Need help identifying feed screw threads on Smithy AT-300

This forum is dedicated to those hobbyists with the 3-in-1 metalworking machines. Mill-Drill-Lathes. Tips, techniques, modification and use of these machines is topical.

Moderator: Harold_V

diebog
Posts: 6
Joined: Tue Oct 13, 2020 4:48 pm

Re: Need help identifying feed screw threads on Smithy AT-300

Post by diebog »

Thanks for the detailed reply! I have allot to learn and I appreciate you taking the time to explain some stuff to me. Maybe I would be better off buying one of those DRO kits of Amazon and then I would be able to get more precise with my measurements sense the marks are so coarse on this dial. I could make marks in between each one but don't know a precise way to scribe them.

I have a nice digital Starrett mic but it stopped working a while ago. I have some older older mics I got from a friend but they don't have the calibrated knob on the end that spins when the correct pressure is obtained for a correct reading. So I am not sure how they work or to be honest how to read them. lol I wish there was a machinist class I could take but there inst much around me. Everything I know is self taught from trial and error. I bought this 3-1 combo and with no experience with lathes or mills, I just jumped right in and slowly figured it out. Had a few things break (cutters and such) but eventually I got to a point where I am comfortable. YouTube also is a great help. Back years ago YouTube wasn't as popular and the ones that did post didn't know what they were doing so watching a video could do more harm then good. Then as years went on more and more professional machine shop guys started channels and added good content that I could trust was correct. So recently I have been watching more and more videos and have been learning allot. Problem is I see so many things done I wish I could do, but aren't possible with this little lathe. Ive looked over the years for another lathe but there is just not much near me. I know a separate mill and lathe are recommended, I just don't have the space for that. If anyone has a suggestion on a good 3-1 brand/model I sure would appreciate it.

So as far as the thread goes, is Metric Trapezoidal a common form that the Chinese makers would of used? What other metric threads with this similar form are used? If I did buy that tap and die I posted would threading a 10" rod be to much to ask from it? Ive never made threads that long before so I don't know what to expect. I haven't been able to find a TR12x2 left hand prethreaded rod so I am looking to make one.
Torch
Posts: 1684
Joined: Wed Mar 17, 2010 7:58 am
Location: Muskoka

Re: Need help identifying feed screw threads on Smithy AT-300

Post by Torch »

If you are worried about the performance of the die, you could rough out the thread by single-pointing on the lathe and just use the die for the final clean-up pass.
User avatar
Harold_V
Posts: 20226
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Need help identifying feed screw threads on Smithy AT-300

Post by Harold_V »

I just noticed I'd made a mental error in regards to the dial feed. I suggested double what is reality. Please re-read that post if you wish to see the correction.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
pete
Posts: 2518
Joined: Tue Feb 10, 2009 6:04 am

Re: Need help identifying feed screw threads on Smithy AT-300

Post by pete »

Accuracy costs, the more you want the more it's going to cost. However that dealer has already proven the supposedly interchangeable items there stocking and charging you for aren't worth buying if the fit between those critical parts is that bad. If it were me I'd be thinking of how to do this right. And I'd start looking for an independent and trust worthy feed screw and nut supplier. Nook would be just one of them but a Google search will turn up a lot more. And there's not a hope I'd be buying by the lowest possible price or from an unknown source in China. If your not happy with what your already buying it's not where you want to try and be pinching pennies.In one way or another those factory made and guaranteed accuracy feed screws can be adapted to fit your existing lathe parts that aren't a part of the thread itself. And the nut blanks are left well over size so they can be machined to fit your exact application. Yeah it might cost you $100 - $150, but both the screw and nut will be accurate, the correct materials that are compatible with each other and cost you multiple hours and many many failures less.

This isn't meant to be critical, I'm only being honest and trying to save you some wasted money and frustration. By the sounds of it neither the lathe or yourself have the accuracy nor skills required to machine your own feed screws and nuts. And even the very best and most expensive ACME die ever made WILL NOT and can never even hope to make a proper thread of that length. Dies are a whole lot different than taps. A tap is guided and driven forward by the thread pitch it's already cutting, it it's a well mae and accurate tap then the threads it cuts will be also. And few tapped threads in a home shop are much over an inch or two long.A die has little to keep it aligned and 100% square as well as completely concentric to the major diameter of the part it's cutting the threads on. Standard dies meant to be used with a hand held die stock are at best only good for either short and fairly low quality threads or if desperate chasing lathe cut threads that have a very closely controlled pitch already 95%+++ formed. Even then I'd have large reservations about the accuracy and fit of a die finished feed screw thread of that length. And it would be highly doubtful any die cut thread would ever operate correctly in a decent ACME tapped part for the nut unless it was cut well under size.It takes expensive and generally specialized high accuracy cnc machines with talented operators to machine half decent feed screws, and even then there guaranteed pitch accuracy can vary 2-4 thou per foot of thread unless you willing to pay for ultra accurate and very expensive feed screws and nuts. Building your own isn't impossible in a home shop, but you need to know and have a whole lot more experience before you should be attempting a job like this. Buying the pre machined parts and doing the fairly simple but still critical machining to adapt them to your lathe will be more than tough enough at this point. Once you do that, then you can get some proper free machining material and just try single pointing your own ACME screws over that length and you'll see what I mean. Even grinding your own high speed steel cutting tool to the correct tolerance and still have enough clearance for the pitch angle to fit a decent ACME tap is not an easy task. And just cutting that thread requires accurate measurements until you know it's correct. At the minimum that's going to take a decent micrometer, trustworthy thread wires and some more skill. With machining you need to learn to walk before your going to run. And I'd have to say ACME threads are at least a bit above jogging. :-)
Torch
Posts: 1684
Joined: Wed Mar 17, 2010 7:58 am
Location: Muskoka

