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Simple manual engine lathe accuracy questions

NC Rick

Aluminum
Joined
Oct 7, 2007
Location
Asheville, NC
Hello folks,

I'm hoping I can indulge myself here and ask for some of your experienced input. I'm attempting to learn from a very simple project which is outside of my experience. Honestly, I felt this was so basic, I posted on the hobby machinist site with some questions and details of what I was up to. It seems I may be even worse than I thought at this stuff and from what I was told, if my technique isn't way off, I may have a problem with my lathe. Although my equipment is 50+ years old, I'm more suspect of my skill level.

Anyhow, my objective was to make an adapter plate to allow me to mount an 8" three jaw, plain back, scroll chuck to my 12" Yuasa rotary table. I purchased a 10" x 1/2" gray cast iron disk from McMaster Carr and wanted to face it off to be reasonably parallel between sides. I used my 12" 4 jaw chuck with some adjustable stops to enable me to grab just a little of the O.D. at a time. I think I would have been better off with a 3 jaw chuck...

Anyhow, after some screwing around I got the disk to within under a 0.001" in all the ways I could measure it. I was pretty happy. Measuring shows I managed to turn a bit of a cone (by about .0005" over the 5" radius of the part). I feel certain I could do a better job on the flip and turn methodology next time and I think it took me too much time learning to get this faced.

I am being told that the lack of accuracy I produced is bad enough that no one would be able to use a lathe of such low standards in accuracy. I really don't know what I should expect. I would have been pretty okay with anything inside of a couple thou.

Can you folks clue me in to what kind of parallelism and accuracy I should expect from my machine and from my limited experience? I do know that having the Chuck on the rotary table is proving to be a very nice feature for what we do.

Thanks!

The included photo is a quick map of some micrometer readings I made on my disk this morning. I did put it flat on a surface plate as well and it confirms the outer edges being "high" fairly equally.
DB501FAD-4C4E-40E7-A802-5FD14A65F74D.jpg
 
So, if I am reading your picture right, you are getting a thickness difference of .0007” or .0008” between the outer and inner edges in a 5” radius?

This looks like a relatively thin disk? What material? What speeds and feeds and tooling?

You are fine. Discrepancies of this amount on a thin disk off the lathe can be easily faulted to warpage under clamping pressure, tool dig in or part flex (remember, the part is only held rigidly on the edges using a chuck), or thermal warpage as you cut. Did you lock your carriage? Tool pressure could have easily shifted your carriage a bit during the cut.

If you were tasked to make a thin plate within tighter tolerances, you would expect to spend some time on a grinder—though avoiding warpage is still a concern.
 
You're doing great and you'll only do better with a surface grinder. FWIW, it's considered a serious flaw if a lathe turns a convex face. Thus, most lathes will turn a very slightly concave face because that's the direction the manufacturer will tend to err in.
 
So, if I am reading your picture right, you are getting a thickness difference of .0007” or .0008” between the outer and inner edges in a 5” radius?

This looks like a relatively thin disk? What material? What speeds and feeds and tooling?

You are fine. Discrepancies of this amount on a thin disk off the lathe can be easily faulted to warpage under clamping pressure, tool dig in or part flex (remember, the part is only held rigidly on the edges using a chuck), or thermal warpage as you cut. Did you lock your carriage? Tool pressure could have easily shifted your carriage a bit during the cut.

If you were tasked to make a thin plate within tighter tolerances, you would expect to spend some time on a grinder—though avoiding warpage is still a concern.
Thank you much to everyone! I do watch Tom Lipton's videos but hadn't seen that one. I used a tpg 322 insert and wasn't super confident in spinning the 10 inch diameter chunk held by about a quarter inch of the edge by more than 90 rpm. I wanted to be able to out-run it if it got loose :-). I had a slow feed too, I'm not sure but I think about 3 thou. per rev maybe less (the feed is halved when cross feeding). On my first cut of about 6 to 10 thou. I noticed all sorts of "issues". When I clamped the part hard it did weird stuff. I had made backup stops like Tom Lipton has (an awesome friend of mine gives me crap for using the four jaw, saying I picked up some "strange YouTube stuff" but I was scared of the 10 inch chunk in a 10" chuck. If I were doing it again, I'd make soft jaws for the scroll chuck. Anyhow, after flipping it, I skimmed the second side and could see it wasn't good even tho I hadn't cleaned it up entirely I say like 4 thou out of parallel. I had drilled a hole on center to indicate to so I stuffed my live center in there and some Tig wire between the jaws and the part and employed a special chant while tapping the disk with a hammer trying to read what the indicator on the wrong side was telling me.

