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Robin2
20-10-2013, 10:54 AM
Hi, I'm new here.

I am planning to add stepper motors to a Chester Cobra / SiegC1 lathe with a view to experimenting with CNC and creating a 3D printer. (I don't have space for a larger lathe or for a lathe + a 3D printer).

I haven't seen it explained anywhere how to set the lathe tool tip reliably and accurately at a known distance from the centre of revolution of the lathe - I think this is the X0 position.

I can imagine putting a piece of silver steel in the chuck and moving the tool until it is touching the bar. One would then know that the tool was half the diameter from the lathe centre line.

But you can't do this if you want to change a tool after you have a piece of material in the chuck.

I suspect there is a blindingly simple solution but I haven't figured it out?

What is the proper procedure for establishing a known position on the X axis?

Thanks

...R

thomashomer1986
20-10-2013, 11:54 AM
put some old stock material in the chuck, take a cut on the diameter, measure the size accurately and then input half of that size into your x offsets. make sure your tool is set at the correct center height.

Jonathan
20-10-2013, 01:03 PM
Touching the tool on a piece of silver steel is unlikely to get great accuracy as it doesn't account for the runout of the chuck. It's also hard to judge when the tool is just touching.

You could make a light test cut with the tool, measure the diameter of what it just cut and use that value to set the tool distance from the centerline. The problem with that is, as you say, once you change the tool you need to do it again. That's why people get accurate toolposts which allow you to change the tool holder, go back to the first one and still have the tool in the same position to within a decent tolerance. Clearly you need one holder for each tool.

Another option is clamp lots of tools to the machine on a plate, then move/offset to each tool position. It's probably only worth it if you're making lots of the same part.

You might be interested in my CNC lathe build log here:
http://www.mycncuk.com/forums/lathe-build-logs/4497-micro-lathe-zero-backlash*-conversion.html

One day I might actually use it!

Robin2
20-10-2013, 05:25 PM
@Jonathan, thanks. I had already found your blog - that's what drew me to this forum.

I guess I need to keep thinking about my problem. Having several preset toolholders is not an option - I don't plan to use it so much that would justify them.

I thought I had created the thread under the heading "help" rather than "build log" - is it possible to change it?

...R

Robin2
20-10-2013, 07:33 PM
Somebody seems to have responded to my request to change the title from "build log" to "help" - but in the process this thread seems to have moved to an altogether different place. I hadn't expected that?

...R

m_c
20-10-2013, 07:49 PM
Without having various preset toolholders, there are only two reliable options.
1) take a skim cut and measure.
2) use a probe. However for the cost of an accurate probe, you'll be able to buy lots of toolholders!

Getting a QCTP can't be recommended highly enough. They save lots of time, and once you've got one you'll wonder how you ever managed without one.

Robin2
20-10-2013, 08:18 PM
How do you preset the tools in the quick-change toolpost?

I thought all a QCTP did was to install the tool at the correct height? Can't you rotate the toolpost holder to change the angle of the cutting tip?

I presume if you take a skim cut it is essential not to move the tool until after you have measured the effect?

With apologies to everyone who already knows all this ...

...R

m_c
20-10-2013, 11:37 PM
How do you preset the tools in the quick-change toolpost?
The same as you would in a normal toolpost

I thought all a QCTP did was to install the tool at the correct height? Can't you rotate the toolpost holder to change the angle of the cutting tip?
Once a tool is setup in a QCTP holder, it's just a case of swapping the holders, and the tool should remain in the same position.

I presume if you take a skim cut it is essential not to move the tool until after you have measured the effect?
Yes.
Using a QCTP, you'd setup each tool in it's own holder, take a skim cut with each tool, and store the offsets for that tool within the software. Then it's just a case of telling the software what tool is installed.
The only issue is on startup, you need to reference the X-axis, which can be done in one of two ways.
Either you setup one tool as a master tool, which all other tools have their offsets stored from, so when you startup/need to recover from loss of position, you take a skim cut with any tool, measure and use the measurement to set the axis position;
The alternative is install a home sensor (highly recommended as it makes starting up or recovering from crashes easy), and then it's just a case of homing the lathe at startup/after loss of position.

With apologies to everyone who already knows all this ...

No need to apologise. Setting up lathe tools initially can be a bit intimidating, but once you've got a couple setup, things start to make sense.