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Thread: Acme Thread Cutting Question
05-25-2009, 08:23 AM #1
Acme Thread Cutting Question
I am preparing to cut Acme threads on the new shaft for my antique grinder some of you may remember from a previous post (subject of that one was babbitt bearings). I have ground a tool bit and fitted it to the original threads and am now getting ready to make a trial cut, but I'm not sure how to set the compound. HTRAL instructs when cutting threads to set the compound at 29o, which makes sense, since the typical thread angle is 60o. Using that same ogic, should I set the compound at 14o? One less than 1/2 the included angle of 29o? I may end up having to wait several days to make my trial cut since I do not have a thread tool gauge. I may be able to borrow one if my buddy has one, but otherwise, I'm going to have to wait til I can get one from MSC in order ot set the tool to the work properly...
05-25-2009, 08:53 AM #2
You are correct on the angle but posing the question suggests you have not cut acme before.
I suggest some trial runs; acme threading, even when you go in on the angle, cuts across a wider face than the same pitch 60 degree thread and sometimes chatter is a real problem. Occasionally the only solution is to cut a vee thread 'inside' the acme then open it up with the acme tool. On large pitch acme, probably not applicable in your case, it is sometimes necessary to do each flank of the acme separately.
Really tedious and requiring careful alignment of tools on a manual lathe, even tedious on CNC.
05-25-2009, 09:26 AM #3
You are 100% correct, I have not cut Acme before, and only limited 60o threads; I am very inexperienced in thread cutting. I have had trouble getting a good finish and your mention of chatter makes me a bit nervous. The shaft is 1", the threads are 8 TPI. You think I ought to cut 60o threads first and then use the Acme tool to finish?
05-25-2009, 09:49 AM #4
According to my friend John, who used to cut the threads on feed and lead screws for CVA tool room lathes, best practice with acme threads is to use a tool narrower than the thread groove width with the top slide set parallel to the bed. He says to start off by cutting a spiral groove to thread depth with a plain square ended grooving tool. Having produced the groove he switches to a tool with the right angles for the acme flanks and creates a thread of correct form but narrower grooves. By feeding the top slide he then opens up the grooves to exactly fit the mating thread. Apparently if you are careful its quite easy to get nominal zero backlash. For him maybe. Not me.
I got to see him do an acme thread when friend Mike had a new cross-slide screw made for an old Churchill Cub lathe with the acme thread roughed out for John to finish so it matched the old worn nut. John made the job look easy and apologised for leaving about half a thou backlash due to being out of practice. Given that his small lathe in those days was a fairly ordinary "no-name" flat belt drive device of around first world war vintage, albeit carefully re-worked I figured it to be pretty good going but John said he expected the same class of results from the old lathe as he got from the tuned up Pratt & Whitney C he drove for CVA.
05-25-2009, 09:56 AM #5
That is a larger thread than I was picturing, it must be a big grinder.
You have a learning curve ahead, not really steep but it might prove frustrating at times. Presumably you have turned the replacement shaft so you want to be sure of success before attacking it.
What material did you use? Medium carbon steel something like 1045? Getting a smooth finish on 1045 or 1018 on a manual lathe running slowly can be a real bear; these materials will tend to tear when cut slowly which is the case with threading. I suggest turning some replicates of the thread stub on the shaft and practising.
Grind your own tools out of HSS and put on a top rake of maybe 5 degrees; I am of two minds whether to suggest you do the two flank approach so I will describe it with the caveat that maybe this is getting more complicated than necessary.
As I mentioned for doing the flanks separately you first cut a vee thread that fits within the acme profile and of course you don't worry too much about the finish on this. Then you grind 'half tools' for the acme and you make the top rake perpendicular to the flank; this is different to a full profile tool which by necessity has the rake perpendicular to the axis of the thread.
When doing the two flank approach you set the compound parallel to the axis of the thread and use the cross slide for the infeed. With only one flank being cut using the compound for infeed at the half angle is not needed but what is needed is a way of tweaking the half tools sideways so you can center them on the previously cut vee thread.
