Extension shaft to x-axis feed screw for stepper motor drive

[Adam HarrisParticipant @adamharris13683]

The lathe is an Emcomat 20 which is fairly heavy duty (125mm centre height over cross slide, 4.5kw motor). I am fitting an extension shaft to the far end of the cross feed screw to couple with a Nema 34 stepper motor driveshaft in-line. The feed screw is trapezoidal TR16x3. The extension shaft is bright steel 13mm diameter and 120mm long with 25mm at end threaded to M8 . I have tapped the end of the feed screw M8 with a depth of 25mm. My plan was to put a deep chamfer on the threaded end of the extension shaft in order to fill with weld after screwing into feed screw. The issue is that I have no idea of the spec of steel of the feed screw, but it did drill and tap surprisingly easily so definitely not hardened much. Emco are always extremely unhelpful on anything to do with what they regard as proprietary specs (terrified they may lose a hugely profitable sale of an Emco part to a Chinese maker). On Google AI I find “Lathe feedscrews and leadscrews are typically made from medium-carbon alloy steels or free-machining steels that offer a balance of strength, wear resistance, and machinability. The most common materials used for making new feedscrews are 1144 Stressproof steel, 4140 alloy steel, and 12L14 leaded steel”.  Assuming it is one of these three, further googling says for each one that it is not at all good for welding without specialist equipment. So the question is should I ignore Google AI and employ my trusty Oxford Oil Welder, or should I rely on Loctite Red 271 thread sealant (or similar/better?). I’m not sure about putting a pin through the threaded joint as I don’t want any interference with the feedscrew nut and if I keep it short to avoid that, the thickness of metal between Minor Diameter of the trapezoidal thread and Major Diameter of M8 thread is no more than 2mm (in radius). Any thoughts much appreciated.

[not done it yetParticipant @notdoneityet]

25mm is presumably hardly, if anything, through the lead screw support bearing?

 

A suitable permanent thread locker should be sufficient if used properly over a 25mm thread, I would have thought.  I loctited a stud into the end of my

milling machine when I added a power feed.

[Adam HarrisParticipant @adamharris13683]

The rotational force of the stepper motor will be tightening the thread in one direction and loosening the thread in the opposite direction, repeatedly and frequently albeit without much force resistance from the cross slide (the Nema 34 size is massive overkill but it is what I have in the cupboard)

[Adam HarrisParticipant @adamharris13683]

The 25mm M8 threaded part will not be in contact with the to-be-fitted feedscrew bearing in the wall at the far end of the feedscrew pocket. This to-be-fitted bearing is for shaft alignment and will not be supporting the weight of stepper motor.

[Adam HarrisParticipant @adamharris13683]

There is no reason that i could not make the M8 threaded connection longer for greater strength using a thread locking glue/sealant. I originally thought 25 mm length sufficient without having to worry that it weakens the leadscrew.

[Adam HarrisParticipant @adamharris13683]

The total trapezoidal thread of the feed screw is 350mm long

[Adam HarrisParticipant @adamharris13683]

There is currently no additional feedscrew support bearing other than at the handle end. The other support is the feedscrew nut itself.

[BazyleParticipant @bazyle]

Radial drilled hole, taper reamer, pin.

[Adam HarrisParticipant @adamharris13683]

The pitch is 3mm, the gap between threads is 1.4mm so drilling for a pin (say 3mm or 4mm diameter) is going to be very difficult in terms of avoiding threads or even just achieving a straight hole.

[Robert Atkinson 2Participant @robertatkinson2]

A 25mm length of M8 with Loctite 270 high strength threadlock will be more than strong enough. Even a long NEMA 34 stepper motor (useless to just specifiy the frame number, could by any type or length of motor). will be less than 15 Nm torque and just a standard nut with 270 will hold at least twice that. Retainer type 601 or 638 would also do the job.

Robert.

[Adam HarrisParticipant @adamharris13683]

Thanks Robert, very nice to hear that! I’ll skip the welding then. The Nema 34 incidentally is 99mm long and 7.7Nm which is more than twice what I need I should think.

[Adam HarrisParticipant @adamharris13683]

Just bought some Loctite 270

[HuubParticipant @huub]

I did the same for both my lathes. Drilled a hole and soft soldered (3/97 silver/tin solder) the extension in to it. So not threaded.
After soldering I put the shaft in the chuck locked the X-axis bolts and used the compound slide (90° rotated) and carriage to turn the extension to dimension with minimal runout.

Nema34 is quit heavy for the x-axis.

Welding may be difficult because not all steel is easy to weld. Due to the temperature stress, the shaft may get crooked.

[Adam HarrisParticipant @adamharris13683]

Huub the Nema 34 is a bit heavy but I happen to have it already sitting in a cupboard and with its driver and transformer. The carriage is heavy duty and capable of carrying a heavy duty mount for it. Rocketronics recommend 3.3Nm for x-axis on lathes over 400kgs and I think they are even then being overly germanic because on the Z-axis they recommend 20 Nm !! My z-axis is driven by a duplicate Nema 34 giving 7.7Nm and works just fine. My lathe is 850 kgs. Incidentally I am using the French ELS which I am very pleased with and is a fraction of the cost of the Rocketronics ELS as well as being a smaller form factor which I like. It does pretty much everything the Rocketronics offers

[Robert Atkinson 2Participant @robertatkinson2]

Good to hear you are happy with the French Electronic Lead Screw. I have the single axis model but have not finished the installation yet. Too many projects..

I forgot to give the reference for the Loctite 270 so here it is https://www.henkel-adhesives.com/ae/en/product/threadlockers/loctite_2702.html

It will hold 33 Nm on a M10 nut according to the datasheet. So >20 mm length of M8 will easily hold 8 Nm. If you do need to unscrew it then heating with a hot air gun is recommended.

Robert.

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