Rotary table calculations

Image from: http://upload.wikimedia.org/wikipedia/commons/archive/2/2c/20080518200048!Terms_involute_gear_engagement.PNG

Rotary Table

For the rotary table we will be making a gear cutter, worm, and worm wheel. I just "figured out" an equation for calculating TPI, and worm gear teeth. Starting with S=r(theta) I derived:

60/TPI = r(2*pi)

Where 60 is the gear ratio of the worm gear to the worm wheel. Referenced below (in the previous post) is what lets you calculate the resolution that will give you on your table. The 'r' in the equation i suspect is the diametrical pitch, or namely the diameter where the force of the gear is being transmitted, not the OD of the gear. Anyways with this equation I can adjust my TPI for the cutter, and worm gear till I find a TPI that matches what the lathe is capable of cutting. Now in my research i did find that 40' was common for this application no the 60' degrees you typically see. So I need to do some more research or simply "test" it out with an ACME cutter that i already have. (I'd prefer to take the simple route)

An example calculation yielded:
8 TPI, would yield a 1.1936 radius worm wheel.

CNC update + rotary table start

I received our first CNC package of THK linear rails, they are about 30" long were used, but in great condition. My only gripe is the wipers on some of the carriages have a tendency to "catch" on the rail a little bit. I simply hand bent it up a marginal bit and it rides smooth as butter. You can see the ground sheen off them. I put electrical tape on the ends of the rails to ensure the carriages don't slide off. Pictures of these rails will come later.

I had done some research on the spindle, and found several examples of folks using treadmill motors to power their mini-cnc's typically in the 2-3 HP range, (DC motor) with an operational range of 0-6750 RPM. I didn't know much about the controller, and Troy instantly thought that was a great idea, because the VFD we were looking at for the 3 Phase 3HP motor he has would cost 300$. Where we found a treadmill on craigslist for 20$. Hopefully we can pick it up tommorow for that price. In any event he found a DC controller for the motor for less than 100$ on ebay on a 'buy it now' auction which puts at a much more cost effective solution for the motor control system.

There was also a discussion about frame materials, cost, rigidity, ease of manufacturing, and ease of transportation. So things like extruded high grade aluminum (6061), welding steel vs. warpage, and granite composite were discussed. No conclusions were drawn up with that except that we want our solution to incorporate all the attributes listed above.

When I met Troy and Bryan at the techshop. They showed me how to do assembly's in SW. This made me tremendously happy because it's like doors are being opened up for my imagination. Since we finished the fly cutter, we are now going to move onto other bigger and better projects, (which involve the use of our previously made fly cutter tehehe.) This time it is a rotary table. So I started working on modeling the rotary table on solidworks. Here is what i've got so far:



One of the problems i'm encountering is I don't know the dimensions for a standard t-slot so i went with an arbitrary depth of 1 inch and 0.25" slots. The beauty of parametric modeling, is I can go back and fix the dimensions later, Yay! I've also been playing with screw placement, clearance issues with mounting bolts and the bolts that hold the rotary table together.

The problem is I'm trying to make use of material that I already have (0.38" thick steel stock.) Which I have plenty of so I'm going to have to carefully play with the dimensions and design internal blocks to bolt everything together since 0.38" steel is not thick enough for me to comfortably drill and tap strong bolts into it. While I could calculate the yield load and fatigue life of bolts that would fit into the stock, I simply am going to use the KISS principle and oversize the bolts, and use internal mounting blocks.

References:
http://www.technologystudent.com/gears1/worm1.htm
http://www.helicron.net/workshop/gearcutting/cutting_gears/
http://www.astronomiainumbria.org/advanced_internet_files/meccanica/easyweb.easynet.co.uk/_chrish/worms.htm
http://www.cnczone.com/forums/showthread.php?t=63992

Fly cutter test

The fly cutter was tested today! Admittedly, I still have to use these silly bolts that "bump" into each other because that is all i could find at Lowe's/Home Depot. Apparently it's not wise to make something 1/4-28 NF thread. I did see quite a bit of 10-32 NF though.



