Mini-Lathe Repair

[egaParticipant @ega]

Presumably, you are using the same material as before? ie comparing apples with apples.

[Ron LadenParticipant @ronladen17547]

Martin, I did run it in after changing the gears but I didnt check it for temperature. Clocking the spindle is a good idea as I clocked it when the lathe was new so have something to compare it with as well as checking it for movement..

Ega, yes the same material free cutting steel and 6082 alu.

Ron

[Martin Shaw 1Participant @martinshaw1]

Ron

As a matter of interest I changed the headstock bearings for the angular contact ones which you can preload and will run better than the originals. It is a bit of a faff stripping the headstock and you should be aware that the bearings are a tight fit in both the headstock and on the spindle, I thought it a good idea to polish the spindle for the rear bearing so that there is a possibility of movement when adjusting the preload. Having said that I'm into the nth mod to get the whole thing usefully productive, never again, nor would I recommend a mini lathe they are just not well enough made.

Regards

Martin

[Ron LadenParticipant @ronladen17547]

Posted by Martin Shaw 1 on 23/01/2019 18:40:39:

Ron

As a matter of interest I changed the headstock bearings for the angular contact ones which you can preload and will run better than the originals. It is a bit of a faff stripping the headstock and you should be aware that the bearings are a tight fit in both the headstock and on the spindle, I thought it a good idea to polish the spindle for the rear bearing so that there is a possibility of movement when adjusting the preload. Having said that I'm into the nth mod to get the whole thing usefully productive, never again, nor would I recommend a mini lathe they are just not well enough made.

Regards

Martin

Thanks Martin, angular contact and preload,? can you explain a bit more. Also the tight bearings..? did you manage in the shop or did you have the use of a press.

Ron

[Martin Shaw 1Participant @martinshaw1]

Hi Ron

I'll do my best. The bearings fitted at manufacture are standard ball bearings, undoubtedly as a cost measure since they're cheap, are ideal for highish speed radial loadings, however I'm sure you'll appreciate that the action of a tool into a workpiece provides a significant axial loading which they are not so good at. This is the cause of chatter because the cage and balls are forced away from the path they would naturally wish to follow. Angular contact bearings use inner and outer races that are sections of a cone and rely on preload to ensure that thay are running true before the action of an axial load. The idea two c nuts on the spindle rear are in effect a preload adjustment and a locking nut, and ideally you want a situation where there is sufficient preload to make the bearings run correctly without too much that would cause them to overheat.

When I changed mine tapping the spindle out took the front one out at the same time, the rear one required some suitably sized tube and since they were scrap I just battered it which worked, The new bearings permit by design installation of the outer races into the headstock by virtue of careful alignment and bits of wood, the front inner race used a combination of tubes and the rear inner was almost a tight sliding fit with everything else in place. Adjusting the preload was pretty much common sense and gut feel rather than anything more technical, difficult to describe but if you've any feel for mechanical stuff, probably intuitive. Best I can do, hope it helps.

Martin

PS Arc have a downloadable bit on their website with pictures that may help.

Edited By Martin Shaw 1 on 23/01/2019 21:30:16

[Chris TriceParticipant @christrice43267]

Martin, I think you mean taper roller bearings. Angular contact bearings are similar to conventional ball bearing races but they're designed to take axial loads.

[Brian GParticipant @briang]

Arc's guide to fitting angular contact bearings. **LINK**

Brian

[Martin Shaw 1Participant @martinshaw1]

Chris

Your quite correct, some vin rouge had been taken.

Martin

[Ron LadenParticipant @ronladen17547]

Thanks Martin and to Brian for the link.

I am thinking it may be as well to replace the bearings with angular contact type regardless though it would appear that the original bearings are ok, well as far as I can tell.

This morning I backed off the two spindle locking nuts just in case I had over tightened when I changed the gears. I mounted a 20mm steel bar in the 3 jaw to check for play or end float but found none. With the lathe in neutral avoiding the drive assy the bearings felt smooth and free, no roughness.

I clocked the edge of the spindle chuck location flange and it was just the same as when the lathe was new 0.01mm for the edge and 0.015mm on the flange face, which is excellent I doubt it gets much better than that on a mini lathe.

So I am at a bit of a loss where the lack of a good finish is coming from, it would appear not the bearings and having checked and adjusted from the carriage up I,ve run out of ideas.

