Monday, 26 December 2005

Project - CNC Mill X and Y Axes Trial Assembly

With the ballnut carriers complete, and after some basic plate work we come to the X-Axis trial assembly.



Tapping the motor mouting plate for the X-Axis. In order to keep the tap vertical and square to the plate, I am using a small block drilled to clear the tap.


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We start off with the carriage plate, registering the X rails with the step that was ground into the plate.


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Using a dial test indicator attached to the master rail we also check to make sure that it runs parallel with the side of the carriage...


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....here is the view from the other side. Looks pretty good.


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Now we make sure the othet rail is parallel to the master rail, again with the DTI.


This is a time consuming process took me a couple of hours to get this far.


200907232110.jpg



Once we have the rails parallel, we flip the whole thing over and screw the rails to the table, at the same time mounting the ballscrew.


You'll notice I haven't done the floating bearing for the X-Axis yet.


The vice is the squarest thing I own so I adjusted its position such that the back surface is parallel to the X-Axis travel and then used the side to make sure that the Y-Axis master rail travel is perpendicular to the X-Axis.


Once this is accomplished, as you can see here I am making sure the secondary Y-Axis rail is parallel to the master.


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The bed and the Y-Axis ballscrew ready for the table assembly to be bolted down.


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Here the table assembly has been flipped and the Y-Axis rails have been screwed down


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Monday, 21 November 2005

Project - CNC Mill Bearing Blocks Part II

Back on the mill registering the plates to size.


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Here are the blanks.


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Find the center, drill with a 12mm drill


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Now bore out to clear the lock nut for the ballscrew


200906291210.jpg


Here I am using a bit of old scrap (one of the scrapped parts of my Steam Engine project). I have milled a register and drilled and tapped a hole in the center to mount the part. This way, I only have to set zero on the DRO once and I can drill and counterbore all three plates identically.


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Here is one of the keep plates mounted in the jig - I have just finished drilling all the holes.


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The housings and keep plates are complete.   I now need to make a spacer ring to keep the center races of the two bearings apart.


200906291211.jpg


Here I have turned the locating spigot off of my stub mandrel that I used to hold the housings for turning.  I have also drilled a 12mm hole 20mm deep in preparation for boring out to 18mm.


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Parting off a spacer ring.


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Assembly - Bearing goes in, followed by space ring, then followed by another bearing....


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The keep plates are then screwed on with M4 SHCS.


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Monday, 14 November 2005

Project - CNC Mill Bearing Blocks Part I

The bearing blocks are what we mount the bearings in to stop axial play in the ballscrew. We need one for each axis.


200906291200.jpg


You can buy them, but they are mega expensive. I bought some angular contact bearings from eBay and decided to make my own blocks.


The blocks contain two opposing angular contact bearings separated by a small spacer - this allows them to be preloaded to stop axial play in the ballscrew.


Drawings can be found here.


I cut some rough size blocks on the bandsaw and I am facing them to size in the mill.


200906291201.jpg


Heres what one looks like after it has been bored for the bearings on the lathe and faced to length. I am getting ready to drill the mounting holes and the holes for the bearing keep plate that screws to the front.


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Finding the edges with an edge finder.


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Holes drilled.



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Back on to the lathe now - part is mounted on a stub mandrel for turning away the bits that don't look like a bearing block.


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One down - two to go...


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Stay tuned for the keep plates and final assembly.

Wednesday, 9 November 2005

Project - CNC Mill Ballnut Carriers

Isel ballnuts look like this:


IselBallnut


So we need a carrier block for them. The carrier block needs to do two things:



  • Hold the ball nut so there is no axial or rotational play

  • Provide a means to take advantage of the ballnut's adjustable preload feature


Blocks for these ballnuts are available from Isel, but neither configuration was suitable due to space constraints. I decided to design my own.



IselBallnutCarrier


As you can see, the block is split with holes for two clamp screws. The larger of the two holes along the side is for a locating screw that mates with a hole on the side of the ball nut to stop axial play, the smaller of the two is a clamp screw that bears on the side of the ball nut and when tightened squeezes the ball nut slightly so that the nut can be preloaded. The other two counterbored holes at the front are simply mounting holes.


Block after milling to size and milling out recess.


http://farm6.static.flickr.com/5140/5541876329_7ac72ec938.jpg


Now we flip it over and bore it. The ballnut is 28mm in diameter so I am aiming for 28.00 -0/+0.04.   I use WD-40 for cutting aluminium - works the same as kerosene, just dosen't smell as bad.


http://farm6.static.flickr.com/5132/5542456738_c25287aabd.jpg


Drilling the holes for the locating screw and the preload adjust screw


http://farm6.static.flickr.com/5058/5541876867_bb23301c58.jpg


Counterboring the mounting holes.


http://farm6.static.flickr.com/5175/5541877173_a7ff2f892c.jpg


Just to prove I still use the Taig - here I am adding the chamfer along the sides.


http://farm6.static.flickr.com/5099/5542457656_2c2be3bf8b.jpg


Taig is also used for slitting the block.


http://farm6.static.flickr.com/5011/5542457934_b84856e164.jpg



All done! Assembled with ballnut.


http://farm6.static.flickr.com/5019/5541877937_7599ec8f1d.jpg

Wednesday, 12 October 2005

Project - CNC Mill Bed and Column

The bed and column - the spine of the machine.


