Thread: Lathe accuracy.

I'm assuming this as been asked before but -
I have a Hercus that I've been using for a while and it's been fine for small jobs I've been doing but I have some stuff to turn that needs spot on accuracy.
How do I elliminate backlash on slides and bed ?
It's turning a fairly large taper on diameters, can I elliminate that ?
Any help or pointers would be great.

So as it has been cutting tapers for a fair while and know you need to try and eliminate them as much as possable.

You may be able to get everything spot on if your extremely lucky,other wise you would need to polish to size or prefferably grind to size.

There is no built in adjustment for backlash on the Cross Slide or Compound Slide.

How much measured backlash is there?

Not sure what you mean about backlash on the bed.

The Carriage runs along a rack attached to the underside of the front Vee way,if there is (backlash here when manually moving the Carriage it is generally clearance between the rack and gear.

There is the possability that the rack or gear has worn over the years.

You could if you wanted place shims under the rack to bring it in to closer contact with the gear.

The bed can wear and also cause clearance between the rack and gear.

I have not yet come across a tolerance or nominated amount of backlash for the Carriage.

Backlash on the Cross Slide and Compound Slide is needed other wise they would be to hard to use.

A nominal amount of backlash for both of these around .004" max from new.

Most of the older machines yould have a lot more than this.

Most backlash is not a real problem as long as it is taken up before machining commences,if its not taken up that is when problems with it can occur.

You can tighten the Gib Screws to make things a bit tighter (this will not reduce the backlash) on the Cross Slide and Compound Slide.

There is no Gib as such on the Carriage.

At the rear of the Carriage there is a keeper plate that rides along with the Carriage,this stops the Carriage from lifting the rear section up while under load.If the clearance under the keeper plate is excessive you can adjust by either machining it or using shims.

Which way is the taper cutting?
Is the lathe level?
What type of tooling are you using?
How deep a cut are you taking?
What feed rate are you using?
What speed are you running?
What length is the job?
What type of material are you cutting?
How are you holding the job?

Originally Posted by yorkshire racer

I'm assuming this as been asked before but -
I have a Hercus that I've been using for a while and it's been fine for small jobs I've been doing but I have some stuff to turn that needs spot on accuracy.
How do I elliminate backlash on slides and bed ?
It's turning a fairly large taper on diameters, can I elliminate that ?
Any help or pointers would be great.

Hi Yorkshire Racer,

please note that backlash has nothing to do with accuracy! I am the first to agree that it is more fun and quicker and easier to do precision work on a machine with only little backlash. But fact is that equally accurate work can be made on a machine with some backlash. In the days before cnc, the machinist apprentice did learn exactly this. How to anticipate machine errors like backlash and correct for on the fly.

Some better lathes have inbuilt adjustments to keep backlash to a minimum. For example my Emco has adjustable (slitted) leadcrew nuts and adjustable handwheels. However there is always some residual backlash, has to bee, between 0.05 and 0.1mm. Even on a brand new Swiss made Schaublin high precision lathe, or on a brand new American Hardinge toolroom lathe. It is unavoidable when using conventional leadscrews. If you tried to completely eliminate backlash, the slide would simply lock up. Ballscrews on CNC machines have no backlash, but these are not self hemming and thus not usable on a manual lathe.

Unfortunately Hercus lathes do not have adjustments to take up wear. Backlash say in the cross slide can get as bad as one half revolution of the cross slide handwheel. Not exactly a joy to use, but it does not affect accuracy of the worpieces. You can replace the leadscrew nut once or twice, before the leadscrew itself wears out.

Turnig taper is another issue altogether. In most cases it is due the lathe not being properly installed and as a consequence the lathe bed is twisted. The proper installation of a lathe always involves "untwisting" of the lathe bed. This is most easily done with a precision level, but there are several other methods too (google for "rollie's dad lathe alignment", or read the Hercus book "text book of turning" that came with all Hercus lathes). Note that on a lathe with roller spindle bearings, insufficient bearing preload can also lead to cutting taper. As can a misaligned headstock, but this is not very common unless the lathe was involved in an accident, or someone took it apart without thought. Chris

Greetings,
Just to add to previous logs.
The Hercus Textbook of Turning can be downloaded from the internet for free if you have purchased a secondhand machine or lost the book.
Stationary