Choosing the right alloy steel - and make a quill

[graham-xrf]

This is about attempting to machine a new quill, in a case of using one lathe to fix up another. I searched the net, and found that quills can be made of alloy steel, or high strength cast iron. My thoughts were that it needs high strength and be able to take impact, easy machining, not ultra-hard, corrosion resistant would be nice. The older South Bend 9C is in good condition, but the South Bend 9A tailstock quill has been greviously mangled. The MT2 taper was partly drilled out off center. Amazingly, what remains of the MT2 still grabs onto a taper, but is not properly supported. It needs a new quill!



Parted-out South Bend tailstocks turn up on eBay, but the risk is repeatedly shelling out for a banged-up old part of unknown internal condition, so I was thinking to use the SB9C to make a new quill for the SB9A. (There is a kind of poetic justice in that)! We get to choosing the steel.

I thought 4140. More easily available in UK is EN19T and EN24T. I found some specifications and set them together as a compare list. The info I have on 4140 mentions "0.2% Proof Stress" which may be yield-related, but the other two steels show "yield" instead. I tried to convert units, but not guaranteed to be error-free. Mpa is the same as N/mm2. KSI is "kilo-pounds per square inch".

The steels all seem very similar, and I guess a quill could be made out of any of them. 4140 seems more tightly controlled when it comes to impurities. EN19T is readily available, slightly lower cost than the other two. EN24T looks kind of similar to 4140, except it has a bigger dollop of nickel, and is a bit stronger.
Enough EN24T to make two quills is £16.50 (about $20).



So where am I going with this? First is to just ask which would be your preference. Would you use something else? Does anyone know what the South Bend quills were made of in the first place?

The next thing, given my relative inexperience, is to seek advice on actually making it. The quill measures 5.125" long x 1.070" diameter, so I guess around 28mm, or 1.125" is the starting out raw size. A precision centered taper and a 1/2" x 10 ACME thread on a alloy steel quill is not a trivial thing, and it may take me a couple of tries before I get it right. I could use an ACME tap, but I was thinking to just use a ready-made bronze ACME nut insert turned to fit and secure into the back of the quill, just like was done for the SB9C.



I don't have a mill, so I have no idea what to do to cut the anti-rotation slot. I may have to find some milling help from a pal. I did see some dude on YouTube use an angle grinder with a cutting disk to hack out most of a quill slot, and then finish with a file (shudder)! Maybe I can clamp a tool sideways, and move the saddle to carve the slot out. Hmm - is that a crazy notion?

 

[john.k]

I made a (missing) quill for a Colchester Bantam out of (I assume) 4140 PH......I made it somewhat like the SB pictured,but the thread nut simply screwed into a pipe taper....Whereas the Bantam has a cross hole with a movable bronze thread.......Anyhoo,it was wanted to use the lathe urgently,no time for frills.....Made a tap for the thread,IIRC was 1/2x10 LH .....still had the handwheel and screw........The little scrote who stole the quill didnt get the wheel.....Would have been an apprentice,so he probably didnt know how to unscrew a hex screw..............edit ,you have a lathe working,use it as a horizontal mill.....slow cut in PH tho,plenty of suds,or cutter will burn.

 

[GrayTech]

If you're going to harden it, which is the most stable? Which will be the essiest to get a good finish on your machines. I'd use the seperate nut idea, you could have a go at making that too without risking the spindle.

 

[benmychree]

Definitely use the separate bronze nut SAE grade 954 would be a good choice, it would be simply lightly pressed in and secured with setscrews as it the one in your pictures. For the quill, any of the choices that you presented would be fine without further heat treatment.

 

[Tim9]

Yeah...I’d do it as Ben says above. 4140 machines nicely. Many more experienced machinist love that stressproof steel. They all say it’s more stable and machines nicely. I’ve never used it but have used 4140 and like it a lot.
https://www.speedymetals.com/information/Material37.html
Only thing I would probably do differently is I’d press the bronze nut in place with a little loctite and then lock in place with the set screws.

 

[benmychree]

Stressproof is a positive joy to machine, but not so hard as 4140HT, but would be fine for a tailstock quill.

 

[graham-xrf]

Stressproof is a positive joy to machine, but not so hard as 4140HT, but would be fine for a tailstock quill.

Guys - thanks so much for the replies. Try not to laugh when I admit to being somewhat "YouTube-Trained". I know there is all sorts of potential for messing up doing that, but there are also lots of eye-openers for one like myself.
Dudley Toolwright has two videos where he explores the cutting of 4140 in 3 stages of hardness, two cutting speeds, for both negative and positive rake cutters.

For great entertainment on a property of 4140, WBF777 publishes how to chop up a big chunk of 4x2 with a 4140 tomahawk, then shave hair with it (the tomahawk)! It makes me think the average Gillette could be made of better stuff!

I don't know how hard a quill should be, but my internal reasoning suggests that drill male tapers are plenty hard already, and maybe want to be set into a not-so-hard quill. The friction in there would be better if one of the surfaces was slightly more compliant. I will be going with the temper standard as supplied.

In my internet searches, I came across a new thing to learn about. From the sentence..
"Blaze or consecration hardening of EN24T can accord a case acerbity of 50 HRc or higher."

OK -so I don't yet know what on Earth is "consecration" hardening, and "acerbity" is a new word for me. The mischief in my cartoon mind is right now showing a guy with a blazing torch in one hand, Bible in the other, offering up a prayer before applying the fires of Hell!
Surface flame hardening - right?

 

[Tim9]

I don’t think a tailstock quill needs to be hardened. I’ve seen quite a number of experienced YouTube machinist with big followings make quills and they just used a decent steel. Nothing that special about quills in my opinion. The MT adapters don’t seem that hard to me. Most of my MT stuff doesn’t appear that hard either.
Honestly, I think you’d be just fine using any decent steel... Leaded steel or 4140. I doubt it will make much difference one way or another for most hobby guys.
My old SB quills steel seem kind of soft also. I wouldn’t overthink this project.

 

[graham-xrf]

I made a (missing) quill for a Colchester Bantam out of (I assume) 4140 PH.....

Decoding "PH". Does that mean "Partly Hardened"? Do I need to know something?
edit .. Ah.haa - would that be "Precipitation Hardening"?

The little scrote who stole the quill didn't get the wheel.....Would have been an apprentice,so he probably didn't know how to unscrew a hex screw....

John, I do feel for you. I know that among folk in this forum we can expect that some thieving git making off with an essential part of a working machine (especially your Colchester), will evoke more than average disgust! He deserves a special spot in that place where "consecration hardening" is done!

..........edit ,you have a lathe working,use it as a horizontal mill.....slow cut in PH tho,plenty of suds,or cutter will burn.

So that would be .. clamp the quill up in V-blocks onto a right-angle plate or something, in place of the compound, and put a MT3 mill cutter into the spindle? A main question right now is, does one complete all the ACME thread bushing, except for final assembly with Locktite and side screws before milling the slot, or can we mill the slot first?

 

[graham-xrf]

My old SB quills steel seem kind of soft also. I wouldn’t overthink this project.

Thanks Tim.
I do agree that the SB quills, though tough, also seem kind of soft, which is why I was asking if anyone knew what they were made of. I am sure that any of the replacement steels we have been discussing would be an "upgrade" anyway.

"Overthinking" the project is the natural compensation for one who starts out not knowing very much. I guess I am also prone to ending up with "over-designed" solutions to most things I attempt.