The one thing I do know its damn frustrating.

[Paul KempParticipant @paulkemp46892]

Ron,

Can you pinpoint when the finish changed? Coüld it coincide exactly with when you changed the gears? I think you did say the lathe was slightly noiser now than previously. Could this be another effect of the gear material? Ie the steel gears giving a different / higher torsional vibration at the spindle? No idea if this would be the case, pure speculation.

Paul.

[KettrinboyParticipant @kettrinboy]

Hi Ron

what sort of bad finish are we talking , vibration marks or surface tearing of the material ? , as the lathe was doing good finishes before the mods you have done to the drive recently may have introduced some vibrations that were not there previously or surface tearing suggests a tool angle or cutting clearance problem, when I grind a tool I get the magnifying glass on it to check the cutting edge as its easy to get in particular the side rake a bit too vertical so then it wont cut well or give a good finish , also after grinding its usual to stone the tool to finish it and that can also lose the sharpness and clearances if not done with care.

regards Geoff

[Ron LadenParticipant @ronladen17547]

Posted by Paul Kemp on 24/01/2019 09:37:26:

Ron,

Can you pinpoint when the finish changed? Coüld it coincide exactly with when you changed the gears? I think you did say the lathe was slightly noiser now than previously. Could this be another effect of the gear material? Ie the steel gears giving a different / higher torsional vibration at the spindle? No idea if this would be the case, pure speculation.

Paul.

Hi Paul,

Thats a good point, though I have had some good surface finish since I changed to metal gears but has something changed now the gears are bedded in..? Although I can find no play or end float in the spindle and the bearings feel smooth by hand I did wonder that if they have deteriorated a touch is that showing up when running..? I dont know.

Ron

Edited By Ron Laden on 24/01/2019 10:04:32

[Ron LadenParticipant @ronladen17547]

Posted by Kettrinboy on 24/01/2019 09:48:12:

Hi Ron

what sort of bad finish are we talking , vibration marks or surface tearing of the material ? , as the lathe was doing good finishes before the mods you have done to the drive recently may have introduced some vibrations that were not there previously or surface tearing suggests a tool angle or cutting clearance problem, when I grind a tool I get the magnifying glass on it to check the cutting edge as its easy to get in particular the side rake a bit too vertical so then it wont cut well or give a good finish , also after grinding its usual to stone the tool to finish it and that can also lose the sharpness and clearances if not done with care.

regards Geoff

Thanks Geoff,

I will try and get a close up picture of the finish, you guys can probably identify the type of problem.

Ron

[Niels AbildgaardParticipant @nielsabildgaard33719]

Posted by Ron Laden on 24/01/2019 09:23:58:

So I am at a bit of a loss where the lack of a good finish is coming from, it would appear not the bearings and having checked and adjusted from the carriage up I,ve run out of ideas.

The one thing I do know its damn frustrating.

A photo or two of setup will help us old wise guys.

[Anonymous]

Plus one on the above. I very much doubt the problem is to do with bearings; summat else has changed. A picture and description of the 'poor' finish will definitely help narrow things down.

Andrew

[JasonBModerator @jasonb]

May also be worth turning it over with the belt off but in gear to see if you can feel any tight spots in the new gears.

[Paul KempParticipant @paulkemp46892]

Posted by Ron Laden on 24/01/2019 10:04:15:

Posted by Paul Kemp on 24/01/2019 09:37:26:

Ron,

Can you pinpoint when the finish changed? Coüld it coincide exactly with when you changed the gears? I think you did say the lathe was slightly noiser now than previously. Could this be another effect of the gear material? Ie the steel gears giving a different / higher torsional vibration at the spindle? No idea if this would be the case, pure speculation.

Paul.

Hi Paul,

Thats a good point, though I have had some good surface finish since I changed to metal gears but has something changed now the gears are bedded in..? Although I can find no play or end float in the spindle and the bearings feel smooth by hand I did wonder that if they have deteriorated a touch is that showing up when running..? I dont know.

Ron

Edited By Ron Laden on 24/01/2019 10:04:32

Ron,

That being the case – you have had a good finish since changing the gears then the probability is this is not related to bearings or gear material / vibration charachteristics. I think another look at tool, speed or feed is the next logical thing.

Paul.

[Ron LadenParticipant @ronladen17547]

A couple of pictures below of the finish I am getting, I had to enlarge the images so they may not be the best.