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The basic design revolves around bolted/dowelled together sections.


You can see there are no dowells on the sub column which is attached in such a way as the bolts can be loosened so the column can be adjusted back and forward. The column proper bolts to the front face of this and can be adjusted side to side. In this way I hope to be able to adjust the column square to the table.


Here is a pic taken while surface grinding the front spacer, rear spacer and lower column parallel in one setting - this will ensure that when the two cheek plates are bolted in place there will be a clean, stress free bed.


http://farm6.static.flickr.com/5291/5540975213_a67b965800.jpg


The bed cheek plates were too long (470mm) for my X3 so I am using the King Rich at the local college - a monster of a machine - 1/3 as big again as a Bridgeport. Here I am facing in preparation for grinding.



http://farm6.static.flickr.com/5091/5541556104_4edc2883fc.jpg


Parts all ground on their mating faces, and in the process of being setup on the mill to drill, tap and dowell the cheekplate to the spacer blocks. You'll notice that the space blocks still have raw lower surfaces. When assembly is complete the whole shebang will be machined all over to final dimensions.



http://farm6.static.flickr.com/5293/5540976097_6c554b8dd5.jpg


The process continues...


The bolts are M10x40mm SHCS and the dowells are 6mm. Everything is clamped, drilled, dowelled and tapped on the mill whilst set up to ensure alignment.



http://farm6.static.flickr.com/5293/5540976539_0d26bc182a.jpg


The finished product!   You can see here all of the mounting holes for the rails have been drilled and tapped and column base has been bolted in. This shot also shows the Y-Axis side of the carriage plate and the underside of the table.



http://farm6.static.flickr.com/5253/5540977451_ea4af602ca.jpg


From the other side we can see the column constructed using the same methodology and the X-Axis side of the carriage plate.



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Tuesday, 11 October 2005

Project - CNC Mill

Research and Ponderings



I have been looking at getting a CNC capability for some time. I want it mostly to carve 'castings' from solid billets as my main interest is in building small engines. Here in Oz it is cheaper for me to buy (or build) a CNC machine and carve from the solid, than to order castings from the US or UK. If you want to do something a little out of the ordinary, like make your own patterns and get them cast yourself. Forget it. If you can first find somebody who will do the castings, you will then find you can't afford it.


Build, Retrofit or Buy


I considered retrofitting my Taig, however, the thought of carving out a bedplate in cast iron on the Taig filled me with creeping horrors. In addition CV 3D contouring with the Taig lead screws would be a problem because of backlash. I also thought about retrofitting my X3 and doing a ballscrew conversion at the same time. Then I saw www.5bears.com and I got inspired so I decided to build my own. I looked at lots of desktop machines and decided like Swede, that if I wanted the rigidity and performance, I would have to build because I couldn't afford to buy.


I wanted a machine of similar envelope to the Taig. ie X axis 300mm, Y axis 150mm, Z axis 230mm (12, 6, 9 inches respectively for our American friends) and a rotary axis. I wanted zero backlash on all four axes, I wanted massive and rigid, as it is a requirement to machine ferrous metals.


On the topic of eBay



If you live in the US, I'd imagine that eBay is a boon for projects like these. I however, do not live in the US so after looking at eBay for a while, I decided that whilst it could be taken advantage of for some things, it would be better if I looked elsewhere for parts, especially motion control.


For example, there has been a dearth of suitable ballscrews on eBay and premium prices are being paid for quite ordinary units. I figured finally that if I was going to spend AUD$200 on a ballscrew and AUD$50 freight to get it here, I may as well spend AUD$300 locally and get local after sales support and a warranty.


Similarly for linear guides. The freight costs to this part of the world are murder, and there never seem to be the right ones available. In addition, lately the prices have skyrocketted. Probably because of people like Swede telling everybody to go to eBay! :-)


I did however use eBay for a few things, like bBearings - 7200B angular contact bearings - for the ballscrews. I also picked up a harmonic drive to use as a 4th axis.


Basic Design Requirements


 

















Massive and rigid

It is a requirement that I machine ferrous metals, the vibration frequencies when machining these materials is quite low and thus the machine has to be as rigid as possible and structures such that it can damp unwanted vibration.


This sort of rules out a moving gantry design.


I had considered something like:



200906291140.jpg


Which is a fixed table (disregard the 4th/5th axes) in a box design, however since cast iron was the material of choice, I would need a lot to construct a frame big enough and I'd never be able to lift it once constructed.


Another design is the fixed bridge design:



200906291141.jpg


This has merits, however, I was not convinced I could get enough travel in the Z-axis whilst maintaining rigidity.


I settled on a C-Frame design thus:



200906291141.jpg


The difference between this design and mine is the large carriage. My design uses a small carriage to reduce the torque requirement for all that mass. I could also contruct in such a way as to make everything adjustable (see accuracy below) and I could lift each piece when dissassembled.