You will likely need to experiment with cutting fluids. The two big problems when running slow are the build up of material right at the cutting edge and the failure of the chips to form a nice curl. In your practice runs experiment with things like tapping fluids, the high sulfur threading lubricant used for cutting pipes threads in a threading machine (I think The Home Depot has it), weird things such as lard or even gear oil. Come to think of it lard may be historically appropriate for remaking an antique.
With luck an perseverance you will finally get the conditions to cut a thread with flanks that almost look ground. When grinding your toolbit use a trick I was taught about half a century ago; grind on the side of the wheel so the grainding action is parallel to the cutting edge. This avoids creating minuscule grooves in the cutting edge which happens with conventional grinding from the grit. It does require a bit of practise to hold it correctly and safely against the wheel especially for the top rake and you have to be sure nobody from OSHA is watching because side grinding is frowned upon.
Good luck. Or should I take the attitude that luck has nothing to do with it because it is dedication and practise that is needed?
EDIT: after uploading my post I see Clive603 suggesting a similar approach. I prefer to use a vee tool for the 'pre-thread' simply because chip clearance and leading edge clearance can be problematic with a square tool but either approach can work. Similarly the single acme tool for finishing works and the only possible benefit from my approach is a better orientation of the top rake; also you don't need to grind the 29 degree include angle because you can tweak the toolbit to get the cutting edge correctly aligned.
Last edited by Hdpg; 05-25-2009 at 10:01 AM. Reason: Added comment
05-25-2009, 11:32 AM #6
As hdpg says, you can't just use a typical cutoff tool for the spiral groove (or square thread). Trying that is going to cause you all kinds of trouble, as the tool is going to bind up in the slot, due to the lead angle. Been there, done that cutting an internal square thread. Your grooving tool will be laid over at what ever angle, with a little extra on the leading edge for clearance. Good tool grinding lesson.
I have actually had good luck cutting even internal acme using the compound at 14 degrees. Take small bites and it goes fine. The Acme threading tool has such a wide nose that it is almost like a cutoff operation.
05-25-2009, 01:41 PM #7
Best approach for the initial grooving tool is to make it from round stock (something to use one of those slocombe centre drills with the tip broken off you've been keeping "in case they come in handy" for). Make a holder on the lines of an Armstrong but with a round hole and lay the tip over to match the helix / lead angle. Which isn't that bad on 8 TPI at 1" diameter. When you have made the holder you might as well do all your thread cutting tools that way.
John sees no point in starting with a Vee thread as you have to use something similar to normal thread cutting techniques to cope with chip crowding and clearance which is slower than going for a straight in groove. For 8 TPI thread and groove depth is only a bit over 62 thou so you don't really need any back taper on the grooving tool so long as you keep things well lubricated. (John says that acme threads only start getting hairy at 4 TPI and below. Last time I spoke to him he'd just done an internal one 2" diameter and 6" long at 1/2 TPI which was apparently interesting.) To save wear and tear on the acme tool proper its quite acceptable to angle over the top slide after cutting the square groove and chisel out the sides at close to the correct angle.
If you are using the side to side feed of the top slide to finish the groove width then the acme tool is acting as a form tool so honing prior to use, mirror finish if you can get it, is advisable. Material choice is also important. Something with poor cutting characteristics is not going to help.
Finally you do need to be firm with the feeds making sure that the tool cuts. Trying to sneak up a tenth of a thou at a time just wears the tool edge off. The big difference between a pro and an amateur is in the confidence to take decent cuts when roughing and the ability to come down to size off the dials with a single finishing cut. Have a good practice first to get your confidence up.
Like so much to do with machining its not that hard if you have a viable technique and a bit of practice.
05-25-2009, 07:10 PM #8
Acme Threading Video
Does anyone have any comments regarding the Acme thread cutting technique demonstrated in the old film on Steve Wells' site?
It's the one at the bottom of the list:
05-25-2009, 07:56 PM #9
Thanks for all the detailed helpful instructions. Being impatient I was hoping to get a simple answer and then go cut expert quality threads on the piece I have chucked in my 4-jaw chuck so I don't have to set it up again. Sounds reasonable, right?? (If I know how to put a "smilie" in, I'd put a big-toothed grin right here).
