Design Advise: Variable Stroke Crank

.....snipnagin

There are a number of interesting designs for this in "Ingenious Mechanisms for Designers and Engineers", a four-volume set. Most any good engineering library should have the set.

I used to own an old Logan shaper, but never looked inside (duh!).

LLoyd

The gizmo that sets the timing on a steam locomotive is one. It has a name which slips my mind at the moment, however.

I can think of one or two ways to do this that involve rods with slots and pins that slide -- all share the common characteristic that they have way to many parts and would be maintenance nightmares. I think I'm showing my German heritage.

If I were to make a shaper I think I'd move the head with a hydraulic ram, actuated by a solenoid valve and with position feedback running to an electronic control. This way I could set everything ever so finely by a beeper box with an LCD display and maybe even twinkee lights like a computer from a '50s movie (Spencer Tracy as computer consultant vs. Katherine Hepburn as librarian -- remember that one?). This would be easy for me 'cause I build embedded systems for a living, the all-mechanical approach may be best for you.

"Artemia Salina" wrote: (clip) Any other suggestions are welcomed too. ^^^^^^^^^^^^^^^^^^ How about embedding a ring or a disk in place of the slot and threaded rod. Attach your post to this disk--make it eccentric and make it adjustable.

What about using a lever to change the stroke length? The crank would operate one end and the other would actuate the shaper bit. The downside is that it takes up more space. The upside is that you can more easily adjust the stroke at the lever, and since you wouldn't have to worry about designing the crank with a bunch of fiddly bits, you could potentially make the crank (and probably the whole assembly) stronger.

Just a thought after a full thirty seconds of thinking about it.

Peter

If your crankpin was fitted into a tee slot in the disk,with the adjusting screw threaded through it and by fitting a shouldered bush to take the conrod, it would be then possible to lock the crankpin securely to the disk, often seen on shaper feed devices..

As for ascertaining the stroke length, for many years, some shapers had a removable cover to access the stroke adjustment, practicality over sophistication, also known as KISS.

However, do you really need an adjustable stroke? Many slotting attachments for horizontal mills didn't, I have a large one myself if you'd like a pic. the only part enclosed is the geartrain which raises it to increase the daylight.

If you want to go down the dialup scenario, have a look at this patent: 2551359

Cheers Tom

Do you want to have a scetch of Deckel's slotting head? I can scan and put it on my HP, if you want.

Nick

"Artemia Salina" wrote in message news: snipped-for-privacy@sheayright.com... | On and off I'm designing a shaper head for a horizontal miller | which is driven by the miller's spindle. I'm at the point where | I'm working on the variable stroke length adjuster mechanism. | | What I've got is a rotating disk, which serves as the crank arm, | with a radial slot in it. In that slot is a captive screw, with the | head end at the outer circumference of the disk. On that screw is a | nut, and finally on that nut is the crank pin. Turning the screw causes | the crank pin to move radially toward or away from the center of the disk, | thereby shortening or lengthening the stroke.

The first thing that came to my mind was two discs on top of each other. The first one has the radial slot like you have, the second has a spiral sort of slot in it. The spiral disc turns independent of the other, and as it turns it moves the pin with the spiral. Clamp the two plates together as you see fit and you have variable stroke. Another thing I've seen elsewhere but doesn't come completely designed inside my head is something a bit different. Rotating crank with a pushrod. At the end of the pushrod is two more pushrods. One operates the slide and the other is attached to a fixed pin, which can be moved along a row of holes or something. The point where the two pushrods meet will be a function of the location of the fixed end of the pushrod and the point the crank is at. The third pushrod goes to a linear mechanism which is the shaper end. I think in order to make this work in my head I'm gonna have to visit the Lego Technics stuff I gave my sons and put something together. I seem to recall seeing this on a drawing of an old steam engine to control the output stroke or something.

Actually you've just about got the standard shaper stroke adjustment right there. The addition of a miter gear in the middle of the screw and another on a shaft going through a hole in your crank drive shaft is used to bring the adjustment outside the shaper. Most shapers do have a pair of bolt on gibs on either side of the adjusting just to trap the square part of the nut in the slot on the bull gear.

A simpler approach is to just drill and tap several holes in the disk and to change stroke length just unscrew the crank pin and put it in another hole.

Wayne Cook Shamrock, TX

The Bridgeport shaping attachments have a rack and pinion arrangement that adjusts the throw. The pinion shaft is runs thru the center of the crankshaft and the rack is attached to a block which rides in a box way. The crankpin is attached to the block. The knob you've seen is on the end of the pinion shaft. The details are clear in the parts list in older BP manuals. I have a 1966 manual and a 1981 edition. The section view in the older manual is easier to understand than the exploded view in the '81 version.

If you don't have access to a BP manual and would like copies of the relevant pages email me your mailing address. I don't have a scanner and doubt that a fax or photo would pick up the details.

Ned Simmons

Logan Shaper is exactly what you are describing... I could send some pictures. Ken.

This idea was so intriguing that I went on a search for more info. I can't say that I've found anything on it, precisely yet, but I did find this wealth of mechanical design books online, and I wanted to pass the link along to you all.

Fantastic resource!

Gotta run right now, but I'll post the link in a separate thread tonight.

Thanks for all the suggestions from everyone. Much food for thought!

I had an idea. Imagine a bar with one end anchored at a pivot with a perpendicular bit sticking out with a secondary pivot. Now attach a fixed-stroke crank mechanism to the second pivot. If you locate the crank assembly perpendicular to the main rod, you have minimal stroke, but if you swing the crank assembly off to the side, toward the main pivot, you lengthen the stroke. You'd only need a small movement of the assembly to get a large variation in stroke. Well, depending on how long the perpendicular projection is. If you eliminate it altogether you'll get a BIG difference from a little movement, but if you make it so it's proportioned like the letter 'T' you'll need to move the crank a lot further to vary the stroke as much.

Crappy ASCII:

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