First picture is 20mm free cutting steel, brand new carbide insert, .005" cut, speed 1600 rpm, didnt measure the feed but it was good.

Second picture is 20mm 6082 alu, HSS knife tool freshly sharpened and dressed on a diamond card, 003" finishing cut, speed 800 rpm.

They both appeared to cut really well but not the finish, I have just noticed that despite the different material and different cutters they look very similar.

[SillyOldDufferModerator @sillyoldduffer]

Looks like something I made!

Some suggestions:

• the tool is too high or otherwise rubbing rather than cutting,
• tool overhang is too big causing slight chatter,
• swarf is failing to clear and getting trapped between the edge and the work.
• lathe is vibrating – I suppose the bearings might have been damaged during installation, but your posts don't suggest you're a cack-handed brute with a big hammer! So I doubt the bearings are shot, but you might have been unlucky. Try holding an oily finger against a smooth rod while the lathe is running – you might be able to feel any untoward vibration. Also, put a mug of water on the bed and bounce a torch beam off it on to the ceiling. Then run the lathe up and down the speed range. The amount of ripple seen on the ceiling may be obviously excessive or become much worse at particular speeds. If so, may indicate a problem in the motor, drive belt, gear train, or a resonance due to a mounting problem. (Anyone know a better way of detecting vibration?)

Might help to post some close-ups of the tools as well.

Dave

[Neil WyattModerator @neilwyatt]

For carbide, if it is a CCMT insert you want the depth of cut to be at least 2/3 the tip radius.

You can take smaller cuts with CCGT

Neil

[David Standing 1Participant @davidstanding1]

Ron

Are you using the tool slide or carriage wheel to advance the cuts?

I note you have been playing with the gib strips

Try locking down whichever axis you aren't using to see what effect that has – if you are advancing with the tool slide, lock the carriage, if you can, and vice versa.

Also try winding on your next depth of cut, and then locking the cross slide before advancing the cut.

Just takes some of the variables out of the equation.

[Anonymous]

The torn finish on the low carbon steel is exactly what I'd expect for an insert with too low a DOC and feedrate. Similarly for the 6082 I'd say tool geometry is the problem.

Andrew

[Ron LadenParticipant @ronladen17547]

Posted by Andrew Johnston on 24/01/2019 16:28:18:

The torn finish on the low carbon steel is exactly what I'd expect for an insert with too low a DOC and feedrate. Similarly for the 6082 I'd say tool geometry is the problem.

Andrew

Andrew, the insert is a CCMT and I know it goes against all the theory but I have cut .005" with the same settings on low carbon steel previously and not had an issue. I will try a .020" cut, the material is 20mm, what settings do you suggest and I will give it a try.

The knife tool I used on the 6082 has 5 degree angles all round except I increased the top rake to 10 degrees for cutting aluminium, again it has not been a problem previously and always produced a good finish.

Ron

[JasonBModerator @jasonb]

Ron if your tools have not changed and you got a good finish after changing the gears could it be something to do with the new belt/pulleys?

Are they a good fit on the shafts and running true? May also be worth playing with belt tension a little either side of where you have it now.

Having said that it does look more of a tool issue. On the steel the brighter bands are where the tool is cutting but then it is getting pushed off the work or the work is getting deflected and that gives the poor areas. The burrs on the aluminium also point to the tool not cutting well. Try a facing cut with the CCMT yo check it is on ctr height then a deeper cut along the length say 0.25mm

Edited By JasonB on 25/01/2019 08:19:30

[Ron LadenParticipant @ronladen17547]

Posted by JasonB on 25/01/2019 08:01:07:

Ron if your tools have not changed and you got a good finish after changing the gears could it be something to do with the new belt/pulleys?

Are they a good fit on the shafts and running true? May also be worth playing with belt tension a little either side of where you have it now.

Hi Jason, the new pulleys are not fitted just yet, I have just received the 3mm tool steel so I can now cut the motor pulley keyway and get them fitted. I have also done as you suggested and removed the current drive belt to see if the gears had a tight spot but they seem fine.

I just know something has changed but as yet I cant fathom out what. I accept that I am probably not using the insert cutter at its best settings but my old faithful knife tool would always give me a superb finish on aluminium but you can see from the pics what I,m getting now.

Ron

[Author]

Posts