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Thread: machining acme threads
10-07-2007, 11:07 AM #1
What is the recommended procedure/cutting tool for cutting an internal 5/8"-8 acme thread on the lathe?
Do acme thread taps cut threads ok?
10-07-2007, 12:02 PM #2
Good quality ACME taps cut just fine. Good ones cost more and are worth it.
I have never tried cutting an internal ACME thread that small. Not sure if I would bother trying to.
How long is the threaded portion? How many parts do you have to make?
10-07-2007, 12:07 PM #3
It can be done but it would take a very small rigid boring bar and not really worth the effort and time.
You can do a faster and better job with an Acme tap.
10-07-2007, 12:15 PM #4
Bruce,Hi. I was just in your city for the Plaza arts fair. You need to have a gauge to grind a threading tool. Gauges are made for such purposes,but if you have the male thread you can use it as a gauge. the acme thread sides are 14 1/2 degrees. you must set the compound accordingly. Use as large a boring bar as possible to hold the threading tool. How deep is the hole? You may be compelled to go to an acme tap if it is very deep. Also,what material are you threading?
you could just make a tap and use it rather than trying to thread an acme thread hole.Taps aren't really that hard to make. If you want details on tap making,ask.
10-07-2007, 12:51 PM #5
its been my experience when using a Acme tap they are tandem taps which rough out the majority of the material then the final form finishes and sizes the thread.
be very careful if you are trying to hand tap with a tandem tap as the section where the transition betweeen the rougher and finish forms is smaller and also weaker so breakage is a very real concern...jim
10-07-2007, 12:53 PM #6
one time i had to make one that size i was able to grind down a 1/2 square hss tool bit work ok had to take allot of cleanup passes and it was in bronze so that helped. took a few hours just getting the relief ground on the tool bit
10-07-2007, 03:51 PM #7
Something that worked for me was to grind a tool on a small boring bar at the 14.5* angle only narrow, kinda like a grooving tool. Then set the compound at 0degrees. (parallel to ways) Take several passes to get out to depth, then use compound to get threads out to full WIDTH until gage/mating part fits.
I hope I explained this well enough so you can make sense of it.
10-07-2007, 07:40 PM #8
I would machine the acme thread out of bearing bronze. I have that on hand.
WA toolman, I like your idea of grinding a narrow cutting tool and then moving over each way (or one way) to make the thread. I guess I could use the acme feed screw for the gauge.
Seems like making tooling is all I get done. Maybe a tap is a better way to go or just buy the feed screw nut.
If I tapped the thread, will I need to use cutting fluid in 932 bronze?
You all have been very helpful
10-07-2007, 07:45 PM #9
I would give it a try with a boring bar and a HSS tool ground to fit. I did a 10tpi acme feed nut in SAE660 that way- ground a HSS tool till it fit into the thread of the screw with no light showing, then set up for internal threading as normal except put the compound at 14.5 degrees. Maybe 8 tpi is getting coarse enough that you'll have chatter problems though. It might be worth a try doing it in a piece of scrap first- I did 2 practice runs which helped a lot.
10-07-2007, 08:11 PM #10
Found some in McMaster Carr at under 30 bucks they aren't too bad, screwcutting them from where you're starting is going to eat that pretty quickly,.........unless of course it's for you.
The core on that size is .500" or .510" depending on clearance, either way there's not a lot of room to get a bar with 1/16" plus cutter protrusion in without losing stiffness.
When I make threading bars for that sort of job I use a bar say .025 smaller than the hole, then eccentricly turn it (it comes out sorta oval) for the cutter depth clearance, braze or clamp an inserted cutter in the end and use the parralell topslide method.
The bars seem that bit stiffer, needs better math than mine to yay or nay it though.
Take care. Sami.
10-07-2007, 08:14 PM #11
Unless you crave this job you might look at MSC's selection of bronze acme thread nuts and see if you can adapt one to your purpose. I also once had an old line shaft lathe that had the half nut liner poured from babbit around the lead screw....Joe
10-07-2007, 11:20 PM #12
I once made a new cross slide nut for a 12" Atlas---lathe looked absolutely like new but I decided I could reduce backlash. The nut was relatively small acme thread--I don't recall its size but smaller than 5/8".
What I do remember is that it took me multiple attempts before I suceeded in improving the fit---(due to flex in the bar kept getting tapered "fit" and the nut was probably only 1/2" or 5/8" long)---eventually I suceeded--to get it rigid (if any set up on an Atlas can be described as rigid) I ended up grinding a "boring bar with integral Acme cutter" from probably a 1/2" HSS lathe bit (and that necessitated making a new tool holder--but darn it I was going to win that battle). That project probably ended up being a pretty good learning experience.
Meanwhile that was the last time I went this route---since then I have made perhaps 4 more cross slide nuts and several compounds nuts and now use acme taps whenever these projects come up. The nuts are quick to make with a tap.
PS NUTS only usually improve the situation but if you can see the flats sre smaller (more V shape)on the most used area of the cross slide thread consider replacing both. I have made the male from scratch but several years ago I started using McMaster Carr acme threaded rod instead of cutting threads on ordinary cross slide and compound threads. I used this rod sucessfully on several South Bend Lathes. With bronze nut fabricated with acme tap you get reasonable fit with less work than threading a new cross screw---(even can be used to make SB taper cross feed screws by cutting a key way and replacing just the threaded section by splicing). But if you insist on "minimal backlash" make the nut with a acme tap and use it to "fit" the cut male thread of the new cross slide---but this is overkill for many home shop machines
10-08-2007, 11:10 AM #13
The only way cutting that thread on a lathe is for the experience of doing it. It will be very time consumming as others have pointed out. However, if this is to learn how to single point cut an acme thread go for it.
One thing I will suggest you don't try is to put the compound parallel to the ways or 90deg to the ways. Doing so will cause more chatter than you will want.
If you set the compound at 14.5deg and use the compound to advance the tool in and the cross slide to take the tool out of the cut and return it to the cut you will get better results as the cutter will only be cutting on the leading edge.
10-08-2007, 01:46 PM #14
I cut a 1/2" 10 tpi acme for my Clausing cross slide nut. It wasn't that hard plus I got a perfect fit to the new screw.
10-08-2007, 02:08 PM #15
I made a #4 acme internal once for a turret lathe, the leadscrew had a keyway...the keyway had a burr, if I had not deburred the keyway the new nut would have been toast real quick
10-08-2007, 02:51 PM #16
Leave it for the night shift.