Cross Slide Alignment

[Larboard]

Hi All, I was hoping you could help me. I'm a beginner in model engineering and bought one of the Chinese (Speedway) lathes. I am starting one of the Donkey engine projects, but noticed when I turn small parts the left side is .003 smaller than the right and am getting a taper. Ever since I had a rocket land near me in Iraq, my logic center doesn't work well anymore. I know this is real simple, but I sure could use some help. Thank you.

Richard

 

[goldstar31]

I'm not sure whether you have a problem with your crosslide. All that could be wrong is that the tailstock is not parallel to the lathe itself.
If that is the case, it is a simple matter to correct.

If it is a new lathe, I would go to what should have been done and that is to see that the lathe is bedded down correctly with couple of spirit levels( at least) and with the lathe correct, go through the series of normal tests- one of which is checking the tailstock.

I'm a bit of an old tosser who was War disabled and Goldstar31 refers to the RAF. Please write to me direct if it will help.

Meantime, I wish you well

Norman.

 

[portlandron]

Would only be a problem with the tailstock if you are turning between centers.
If you are just using a chuck to hold the piece it could be the cross slide or more likely the head stock is not aligned with the bed.
As Norman stated also check the for twist in the mounting of the lathe to the bench.

 

[Cogsy]

You say it's when you turn small parts. Depending on how small you're talking, you could be getting that much deflection (bending) of the unsupported end of the part, but less where the part joins the larger stock. It's one of the problems encountered when making small I.C. valves.

If it's not happening on larger parts then I'd say it was deflection.

 

[goldstar31]

As Cogsy says! Deflection of 3 thous is actually only 1 and a half thous and that could be simply leaning on the tailstock. Yea man, done it!

Again, it could be deflection if the work is not being supported by a running centre in the tailstock. Your lathe tool could be a bit blunt and bring pushed away at the loose end more than the chuck end.

Importantly, try to get someone- say another vet. to monitor what you are doing. Always remember that small lathes are not built like brick sh1thouses but can do surprisingly good work if treated kindly.

Cheers

Norman

 

[Swifty]

You say that you are a beginner, as has been stated by others, it's probably deflection. I just have to make sure that you are using the saddle to move the tool and not the top slide. If you are using the top slide, it only means that it is not aligned square to the bed ways.

Paul.

 

[goldstar31]

Good point, Paul. A lot of us try to avoid top slides in small lathes. I've got a titchy little plain lathe far smaller than this Chinese affair. My larger lathe- with a 7" swing - and it was chattering with one of these confounded Quick Change Tool holder thingies. I should be led away by a couple of guys in whitecoats for being such an idiot.

Hangs head in shame

Norman

 

[Swifty]

Norman, in my first week as an apprentice, hardly ever having seen a lathe, let alone used one, I was given some silver steel and told to turn a centre punch. After quite a while spent trying to turn the tapered end using the saddle and cross slide, the Forman came over and pointed out to me that I can swivel the topslide to the angle I wanted and use that to turn the taper, I felt really stupid that I hadn't worked it out.

Paul.

 

[goldstar31]

Ah Paul, the joys of being an apprentice. I missed the joys of having a hat full of nails and a bucket of steam and tartan paint.
There were, I gather, certain initiation rights as well. I grew spinach from a packet which cost pennies and sold the produce to buy an education- whatever that is. So I muddled on, made models, played with a friend's little watchmaker's lathe and knew who Mr Sparey was. Family was engineers/blacksmiths/ farriers and I never knew that I was learning. I picked up tools and that was that. My son learned the same way. He might have a couple of Mercedes and a Lotus but he can mend most things. My daughter is like her Mum, a dental consultant and plays the clarinet, saxophone and piano but not always at the same time!

Somewhere in the scheme of things, a nice old boy taught me religion and the necessity to have and know two jobs. So tonight I have audited the accounts and last night was playing with a bit of metal. I suppose that I could not have had a better teacher.

Regards

Norman

 

[portlandron]

In his post he stated the "left side is .003 smaller". The left side I think would be the head stock end if so then it would not be a deflection problem. If it were deflecting the right end would be smaller.

 

[Cogsy]

In his post he stated the "left side is .003 smaller". The left side I think would be the head stock end if so then it would not be a deflection problem. If it were deflecting the right end would be smaller.

We'll have to agree to disagree on this point (unless you want to change your mind ). Let's say you're turning a 2mm valve stem from a piece of 6mm stock for 20mm length. As you touch the cutting tool to the 2mm unsupported end of the stock, the stock flexes away from the tool and very little (if any) cutting occurs. As your 'cut' moves towards the headstock, the stock becomes more rigid as it is supported by the thicker 6mm diameter being held in the chuck. So with less 'flexing' you are cutting more then when you started. The closer you get to the supported end, the better or 'deeper' the effective cut. What you end up with is a taper with the smallest diameter to the left.

 

[goldstar31]

Cogsy,
You are right on the ball. I suggested that the poster went even further and did the classical alignment checks.

There is an interesting thing though. I was fannying on with my 0-1" reading tenths and my old Chinese vernier. The batteries were flat, the temperature was the square root of bugger all and the figures were romping around. How does the poster know with a cheap mike or vernier whether the thing is anywhere near accurate to talk in a variance of 3 thous-- plus or minus - we don't know.

Yea, I have a nice set of J Blocks but- well, who knows whether this is all an inspired guess.

Kind regards and thank you for exciting my dormant grey matter

Norman

 

[portlandron]

Cogsy your are correct, not sure what I was thinking when I posted that response. My only excuse is being up most the night with a my sick dog.

 

[Cogsy]

No worries at all, happens to all of us. Hope your dog is feeling better.

 

[werowance]

ill second cogsy on the deflection probability. week or so ago i was cutting a crank pin down to .156 out of drill rod for the engine im working on, i kept cutting a taper and after 3 or 4 attempts with very light cuts i got a straight pin. actually ended up chipping a carbide bit and it had a very sharp tip after that and that cut better than a brand new one so the light cuts and that chiped bit allowed me to cut it straight.

the taper was smaller on the left (chuck side) than the right - not supported by tail stock on all the failed attempts

 

[B-Bee]

Best bet is to use a centre in the tail stock to reduce push off (taper) in your job if the job is sticking out to far
Or is a little thin, you can leave a little metal on the end of the job so you can put the centre hole in then turn it off at a later time.
Oh the tail stock will need to be checked for correct positioning, run a dti round the centre while fixed in the chuck.

Hope this helps

 

[goldstar31]

The best bet is surely to find out if there is obvious misalignment. If I was doing the job and before I got out my boxes of measurement tools, I'd get a safety razor blade and nip it between the centres.No, it isn't classical engineering but merely an indication of things. There are lots of things before getting out my copy of Schlesinger.RegardsNorman

 

[B-Bee]

We used to use a large parallel test bar between centers with a dti on the tool post but with out a test bar spinning the dti round in the chuck while on the centre is a good indication of trueness.

 

[Blogwitch]

In his post he stated the "left side is .003 smaller". The left side I think would be the head stock end if so then it would not be a deflection problem. If it were deflecting the right end would be smaller.

Totally incorrect statement.

If it is deflection, which I personally think it is, then the piece part is being pushed away on the RHS as it is unsupported, so it will be larger in diameter as less material is being cut off than when close to the headstock, where the piece part is being held more rigid so more material is being removed.

John