Thread: Harrison M250 Turning Taper

The guys on the yahoo Harrison forum may help you

Hi John,

I'd stick a DTI on and bar at center height, compare both ends and see what it tells you.

I wouldnt be moving the headstock just yet.(but then you said that already)

Stuart

I would not make any assumption based on one cut..

I would be doing multiple cuts and looking for a pattern first...

Or maybe I am just a crap machinist and when doing more precise work with a lathe, I sort of struggle and in all cases so far when the lathe turns a taper it turns out to be me...

Originally Posted by electrosteam

Hi all,
I have finally got around to re-arranging the workshop with my M250 mounted on proper feet, levelled and have taken a first cut to see how it performs.
Dia 40 mm black MS bar in the 4-jaw with 100 mm protruding and unsupported.
A worn tangential HSS tool without coolant at 240 RPM with 0.1 cut at 0.07 feed.
I had planned to machine a boss at either end, then change tools, add coolant and do a final finishing cut on just the bosses to assess the parrellism.

But the finish was much better than expected, so I just measured what I had - 0.05 over 100 mm taper with the outboard end smaller.

My understanding is that such a taper means the headstock needs to be rotated on the bedways.
I won't be doing anything about the problem anytime soon as a negative taper is in the correct direction when checking bearing fits.

Can anyone comment on:
- my diagnosis,
- taper not excessive for typical ball bearing fits,
- and how a similar problem was corrected?

Happy machining for the season,
John.

Do not, under no circumstances, try to align the headtock. The headstock alignment is to 99.9% certainty NOT your problem. If you disturb the headstock alignment you open a can of worms, as other alignmnets depend on it and will be out.

Your problem is simply a very slight bed twist. It is not unusual for such a small error to remain after levelling the bed. It can be corrected by adding a shim between bench and lathe bed at the right rear end (rising the bed at the rear under the tailstock end).

Originally Posted by cba_melbourne

Your problem is simply a very slight bed twist.

I would hesitate to be so certain of that..

John

Can you describe how you levelled the lathe ? Do you have a precision level ?

The 250 is a solid lathe . Is it sitting on the factory base ?

I think I spotted a Harrison 250 on EBAY/GUMTREE in Kingswood ? A couple of years ago .

Mike

The first thing I would do is sharpen that worn HSS tool then try it again but relieve part of the shaft to leave 2 journals/collars.

Well guys,
I have to admit I screwed up, the bed is actually twisted.

I had levelled the bed longitudinally and transversely in the middle of the bed, not bothering with a separate check of the transverse at each end, and finished by checking by hand that the jacking screws were all equally torqued.
I had been warned to never separate the lathe bed from the stand as they were matched at original manufacture and the resultant assembly is extremely stiff.

A more thorough check tonight had the lathe stand balanced on the diagonal, headstock front to tailstock rear, with the precision level still showing the tailstock front as too high.
The measured bed twist can easily explain the measured work taper.
The only remediation would be to bolt down to the concrete floor and pull the stand to get bed alignment, something I am loathe to do.

I think I will do nothing about it, other than just use the lathe with only light finger torque, checked for each job, on the tailstock front jacking screw.
The bed/stand assembly may eventually settle down into a different (better) state.

Apologies for the earlier misleading descriptions,
Happy machining,
John.

You started out with a not kown to be perfectly round and perfectly straight bar in a 4 jaw.
No way that is going to give you any indication upon the alignment of the headstock.
Forget about the level, turn a bar between centers and you will know here to go.

Originally Posted by janvanruth

You started out with a not kown to be perfectly round and perfectly straight bar in a 4 jaw.
No way that is going to give you any indication upon the alignment of the headstock.
Forget about the level, turn a bar between centers and you will know here to go.

If you turn between centres and it shows a taper, how would you know if it's from a twisted bed (or other issues) or from a mis aligned tailstock? Adjusting the tailstock would remove the taper but you could still have other alignment issues.

I could be wrong, just a thought?

Simon

- I believe that in over 95% of cases where someone posts a question suspecting lathe headstock misalignment, the problem is really not the headstock but elsewhere.

- I also believe that 95% of people that actually went to adjust their lathe headstock alignment, came to bitterly regret their decision soon after.

It is just too easy for a beginner to blame headstock alignment. To get an understanding of alignment interdependances, it probably takes several nights of thinking all lathe alignmnets and their sequence through. A lathe inspection record helps immensely in understanding. A copy of Georg Schlesinger's book "testing machine tools" is highly recommended - as this has been the standard reading on this subject for any engineering student since the mid 1930's.

Originally Posted by simonl

If you turn between centres and it shows a taper, how would you know if it's from a twisted bed (or other issues) or from a mis aligned tailstock? Adjusting the tailstock would remove the taper but you could still have other alignment issues.

You're right - you don't. Turning between centres is primarily to check tailstock alignment after you know the bed is not twisted. Before doing that you should be turning a piece of bar just in a chuck in the headstock. Lots of spring cuts to minimise work piece deflection. That of course relies on the lathe bed being straight (ie no wear that will move the carriage off line). To check for that...



At the end of the day it again depends on what you are making. John had 0.05mm (50 micron) over 100mm taper. If you assume a ball bearing is 15mm wide, that will be 7.5 micron taper over that length.

SKF suggests these fits
http://www.skf.com/au/products/beari...its/index.html

A j5 fit for a 20mm shaft is -10 to 0 micron - a 10 micron range. On that basis you achieve the recommended fit, so it should not worry you (if you can hit those tolerances!). These days bearing glue is used to avoid these issues so much.

Michael

Originally Posted by janvanruth

You started out with a not kown to be perfectly round and perfectly straight bar in a 4 jaw.
No way that is going to give you any indication upon the alignment of the headstock.
Forget about the level, turn a bar between centers and you will know here to go.

Its this sort of information that simply confuses and misleads so many. Leveling a lathe should be an easy task, once you have the facts in hand.

The easiest thing to do is go and buy a length of acetyl/delrin. Take nice light cuts on it with a razor sharp HSS tool, just in the chuck without the tailstock. It will not deflect or give you poor surface finishes like steel may. Rigidity of the lathe should not be a factor cutting plastic either, not that the 250 is a lightweight machine. The only thing to be careful of with the plastic is it can pull into the cut if you take heavy DOC's with any kind of positive geometry tool.

John,
Your description of which feet are touching the ground sounds exactly right for the problem you have. These lathes with big bases (is the M250 cast or fabbed?) are pretty rigid and will lift a corner without twisting too much. Leave it jacked up and see if it settles. I would not be happy with .05 over 100mm at all. It may be going the right way for a bearing pocket but what if your machining a shaft for a bearing?

Cheers,
Ew

Thanks guys for turning my incorrect premise thread into a good discussion on all the issues, I don't feel so embarrassed now.

The comment on machining recesses by Ueee is spot on, my solution will be to do the final recess finishing for critical fits on the backside, possibly with the lathe in reverse.
Shop made boring bars with HSS tools can easily be arranged to cut either direction, up or down.
I have played with this approach previously, now perhaps I have the need to hone this skill.
John