I really have learned a lot from this community. On the hobby forum, this place is known as the "unfriendly place" and I have only had people be super friendly and helpful to me (except maybe @Thermite but he was speaking the truth... just kidding).

I'm 65, still making my living with all these tools but as machining isn't my first profession, I really enjoy being on the steep side of the learning curve. I fully expect to make all these tools be my hobby and my "job" over the past 19 years has been a little like retirement as I like it so much better than my old "real job". The help I get here is great as it all combines to help me do more things.

The other forum really only provided the one critical comment which I don't mind but I'd have expected others to support what I said (I mostly felt I got this as good as I expected). No one else spoke up. I need my better friends over here, obviously.
Tom L. Wanabe stops attached (lol)
 

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By the time you bolt on the chuck, that cone will probably be gone. As others have said, you're fine.

I was never too concerned of it being good enough for me but I was a little worried I didn't know what I thought I knew, if that makes sense!

Not safe for work photo, sorry!
 

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@blackrainstorm I didn't answer... yes you are reading correctly that there is maybe 7 tenths in that center to edge measure. Measuring to a couple tenths is an issue for me, handling the micrometer and the part will change it by close to that.
 
If you are having to run slow due to machine design or part/workholding limitations, you can often do finer work with a HSS cutter sharpened to a razor edge, as opposed to a carbide insert. HSS has more tensile strength than carbide, so you can really increase the rake and have a thinner edge without it chipping. That means lower pressure on the work and the work won't flex as much where it is unsupported.
 
Thank you people!

My project was a success and I learned stuff, can't beat that! I'm making a few sets of soft jaws since I realize I won't make them when I need them. Having the blanks ready to go will be liberating. I can't find where to buy them for my odd-ball two piece jaws.
73CD8329-D0ED-46D3-BABD-BD6FF8DDEAF6.jpg
 
170A877A-7611-48D2-80AD-2354848FAC28.jpgI can be really easy to confuse. Am I wrong in saying that the "key" is the wrong sex? I don't know much about this, looks through catalogs and couldn't find the match. Metric dimensions and as far as I know, 1970's vintage I believe.

I started the soft jaw project this afternoon. It would be good to buy.
 
Yes, you're correct. They're the opposite of american standard tongue and groove. Buck chucks had that style of jaw, and I think there's at least one import brand that still makes them that "sex" but I can't remember the name atm. Kalamazoo chucks are us made and made by former buck employees IIRC, bit I don't know of their jaws are interchangeable with buck.

I have a couple of buck 6" set tru 3 jaw chucks that are missing the hard top jaws. I'd be interested if anyone can suggest where to find some :)

Lee

Sent from my SM-N960U using Tapatalk
 
I am being told that the lack of accuracy I produced is bad enough that no one would be able to use a lathe of such low standards in accuracy. I really don't know what I should expect. I would have been pretty okay with anything inside of a couple thou.

Sounds like someone is living in a fairyland or doesn't know what they are doing, or maybe both. Good parts can be made on worn out machines. It just requires you to know the machine, know what you are doing and work around it's shortcomings.

5 tenths is plenty good and I bet with some playing around it could be improved. First thing is to see where the taper is coming from. I would run an indicator across the face of the chuck while moving the cross slide. If the taper is coming from the X axis you can likely play around with different amounts of torque to lock the carriage and or applying slight z + or - pressure against the locked carriage.

If it's warpage several flips of the part will likely help using less and less chucking pressure as the cuts get lighter.
 
Top machines and 50 years experience can still stuff thing up......I made some large parallels ,had them hardened ,and took them to a friend with a new big surface grinder for finishing..............when I got them back ,they were noticeably waisted by about 005".......took them back ,guy got very agitated at the suggestion he d stuffed up........IMHO reason for the fail was he did not use any coolant ,he ground dry and the pieces bowed up into the wheel......he would not accept that.
 
Nobody commented on the 3jaw v. 4 jaw issue... You did exactly the right thing using the 4 jaw, your instincts were correct.
 
To check if the crossslide is perpindicular to the mainspindle you face of a big piece of round and then indicate from the middle to the back of your lathe From the front to the middle will always indicate nicely off course
To the back will give you the dubble amount of taper
Indicating on a chuck is indicating on a unknown part /feature You first have to measure/indicate it to be of any value

Peter
 
When facing in a lathe,an error is encountered caused by the variation in cutting speed ,where the tool cuts differently at different speeds..........unless you have a CSS machine.........In a new machine ,the cross slide may be bowed slightly towards the chuck,as a compensation for wear.
 








 
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