THK Linear Guides - Rigidity For Lathe

[yngndrw]

So I'm looking to build a lathe and I'm basing everything on the Emco Compact 6 / 120 / 220 lathes. I can't just pick up a used lathe as they are too big and heavy for me to bring get into my garage, so I'll just build one in situ.

To give some context, at a high level I just intend to buy a spindle and turret from one of these lathes. (Already have the 220P spindle on the way and I'm currently looking a either a turret from the 220 or a slightly larger turret from the 320.) To give an idea on size, the larger turret has a mounting base of around 120mm tall by 85mm wide and the work area for the 320 lathe is 176mm for the Z axis and 56mm for the X axis, so only small. The originals use ballscrews and box ways for both the Z and X axis, with the X axis mounted "upside down" with the longest part moving up and down.

My plan is to mount everything to a cast iron surface plate (About 24" x 18") which will give me a nice flat ferrous surface (For magnetic indicator mounts) and proper metal to drill and tap for mounting, then I'd cast some epoxy granite under this surface plate to build the slant angle and add mass / rigidity. For the Z axis, I'll use a ground ballscrew and some preloaded linear bearings, but for the X axis I've found a THK KR46 (https://tech.thk.com/en/products/pdfs/en_a02_116.pdf) which happens to be just about the right size. (It's not a standard length so it must have been cut down or custom made) I'd mount the THK KR46 upside-down (I.e. With the sliding block mounted to the Z axis and the body / rail / X axis motor moving with the turret) so that coolant / chips drain out, but it would be covered anyway. I can't use a larger THK KR linear guide as they are all far too long.

My question is whether or not the THK KR46 will be rigid enough for the X axis ? For reference, the standard Emco 220 spindle motor is a DC 2.6kW motor with a maximum spindle speed of 6,300RPM. It is meant to be constant torque upto 1,700RPM and constant power for the rest of the range, which by my calculation suggests that the maximum torque is 14.60Nm. (2.6kW @ 1,700RPM)

Here's the motor nameplate from the 120:
https://vignette.wikia.nocookie.net/...ns120motor.JPG

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[hanermo]

Yes.
No.
Depends.

A fast look did not show good details on the linear guides.

You want rigidity, that comes from size.
Definitely the lathe will work, but sf quality will depend on cut depth.

My 12x lathe with 2.5 kW industrial ac servo, is now fitted with 25 mm wide linear guides on x.

[yngndrw]

Apologies, I tried to be clever by linking to the size-specific .pdf but forgot that it doesn't include all of the load information. Here's the full document: https://tech.thk.com/upload/catalog_.../cat_kr_en.pdf While I'm into linking documents, here's the specs page out of the Emco 320 manual: https://image.slidesharecdn.com/6869...?cb=1432101738

I suppose the issue I'm having is translating the existing box ways into linear rails. For the X axis all I know is that the box ways on the original are about as wide as the turret base, so ~85mm with space in the middle for the ballscrew. The Z axis box ways aren't much bigger (~100m) but I can space mine out further as the space is available.

The experience you have with yours is very helpful for sizing the linear guides, how would you rate your 25mm linear guides ? I suppose the question is how is the balance between spindle power and the rigidity of your guides, is the spindle motor about right or do you need a more rigid machine to take advantage of it ?


Here are the details of the KR46 for reference:
Outer rail characteristics:
- Moment of inertia around rail X axis (Z axis in the lathe): 240000 (mm^4)
- Moment of inertia around rail Y axis (Y axis in the lathe): 150000 (mm^4)
- Rail mass: 2.5kg
Load ratings:
- Guide unit / Basic dynamic load rating C: 27400N
- Guide unit / Basic static load rating C0: 45500N
- Ball screw / Basic dynamic load rating Ca: 3140N
- Ball screw / Basic static load rating C0a: 6760N
Static permissible moment:
- Around lathe Z axis: 547Nm
- Around lathe Y axis: 547Nm
- Around lathe X axis: 1400Nm

[hanermo]

My saddle plate is 200 mm (or 180 mm) wide, 520 mm deep iirc.
F1 acero-calibrado tool steel, 20 mm thick.

12x lathe, spindle originally 1.5 kW industrial 220V EU, now 2.5 kW ac brushless servo.

I think my linear guides are the minimum useful, and know they are the maximum possible if I want to use my 2 toolchangers, even with custom offset reduced-height tools.
Yes, I will need to hang my tools below the 4-pos toolchanger plate height, by about 10 mm.
20 mm tool size, base, iirc.

The whole new-saddle assy has been a nightmare.
21 fitouts so far, and endless issues.
More than 300 work hours, for the saddle-assy, so far.

I considered junking the assy, and just fitting bigger rails and guides below/around the z bed, done.
And I still might do it.
Endless aggro and vastly hard fitments into blind mounts.

[yngndrw]

Thanks, it's great to have some real numbers to work with and to know your thoughts on how they perform. (Or don't !)

I'm trying to pick up a complete Emco 220/320 bed assembly so I can use the original ways and don't have to worry about sizing my own at all, but if I do I'll definitely make sure I go for something larger than 20mm - 30mm would probably be a good start from what you've written.

Good luck with your upgrades, hope you find a configuration that works well.