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PaulG
06-27-2005, 10:16 PM
I know this topic has been repeated many times, but I am just getting around to figuring out how to thread on the lathe.

I must be doing something wrong but I can't figure out what. When I cut threads the diameter gets mush larger and the thread form doesn't look symmetrical.

Just tried to thread a 1/4-20 rod and after I am done the threads are .285. So of course it's too large for a nut to fit. Also the threads don't look right. Compound set at 30, the tool square with the work, feeding in with the compound. The pitch gauge matches the threads.

My guess is that whatever I am doing wrong is causing both the threads to look asymmetrical and the diameter to get larger.

Any ideas on what to try next?
Maybe a tap! ;-0

mochinist
06-27-2005, 10:28 PM
<font face="Verdana, Arial" size="2">Originally posted by PaulG:
I know this topic has been repeated many times, but I am just getting around to figuring out how to thread on the lathe.

I must be doing something wrong but I can't figure out what. When I cut threads the diameter gets mush larger and the thread form doesn't look symmetrical.

Just tried to thread a 1/4-20 rod and after I am done the threads are .285. So of course it's too large for a nut to fit. Also the threads don't look right. Compound set at 30, the tool square with the work, feeding in with the compound. The pitch gauge matches the threads.

My guess is that whatever I am doing wrong is causing both the threads to look asymmetrical and the diameter to get larger.

Any ideas on what to try next?
Maybe a tap! ;-0</font>

I would offer to help you out, but your post confused me. I don't know how you ended up with a bigger diameter than what you started with. My suggestion is to get a detailed book on the subject or maybe take some pictures of what you are doing so we can better understand what you are doing wrong. Also try using the search button, I am sure this has been went over in detail by someone like forest in the past, and he seems to be a pretty good teacher and can explain how to do it better than I can.

rkepler
06-27-2005, 10:52 PM
Are you sure that the cutting edge is on or below center? What you're seeing can happen if the edge is above the center, it'll sort of mash things around forming something that looks a bit like a thread. It's also possible that you're using a left hand tool for aright hand thread, or the leading edge is simply gone. Check those things.

You should be seeing swarf coming off the edge of the tool, the first few passes will look more like a wire, later passes will look more like a real chip is being formed.

[This message has been edited by rkepler (edited 06-27-2005).]

J Tiers
06-27-2005, 11:12 PM
What rkepler said,

plus, if that is hardware store rod, it is generally made apparently of annealed silly putty, and will throw up a huge burr anyhow.

Carl
06-27-2005, 11:56 PM
The compound must be 30(or 29.5) degrees from perpendicular NOT parallel to the turning axis of the lathe. Some lathes, including mine, read 60 degrees when set properly.

PaulG
06-28-2005, 12:53 AM
Thanks for the replies.

I tried a 1/4 6061 rod and a 1/4 drill rod.

I'll go back and check the center line of the tip but I had checked it before I started and it was dead on. I am using a new 60 pre-ground HHS bit.

My goofus lathe of course has the compound marked with 0 as parallel to the bed so it would need to be turned 60 from that position. But of course it's only marked to 40 degrees. So I had to use a protractor set to 30 and measure that from perpendicular to the bed. I suppose it could be a little off.

The diameter of the threads after advancing the compound .035 is about .285. Measuring a 1/4-20 bolt across the threads, it's just under (.246) 1/4. I guess it's not cutting much just displacing the metal.

I'll try just cutting at 90 tomorrow and see if there is any difference.

For some reason I thought this might be easier. I'll get it figured out.


[This message has been edited by PaulG (edited 06-28-2005).]

tattoomike68
06-28-2005, 01:42 AM
hit it with a file is what I do, dont fear making the od a little small &lt;.248- .246 is great. for 1/4" 20 tpi.

is the tool sharp? touch it up with a hand held diamond hone and oil the part big time.

we all make some rough threads at first, after you get the tool grinding down and get faster they start looking and working great.

threading is one thing that makes for a good machinist, you want to get good at it.

best of luck http://bbs.homeshopmachinist.net//smile.gif

Your Old Dog
06-28-2005, 08:08 AM
I had trouble on my SB9 because someone had the spare gear (48tooth stud gear) was interchanged with the screw gear. It's supposed to be stored there but if you switch them out left for right then all is messed up. It cuts threads but not any that will match the rest of the world !

Discovered the problem while on the throne with a fresh copy of "How to run a lathe". It was a very productive morning!

