Lathe Rigidity Issues - Modification Opinions

I meant to add - how do you keep swarf out of the ballscrews?

Coming in LATE without having read all of the posts.

Has your lathe got Back Gear?

Unless my calculations are off, turning a 5" diameter at 150 rpm gives surface speed of 196 fpm

To my mind, that seems much too fast for steel, particularly with a wide tool, hence the chatter.

If no Back Gear, but belt drive, make up new pulleys and new belt o slow the speed to under 75 rpm?

What are your thoughts?

Howard

Another update on the build, finally got the top slide done, although I need to cut the bolts down. Note that the holes on the slide are offset to one end because I milled through some original holes to make the bolt pattern symetrical. I used an old HSS chamfer tool which left huge burs on the slots, they were not fun to file. Overall it looks pretty good, I turned about half the original mass into chips. The lathe also has a huge amount of travel on the slide and plenty of space for front and rear tooling.

To address a few questions and comments. Joes build on his CNC is absolutely amazing, can't wait to see it running, and really interested to see his tool changer design. Looks like he used the same supplier for his servo's as me, and that is the same spindle / servo I'm thinking about for the drive. Not worried about swarf on the screws, they have wipers, would be a little more concerned if I were doing iron or grinding. I might make a completely new head for the lathe as Hopper suggested and leave my own lathe build for another time. I'll make an adapter plate so I can swap this new head over to the other lathe in the future. I'm more concerned about spindle bore, which I need to pass 2 inch down. So my current design uses a 2.6" bore, 8 inch chuck and two 80 x 135mm taper roller bearings.

I have been thinking very hard how to make my next 62mm bearing bore lathe more capable.Main problem is to make a system of mandrels holding front bearing holder while Chokfast,Non-shrinking loctite or epoxy sets without locking up tool as well.

On the other hand it is only for the brave or rich as it will not be possible to go back.

You can se on picture that the bearing bore stop on front bearing hole has to go.Not a precission job but non recoverable.

While I am at it it is no big deal to show how it can look on a 290 lathe with 75mm bores.

Edited By Niels Abildgaard on 21/03/2021 08:38:55

Quick update, took me all week to find a bandsaw, so I could finally cut some blocks for the holders. Also waiting on some SKF spindle bearings, surprisingly the stock ones were NSK, but full of paint flecks. Might have gone a little over-kill on the holders, but will allow most tooling setups and a second spindle. Will be keeping the stock spindle and build a second spindle to bolt to the other end of the bed, for larger stuff. Waiting on my encode too, but hopefully in another week I'll be threading. I also really hate hand tapping holes now!

Interesting choice in mounting methods. I would have thought the slot for nut access reduced rigidity of the tool holders when the thread seems to be about increasing rigidity. Would through holes on the tailstock side of the holders for HT bolts or cap heads have been a better option?

Forgot about a Drummond mod which noticeably improved spindle stiffness

I hate to rain on Ed's parade but Jason is spot on.

Tony

I did fear the slot being close to the bottom would reduce rigidity, counter intuitive to what this thread is about. I chose studs over bolts at the back of the holder to reduce wear, / cheaper to replace, but in reality I probably would never wear them out. The base of the holder is 0.5 inch thick by 4 inch long, that would take a huge amount of effort to flex it, especially on a hobby lathe. Also consider a QCTP holder has about the same thickness underneath.

Worst case if this is a mistake then I will modify for long bolts and fill in the grooves with lead to dampen the holders. Might even do one as a comparison for my own curiosity.

.

Having worked in a vibration test-house ... might I be permitted an observation ?

Shorter bolts [HT Cap-heads] down deep counterbores, generally give a stiffer fixing than long bolts.

MichaelG.