Here's a picture:



200906291141.jpg


AccuracyWhoa - here is a can of worms. I'm using ballscrews aren't I? 'Course it'll be accurate!

 


Wrong. Well at least not necessarily correct depending...After a lot of reading the secret appears to be something like this:


Design for repeatability. Accuracy can be achieved through calibration or compensation.


What this means is that since machine tools are deterministic and their errors display cause and effect relationships, if I can build compensation into my design I can make my machine tool accurate based on an observation and correction cycle; ie I can calibrate it.


Thus my final accuracy is not necessarily dependent on the accuracy of the components or my ability to accurately machine.


We'll see how this plays out....


CoolantThe ability for this machine to be enclosed and doused liberally with flood coolant during operation is mandatory. I don't want it to be an afterthought. This means protection for all the motion gear and motors, and probably some sort of enclosure so coolant doesn't end up everywhere.


Basic Design Walkthrough





























Bed and column - Structural members


200906291142.jpg


I chose cast iron for all structural parts of the machine. Cast iron is a good machine tool material.
The material I am using is 4E gray iron that is continuous cast in various sizes.


I got it from Flocast Australia (www.flocast.com.au). The people there were very helpful given I ordered such a small lot of material.


The weight of my order was approximately 90 kilograms (198lbs), the CAD software reckons that the machine will weigh about this after all machining. Bear in mind that this machine is only slightly larger that a Taig or chinese Mini-Mill.


The pictures show the two major assemblies, the bed and column. All mating surfaces will be ground, then will be permanently assembled using Loctite and dowels and then final machined as an assembly.


Ways


http://farm6.static.flickr.com/5131/5540957953_e8a848d437.jpg


For the ways, I am using linear guides. I tried eBay, but I ended up with these from a local supplier, at a landed cost similar to what I would have paid on eBay after freight. The good bit about getting them locally is that I know exactly what I am getting and can spec exactly what I want. I also get the after sales service and warranty.
These particular linear guides are HiWin HG series rails and blocks in 15mm sizes. HG series blocks are a 4 row system that can take equal load in all four directions. 15mm rails are plenty big enough for this application. There is no chance of exceeding any of the static load ratings of these rails.


In addition these rails are a medium preload, so to move by hand they are a little stiff. This will help rigidity when machining harder materials.


I got these from Bearings Incorporated (www.bearingsinc.com.au) Neil Day is the guy to speak to - their service is excellent.


Ballscrews


http://farm6.static.flickr.com/5131/5541538626_fcb7c1c223.jpg


Rolled or ground? How much do you want to pay? These suckers are very expensive in ground form. Hence a trip to eBay, however I couldn't find any screws to suit (16mm diameter, 5mm pitch, correct length) so I again went on the hunt locally.
I ended up with ISEL rolled screws. These are class 7 screws (0.003"/12"). Doh! Dude your accuracy is gonna suck!


Not necessarily so - though if anybody wants to donate some class 0 screws to this project feel free.


Remember above all things this: The lead error in a ballscrew is a cumulative error over the stated length. The reality with rolled ballscrews is that over short lengths the actual absolute lead error is much smaller than this. ie you may travel any given 12" of a ballscrew and not get 0.003" error but if you travel 36" you may have anything up to 0.009" error ie it takes a longer travel before the error accumulates enough to be significant.


In any case we are not relying on the ballscrews for their accuracy, only their repeatability. If they are repeatable (which they are) we can compensate for lead error.


This is what manufacturers do in big machines anyway, the machine is mapped by calibrating it with a laser. When the software is asked to move an axis to a particular position, this position is compensated for based on requested vs actual motion when the calibration is done.


Backlash in these screws is not a concern because ISEL nuts can be squeezed slightly to preload them.


I got these screws from Applied Automation and Engineering (www.appliedauto.com.au). The guy to speak to is Andrew Shaw - again excellent service and support from a local company.


Another important part of the motion chain in a CNC machine is bearings for the ballscrews. The captive end of the screw needs to be held by a preloaded bearing such that there is no axial play.
In order to do this I am using angular contact bearings in machined housings. These bearings will be squeezed together in their housing to preload them.


TableI took Swede's lead here and bought the same table that he used on his mill. The surface is excellent, and the cast iron is good quality.
Controller and Drives

I have chosen a stepper system. I wanted to get a servo system, but I blew most of my budget on motion hardware.


In addition, the type of machining that I envisage doing with this machine will dictate slower feedrates anyway so a stepper system will be not that much of a compromise.


The steppers are 300oz-in NEMA23 units and the controller is a 10x microstepping 4 axis controller with external power supply.


Since first time around I didn't want to muck about with electronics and I was after something turnkey - the stuff from www.lowcostcncretrofits.com fits the bill nicely - again a company with excellent service.


4th Axis


http://farm6.static.flickr.com/5177/5541537636_7f51b40f08.jpg


A Harmonic Drive 50:1 zero-backlash gearhead will be the heart of the rotary axis.


Got this on eBay for US$105.00 - Bargain.


The motor is a NEMA23 stepper. I haven't finalized the design of this part yet.