View Full Version : ohmnic sensing for tool height?

Black Forest
12-25-2013, 04:08 AM
Is there such a tool? I was thinking that a ohmnic sensing torch height controller for a CNC plasma table might be adaptable to a mill's Z axis. One that uses the actual cutting tool to sense the top of the material. Somehow connect the sensor to the spindle so regardless of what tool is mounted if you slowly lowered the tool or brought the knee up it would sense the top of the material and register on a led readout.

I did a search but didn't come up with anything that matched my search queries.

The Artful Bodger
12-25-2013, 04:20 AM
I believe that has recently been discussed here but maybe for a lathe.

How about taking a look at the method used by the 'Dam Busters' in WWII to determine when the bombers were at the required height maybe using a couple of lasers?

Yow Ling
12-25-2013, 08:32 AM
So want to measure the gap or just know when the target is there. measuring resistance of the air is going to be tricky.
There are plenty of proximity sensors that can work, optical, magnetic capacitive and ultrasonic, they are already made tough for the harsh enviroment
Check out Erwin Sick they have a full range

Weston Bye
12-25-2013, 09:08 AM
Try searching for touch off sensors. Typical practice is to use a piece of copper-clad circuit board of known thickness placed on top of the detail. The tool is touched off on the plate, and then the thickness is accounted for when setting the tool zero position.

When I get around to it, I may use a brass plate machined to a nice round number dimension with Kapton tape on the bottom for an insulator, even though I have a lot of copper-clad board around.

Paul Alciatore
12-25-2013, 09:30 AM
If you are talking about using some kind of insulator, then this is easy: Voltage source, LED, resistor, and just connect to the insulated part of the tool and to the base of the machine/table.

If you have fairly loose tolerances, like +/- 0.003" then some kind of switch can be made that can be inserted between the work and the cutting tool. You can probably get better than that with some work.

If you want to work with just the normally mounted tool, with no insulation, and rely on the change in resistance when it touches the work (short path vs. longer path through the rest of the machine) then things get a lot more difficult. You are trying to read a very small change in resistance when the tool makes contact. And there will be all sorts of contaminants present which will complicate things. Also the resistance through the spindle can change as it moves up and down and this can be a confusing factor. You will probably need to make one of your connections above the spindle bearings to avoid this. I am not saying that this is not possible, just more difficult. There have been discussions on this and I know of one person who has made a personal project of this. He seems to be making some progress. I do not know how this would work out in a real shop situation. Much testing and refinement would be needed.

12-25-2013, 12:13 PM
There are tool height sensors, they are small touch probes mounted to the machine someplace where the tool can touch it to measure it's length. Though most shops only use these to check for tool breakage after a operation.

I still recommend the Mitutoyo tool height setter.

Black Forest
12-25-2013, 02:16 PM
There are tool height sensors, they are small touch probes mounted to the machine someplace where the tool can touch it to measure it's length. Though most shops only use these to check for tool breakage after a operation.

I still recommend the Mitutoyo tool height setter.

I know you posted a tool height setter before but I don't remember if it was a standalone unit or it required to be connected to a cnc?

Forrest Addy
12-25-2013, 03:33 PM
I don't want to rain on your parade but your proposal sounds like a solution in search of a problem. Tool height (axial referenceing) is very simply accomplished with a bump or a feeler. Tool-end setting on a CNC machine can be done in a variety of ways ranging from off-machine tool presets, on-machine tool probe, a tool end setter, or by techniques used on manuel machines.

Have you seen these: http://www.ebay.com/sch/i.html?_trksid=p2050601.m570.l1313.TR0.TRC0.Xled+t ool+probe&_nkw=led+tool+probe&_sacat=0&_from=R40

The classic technique for touching off is to interpose a slip of paper between rotating cutter and work. When the paper is tugged from your finger you are within the thickness of the paper of contacting the work.

Alternatves from the old school include:

1 - layout paint or felt tip mark on the work, approach and stop at the first scrape of the cutting edge,

2 - stationary cutter and feeler,

3 - calcuation from known references, cutter end position, and cutter diameter.

4 - On lighter machines with sensitive "feel" at the handwheel, you can physically touch the work with the stationary cutter, feel the changed resistance in the handwheel, and set references from the first barely perceptable contact. Very small cutters (sub 1/8") are easily broken so this method has lower limits.

5 - A dowel pin in a collet or endmill holder also gives a good bump reference. THen there are wigglers, LED conctact sensors and a variety of other gimmicks.

The CNC world uses contact probes of difference type but these are very expensive to implement in the home shop.

6 - The LED sensors shown in the abive link are surprisingly handy and sensitive.

Needless to say you ned a calculator to get from here to there but you spend the whole trade figuring one thing or another so what difference does one more make? There are probably a few more tricks leaked out of my tiny brain. All are slow, tedious, and some can be unsafe for the fumble-fingered.

If the tool is electricaly insulated from the work, then electrical sensing becomes very practical. As soon as the tool touches something lights up, beeps, of triggers the display or control and you are off to the races.

I use a mix of the old school tricks becuse it's impractical to insulate the tool from the work without some pretty fancy retorfit or tricky tooling.

oil mac
12-25-2013, 06:44 PM
I am always somewhat nervous with a revolving cutter tugging a piece of paper out of my fingers (fingers & cutters do not pair up without extreme suffering!)
What I tend to do is to have a slightly oily surface on the work piece, on which is laid a piece of cigarette paper, With the machine stopped, Start up the machine & when the cutter takes in your paper you are within approx. 0.001"

12-25-2013, 07:33 PM
This guy.


Worth every penny. Can be used with everything from tiny endmills to face mills. Less chance of overrunning and damage a cutting edge.