Threading on Old Mitchell Lathe

[s.d.nano]

We purchased an old Mitchell lathe a while back and have been running it as a general purpose maintenance lathe. I am now at the point where I want to do some slightly more advanced things and am running into some difficulties. I am research engineer and not a machinist so apologies for my gaps in knowledge. I spent a lot of my free time at university in the machine shop hanging out with the staff there but I have no formal training. Yes, it needs a clean, yes, a three jaw chuck held in the 4 jaw is janky and has a lot of runout, I am not the only user of this guy...



To the issue: The threading chart seems to bear little connection to what actually happens on the workpiece.


The change gear arrangement is as follows:


I am trying to make an M16x2mm drawbar for the mill I just purchased (It has a 40 taper and 5/8"-11 drawbar and would like to run cheap import tooling). The change gears that came with the lathe seem to be 12DP with a 3/4" bore. I only have those pictured available currently which are 40, 40, 42, 42, 48. I imagine these can be found used or from McMaster etc. if I need more, or someone on Alibaba sells sets I could substitute in.

Anyways, the chart lists 4 gears which is my first confusion, I see only one driver and one driven gear in this arrangement. The second problem is that of course I don't have a 45 or 60 tooth gear available. I tried running it with 42 and 40 and it wasn't even close, roughly 2.6mm pitch. I messed around with settings and couldn't get it close. I tried with 48-42 which got me close enough (levers set A-D but in position 6) that I could probably run a die over a shallow thread and get something workable.

Time is on my side here so if possible I would like to understand what is going on at minimum and hopefully grab a gear off ebay to get me 100% of the way there.

Thank you for the help.
-Stephen

Also, if you have battery or chemistry questions I am the person to ask.

 

[Tyrone Shoelaces]

The three jaw in a four jaw is possibly because the gap piece appears to be missing. Why are you bothering to single point an M16 thread ? A stock and die would do fine.

By the way, I’d get all that crap out of the tray.

Regards Tyrone.

 

[s.d.nano]

The three jaw in a four jaw is possibly because the gap piece appears to be missing. Why are you bothering to single point an M16 thread. A stock and die would do fine.

Regards Tyrone.

It it definitely an option but my history with cutting threads even remotely straight with a die is not great. I guess I could set something up with the tailstock?

Three jaw in the 4 is because my coworker can't dial in a 4 jaw chuck. I didn't even know there was a piece missing until you pointed it out. That makes a lot more sense now. It would be long gone, although maybe a fun project if I have a lot of down time and can find some material to make one out of (so probably not).

 

[Clive603]

Mitchells are a bit "odd" when it comes to threading and feeds. Can't think of any other make that has fractional and diametrical pitches on the plate.

With what you have closest you can get to 2 mm will be 12 tpi using a straight 1:1 ratio drop gear train of 42-48-42. Maybe good enough to mate with a cheap import nut but the pitch error will be too great for a strong drawbar thread and it won't clean up properly over any useful distance with a die.

Realistically you need to find 45 and 60 tooth gears which will complete the normal set and give you 2 mm pitch directly off the box.

I suggest you figure out what the feeds are for various set-up and make up your own chart. Trying to do it on the fly from the plate data will likely make your ears bleed!

Clive

 

[s.d.nano]

Mitchells are a bit "odd" when it comes to threading and feeds. Can't think of any other make that has fractional and diametrical pitches on the plate.

With what you have closest you can get to 2 mm will be 12 tpi using a straight 1:1 ratio drop gear train of 42-48-42. Maybe good enough to mate with a cheap import nut but the pitch error will be too great for a strong drawbar thread and it won't clean up properly over any useful distance with a die.

Realistically you need to find 45 and 60 tooth gears which will complete the normal set and give you 2 mm pitch directly off the box.

I suggest you figure out what the feeds are for various set-up and make up your own chart. Trying to do it on the fly from the plate data will likely make your ears bleed!

Clive

Thank you for the tips. I think I can track down the gears, it is just a matter of finding them at a reasonable price. I was quoted $3000 a gear by a company called rush gears (who do have an excellent gear design tool online) which is several times more than we paid for the lathe.

I see them come up on ebay and kijiji from time to time so hopefully the correct sizes come up. I was also mistaken about the 3/4" bore, it uses a 1" bore with a 1" face, slightly harder to find unfortunately.

