Dovetail Lapping or Scraping
The mills and lathes that come from overseas most times don't have the best fitting gibs.
A lot of them, especially Sieg products, suffer from dovetails that are sloppily machined
which leads to binding at one end of travel or both with table slop in the middle. The G0704
has better tapered gibs versus the flat gibs on most other machines.They can still suffer from
some of the same problems, ususally because of a warped gib strip. All these troubles can be lessened or eliminated by lapping, scraping or both. Which to do, which to do. You'll find many arguments for/against both such as in this long thread on Cnczone. It's my thought that if the amount of material to be removed is small, then lapping should be good enough.
Basically, Lapping invloves loosening the gibs, pulling the table for example off, applying a gritting paste to the dovetails, reassembling, and pushing the table back and forth by hand many times
until you feel the binding areas no more, apply more grit paste and adjust the gibs a little tighter, and repeat. Generally starting with a coarse grit (240 or 320 if severe) and then with a couple finer grits, 600 and 1200 for example. that's what I used on my X2 years ago. Follow up with a thorough cleaning and reassembly. Scraping can be more involved and requires more skill, basically you apply a layout fluid such as Dykem to the mating part, assemble them and push them back and forth against each other. This will show you the high spots that you can then scrape down with a flat tool. Repeat the process until you have a uniform transfer of dykem over the entire length.
Don't forget some Layout fluid remover for cleaning up. Mcmaster.com #19215A21
You'll find some links below for more details, some are for lathes but the process is very similar.
Lapping with Wet/Dry Sandpaper - from mini-lathe.com
Lapping the Dovetails - Mike Taglieri's Procedure
Basic Hand Scraping books and videos - machinerepair.com
Video - Scraping in a dovetail - detailed
Video - Scraping in a lathe's top slide with tapered gib
Machine Tool Reconditioning by Connelly - The bible to hand scraping.
Scaping Why and How - pdf by Ron Gerlach
American Lap Company
Newman Tools Timesaver
MscDirect Clover Lapping Compound
MscDirect - Machinists Scrapers
T10086 3 Pc. Machinist's Scraper Set - Grizzly
Alibre to DXF for CAM.
Watch this small tutorial for showing how to export an Alibre CAD part to either
.stl for 3D CAM work or .dxf for 2/2.5D CAM work.
Lots of free or very affordable CAD and CAM programs here.
G0704 CNC Conversion Tooling
Below you will find the minimal list of tooling needed to machine the
G0704 Phase 1 and Phase 2 CNC Conversions.
#29, #21, #7, F, 13/32, 7/8*, 1 1/16*
8-32, 10-32, 1/4-20, 5/16-18, 15/16-16*, 1.125-18*
5/8 90 degree **
5/16 double ended **
The majority of the items are drill bits and taps. You should pick up a complete set of each anyway for your shop, they are basic, must haves. Don't buy the cheap imports, spend a few more dollars for decent US high speed steel or cobalt bits, with a Drill Doctor, they should last for years. Drill bits size 1 - 60, Drill bits size A -Z, Drill bits size 1/16 - 1/2,
Tap and Die Set.**
* If you can cut internal threads on a lathe or get the C7 ebay ballscrews, you won't need these drills and taps.
** optional tools not absolutely necessary.
CNC Conversion Costs
The following are some ballpark CNC conversion costs for converting the G0704.
You'll find many links to the particular items on the Electronics Page and under "Accessories" on the Main page. This is what you'll need to spend to make a mill that can machine parts. Tooling and software will add to the costs as your needs increase, but this basic package will allow you to make chips. You'll be years adding to your tooling accessories. Software can cost more than the mill easily but there are plenty of free or very affordable programs to get you going.
Note: The mechanical side of the DIY CNC conversion with ballscrews can be done for less than $450 which is less than HALF the cost ($975) of comparable ready made kits out there. Sometimes it pays to Do-It-Yourself.
G0704 Mill - $1294 shipped
CNC parts (stock, hardware,bearings, couplers or pulleys, plans) - $200
Ballscrews and nuts for Phase 2 (Roton -not machined) - $185
C7 Ebay Ballscrews and nuts for Phase 2 (machined) - $225 shipped
Electronics (Motors, Drivers, Power Supply, BOB etc.) - $400 - 700 *
Computer - $200
Mach 3 (to run the mill) - $175 **
Enclosure, wiring, E-stop etc. - $200
Tooling Package - $300
Measuring Package - $60.
