Shopsmith Forums

I was always intrigued about the speed of my Mark V at the different Letter settings. (Yes I know that the speeds appear in a table in the PTWFE for 50 Hz and I can find the rest throughout the book, but how can I be sure?)

So I built a stroboscope to attach to the SS and read given speeds as I increased and decreased the RPM with the speed dial and checked the belts were properly set.

I took a discarded CD and painted a thick black bar - the width of the center hole of 5/8" - from the center to the edge on one side of the disk.

Having expanded the center hole to 5/8" I could mount the disk on a 5/8" arbor, something I did.

Illuminating the disk with a fluorescent tube - which oscillates at the frequency of the power source of 60 Hz - this half diameter bar will appear still in synchronization with the light source configuring the following patterns (mid speeds can be determined using the letter scale):

Top auxiliary spindle
3600 RPM = 1 spoke, a half diameter bar (R-S)
1800, 5400 RPM = 2 spokes forming a diameter bar (I-J)
1200, 2400, 4800 RPM = 3 spokes (E-F)
900, 4500 RPM = 4 spokes forming a cross (B-C)
600, 3000, 4200 RPM = 6 spokes (P, T)

Bottom auxiliary spindle - Jointer
5400 RPM = 2 spokes forming a diameter bar (Q)
4500 RPM = 4 spokes forming a cross (O)

Other speeds can be read as multiples of the above.

Wow! Let me be the first to commend you! A very, very different approach. Innovative to say the least! I like It!!

FlashbacPT:)

Holy Cow! You've done it again! Another innovation that is just plain fascinating. Thanks for sharing this invention!

What would happen if the painted line were only 1/4" wide? Would the spokes just be narrower?

A stroboscopic disk is patterned so when a given speed is attained and it is subject to a given frequency of light (Hz) the marks on its surface appear immobile.

The traditional stroboscopic markings that surrounded the record turntable were dimensioned so they appeared immobile as the turntable turned. You could also purchase cardboard disks that could be placed in place of a record for the same purpose.

[Note: the principle is based on the fact that every time the light illuminates it catches the bar in a given position on the rotating disk. When the bar is always caught in one spot it appears to be immobile. If the bar appears to rotate clockwise it is an indication that it could travel more between flashes and therefore is rotating faster then the expected speed. If it appears to rotate anti-clockwise then the disk is rotating sower than the expected speed. Hence the patterns corresponding to fractions of the standard speed of 3600 RPM at 60 Hz showing an immobile half diameter bar]

A multitude of markings is not necessary. In a way they can confuse. If you only want to read one given speed then multiple markings might make sense, but when you want to read many speeds, one sole marking is best because the frequency of the light source will create visual patterns based on image retention of the eye.

Associating these still retained images with the speed of rotation and the frequency of the light provides us with an indication of the speed the disk is spinning.

I use one bar 5/8 thick just because. It could easily be a pencil line but would be difficult to see. Any thickness is valid.

BUT you must be sure that this bar is drawn as a radius from the centre of the disk to the edge, otherwise you might find that the stroboscopic effect could become fuzzy. Not that it necessarily would.

Have fun and recycle CDs.

I bought a LASER tach on eBay, it works sometimes but picks up a lot of other marks so I have to know about what speed it should be so I can be sure about the reading. I wonder what I really should have expected for less than $30.

I found out that my SS is running a little fast, slow was about 770 and fast about 5400, and the motor was about 3600. I checked the tach's accuracy with a fluorescent light and it was only a couple of numbers off of 7200 which is real close.

Several years ago when I was in the Air Force I used a digital tach and it was very accurate and easy to use.

I'm not sure I understand the technical requirements. Would a white line on a 10" sanding disk and a florescent trouble light fill the bill?

Yes, it just has to be visible under a T-12 type fluorescent light, T-8 won't work because it flickers at a much higher rate. I would prefer to turn something smaller than a sanding disc at top speed and I wouldn't stand where I wouldn't get hit by a piece of anything when it is going fast. Remember that you have to have the entire disc open to see the whole surface for this to work.

Good posting! Love to see science being put to good use!

It also reminds me of the old days when I had my PE turntable and setting the "pitch". Of couse the days of heavy platers are long gone and with electronic control and stepper motors that is a none issue.

I've always used the belt location in the high speed setting to establish some sort of reference point but I may build one of these for next summer when the neighbor kids stop by to see what I'm up to and I sneak in a science lesson. And yes a few wood working tips and some scrapes to play with.

Ed