I hand ground a HSS-cobalt blank on the cheap Ryobi 6" grinder at the shop. I noticed the wheels were pretty dirty from dust/aluminum. So I located the dresser, and dressed both wheels (they were in bad condition.) From there I found a bucket of water and got to work using cnccookbook.com as a guide. The rounded edge for cutting was the hardest part. I think i took a little too much off so it likes like 10-20 thousands wide, where the guide i read said you should go for 3-6 thousands. Oh well. It worked!

Here is a v-block i used earlier, and i faced one side with the fly cutter in a jiffy, hand feeding around 4ipm at 660 rpm. (High speed i know - but there is no lower gear on that machine.)

CNC update

I have successfully purchased all the linear rails i'll need for the CNC. They all are THK brand, the Z-axis has HSR20 carriage blocks for the moment load. The x and y axis are on SR20 and SR25's. We'll use the SR25's on the X-axis to take the weight of the Y-axis. The lengths for each axis are:

x-axis = 32.25"
y-axis = 29-7/8"
z-axis = 20"

Now these linear rail length's are the total length not the travel length, because the carriages when bolted up to the table mounting plate will be taking something like 8" off the total travel. So a rough guess of our work envelope will be something like:

x-axis = 24"
y-axis = 22"
z-axis = 12"

We now are working on locating screws and purchasing screws. I also purchased http://www.hobbycnc.com/products/hobbycnc-ez-driver-board-kit/ for 64$. We are holding off on the steppers for just a bit to finalize our torque requirements. Troy has been busy on the spindle design. He sucessfully modeled it up in SW, and he is performing FEA calculations on it. While I did find him some angular contact bearings that dimensionally work, and financially work, they did not have specifications on the tolerances. Troy has been working on some equations he can apply to ensure everything falls into our safety factors.



I have been keeping a spreadsheet on expenditures... Oh man it's starting to add up. I alone have purchased 450$ worth of parts specifically for our CNC alone. Troy special ordered some 4340 for his spindle for 50$. He also needs to make a tool post grinder, take a safety milling class, and buy angular contact bearings, and a VFD (300$) all for the spindle. This is going to hurt our pockets. This is really going to start being the "test" on us, we are on a tight budget (our own pocket money no less.) Our wits, our ability to scrounge, our courage, and our vigor are all going to be necessary to get through this project. The last project that i have taken part in that is of this magnitude was the 2008 HPV project. That said back to research!

Progress on CNC design

After wasting most of the day with friends, I got down to work late at night. I have been writing a program to effectively choose ballscrews, stepper motors (particularly this), and gear ratios (if they are necessary.) After doing some reading for the past few days, i've come to have a much better understanding of stepper motors, torque, pulses per second, microstepping and the relationship between these design variables. The following are good reads to get you on your way for selecting a stepper setup for your machine.

http://www.cnczone.com/forums/showpost.php?p=242497&postcount=7
http://www.geckodrive.com/faq.aspx?n=8792
http://www.kelinginc.net/SMotorstock.html
http://www.cnczone.com/forums/showthread.php?t=5780
http://www.cnczone.com/forums/showthread.php?t=5866

Here is a screenshot of the program i've been working on to optimize the design features of our CNC machine build. In this case i've used it to determine the maximum feedrate of 295 IPM, according to the Torque vs. PPS chart the stepper should be yielding around 0.3 N/M of torque in a direct drive configuration. Using gear ratios, and manipulating the microstepping ratio I may be able to increase the torque for a mid to high range feedrate. As it stands 300 IPM is a bit unncessary for the metal working we'd be doing. I'll have to go over cutting forces to see if everything falls into spec. As it stands i know the linear rail system will certainly take the required loads, so the only question is if the stepper's can maintain enough torque, and if the ballscrews can take the required axial loads. (We looked up dynamic and static loads for some example ballscrews at THK and NSK)

OMG It's HOT

So we went to Canby, and bryan, bought a saw! Not just any saw, but none other than the great CHICAGO harbor freight high quality product covered in dust and cow manure. But as bryan so eloquently put it, "It cuts pretty well."

