Lathe spindle alignment

Hang the expense … try turning bit of nice Brass

MichaelG.

Thanks Howard,

I meant to say in the previous post that this was attempted with both HSS and carbide insert (the sharp aluminium type). Also with various speeds from 500 rpm up to 2000. I've also swapped out the insert and I've tried cuts from wafer thin to 0.5mm.

The insert bar has worked well in the past and is my goto bar for boring. What I'm doing is duplicating a piece (45mm long, OD 20mm, ID 16mm) which I'd made before the changes which work OK. That really was EN1A, but now run out .

I've ordered some more EN1A Pb and will re-try with that before I consider stripping the lathe again.

Iain

I think we swapped Michael. I will be patient and try the EN1A.

Iain

The 45 mm long part has a 2 mm wall thickness. That results in a lot of flex especially when turning the inner diameter. Not an easy part to make!

If possible, I would turn the inner diameter first and mount the part in a 4 jaw chuck (no part stick out) to reduce the flex of the part. Bore the hole 15 mm thick and finish using a 12 mm inner turning bar. Let the inner turning bar stick out 47 mm max. Select a feed/rev as slow as possible. On my mini lathe in manual mode, that is 0.1 mm/rev. Bring the stock to final diameter using a cutting depth of 0.05 mm or even less. A sharp insert (for alluminum) should work fine.
If there is chatter, reduce the RPM and cutting depth if possible.

On my mini lathe, I only have a 3 jaw chuck that has a 16 mm bore. So the part would stick out quit a lot. I would follow the same procedure but turn CNC at a feed of 0.02..0.05 mm/rev.

Hi, Huub.

The one I've made was OK apart from narrowing towards the chuck by about 0.03 mm. I 'fixed' this with some emery and it was at least good enough for a prototype (a high speed milling spindle).

What I'm actually doing is drilling out to 14mm and then boring the remaining 2mm with the last passes being very fine (helped by some telescoping bore gauge).

Then bring down the outer dia (from 25 / 27 mm depending on the stock) to 20mm.

I'm holding a piece of 65mm bar in my 3 jaw and parting it off after the turning and boring, then turning it round in a 20mm ER32 collet to face off - or perhaps I thought the parting off was good and I just finished on some emery. Can't remember now.

Sadly, autofeed on this lathe is not particularly fine and is a pain to set up. I don't use it. Instead practicing a steady hand - well eventually it will get steady!

Iain

If you can hold a 65 mm bar in the 3 jaw, your lathe and tool post are probably rigid enough to hold a long boring bar.

Putting a stepper on the lead screw solved the gear switching on the mini lathe. My bigger lathe has besides the change gears, a 2 and 3 position lever to select 6 different feeds.

If it need to be accurate, I use the CNC to turn an opposite taper. That requires 3 finish passes and 2 thickness measurements. That is a lot easier and faster than setting up the alignment.
For accurate drilling you still need an aligned setup.

High speed contact angle bearings are pretty expensive and requires very accurate machining. That is beyond the capabilities of me and my lathe.

Edited By Huub on 15/06/2023 18:48:51

If you get 0.03 mm narrowing when using the top slide, then you have to check the alignment of the top slide.

• Put a thick bar in the chuck.
• Mount an indicator on the bar using a magnetic holder.
• Set the indicator on a point of the moving part of the top slide and zero the dial (or note the value). Mark the position of the indicator on the moving part of the top slide.
• Feed the top slide 50 mm and use the carriage to reposition the indicator at the marked position. The indicator will now show the alignment of the top slide.

Is your boring bar small enough for the 14mm hole? If you use too big a boring bar the heel of the bar can rub on the inside of the hole and cause the kind of serrated finish you mention. For 14mm hole you might need a small insert boring bar with 6mm or 8mm shank or so, not a big fat 12mm shank.

Also, slowing down the rpm for boring can help reduce chatter.

Hopper,

You are right to check if the boring bar is small enough. I mark the "critical sides of the boring bar" with a sharpy and check if the boring bar doesn't contact the hole.

I have a 12 mm boring bar (S12M SCLCR06, CCMT060204 insert) that fits a 14 mm hole.

The CCMT0602xx boring bars are the "smallest". For larger diameter holes, I prefer the DCMT0702xx inserts in a Q style holder because chips don't build up so easy on Q type insert holders.

Huub - not using a top slid and I suspect my boring bar is the same as yours.

Which makes me ask a question. Previously I've drilled out to 14mm with a blacksmith drill, but not been happy with the finish or accuracy. I'm asking myself if I stopped at 12mm this time with a 'good' drill. Perhaps that's the root issue. I will put a 14mm hole in and see if that makes things better. I don't think I saw any rubbing (and I think I checked), but don't trust my memory).

That will be embarrassing if that's the case!

Iain

From roughing (drilling) to finishing (turning) it takes 3 passes at an equal cutting depth if the setup (not a rigid lathe, large tool stick out, large thin stock stick out) is not rigid. Getting a decent finish under these conditions is always a challenge. You have to reduce the cutting forces to succeed. Reducing the feed/rev, depth of cut and to some degree, the RPM is the only thing you can do.
The insert is also important if you can't run under the conditions advised by the manufacturer. In general, I use finishing inserts on my not rigid desktop lathes. Large (DCMT11xxxx, WNMG08xxxx, etc) inserts are generally made for heavy turning conditions. Small inserts perform better under light cutting conditions.
I have 3 boxes of TCMT110204 VT15TF of the same brand and supplier and 3 boxes of other brands and suppliers. One of the 3 boxes gives an outstanding finish, a lot better than the other 2 boxes and the best of all 6 boxes of this insert type I have. So I use this box only for making gear gutters. All my inserts are from web shops and even the branded are probably counter fait. Never the less, 8 of 10 boxes perform well for my hobby use.
When nothing works, I use a CCGT060204 H01 (alluminum) insert to do the job.
I use 0.4 and 0.8 mm tool nose radius but I am going to test a 0.02 mm tool nose radius because I think these will do a better job (lower cutting forces) on my lathes.
You could also try a HSS tool, that should also give a good finish under light cutting conditions.

Have you thought of using a reamer. It is hard to match their finish using turning. I only have H7 reamers (+0.02 mm) but there are reamers for press fit holes.

You don't need a press fit for a bearing. You can also glue the bearing using lock tide.

Well. It turns out that I am an idiot.

I started by drilling out to 14mm, but there was still seem screeching. So I checked the tool height and it was a little under. I fixed that and, once I'd worked out that the best speed was around 1000 rpm, I got a good finish.

The lathe still turns to be narrower at the chuck end but is 15 microns or so better. I've tuned the outside with some emery and it's within 2 microns along the length. The bore is good enough for a 'light press fit' at both ends.

Huub I'm using the CCGT060204 (aluminium) and it's pretty much all I use for normal cutting and boring - HSS with zero rake for bronze and brass, though and I have some very small pieces to turn (2mm dia) which I will probably use hss for.

I want a tight fit for the bearings to make sure concentricity is good. If you have a gap for loctite (I use 638 for bearings) then there is the chance of it being a bit off.

I don't think I've finished setting up the lathe, but I've other things to get on with in the short term.

Thanks for all the advice.

Iain

Ian,

Good to hear you have things working for you.

I have checked the minimum turning diameter for my 12 mm boring bar (S12M SCLCR06, CCMT060204 insert) and that is 13 mm.

Regards,

Huub

Glad that setting the sharp tools to centre height has solve the finish problem.

So when time allows bvack to iterating taking out twist and aligning the Headstock.

By the time that you've finished, it might be better tha when first received!

Howard