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

Need help identifying feed screw threads on Smithy AT-300

Post by diebog »

Hi everyone, im new here. I bought a Smithy At-300 years ago (its a 98-99 model) Its always had allot of slop in the lead screws so I finally go tired of it and called Smithy who is now Detroit Machinery and asked if they had new brass lead screw nuts. They had me measure the main lead screw and determined it was a metric machine so both lead screws are metric. The new nut for the main lead screw was really tight but should loosen up over time. But the cross slide feed screw nut was more sloppy then my old one. The new nut measures 10.16mm on the inside (threads) and my old one measures 9.95mm. So the new nut is .21mm larger then my old one. My lead screw measures 9.68mm down in the root/pitch dia of the tread and the outer dia is 11.94mm. I believe this is a M12x2 left hand thread but what I don't know is it square, acme, trapezoidal? I know it is a single start left hand thread. Im looking to buy just 10" of the threaded rod which is brazed into the rest of the lead screw (show in pic)

I might have to send back the cross slide nut because it so much larger then my old one. According to https://www.engineersedge.com/hardware/ ... reads1.htm if this is the correct thread I have it says the max would be 9.5 so technically both nuts are out of spec. Unless the new threaded rod (if I am able to find it) is larger in dia and fits better but I doubt it. Maybe I would be better off buying a left handed M12.2 tap and make my own nut? I don''t know if my lead screw was made a tad undersized or what.

Can anyone tell what type of thread this is off pics? If there is any other ways to measure please let me know. I don't have acme thread pitch gauges to compare this to. I tried to get close up pics with a magnifine glass on my camera and although the threads look somewhat square it does look like there is a 30 degree on each side
IMG_7253.JPG
IMG_7252.JPG
IMG_7249.JPG
IMG_7251.JPG
IMG_7254.JPG
IMG_7255.JPG
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 »

Sounds like a typical Chinese lead screw. Mine is an imperial thread -- but a metric diameter.

If you want to make your own nut, then I think you will have to make your own tap first. You can make a form tool, using the existing screw as a guide, and then thread a piece of O1, gash it and harden it. Should be adequate for tapping brass.
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 »

The thread appears to be an Acme form. It isn't 30°, but 14½° (29° included angle), assuming it is. The pitch of the thread should match the dial on the cross slide. In other words, if a full turn of the dial takes off .200" on the diameter, the pitch is 10. Using a ruler, measure across several threads of the cross slide screw to determine what the pitch is. You may discover the threads are, indeed, metric. What are your dial marks supposed to represent?

Beyond that, I'm no help.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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 »

Torch wrote: Mon Oct 19, 2020 11:09 pm Sounds like a typical Chinese lead screw. Mine is an imperial thread -- but a metric diameter.

If you want to make your own nut, then I think you will have to make your own tap first. You can make a form tool, using the existing screw as a guide, and then thread a piece of O1, gash it and harden it. Should be adequate for tapping brass.
Okthanks for the suggestion, I am not a machinist but just a home hobbyist. What is 01? I'm assuming that's the last digits of the material code?
Harold_V wrote: Tue Oct 20, 2020 1:43 am The thread appears to be an Acme form. It isn't 30°, but 14½° (29° included angle), assuming it is. The pitch of the thread should match the dial on the cross slide. In other words, if a full turn of the dial takes off .200" on the diameter, the pitch is 10. Using a ruler, measure across several threads of the cross slide screw to determine what the pitch is. You may discover the threads are, indeed, metric. What are your dial marks supposed to represent?

Beyond that, I'm no help.

H
So to be honest I really don't know what the dial numbers represent. My lathe isn't very accurate so I don't use or ever figured out how to, most things I make I stop the chuck and measure with calipers over and over. Id like to know how to use them, they just don't make sense to me. Are then numbers MM sense this is a metric lathe (Smithy told me it was and all the fasteners are metric and the main lead screw is 2mm metric pitch,) I bought a cross feed nut and main lead screw nut and bother were metric that they sold me. Both fit the treads but the cross feed one is so sloppy, I think it wad machined wrong, but I am almost positive it is a 2mm thread pitch. 13tpi is close but under my magnifying glass when placing the points of the thread gauge on the edge of one side of the thread they gradually advance into the flats of the threads. It is really hard to take a pic of this but I'm fairly sure it isn't 3tpi. Here are some pics. The last number stamped is 30 so to zero it would be 40. Its stamped 0.002" →||← 0.05mm. I asked Smithy if they sold the lead screw and they said they didn't and asked what size the thread was and they said "As far as the thread, all the info I have is that it is a 2mm thread." My 2mm standard metric thread gauge fits in it perfectly but its not the right angle as its a standard thread pitch.

So if this is a 2mm pitch, is this still an acme thread? I guess the best option is to try and make a tap with a piece of high speed steel formed with the correct angle. Any chance these are sold preground? How can I measure if its 30 or 29 deg? They are so close. According to this site: https://www.natool.com/acme-vs-trapezoi ... ifference/ it says "The American Acme form has an included flank angle of 29-degrees. The metric Trapezoidal thread is at 30-degrees." So would the assumption that my threads be metric Trapezoidal be correct?