Here's a pic of the grinder in question:
It's pretty big, I guess. The wheel on the left is 8", the main casting is about 12" from the outside of the bearing block to the outside of the other bearing block
Unfortunately I don't know exactly what alloy the steel is. I know it's decent quality stuff, a buddy made a spindle for a sidecar out of it (he gave me the piece I'm using), but exactly what it is I can't say. I'll make some practice runs on a scrap piece of the same material I have on the shelf and see how it goes. I have some synthetic thread-cutting oil which is formulated for stainless, aluminum alloys and I forget what else it says on the bottle, I'll start with that, but maybe I will need to get some other stuff to try as well.
What's a good speed to run the spindle? I have so far run it in the slowest back gear speed for cutting threads, with dismal results as far as finish goes. As mentioned above, the metal tends to tear off in chunks and makes for an awful finish. Of course, this was on stainless, I'm hoping with a more friendly alloy of carbon steel I'll get a better finish. I do have a junk center drill, I guess I'll have to make a grooving tool out of it for starters.
I ground the Acme thread tool from 3/16" HSS and honed it with a whet stone so it has a pretty nice finish, but I don't know what the angles are, exactly; I gave it side clearance enough to fit in the threads on the existing shaft, figuring this will keep it from binding. Right now it has no top rake at all, I guess I need to add it. How the heck does one measure angles on such a small piece accurately? Going to have to get a tool gauge and check it before I proceed, though it fits pretty well in the existing threads.
I think I sill get a chance to try it out tomorrow on the practice piece and see how it goes.
05-25-2009, 08:04 PM #10
The object of this project is to replace the existing shaft (obviously), pour new babbitt bearings and replace the grease cups with oilers. The original shaft was machined out of concentricity. By a lot. The bearing journals are about .015" out of synch with the wheel journals. This makes the thing shake and carry on, and "hydraulic" the grease out of the bearings and go all over (you can see the green grease all over the place on the casting). I started out just trying to figure a simple easy way to keep the grease in the bearings and found out (by posting a plea for suggestions on this subject) that babbitt doesn't like grease, it likes oil. So that led to this. I susepect someone may have returned the bearing journals along the way and that's how they got out of concentricity, because I can't believe this was made that way to start with. It's been around a while, when I got it it still had the leather flat belt and cast iron pulley on it.
05-25-2009, 08:12 PM #11
Yeesh, this is a classic example of what a bad idea it is to replace oilers with grease cups or fittings. I'm amazed it would even come up to full speed with grease in there. Bet it certainly got hot enough to liquefy it, lol.
You are taking the right track. Take your time and get it right. You'll never own another grinder if you can get this one fixed up.
05-25-2009, 08:58 PM #12
You oughta see the basement floor all around the grinder! Not to mention my hair after using it. Worst when you first start it... Stand Back! It doesn't get hot because there's so much "clearance". I have to keep cranking grease in there to quiet the thing down, which lasts about a second. I have another old grinder that also has a grease cup on it but I've never had it running. After this one is fixed up I'll take that one apart and see what sort of bearings it has and what I need to do to get it running.
05-25-2009, 09:51 PM #13
In other words how fast are your reflexes for rapidly retracting the cross slide at the same time as you drop the half nuts out?
If you have a nice radiused undercut on the inner end of the thread then you only need to drop the half nuts and you can retract the cross slide when the saddle has stopped.
05-26-2009, 06:38 AM #14
Hmmm... Well, they told me at work to never let anyone see me patting my head and rubbing my stomach or they'd make me a foreman, so I don't know... Radiused undercut? I have an angular undercut for the end of the thread; I used my cutoff tool to make it. Hopefully this is tolerable. I was wondering if I went a little faster if the finish might be better, though I'm scared to break something or screw up (no pun intended) the threads. I guess that's what the practice pieces are for. I'll experiment with that.
