In this video we talk about precision mechanics related to membrane flexures and differential screw systems. We make a few test parts showing these concepts applied to the Rahn Repeat meter collaboration project with NYC CNC.
28:06 - the equation for metric threads is far simpler, just pitch a - pitch b. I designed bed corner adjusters for my 3d printers using this principle, M4 is 0.7mm pitch, M6 is 1mm pitch. Difference is 0.3mm, so 0.3mm per 360° rotation of the adjuster wheel. Very cool!
One of my favorite videos you have made. I would love to see more design iteration like in this video. It really gives a good sense of the reality of designing a product and what problems you have to solve along the way, and how you solve them. Thanks for the excellent videos.
Very nice video. Thank you for sharing your knowledge and showing a design process. I'm particularly happy that you showed how you made some prototypes and tested the ideas. Very important for this type of work. Very cool to see.
Thank you mr Lipton, very educational episode! You might just as well build your own repeat o meter, as you are almost half way. Oh and thank you very much for the metric, screen shots, it really makes it easier to understand, and at the same time get to know the imperial sizes!! Ian South Africa
Another great video, Tom. Thanks for showing the various steps to getting finer and finer adjustment. The trick of using differential threads is awesome. An easy way of thinking of the differential movement is that the 1/4-28 thread is moving ~0.03571" per rotation, while the 10-32 thread is absorbing 0.03125" per rotation, making the resulting movement ~0.00446" per rotation, or about ~0.001" for every 90 degrees of rotation.
I can't afford a shop. Not even a small one (living in a block apartment and the noise would be too much). But I love to watch and learn. And I know I'm watching the right people when, after a while, I get goose bumps finding out I can anticipate the same solutions to the problems they propose as the ones they find themselves. Thank you, Tom!
Awesome video! Tom you are making baby nap time very enjoyable for this engineer! These principles of precision are fascinating and make us all better metal workers!
Thanks Tom, great video. I found a use for the differential threads, I just finished making 3d printer bed adjusters using 0.7mm and 1mm pitch threads so the net difference is 0.3mm. Working perfectly!
The description of the differential screw I found very interesting. I was looking for a way to hold a collet chuck in my pillar drill so that I could use it for light milling. The idea I came up also used two threads of unequal pitch to draw the arbor into the taper. Excellent video.
Priceless information for anyone designing fine adjustment into a device. Once again I have to say that this video alone would have saved me many days of head scratching back when I was designing and fabricating research instrumentation. Before I learned about differential screw mechanisms I used to go to swap meets and buy beat up old micrometers and use them for projects that required fine adjustment. I know the following remark betrays my prejudices and I in no way want to diminish the incredible power of software based design, but it is great to see design done in the shop with physical objects. Many thanks Tom!
There are some great tips in here. Loved the ball bearing /screw combination; that can have soooooo many applications. Detent applications are common for ball bearings but I don't recall seeing a ball bearing applied like that.
This is a clever concept for achieving precision, if I understand it correctly; one screw unscrews, while the other one screws in, so it actually moves at a rate corresponding to the difference in the amount of travel between the two different thread pitches. The part I don't understand is how it could work without one them being a left-hand thread.
When you screw the big bolt in, it wants to push the parallels apart since the top parallel is threaded. However, since it also screws onto the small bolt the push on the parallels is reduced by a less amount since it also screws down onto the smal bolt with smaller pitch.
i know i am late in the game but this has intrigued me to build my own repeatometer etc. So if I am not mistaken, the top bolt is threaded into the bar and the lower 10-32 is threaded into the lower bar as well? Thanks!!!! Geo
interesting stuff tom. i" learnt "a lot of new words today, and thanks to you and google i may even be able to use some to impress my buddies. the world beyond tenths is like looking for quarks to me. i really enjoyed the machining and the creative setups as always. i will forever remember the differential screw. thanks for another great video.
I’m a little late to the party but knowledge like this doesn’t go out of date. Some fantastic elements to apply here. As someone just starting to try this video stuff, I really appreciate the work that goes into these.
Adding... I think Tom deserves an extra-big "atta boy." Cooking up demonstrations and making visual aids takes a ton of time. He's done this for all his hundreds of videos and got little thanks for it. Tom, if you weren't 950 miles away, I'd buy you a pizza and even help you eat it. That's how grateful I am for all your good works.
Hey Forrest, Thanks for keeping us all honest around here. Your comments and insight are always on point. This humble mechanic thanks you. All the best, Tom
Being an obsessive/compulsive type personality, this fine measurement series is fascinating. Especially like the fine thread, within a fine thread, differential screw. Thanks Tom.
love this. this is a great example oh combining simple machines to make a much more complex one. it's one of the things I love about this trade. (note must add this to the build list.
Holy shit Tom, that was very I interesting/brain challenging. Just about to look up my Black Book to get the inside on these differential fittings. Great series, thanks for sharing. Kindest regards. Joe.
I hadn't seen that equation for differential screw pitch before. It's remarkably similar to the equation for parallel resistance, except in the case of right/right pitch there's a minus sign. Now I've got to make something with one of those setups. Cool.
I was trying to do the math and I was putting the actual pitch values in the equation, whereas you really meant just the TPI values and I was getting crazy results!