Thanks for all your guys help! I really appreciate it
IMG_7256.JPG
IMG_7257.JPG
IMG_7267.jpg
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 »

diebog wrote: Tue Oct 20, 2020 2:29 pm
Okthanks for the suggestion, I am not a machinist but just a home hobbyist. What is 01? I'm assuming that's the last digits of the material code?

O1 (Oh one) not 01 (zero one). O1 is an oil-hardening steel (drill rod) that can be hardened by the average home hobbyist with an acetylene torch (or any other means of heating it to cherry red) and a coffee can of used motor oil (any oil, really). In it's un-hardened state, it is quite machineable. Once hardened, it can be honed and the resulting tool can be used to machine mild steels and softer metals. Not as long or as well as high speed steel or cobalt steel, etc. but good enough for 1-off parts.

To make a tap, you would turn the rod down to diameter, taper the end for the starting threads and thread it. Then mill or grind the gashes, harden by heating to cherry red and quench in the oil. Hone the cutting edges before cutting the threads.

Others may jump in with some refinements. I'm no expert.
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 »

Ok thanks for the clarification! I have herd of drill rod. Whats the best place to source this? Ebay?
User avatar
NP317
Posts: 4551
Joined: Tue Jun 24, 2014 2:57 pm
Location: Northern Oregon, USA

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

Post by NP317 »

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 »

OK thanks, I was thinking of that but they are always so much higher price wise when I have sourced parts/materials in the past. But you know you can trust what your getting. I might just buy this: https://www.ebay.com/itm/TR12-x-2-Trape ... 4282424377 as its only 16$ and just wait for it to come as its from China. Or buy this one: https://www.ebay.com/itm/1set-TR12-x-2- ... 3606913284 that comes with a die for around 30$. Probably not great quality but for 1 job it should work. That is if I am able to confidently say this is a TP12x2 left hand tread.
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 »

Well with a tap and die pair, you have the option of making both the nut AND the cross-feed screw with confidence that they will fit each other.
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 »

I thought of that but didn't know if a cheap Chinese tap www as capable of making treads on a 10" shaft. Are lead screws made from O1 steel or is mild steel sufficient?

I'm m still wondering how I can verify if my thread is indeed TR12x2? Sense it is metric is that the only possiblity, or are there other metric threads it could be?
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 »

You don't need O1 for a shaft, although you could use it if you want.

Myself, I would probably use 1144 Stressproof. It is strong, easily machined and resists deformation. If that was not available, 12L14 threads well.
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 »

diebog wrote: Tue Oct 20, 2020 2:29 pm So to be honest I really don't know what the dial numbers represent.
According to the picture you posted, the calibration is metric, with each line representing .05 mm, or .002" (.00196"). The only question that would remain is if that relates to the radius or the diameter. In this case, it appears that the marks relate to the radius.
My lathe isn't very accurate so I don't use or ever figured out how to, most things I make I stop the chuck and measure with calipers over and over. Id like to know how to use them, they just don't make sense to me.
Your technique isn't bad. If you hope to do good work, measuring is very much a part of achieving that goal. However, when you say calipers, that's a habit to abandon. Calipers are fine for rough work, when a mis-reading of a thou or two makes no difference, but if you hope to be able to work to critical dimensions, they are simply not capable. Something as simple as how much pressure you apply to taking a reading can yield variations far beyond the level of precision required. Micrometers are the way to make measurements, but even they are subject to "feel". The big difference is you'd have to be a moron to make errors beyond a couple tenths, assuming the surface being measured is reasonably smooth.

Using dial marks is real simple. Once you know the size that the cut establishes, which you determine by measuring with a micrometer (mic), you then turn the handle the appropriate number of lines (determined by using math) to establish the next diameter. If the screw and nut assembly is not worn beyond reason, and you understand backlash, it's a very reliable method to use. However, you must also understand that depth of cut plays a huge role in the size the setting will yield. If you take various depths of cut, each pass will vary. In order for your dial to read reliably, you must take passes of equal size, so cutting pressure, thus deflection, remain a constant. For roughing, where size isn't critical, that's not as important. The way I interpret your dial is that the amount of movement of the cross slide for each line is .00196", or .002", and that would equate to movement on the radius, so each dial mark would represent the change in diameter of the work piece of .00393". The marks are rather coarse, so doing fine work would be challenging.
Are then numbers MM sense this is a metric lathe
Indications are the numbers are metric, so the thread pitch is 2 mm. The amount of error in lead would be quite noticeable if it was imperial and you used a 12 or 13 pitch thread gauge. Form, however, isn't quite as clear to me. You would be best served if you wanted to determine the form by using an optical comparator. Unfortunately, they are seldom found in the home shop.

One thing to keep in mind. If you make a new screw and nut, the form isn't important. So long as the replacements match each other, any form will work. The only thing that is critical is that you ensure that the lead remains 2mm, so the marks on your dial relate to the screw.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
Post Reply