05-26-2009, 08:40 AM #15
The icons say it all. Cutoff tools have sharp corners, sharp corners are stress raisers, stress raisers can lead to fatigue cracks and failure.
Maybe I am expressing things more seriously than is justified for a grinder that is probably only going to see occasional use but on a commercial machine that was used frequently this would be very bad practice.
On second thoughts it is justified being serious; I would never knowingly use a grinder with the thread undercut made with a cutoff tool so I have to say no it is not tolerable.
I strongly suggest this part be used as a practice piece and you make another and have either a full radius for the undercut or nicely radiused corners.
The answer your question about going faster for a better finish on the thread is yes it can help. But don't push the speed so much you simply cannot react fast enough and finish up scrapping the part.
05-26-2009, 08:59 AM #16
acme cutting thread
Check out forum member Tex-VA's cross slide write up.
The first part has a detailed account of acme threading - lots of pictures.
05-26-2009, 02:28 PM #17
Yes, I forgot about that! He looks like he did the Acme threads on one shot. Did he have success with this method because they are smaller than what I propose? I see he ran his machine on the slowest speed in back gears, same as I plan to do, and got an apparently good finish. He is much more of a machinst than I, that's for certain-sure. I feel like I'm stumbling around in the dark!
05-26-2009, 03:46 PM #18
Cutting Acme Threads
Hello - I think I'll join in here. First, I would try to learn to chase/cut regular threads (60 degree) first. I think you said in one of your posts you have, so the next thing I would go is to make some practice acme threads on some stock you have that you could afford to wast, may be some hard aluminum like 6061. Don't use any soft stuff, it will gum up and mess up what your doing. One of my reasons for success is that I used 12L14, "free machining" steel. That stuff works great. The other thing is to take your time grinding your bit. I used a table and miter gauge to grind mine (see post) which is cheating to some but I'm really not that good and I think you need a really big (12" or bigger) grinding wheel to grind a bit freehand. All of the advise given here to you looks right on to me and should serve you well. Please post some photos of the threads you cut even if you have some problems, I'm sue all here will be glad to help you get it right. By the way, my scrap bin is full of my learning mistakes. Good luck, Tex in VA
05-26-2009, 04:48 PM #19
If you can tolerate a bit more groove leading up to the thread, just go in with a radiused cutter and move the shoulder enough to get rid of the sharp edge.
05-26-2009, 06:35 PM #20
Yes, Mike, I think nI'm going to try to radius the sharp edges I have, maybe grind a tool bit just for that purpose. And Tex, I just now resolved to remove my shaft from the chuck and start practicing on some scrap. I have a small piece of the same material, so that will me my test mule for two things: my thread cutting prowess and whether or not the material is friendly enough for that. If not I'll have to see about getting something better suited. I will inquire about that if the need arises. Also, I just made an important discovery. I do not have an Acme thread gauge as yet, so I ground the bit using the existing threads as a gauge. Not the easiest thing to do, since the depth is so small (relative to a gauge). I measured the tool, expecting to see 29o included angle, and found only 15o. This got me to looking more closely (like with a magnifying glass and a light) to see just how closely my tool matched the threads, and I discovered that it isn't so close. These are square threads, not Acme! So, more careful measuring with the protractor and scale, I now know the angle (helix?) is 85o and the width of each groove is exactly .100". The "peak" of each thread, however, is only .070. I also had the threads per inch incorrect, previously stated 8tpi, is actually 6tpi. So, if I grind a tool bit properly, with 85o angle for the helix (?) and .100" wide, with the machine set for 6tpi, will I wind up with the correct "size" thread? I guess I must. This was obviously made on a lathe to begin with, and it seems that thread cutting has been thread cutting for an awfully long time. Sheesh, I wish I paid more attention in Engineering Drawing in college, we had to draw screw threads, which I found immensely boring at the time. Quit, too. Wish I hadn't done that either.
Anyway, I will get some practice in tomorrow afternoon, assumiong grinding the tool bit doesn't take all afternoon, and post pics of my results. Thanks for all the input so far!
Last edited by northeastconfederate; 05-26-2009 at 06:36 PM. Reason: forgot to add