 

[Clive603]

Three grand a gear. That's outrageous.

Standard DP and standard numbers so just buy two flat gears and bolt together if you really need 1" face. Looking at the plate odds are they are only going to be used once in a blue moon so I'd not bother about face width. Just fix a spacer each side so they line up and fill the bearing part of the stud. Power transmission is very low so the width is wasted anyway.

Or go the uber economy route with cheap plastic gears bolted to a hub. I would get enough to make up the width though. Might not be engineering to a standard that will make Inspector Meticulous happy but it will see you out.

Clive

 

[john.k]

most of these old British lathes use the same size change gears ......so gears from a Lang or a Swift will also fit .........as to gears ,I got a quote from a local gear place for two gears from a Coles crane gearbox .......around $1500 a gear .....stick that ,sez I ,and eventually found a pair of gears in an old (1947) bus gearbox (Albion )..........proper hardened and ground gears for truck duty ,not some crap made of soft 4140PH that the gear cutters wanted to use.

 

[Tyrone Shoelaces]

It it definitely an option but my history with cutting threads even remotely straight with a die is not great. I guess I could set something up with the tailstock?

Three jaw in the 4 is because my coworker can't dial in a 4 jaw chuck. I didn't even know there was a piece missing until you pointed it out. That makes a lot more sense now. It would be long gone, although maybe a fun project if I have a lot of down time and can find some material to make one out of (so probably not).

You can buy die holders with a Morse taper shank that will go in the tailstock barrel. The only problem is the amount of travel of the die is limited to about 6” . Good enough for what you want though.
In the past when I’ve been stuck for the correct holder I’ve trapped a conventional die stock with the end of the tailstock and rested one of the die stock arms on top of the top slide/compound slide. Run slowly and follow the die stock down the bar with the tailstock barrel to keep the die running straight. Use a cutting compound.

When you’re smart enough to make a gap piece that actually fits you won’t need to ask questions on here, you’ll be answering them !

Anybody who charges $3000 for a gear you can hold in one hand needs some jail time, you can quote me on that.

Regards Tyrone

 

[4GSR]

I bet somebody there has a 3-D printer. Get the STL file from Rush Gears website, extrude it to the width needed. Then run it through the 3D printer slicer and print it. For a one time use it will work just fine. If you don't have a 3-D printer available, I bet someone here can print you one for a small price.

 

[s.d.nano]

I bet somebody there has a 3-D printer. Get the STL file from Rush Gears website, extrude it to the width needed. Then run it through the 3D printer slicer and print it. For a one time use it will work just fine. If you don't have a 3-D printer available, I bet someone here can print you one for a small price.

Good point, I am sure I can get a model of a 12DP gear pretty easily. McMaster has the 60 tooth on there already so it is just the 45 I have to put some effort into finding.

You can buy die holders with a Morse taper shank that will go in the tailstock barrel. The only problem is the amount of travel of the die is limited to about 6” . Good enough for what you want though.
In the past when I’ve been stuck for the correct holder I’ve trapped a conventional die stock with the end of the tailstock and rested one of the die stock arms on top of the top slide/compound slide. Run slowly and follow the die stock down the bar with the tailstock barrel to keep the die running straight. Use a cutting compound.

When you’re smart enough to make a gap piece that actually fits you won’t need to ask questions on here, you’ll be answering them !

Anybody who charges $3000 for a gear you can hold in one hand needs some jail time, you can quote me on that.

Regards Tyrone

Thank you for all the help. I checked, $3075 actually, lol.
I'll grab one of those threading set-ups, it is cheap enough the boss won't question it.


Thank you everyone for the help, I think I have a pretty clear direction now both short and long term.
-Stephen

 

[jim rozen]

You start the die straight and true by leaving a portion of your stock just over the minor diameter of the thread. You could use a regular die stock for this. After the thread is cut, you trim off the extra portion.

You're probably over-thinking, wanting to use the lathe to make this.

 

[strokersix]

Source your threads by cutting off an m16 bolt and welding it to your drawbar. Select a longish bolt so you can use the unthreaded bolt shank for alignment and to keep the heat away from the threads. An option to consider if you feel confident with the welding.

Friction weld would be neat but would require some trial and error to get it right.

Bolt metallurgy might be a wild card for welding.

As always, just a suggestion. You must decide and accept risks!