Al Messer
06-28-2005, 09:01 AM
What spindle speed are you using and are you using direct drive or back gear? I have found that small stuff like 1/4" dia. likes a fairly fast cut to cut clean and as has already been mentioned, use plenty of cutting oil. I like dark Sulphurized pipe threading oil myself. It may not be kosher, but I try to rough in the threads with the lathe tool and finish them off with a good quality die.

SGW
06-28-2005, 09:10 AM
Drill rod can be uncooperative, but the 6061 ought to thread reasonably well.

The compound ought to be angled this way \ relative to the work, not / that way. I assume you have that correct, but if you don't it would likely cause all kinds of trouble.

Do you get a chip coming off the tool?

jimmstruk
06-28-2005, 09:34 AM
i had same problem. trouble was using cold rolled jim

topct
06-28-2005, 10:11 AM
Our old friend Thrud knows about this I bet. http://bbs.homeshopmachinist.net//smile.gif

------------------
Gene

Paul Alciatore
06-28-2005, 11:12 AM
Dead sharp tool. At or just a mite below center. Good steel. Good cutting fluid or just plain motor oil would work. Start out with fairly light cuts, perhaps only 0.005" to 0.010" deep per pass. Start off as slow as the lathe will go - well, at least 25 or 50 RPM.

You should make a nice chip. If not, something's wrong. After you can cut at this crawl, then go for slightly faster speeds and perhaps deeper cuts.

Some steels (perhaps other metals also) will have a raised edge. After cutting the thread to the proper depth, a finishing cut may be needed on the OD to bring it to size. Don't try to cut deeper with the threading tool as this will make a very loose fit on the nut. You can do this manually with a broad faced tool to span several threads. I say manually because you should not disturbe the threading set-up until you have a proper fit on the nut as you may have to do a final pass or two with the threading tool after cleaning up the OD. A quick change tool holder that provides positive repositioning of the tools when swapped is a must here.

Paul A.

imakechips
06-29-2005, 01:34 PM
So that we are clear on what I'm about to say- the compound is what the toolpost is attached to and the cross slide is what the compound is mounted to. You feed in with the cross slide, NOT the compound for removing most of the stock. After feeding(in small increments per pass) in to full depth with the cross slide, check the fit. If the nut won't go on or the ring gage doesn't fit, feed in a thou or so with the COMPOUND and then check fit again. Run a file over the threaded portion with the spindle turning after each pass to remove any burrs, stop the spindle, and check again. Repeat as necessary to obtain the proper thread O.D. and depth.

You are turning the stock to .250 before attempting to cut those threads right? How long of a section are you trying to thread? Are you using a center in the tailstock to support the end of the part? If the section is very long, more than a couple of inches, it may just be pushing off, the same thing will happen if the outside end of the part isn't supported by a center.

Also, even threads(10,12,16 tpi) can usually be cut by engaging the half nut at any line on the threading dial, odd threads(7, 11, 13 tpi) can only be cut on either odd or even numbers-once you start on a number, say 1 or 3, you must use either 1 or 3, not 2 or 4, otherwise the leadscrew will not pick up in the same place and will ruin your threads. There are exceptions to this- I ran a lathe recently(with an inch/metric quick change threading gearbox and inch/metric dials) that wouldn't pick up even threads on any number on the threading dial, only odd or even. I was cutting an even thread and started on 2, finished the pass, went back and engaged on 1, then it tore the first two or three threads off before I could disengage the half nut and back the tool out. I went back to the starting point and restarted on 4, leaving the tool just far enough back to avoid any cutting and engaged the half nut, let it run about half an inch and then turned off the lathe. The tool was back in the proper place as verified by me dialing in the cross slide to just touch the tool to the part. I really don't know why this happened, my only guess is that it was because the lathe was an import(pretty good lathe except for the threading problem), the brand of which I cannot remember.

Anyway, I hope some of this is helpful. I sure made a few mistakes when I was learning, though nothing quite like you are describing. Good luck!

SGW
06-29-2005, 01:37 PM
imakechips, I think you've got the use of the cross slide and compound reversed. My understanding is you want to feed with the compound, set at a 29 1/2 degree angle to the work, so nearly all the chip is taken on one side of the toolbit. If you feed with the cross slide, you're trying to cut a chip with both sides of the toolbit, which leads to chatter.

Norman Atkinson
06-29-2005, 02:14 PM
Hopefully, I have returned to a more level of discussion.

