lathe alignment - again

Sorry to raise this old chestnut again, but I have just bought my first lathe and need a bit of guidance regarding reducing concentricity error. I have installed a new Warco WM280V onto a Warco metal stand, which is in turn bolted down to a concrete floor, see pic. The floor seemed pretty flat, and the stand did not rock. I made a dumbbell test bar using 25mm mild steel secured in the 3 jaw chuck, with a 120mm central section reduced by 2mm, see pic. I then made a 30mm long 0.05mm cut on each of the dumbbell ends using a carbide tool (SWGCR1215H06) using powered feed at the lowest speed rate (0.07), and spindle speed of 400 RPM. Note the total length of bar beyond the chuck jaws was 40mm (to clear the chuck guard) + 30mm + 120mm + 30mm, so 220mm in total. I then used a digital vernier to measure the resulting diameters, with the result that the tailstock end dia. was 0.15mm larger than the chuck end. I repeated and got the same result again (yes, the definition of madness!). I then tried relaxing the fixing bolt at the front tailstock end, with the same result. Before I start shimming and so twisting the bed, I want to make sure I'm not doing anything fundamentally wrong. Key questions are: 1) is the overall arrangement OK, 2) is the bar too long 3) would it make much difference if I used an HSS tool. Another issue that I don't understand is that I used the DRO to measure the 0.05mm cut, so expected 0.1mm to be reduced from diameter, but the actual reduction was over twice that - so maybe I need to calibrate the DRO (thus far I've only switch it on), and may inadvertently taken taken cuts in the region of 0.1mm. I have had to stop tonight for reasons of domestic harmony, and plan to crack on tomorrow - any views / guidance / suggestions would be gratefully received!

Hi Steve

I would start by ensuring there is no twist in the cabinet before adding shims below the bed, use that as a last resort to get the lathe cutting the dumbell parallel.

Emgee

Thank you Emgee, another check to add to the list! Steve

Note that the DRO measures distance and therefore the cut depth around the circumference ie. twice the amount is removed across the diameter.
Before getting too involved you must allow for possible flex in the bar being cut. Some folk claim no more than 4 x diameter should be sticking out unsupported. A highly sharpened HSS cutter taking a minimum depth of cut would be a better tool in this instance....or a way thicker piece of material.

pgk

Steve,

Beware that the 0.15 mm error also includes the bending during turning. That can be significant when turning such a long bar.

I prefer using an aluminium tube (thick wall), an insert for aluminium (sharp) and a shallow cut of 0.02 mm.

I have made a few videos on measuring the alignment of a lathe. I hope it is of any help.

Measuring lathe alignment

Huub

Edit,

I have a similar lathe (290 kg) that rests on 2 stands (Left & Right). Assuming the bed was grinded on these stands, placing them on a flat surface without bolts the bed should be straight. So my lathe is not bolted to the table (just secured loosely) and the misalignment is about 0.016 mm/ 100 mm.

Edited By Huub Buis on 06/02/2022 22:02:54

The Myford user's manual recommends the use of a piece of 1 inch steel bar sticking out of the chuck 4 to 6 inches. And I would use a sharp HSS toolbit for fine cuts with accuracy.

And if you can;t machine up to the chuck with that guard in place, the guard is improperly designed and creating hazards of its own from excessive stick-out. Disable the switch on it and have the option of operating with it hinged up out of the way.

pgk and Huub, many thanks for your very helpful suggestions and advice. The chuck guard is a key factor in driving unsupported length, it clashes with the top slide - I think I need to operate without it in place. a new plan of action is forming!

Once again, lathe alignment. Things getting bolted down.

My lathe is supported on 4 rubber feet and is not bolted down to anything and I have no problems.

It doesnt move or show any variation in use and I have cut 12" between centers.

Thanks Hopper, your post confirms my thinking regarding the chuck guard interlock, it will be disabled!

Clive, I had wondered about the possible effects of bolting down and have contemplated running a test with the mounting bolts all relaxed. With the helpful suggestions from others regarding the bar and tool, plus this interesting thought I now have several options to persue tomorrow - thank you.

If bolting the lathe down changes the lathe's alignment, you are bolting it down wrong.

It is essential before bolting down to sit the lathe on the cabinet and the cabinet on the floor with the bolts loose. Then use a feeler gauge to find if there is any tiny gap under any of the cabinet feet. If so, slide in a shim of that thickness to take up any gap, then tighten down the cabinet foot bolts.

Then do the same where the lathe mounts on the cabinet. Shim any gaps then tighten the bolts down evenly. That way you are starting out with your lathe in the relaxed natural position without any strain or twisting on the bed. If you are lucky and have a good lathe like Clive's, the lathe will cut true in its natural unstressed condition.

If it doesn't, then shimming can be used to correct it.

I had my lathe bolted down for a couple years before I realized it had twisted the bed. Granted, your lathe is beefier than my 7x14 mini-lathe...But might it still apply? I slacked off the bolts so that it's only preventing the lathe from sliding off the stand and no force is being applied to the bed. My stand just sits on the floor, isn't bolted down at all. Your 210kg lathe isn't going anywhere.

That's a nice lathe, by the way.  Congrats!  I hope to have one that big some day.

Edited By MikeK on 06/02/2022 22:39:44

Thanks again Hopper, this will be step 1 tomorrow. In hindsight I should have asked before starting - but this is what happens when electronics engineers get involved with things mechanical!

Thanks Mitek, did you find that relaxing the bolts resulted in acceptable alignment?

Hi Steve. Yes, I did.

Thanks MikeK, I clearly need to go back to lathe / stand installation before making more test cuts. will let you know how things work out tomorrow.

Wot? Not all electronics engineers are mechanically incompetent!

As stated above a combination of the overhang of the test piece and a carbide tool isn't going to tell you anything about the alignment of the lathe. I'd unbolt the stand and just let it sit on the floor. Then I'd turn a short length (~100mm) with a sharp (large rake angles) HSS tool and see what the results are. You might be surprised.

In 20 years of operating my lathe I've never felt the need to machine test bars. I just make parts, but what do I know. I'm just a dumb electronics guy!

Andrew

Google Rollie's Dad's Method, much easier. If you can't find it pm me

I happened to have the link for RDM:

Rollie's Dad's Method PDF

If this is my lathe ( an Emco Compact 8 ), I will definitely say it's due to flexing. Having said that, your lathe is much beefier so there may be other reasons. To minimize flexing, I need to reduce the cutting load to absolute minimum so the rod I have made for such purpose is composed of two ABS plastic discs glued to the ends of a steel rod. In this arrangement, the cutting load is almost zero. You may try using aluminium in your case and make sure that the tool is super sharp. ie, either HSS or grounded carbide tool ( instead of molded )

Edited By Y C Lui on 07/02/2022 02:05:21