Why I was looking for info about strain gauges and the like

I have been designing a new type of tapping head aimed at small shops with older equipment. It is designed to keep taps from breaking, it detects missing, short or undersize holes, and it does windows. Well, it doesn't really do windows. Anyway, I thought that as long as I was making something that did all that it would be neat if it could report on dull taps too. I may be able to use clutch pressure to arrive at some torque number but I don't know how the clutch material behaves over time, if the pressure will change significantly over time for the same torque. The tapping head will easily handle #2 to 1/4 inch taps in soft and hard materials. I think it will even handle taps as small as #0. Experiments show that I am able to use it to tap #0-80 holes in steel without breaking taps but I am not yet convinced that a machine tool will be able to. When I'm tapping there is some feel whereas a CNC machine will only feed the tap at the programmed speed. I was hoping that some sort of strain gauge applied to some sort of rotating shaft subjected to some amount of torque might be pretty straightforward and cheap. It is not. But thanks for the replies anyway, I really appreciate it. Cheers, Eric

Reply to
etpm
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I see you are dealing with small taps..

If you drive the head/holder directly via a Stepper motor system, you can regulate maximum current to the stepper. Measure the time it takes to complete each step.

A longer than normal time to complete a step will indicate that the tap is getting a little used up or needs lube. Depending on the size of the tap you're driving, you can set the maximum motor stepping current and also need to monitor the time it takes for the step to complete before doing another step.

Not sure how a stepper motor would work attached to existing equipment? it may do just fine. The concept would be the same.

You could also use a servo motor but that starts to get a little expensive.

Jamie

Reply to
Maynard A. Philbrook Jr.

Hey Eric:

That changes things. I think everyone was assuming the same thing that I was -- that you were working on a one-off, and thus that a bunch of development work would have been downright stupid.

But hey, you're not! I think that wireless is probably still too spendy, unless you're going to make 10,000 tapping heads. However, I think you could make a battery-powered unit that goes on the tapping head & uses a "naked" strain gauge, that holds the maximum torque exerted so the operator can look at it after the fact. And I think it could all be done for a BOM cost that's reasonable.

Now, whether we could coach you through designing all of this -- I dunno. Several of us do consulting and could do the work, but we'd want to get paid a fair wage for our labor, which may amount to more than you can afford -- but again, I dunno what your budget is.

Personally, and in spite of the fact that I'd love to talk you into having me do the design for you, I think that in the larger business sense you may be better off to concentrate on getting your tapping head working and on the market in the minimum amount of time. Save the strain gauge for a year or two down the road, when you have revenue coming in and you want to have something to differentiate you from your competitors, and something to tempt your existing installed base to upgrade.

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Reply to
Tim Wescott

If you're putting a battery in it wireless is cheap, probably cheaper than a display. eg: bluetooth. IRDA could also work.

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umop apisdn
Reply to
Jasen Betts

AFIAK, simple 7-segment LCD glass is cheaper than a Bluetooth module, and can be driven from a microprocessor. I don't know if you can do Bluetooth with ultra-low power dissipation: I know that with the right microprocessor choice you could do it with a display in such a way that you could run off of a coin cell for a year if the thing stayed in a drawer, with diminished battery life if you use it a lot.

Assuming that the battery life issue could be dealt with, if he's going to make 100,000 units, or even 10,000, then some custom Bluetooth implementation is probably best.

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

You will have a rotating shaft passing through the middle of the gadget... stick some neodymium magnets on the shaft and put some (stationary) coils next to them. You have to deal with the RPM not being much and that it can go either direction... maybe you use the coils to charge a supercap or NiMH AA or something. Also you have to keep coolant/oil and metal chips out of the (hopefully small) air gap between the magnets and the coils. For tapping, say, 0.25" or M6 or bigger, the generator probably won't mess with your torque readings, but for smaller holes it might.

If a Bluetooth phone earpieces work OK in a machine shop, then this probably isn't a big deal, but: some shops might be a little noisy at RF. Mostly arc welding, but perhaps also noise from the control computers, stepper motors, etc.

At a previous job, I used a Haas mill. It had encoders on all the axes, but if you wanted to get really precise, it had a probe from (I think) Renishaw to help. Half of it went in the chuck where the mill bit would normally go - that half had a battery, a bunch of IR LEDs in a circle, and a spring-loaded plunger sticking out of the end. The other half was a box with an IR reciever that bolted to the (fixed) splash shield on the mill and had a wire to the mill's computer. If I remember right, you basically told the mill computer to listen for the probe, and then used the handwheels to move the mill head until the plunger juuust contacted the surface. That would send a signal via IR, and the mill computer would accept that as the new zero / starting point.

I don't know what the battery life for that probe was like, but driving all those IR LEDs has to take a little juice. It wasn't a commercial production shop, so we didn't use the probe that often. I get the impression that a probe like that would get used less often than a tapping head, though.

Matt Roberds

Reply to
mroberds

What about a shaft encoder at both ends of a torsion shaft? The torque showing up as the difference in angular position.

And it would have the advantage that the electronics could remain stationary.

Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)

Reply to
Mark Zenier

I'm assuming that the whole head is turning, with no stationary parts. Hence the thought of battery power.

If you could have a part that could be fixed to the mill then yes, you could power it from rotation. Or maybe I should say, maybe you could power it from rotation.

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

That's what I was thinking, it could just be two magnets, or optical.

George H.

Reply to
George Herold

Greetings Tim, Yeah, mission creep can be a bad thing. I do already have a few tiny transmitters that can be had pretty cheap. If I only want to send a string of pulses they would work. I already am planning on using a transmitter to send a signal to an alarm box outside of the machine to let the operator know when a hole doesn't get tapped. I have breadboarded the electronics and they work fine on the bench. If I do add some sort of torque sensor it will need to be after I have produced the first model. So I can afford to pay someone to design the electronics if need be. Thanks, Eric

Reply to
etpm

I have decided to use a battery for at least the alarm part because no power would be drawn until the head detects a fault. I thought about a generator but it seems to me that adds too much complexity, at least in the quantities I might be able to sell. I don't think encoders would work because I don't think the shaft would twist enough when tapping very small holes. But this is just seat of the pants figuring. I could make a setup on the lathe to test your idea. I have encoders with 2500 lines which means that read in quadrature there would be

10,000 pulses. And maybe the shaft would twist up 1/10,000 of a revolution when tapping 2-56 holes. Thanks, Eric
Reply to
etpm

Hi Eric, Marks idea includes a torque shaft..which I assume is a section that's designed to twist more with the torque.. (Do they make such things?)

George H.

Reply to
George Herold

If I was designing a device for tapping only a small range of sizes then a "torque shaft" could be used. But the head must tap between #2 and 1/4. So a shaft that would have much deflection with a #2 tap would probably just twist off with a 1/4 tap. Eric

Reply to
etpm

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