I have the opportunity to buy a 1-7/8" right handed tap. I have it in mind to use it to tap threads for a new lathe back plate but due to the enormous size I am concerned that the force required to tap it will be huge as well. Any idea if using this tap is feasible or is single pointing the threads the way to go? Also is right handed the thread I need or does it need to be left handed to prevent it coming off the lathe spindle during use?
Southbend (and clones) all have right hand threads for spindles. Got to be super carefuling when cutting left hand threads on them or you ware the chuck and work piece on your face.
Should I get the tap? Does the force for large taps go up exponentially or is it fairly easy to use massive taps?
If you have the right tap wrench it's ok. But big taps = big cutting force. Never used one that size so I can't say. I thought it was easy enough to single point my backing plates.
If you need a 5" backing plate or less these have been pretty good to me http://cdcotools.com/item.php?itemid=369 Got the proper threads precut in them.
When in doubt - look it up. Looks to be in the 3500 inch pound range? Could not find it in my Machinery's Handbook
Hmm seems like a lot of force. How do I figure out the settings on my lathe for 8 tpi 1-7/8" thread? Is there some chart or calculator I need to use? There also seem to be a lot of options for the premade tap. Not sure how to tell which threads are the right ones to match with my spindle. Is pipe thread the right type?
Never tried threading before on a lathe. Time to learn i guess! I want to remake the 4 jaw back plate since my first one is kind of shit.
Start with some aluminum or get some PVC couplings to practice on. Run insert tooling and remember the inserts are different for internal vs external threading. I dumb f'd that one up already. If you can thread all the way through or towards the tail stock, it removes some of the tension off you. And F all that using the compound shit, just use the crossfeed. Get a fishtail thing and be on center height. Pretty deep passes at first, but then take less and less each pass and tool pressure goes up. If you are going to cut an internal, I highly suggest you cut a male end first so you can test your internal thread. Or else you get to leave the part on the chuck and flip the chuck around. And NO reverse direction threading for you buddy, you could spin your chuck right off the headstock and onto your foot. (another reason to switch to a D1 style chuck mounting system) If that didn't make any sense, I'm fighting a head cold right now. No it's not covid, i can still taste and smell, been burning the candles at both ends lately! Heard a funny one today, someone told me they had the flurona or some stupid shit like that. Funny how the flu and the common cold HAS COME BACK! I'm OUT!
Damn! I just bought insert threading tooling but didn't realize there were seperate ID vs OD tooling.... of course there is.... ugh. I'll have to buy some ID inserts and a bar. Will see if I can figure it out. There's probably a threading table somewhere with the feeds and speeds I need for 8 tpi on that bore. Good tip with starting on pvc or soft stuff. I hope you feel better soon. Being sick is no fun.
I don't feel so bad buying inserts for only the external. I NEVER gave it any thought they were different, but they most certainly are. Your feed and speed is whatever you feel comfortable doing. START SLOW, say 60-70rpms until you get a feel for the timing. My lathe has a brass pin on the lead screw that should break if I f things up. Engaging the half nut takes a little practice to so you hit it in the exact same spot every time. I can push mine in about half way and drag it, then wham it fully when the mark lines up. Good luck, you can do it!
You can find tooling that uses an external insert and run it on an internal bore (I'm telling it works because I did the same thing) One of the best options other than Youtube (look up this old tony, Keith Rucker for single point thread tutorials) Is SouthBend's How to run a lathe http://vintagemachinery.org/pubs/detail.aspx?id=17260
Thread cutting on the lathe is easy, but it really helps to have the handbook for the lathe and legible plaques on the lathe, so that you can set up the change-gears, etc. If you can't access any of that, it no longer becomes an easy process. Before tungsten-carbide inserts became popular, the threading tool was just a suitable HSS tool bit, ground to the correct thread angle with neutral top-rake. Use the slowest speed possible and take light cuts.
I made a face plate for mine by pouring the whole thing and using the pouring shaft as the taper....Yah, took a LOT of lathework to cut to #2 taper, but that is just FUN. Setting the offset precisely on the tailstock manually is a thrill. Incredibly, I have 89 pictures of this process. I am normally too busy to take pics...
Huge faceplate. Very interesting. Aluminum? I found my machinist handbook and found dimensions to use but will need to experiment. I had another look at my 4 jaw back plate that I made. I found that it is completely off. Like crazy out of center. The strange thing is it seems like the back plate when it is disconnected from the chuck is warped somehow. Doesn't make much sense because when I made it it was definitely running true. I don't understand how it is so many thousandths off. I also can't find any debris in the threads to be misalignment it so badly. I'm a bit stumped. Its cast iron so it shouldn't have shattered not bent if there was too much force on it.
Cast iron is fun. It can warp from internal stresses that were there from casting. When you machine it it releases some of the stress in parts but not others. Then over time it warps. If you're sure there's nothing in the threads. Re-machine it true again. About the only thing you can do now. Raw Castings that get machined a lot need stress relieved. By letting them age and destress naturally or by heat treating them or both. In some cases rough machined, heat treated, finished machined. I'm sure there's things you can add to the molten cast Iron to minimize the stresses but I'm not sure what they are. (Iron casting guys speak up here please.)
The question of stress telieving cast iron was definitively addressed by the US Navy in 1948. That study disproved “aging” iron, storing outdoors in weather extremes, and other until then common, and still unfortunately persistent claims about stress relief of cast iron. https://apps.dtic.mil/sti/pdfs/AD0620556.pdf I use the accepted methods of thermal stress relief of cast iron on my castings and on many of them I machine them very carefully to close tolerance. The castings in question are to then be scraped to extreme flatness. I can routinely machine .125” from one face of a prism-shaped thermally stress relieved casting and then flip it and machine the other face. In the forty or fifty castings I’ve done ranging from 8 inches to 26 inches I check both faces after machining. I have yet to have one be out of flat by .0005” over 26”. If the castings were moving around due to machining the first face would not maintain flatness. Add to that one of my customers first machined and then precision scraped to a granite surface plate reference one face of his 18” prism. He then machined the second face. He reported no change in the ink print pattern on the first face after machining the second. I was amazed by that degree of stability. Here are a few pics I could find of the 8” castings with faces and pockets cut: The bottom line is, despite much persistent lore to the contrary, properly thermally (there is no real science in so-called vibratory stress relief) stress relieved cast iron is very stable over time and when machined. Raw castings not stress relieved are another matter and will deflect as stresses are relieved through machining. But time does not stress relieve cast iron, nor does weather or thermal cycling at only warm (950F or less) temps, nor does vibration or ringing. The crystal structure of cast iron is just that. It will only “move” if heated to about 1150F. I have heard of no additives to be inoculated into cast iron to reduce stress in castings. There are many inoculants and variations in alloy to reduce or increase hardness and to improve liquid flow characteristics of iron. Denis
Thanks Denis for the info and the corrections! I knew someone here would correct any short falls in my post. (There's so much practical Knowledge here it's amazing!)