Re: Need help identifying feed screw threads on Smithy AT-300

Post by Torch »

pete wrote: Wed Oct 28, 2020 10:36 pmIt takes expensive and generally specialized high accuracy cnc machines with talented operators to machine half decent feed screws,
I don't disagree with anything in your post, except perhaps this one point. Surely there were reasonably accurate feed screws before CNC machines?
User avatar
Harold_V
Posts: 20226
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Need help identifying feed screw threads on Smithy AT-300

Post by Harold_V »

Yes, there were accurate screws. They were created by thread grinders, not lathes.
Moore used to hand lap their screws to achieve a level of precision that boggled the mind. If memory serves, they were accurate to something like .000020" over 12". Please correct me if I'm wrong, as I'm trusting to memory of long ago.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
Torch
Posts: 1684
Joined: Wed Mar 17, 2010 7:58 am
Location: Muskoka

Re: Need help identifying feed screw threads on Smithy AT-300

Post by Torch »

Wow. Just wow.

If there was someone still around with the skill to achieve that level of precision by hand, I imagine the product would cost more than an entire Chinese lathe.
pete
Posts: 2518
Joined: Tue Feb 10, 2009 6:04 am

Re: Need help identifying feed screw threads on Smithy AT-300

Post by pete »

Yes like Harold said there were very accurate screws made on manual equipment, and if you go looking, the high accuracy ones and some of the better lathe and machine tool manufacturer's verified there's on specialized test equipment that could measure lead/lag errors in the screw pitch. If I recall correctly, Moore mentions there efforts in producing "perfect" feed screws down into the low millionths for pitch and thread form with those hardened, ground and then lapped threads Harold. Even they couldn't get what they were wanting no matter how much time, money and effort they put into it. But it would have been almost wasted effort anyway since at those levels as soon as the machine was put to work the feed screws and nuts would start to have measurable wear fairly soon. All that would have pre dated there use of high accuracy indicators and extremely accurate distance rods. Those were pretty much micrometer setting rods in one inch increments, but a whole lot more accurate than what those are today.

Once they started using an independent method of measurement that didn't depend on the feed screw and nut accuracy, those perfect feed screws and nuts became much less important. My Newall dro is set up to allow an error compensation to be programed in for high accuracy work where the actual scale and reader heads are verified for there measurement accuracy in there working position if those tests can be run using something like laser interferometer etc. How to afford to have that done isn't mentioned in the manual though. :-) So at best I could run basic double checks with my gauge blocks. But afaik most decent dro's today will have that error compensation feature and I'd guess it's already programmed into the better cnc machines right at the factory for there ball screws. I watched a YT video last month that showed a new cnc machine at a feed screw manufacturer that now tests and straightens there feed screws after all the manufacturing processes are completed. According to that video there previous and highly skilled hand straightening and now done by cnc is also a critical part of producing decent feed screws. My assumption is that it's likely only the very high precision screws would get that treatment. And that's part of what your paying for with those. All of this is well outside of what the OP's thread is about, but it's probably still worth knowing about just for general information.
daGrouch
Posts: 25
Joined: Wed Oct 02, 2013 2:17 pm
Location: Detroit, MI. USA

Re: Need help identifying feed screw threads on Smithy AT-300

Post by daGrouch »

I had the same issues. The replacement cross feed nut was way looser than my old one and the threads on original are down to about 50%. The replacement nuts are old stock and made with the same lack of precision as the rest of the machine. The brass wears down fast when adjusted to snug tolerance all the time. When I searched for TR20x2 and TR12x2 rod I only found a chap in Germany (ebay) that had some. I never pulled the trigger and don't know if he still has any. I use my machine both manually and CNC "assisted" so I want as little backlash as I can get in both directions for millwork.
I ended up making an acetal nut for the TR20 (works great) and only because there was room to fit a re-designed nut carrier. The cross feed is a different matter. I will make another acetal (Delrin plastic like material) nut and carrier when I can work up the ambition.

Good luck!

p.s. I suspect the old nuts felt tighter because they are worn to the point that the screw has cut into the threads of the nut so much that it's hugging the crests of the screw tread.
Ed
My YouTube channel
___________________________________
Smithy CB-1220XL / Clark CL500M
Post Reply