The problems with what you call "drill rod" pops up in the UK too. We call it silver steel is anyone's guess. It can machine with difficulty and one suggestion is to use Shell Garia H which is a neat cutting oil. There is little merit in recommending something which cannot be found and the following or something similar would also be ideal for deep drilling
BP CFS 35
Castrol Ilocut 173
Duckhams Sulfcut 300
Esso Dortan 12
Fina Vulcit X C 12

I hope that something from this list will assist

Norman

Shuswap Pat
06-29-2005, 02:44 PM
Threading takes practice. The tools must be sharp, and on center. Also the closer you can get to the optimum cutting speed, the better the finish will be. This has a direct correlation with your reaction time for engage/disengage the half nuts. Also 'Rapid Tap' or a 'chlorinated' ( and nn-healthy) cutting oil will give you optimum results.

Pat

Randy
06-29-2005, 03:48 PM
<font face="Verdana, Arial" size="2">Originally posted by PaulG:

My goofus lathe of course has the compound marked with 0 as parallel to the bed so it would need to be turned 60 from that position. But of course it's only marked to 40 degrees. So I had to use a protractor set to 30 and measure that from perpendicular to the bed. I suppose it could be a little off.

</font>
My lathe is marked the same way. (Well, it goes to 45*.) I put a secondary mark om the compound 45* away from the primary mark, so for threading I set the secondary mark at 15.5*. It sounds confusing, but it's really not. I just take a bird's eye view of the compound and swing it to where it looks about right and say, "Oh, I remember now. I need to be a bit off 15*." Thinking visually about what I'm trying to accomplish tells me which way to nudge it, and I wind up at 15.5*.



[This message has been edited by Randy (edited 06-29-2005).]

jimmstruk
06-30-2005, 08:14 AM
Back to material growing larger afer cutting on it. Cold rolled is actually drawn thru a die and is under stress, then when the outer surface is cut on the metal can expand. How about the ,6061 maybe extruded and under tension? Guy Lautard had long discussion on this in one of his books.

Michael Az
06-30-2005, 09:14 AM
So that we are clear on what I'm about to say- the compound is what the toolpost is attached to and the cross slide is what the compound is mounted to. You feed in with the cross slide, NOT the compound for removing most of the stock. After feeding(in small increments per pass) in to full depth with the cross slide, check the fit. If the nut won't go on or the ring gage doesn't fit, feed in a thou or so with the COMPOUND and then check fit again. Run a file over the threaded portion with the spindle turning after each pass to remove any burrs, stop the spindle, and check again. Repeat as necessary to obtain the proper thread O.D. and depth.
==============================================

imakechips, yes, like somebody else mentioned, this is wrong. Lets not add more confusion. The cross slide should come back to a zero setting after every pass and then feed in with the compound.
Michael

Swarf&Sparks
06-30-2005, 09:41 AM
Don't wish to muddy the waters further, but at 1/4" over any significant length, I'd be using the travelling steady.
Rgds, Lin

kap pullen
06-30-2005, 12:21 PM
Here's something I did on threading a while ago.

Maybe it will be of help.

http://www.bluechipper.net/ThreadCutting.html

Good luck!

kap

BillH
06-30-2005, 12:46 PM
Thanks Kap, I didnt know about the final .001 move with the cross slide.

Randy
06-30-2005, 03:39 PM
And I've never heard of "spit polishing" the final passes. I'll have to try that. I think I'll make a tool grinding fixture like the one shown, too.

Derek13
06-30-2005, 11:19 PM
Just a thought... i have almost no threading expierienc.e..

but opposed to engageing/disengaging the half nut... could you simply reverse the lathe keeping everything still engaged? This assuming you have a 'shifting' type action lathe where you can forward/reverse it immediately.. never tried but just a thought.

Jim Hubbell
07-01-2005, 01:51 AM
Lots of good tips above. I never heard of a " spit polish eather ". If you reverse spindle direction after the finish of a pass you must back out cross slide. Gearing backlash will crash the tool into the work if the tool is left in.

Randy
07-01-2005, 05:28 AM
Yes, Derek, you can reverse the lathe to return to the start if, as Jim points out, you back the tool out for the return pass. In fact, that's the normal way when cutting metric threads on an inch based lathe. (Or conversely, cutting inch pitch threads with a metric leadscrew.) In this case you can't readily re-establish the relationship between the carriage and spindle if you open the half nut, so you simply keep it engaged and run in reverse to return.

PaulG
07-01-2005, 04:12 PM
Thanks to all for the help. It's embarrassing but the problem turned out to be a tool that was just a little high and on a 1/4 piece of course that can make a big difference in how it cuts. All my threads work great now.

Thanks to all.