Device to measure axial forces on lathe ?

If you simply want to maintain a constant "axial force" and don't need to know the exact value in inch pounds or whatever, just use an AC clamp-on ammeter on the input to the drive motor.

Pete Stanaitis

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snipped-for-privacy@comcast.net wrote:

thanks for the reply. I'll look into an ammeter that can be affixed around a cable.

What you are asking for is called a dynamometer, but basically it is a beam with strain gauges on it, and a readout box. A commercial setup will cost at least 10x what the Sherline cost. You could make it, though.

Jon

Hi Jon, can you enlighten me on how one can make such a device ? Marcel

Marcel, I assume, by axial force, you mean the force toward the headstock form the tail stock end. The pressure on the sandpaper.

If so, then a postal scale can do the job. The little mechanical ones for up to 16 ounces. Under $10 I think. If you need more force, then an electronic postal scale would work. (Some can be disassembled to remove the load cell. It's usually an aluminum or steel gadget, about 2" square and 1/2" thick.) You could also use a fish scale, perhaps 8" off center provide a pivot, half way to the spindle a hole for fish scale, with your sanding pad on center. The fish scale will read double the actual pressure. The easiest, but most work, would be to build a compression type fish scale. It could mount in the tail stock. Bore a tube, insert a spring, then a plug, lines and numbers on the plug, might need a way to keep it from rotating.

Dave J.

You can probably buy strain gauges on eBay, or get new ones from some outfit like Omega. They are thin, stick-on pieces that have a resistance element that changes under force. If you put two on the top and two on the bottom of a beam, and wire them up in a criss-cross manner, they make a 4-arm bridge. You want this so that an electrical drawing would look like a tilted square with the top left and bottom right element are on the same side of the beam. This maximizes the unbalance of the bridge under force. You excite the bridge with a constant current (or voltage) from the top to bottom, and read the difference with an instrumentation amplifier such as the Analog Devices AD620 taking its signal from the left and right junctions on the diagram. You could read out the result with a DVM, and create a calibration table with weights or a fish scale. Some reading on Google or WikiPedia might help.

Jon

--For more on this way of doing it google "prony brake".

While "prony brake" is the historically correct term, "pony brake" gets 32 times as many google hits on the same subject (ie, dynamometers).

Umm... Some confusion on exactly what you are doing but if I'm reading between the lines correctly,

When you say "polishing the face of a large lot of cylinders" I'll assume you mean the "round part" and since you didn't specify inside or out I'll assume out...

With all those *assumptions*.. uh why not make a real "prony brake".

Build some type of hinge on your tool post.

Measure distance from hinge point to centerline of work.

Make bar 6 time longer than distance to centerline (or 3 times or what ever you like) and hook to hinge so one end is just under cylinder and other end hangs over tool post.

Put 2000 grit on short end of bar under cylinder facing upwards.

Place weight of choice at other end of bar and polish...

Adjust weights till you like the finish...

Exact force can be calculated by multiplying weight by bar length... e.g. 3 foot bar hinged at 1 foot point, with weight out at 2 foot end from hinge is multiply weight by 2. 6 foot bar would have weight 5 feet out, multiply by 5....

--.- Dave (who could design a computer controlled servo with several force gauges to do this with a snazzy readout and keypad for data entry and ethernet link so you could control it from around the world, but... why?)

A cylinder need not be hollow -- unless you are talking about part of an engine (steam or IC). Consider a bar which has been parted off, and plan to polish the surface where the parting off occurred.

(Of course, I am not the OP, so I could be wrong.)

:-)

Enjoy, DoN.