Oh and we bought some steel with troy. Purchased some square stock for my ball bearing machine frame, and bryan bought some 3" diameter steel tubing for his stirling.

TAta time for smash brothers!

Correction + CNC progress

Forgive me, I seem to have overlooked a major thing, I won a test indicator NOT a dial indicator. There is a difference, the test indicator functions as a "finger" and can be used inside holes. The Dial indicator is very sensitive to direction of setup. A finger indicator you press on from the side. I can't believe i didn't think much of it and simply "knew" what i was looking at was what i wanted, and had all the features i was looking for.

CNC progress report:

Troy and I made substantial design decisions again. I did some calculations based upon the rail and carriage dimensions determined the usable travel on the rails we are "suspected" to get. I bought one of the rail sets with the 'buy it now' feature on ebay. This included 2 used rails, and SR20 blocks from THK. Got it for 100$. We are waiting on two more rail sets to finish the deal. I really hope we win them, because if not i'll have to redo calculations. For our z-axis i'm going for some HSR carriages to ensure they can take the forces on the head.

Using the dimensions of the rails, I calculated the theoretical range of ballscrews we needed to make the most use of the rail travel. We sent off the request for these appropriately sized ballscrews to our "supplier" more info on this later.

We started to evaluate what kind of steppers we want for our setup. It looks like the 282 oz-in steppers will yield a max driving speed of 73.82 ipm. (We had to assume that the max pulse frequency is 10000 pulses per second based upon the specification chart http://www.kelinginc.net/KL23H276-30-8BT.pdf ) To do the calculations was a bit annoying because the charts we had were based in pulses per second. Using the microstepping ratio we were able to compute the approximate RPM's at a given pulse rate. Using the generated torque's on the specification chart we can compare it to the required torque for a cut a specific feed rate.

With all these calculations I've decided to start writing a program to make the process easier to repeat, and manipulate the variables for design considerations. I'll put that up later.

Won a Tenth's Dial Indicator!

I won! I got it for about 60$ shipped. Happy day, cause they typically can be anywhere from 90 to 350$ for a tenths indicator. This one comes with a few accessories, and looks like in good condition.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWNX:IT&item=190322734575

Dial Indicators

As I've been looking for a .0001 or otherwise known as a tenths indicator i inevitably decided to do some research on the given features. Apparently you want a Jeweled indicator because it won't lose precision as badly as one without it being jeweled. You also want size 2 if you have a choice, the size of the indicator has no impact on the accuracy of the indicator.

Reference:
http://longislandindicator.com/p139.html

Fly cutter complete, Scouting materials

I love scouting for materials for making things in the future. While I was looking for the appropriate sized screws at home depot and lowes for my fly cutter,

I snapped a few shots of some interesting things. Like the really low cost ryobi 8" grinding wheel for only 64$ at home depot! I've used the 6" version at the techshop and think it is indeed worth the money. Then of course there is the fellow who parked his toyota truck in classy style (namely on the curb over, and filling two complete parking spots in the middle.) Lastly this 1000 feet of seran wrap was interesting...

Troy and I have been heavily researching, and making serious design decisions about the type of ballscrews, geometry of ballscrews, spindle dimenisions, spindle bearings, dynamic loads for various parts. Grant will do the breakout board once we decide on the stepper holding torque we want. We are feeling 280-425 oz-in. The breakout board we have been looking at is rated for the current of those motors. We just wonder if we could simply pre-upgrade them from the get-go. Basically we are following the "get it right" the first time approach and researching heavily.

Hopefully we will make some more major decisions tommorow.