Good stuff Tom. One of the benefits of your design (for me, at least) is that, since the flexure can be disassembled at will, there is a lot better chance of a home shop machinist getting through the fabrication without destroying it. If I were building the original design I would have to save the saw cut as the last op, and I would still probably wreck it before getting the limit straps in place. and the differential thread idea is brilliant! Regards, Dan
Sneaky? How about alternatively derived, I say your "Tomiszims" are just one of the reasons I enjoy your productions.You have a truly unique mindset.Thank You for the adventurous journey in machining, and satisfying culmination thereof!!!
Excellent information. That differential screw and parallel flexure mechanism would be a great improvement to the adjustment on a cheap comparator stand I have. Great, another project to do before finishing another project.
Man, I listened to the first minute of what you had to say, and thought it would be interesting to see you review/dissect the mechanics/precision of some home/hobby grade "precision equipment" - I'm thinking like 3d printer, or cnc machine. A lot of people don't understand the tolerances of their machines, and I just think it would be interesting to see you dissect one at that level.
This and the last episode were both great. Your indepth analysis of the subtleties behind the mechanics of the tooling and design improvements were both damned interesting. You should do a whole series of "Tom Redesigns [foo]" where foo = industry standards like the Kurt vice (which is no doubt fairly good, but has it's flaws, i.e., overtorque will lead to canting your workpiece vertically, rather than keeping it flush).
Great content, I feel like I should be prepping for a midterm though :) . With the new knowledge I quickly tried to figure out how to get you a one-one turn with the tenths indicator you had even though you were close with the P(effective) of 224 as set up. With 0.0008" total indicator travel for a 360 degree turn you would need at P(effective) of 1250 which could be done with none standard hardware but that was not your point... so with standard gear it looks like that 224 is a good result that can easily be achieved. Just messing with some combinations and an formula, it appears the P(effective) increases as the individual TPI(s) increase and the difference between them decreases. With a custom set of hardware, 80 TPI and 79 TPI, you can get to 6320! Great lesson Tom, Thanks
A very informative feature again. I would live to see your solution to increase the sensitivity of a 1/10000" indicator so we lesser mortals could look to build the meter for home shop use and negate the hi-end indicator requirement. Keep up the great series . All the best. mat
Great demo of the differential screw! Was playing with something similar recently, they're really neat things. You might know them already, but Thorlabs sells stock adjuster screws in various sizes (1/4"x80tpi is popular) along with drills+taps. Not the cheapest, but maybe handy to have.
Hi Lindsay, Somebody else sent me a link. I didn't realize they were so cheap from Thorlabs. Used to New$$port pricing I guess. Thanks for the comment. Cheers, Tom
When you finish this project, let the gentleman who calibrated your surface plate borrow it. Of course, if it performs better or equal you may not see it again. Your videos are really cool and show a wide range of interesting topics! Keep it up!
30:00 ok you got me :-) I wonder however: the threads aren't smooth, how bad is the "rumble" noise still present in the differential screw arrangement?
A drop of weld on the tip of the screw also works. I've done that with an adjustment screw for an indicator base. I don't remember what filler i used tho. Hasn't lost its tip in 10 or so years i've done it.
Very neat! I like to see alternatives explored with reasons why. I have one concern. When you tap the flexure and the base then the screw threads set the any spacing when then two parts are put together. I have been bit by that in the past. Is that what you want in the end?
Tom, fantastic video. Is there any need to create a vent hole in top portion of the differential screw to avoid air/lube compression ? or are the threads course enough to let air/lube to pass? (i assume some oil could be added to the threads to help any friction)
Instead of the bolt in the milled slot, use 1 or 2 metal dowels press-fitted into one side projecting out into larger holes on the other side. The size of the larger holes would dictate how much lateral movement you have. This would be simpler to produce. I hope this makes sense.
I came up with another simple way to increase the sensitivity of the indicator to the adjuster screw by making the lower limb thinner than the upper. After viewing the first video. I could hardly stop thinking about the differential screw. While calculating pitch multiplication factors, I noticed something in Rahn's design which can be further exploited for precision. Notice that both the upper and lower limbs are free to move relative to the base under the force applied by the adjuster screw. This means that the spreading distance from the adjuster screw is distributed between the two limbs in accordance to their springiness. If in the original, assuming the spread is equally distributed between the upper and lower limbs, the upper limb moves about half as much as the change in screw length. Since the indicator measures from base to upper limb, this effectively doubles the thread pitch since the upper limb only moves up by half the spread of the screw. My big idea (in my mind) is that one could further increase sensitivity by making the lower limb springier by making it even thinner. For example, to make an effective screw pitch 4x as fine, make the lower limb 1/3 the thickness of the upper. 1/3 thickness should move 3x further under the same screw force. For a total movement of 4 units, (1 upper + 3 lower) the upper would only move 1 unit. As long as the elastic properties of the limbs is not exceeded, the thinner the lower spring, (or thicker the upper) the greater the thread pitch multiplier. Your proposal (previous video) to make these limbs bolt together in the previous video, makes it easier to try out this idea. Thanks again for this awesome follow up video. I'm vary curious what you think of this.
Hi Goone, It may have some merit. One reason I constructed the analog of the Rahn was to see what effect the two limbs spreading had on the sensitivity needs for the knob. There is certainly some effect but hard to quantify. The other purpose of the screw is to actually unload the indicator from the contact point when not using the device. It needs a bit of travel to do that as the preload of the indicator is something like 1mm. Thanks for the comment. Cheers, Tom
