch-line. IOW the top is normal - at right angles to the helix angle. The clearance angles are the same as for any threading tool - 6 degrees or so symmetrical on the pitch-line helix angle.

If the tool top lies in the axial plane of a thread having a more extreme helix angle, the clearance angles become strongly assymetrical with respect to the axial plane. The more acute edge is ground to the helix angle plus 6 degrees and the more obtuse the angle helix minus 6 degrees - both measured from the axial plane. In the OP case, the more acute angle is ground to 6.5 degrees plus 6 degress = 12.5 degrees. The more obtuse angle = 0.5 degrees - almost square.

In this case the edge angle (the angle between the clearance and top assuming zero rake) is getting aggressive in one case and blunt in the other. If you go straight in, the side thrust may exceed the grip on the tool or lead the carriage away from contact with the lead screw. A clever dodge is to grind (or better, hand stone) a negative seeming top rake on the acute side of the tool to reduce the edge angle to 6 degrees or so. This top angle needs to be only a chip wide. If you in-feed 0.005 per pass x TAN 14.5 = 0.0012" chip. The stoned rake needs to be a strong bright line, no more. The flank angle is nearly unchanged. YMMV depending on material, etc.

The above is mostly sweating petty details but I thought it a good place to preach a little theory. Someday, someone will need to cut a steep helix angle, remember these words, and go on to completing his work with less drama.

 

[LexD]

Quite true Forrest but I personally would rather have the tool cutting cleanly on both sides than worry about a slight flank angle error on a class 2G thread.

 

[Forrest Addy]

Quite true Forrest but I personally would rather have the tool cutting cleanly on both sides than worry about a slight flank angle error on a class 2G thread.

Which is why my remarks about petty details. The details exist and "petty" evolves towards "major" in proportion as the helix increass. The guy cutting the thread has to be aware of how much and when. The knowing is the difference between competence and guesswork.

 

[screensnot]

Off to a bad start.

I finally got my inserts, and I ran a test on some 1018.

imgur: the simple image sharer

Doesn't look good.

I've got a YI3 anvil and milled the shank on 3 degrees to give me a total of 6 degrees helix angle. Then I shimmed to get it on center.

I know I'm cutting too slow at only 210 rpm, but I don't think that is my only problem here.

 

[PixMan]

Are you certain you've got both the angle on the anvil and on the toolholder going the same way? Just asking.

It does look pretty bad, though 1018 at low speed isn't well known for producing nice finishes.

 

[screensnot]

The angles are both in the same direction.

Getting the speed much faster is going to be a problem. With the high lead, I am already moving at 60ipm in the cut. There is not a ton of room to stop or pull out before the shoulder.

 

[Bobw]

Before the insert is engaged on both sides, it actually looks pretty good, which makes me think you have enough angle on the insert and its not rubbing

However. Once the insert is fully engaged, it looks like it all goes to hell, and the thread crests are all mucked up, like something is pushing and rubbing.

Do you have the insert cocked over too far and its rubbing on the backside??

 

[screensnot]

I got some acceptable results by hand grinding a little extra clearance on both the front and the back flank on the insert.

 

[RickRad]

Looks like I ran across the same part...