Total for Phase 1 conversion and cheaper electronics - $2829
Total for Phase 2 conversion and cheaper electronics - $3014 - $3054 ***
Total for Phase 2 conversion and higher end electronics - $3314 - $3773 ***
Total for Phase 1 if you already have a
PC, Tooling, Measuring and use LinuxCNC (free) - $2094 - $2394
Total for Phase 2 if you already have a
PC, Tooling, Measuring and use LinuxCNC (free) - $2279 - $2619
* The cheaper electronics are the package using the KL-6050 drivers and are what I've run on mine, it gives awesome performance with 300 IPM rapid speeds. The more expensive package uses the digital drivers.
** Mach 3 can run 500 lines of code in the free version so you don't have to spend the $175 right away. Most everything will work and you can make parts with the free version or you can use LinuxCNC for free.
*** The price difference is for Roton or C7 Ebay Ballscrews and Nuts.
Note: Costs for the tooling needed to make the conversion parts aren't included as many of them are basic items like drill bits in most homes, others are in the tooling package.
The large taps may or may not be required especially if you use the C7 Ebay Ballscrews and Nuts which won't require threading of the ballnut mounts.
I suggest using ballscrews and nuts from either Roton.com or Linear Motion on Ebay from China strictly as the most affordable options. Backlash for a single ballnut can be reduced to less than .001 by repacking the nuts with oversize balls. Video here. Double ballnut versions of the C7 ballscrews from Linearmotion are available that would be zero backlash.
Rotons have been very popular with the benchtop crowd for years because they are the most affordable not the best accuracy. The chinese C7's are competing with that affordability now and are more accurate than rotons. If you want C7's from a US supplier go to Homeshopcnc.
You can get the C7 ballscrews and nuts for my conversion from Automation Tech now.
One of the benefits of the rotons is their smaller ballnuts for tight locations but you can get
the same size nuts with more accurate screws like Nook Industries XPR screws for example.
The deciding factor is $. Lead error is one of the main specs to consider when choosing a ballscrew. The C7 rated screws for example have an error of .0018/foot or .05/300mm.
This means in that random foot of ballscrew the physical distance of the lead (,200 for example) can vary +/- .0018 in that foot. It accumulates meaning one lead could be .200, the next could be .2001, the next could be .20005, the next could be .20003, the next could be .19999, and so on. The lead wavers around the perfect .200 lead, up and down. It doesn't continue to accumulate always on the high side otherwise the total error per foot would put it in a lower class of screw. If a screw accumulated an error of +.0018 for every lead in that foot it would have a total lead error of +/- .054, they wouldn't be worth the money they charge for these higher end rolled ballscrews if that was the case.
This write up is dated but shows the comparisons of several ballscrew types.
For more info on ballscrew types and tolerances read Swedes nice write up here on Cnczone.
More good reading.
NSK - What is a Ballscrew?
Ball-screw basics: Debunking the Myths
What to Look for When Choosing a Precision Ball Screw
I got a quote from Nook for their XPR ballscrews and nuts that would work for the G0704 CNC conversion. (06-20-2012) Listed are prices for 4 feet of 5/8 ballscrew and types of ballnuts for use on the X and Y axis as well as 2 feet of 3/4 ballscrew and types of ballnuts for use on the Z axis. These are precision rolled ballscrews with a lead error of only +/- .001/foot.
Note: You'll need (2) 5/8 ballnuts (X and Y) and (1) 3/4 ballnut (Z) in the type you like.
X and Y Axis
4 feet XPR6320R48 5/8 ballscrew - $104.05
SGN10083: SGN Non-Preload nut - $504.17
PRN10106: PRN Preloaded Nut w/out Wipers - 408.30
SSN0389: SSN Preloaded Flanged nut - $693.10
SSN10274: SSN Preloaded Internal Return nut - Discontinued
SSN10304: SSN Preloaded Flanged Internal Return nut - $1010.95
4 feet of XPR7520R48 3/4 ballscrew (request custom length cut 2 feet to save $) - 199.95
SSN0390: SSN Preloaded Flanged nut - $883.47
PRN10109: PRN Preloaded nut - $759.21
Cheapest price for full conversion - $1779.84 plus shipping