[peteeng]

Hi Pippin and others - Mild Steel will not change shape over time unless some other factor is brought into play (red heat, more machining, considerable harmonic vibration, sand blasting, mechanical overloads ). The welded structures internal stresses change with change in geometry (machining) with a bit of luck and slowly working each side it may come into tolerance. Or it may not. JIC and others, there is no reason to 100% weld the bits together in a machine part application. So stitching it together is a much better strategy to minimise distortion if stress relief is not available. Hard soldering or brazing are much better alternatives if thermal SR is not available. Peter

[machinedude]

everything on the gantry is the easier part to keep flat and straight. the table and all the linear motion parts that bolt onto it are the hardest part from what i have seen in my build. but i cheated a little and had the bearing blocks and a short set of rails to determine the needed width of my table. so all i really needed was one cross section to fit like a glove and make the rest of them exactly the same. bolted frames are easy to control this way. welded frames are a different story so it will be interesting to see how Jake pulls that part of the build together.

[JakeInCharlotte]

It's a very interesting topic. Before I started doing any welding on my components I went back and had to refresh my self on how I could limit distortion in welding components, as the distortion can also introduce stress into the welded components and cause me to do a lot of extra work to offset/fix the distortion.

I put a lot of thought into the welding. Weld distortion is real and spectacular! When I welded up the frame, I staged my welding so one weld would pull against the other. It seemed to work pretty well as I don't have a banana. I also choose to pull the bead in a single line to minimize heat instead of stitching. I looks kind of ugly, but worked out OK.

Or it may not. JIC and others, there is no reason to 100% weld the bits together in a machine part application. So stitching it together is a much better strategy to minimise distortion if stress relief is not available. Hard soldering or brazing are much better alternatives if thermal SR is not available. Peter

Fingers crossed!

everything on the gantry is the easier part to keep flat and straight. the table and all the linear motion parts that bolt onto it are the hardest part from what i have seen in my build. but i cheated a little and had the bearing blocks and a short set of rails to determine the needed width of my table. so all i really needed was one cross section to fit like a glove and make the rest of them exactly the same. bolted frames are easy to control this way. welded frames are a different story so it will be interesting to see how Jake pulls that part of the build together.

I'm putting a lot of faith in the 3 x 6 surface plate and my homemade power scraper. Also, I'm the lookout for an autocollimator.

[NIC 77]

I put a lot of thought into the welding. Weld distortion is real and spectacular! When I welded up the frame, I staged my welding so one weld would pull against the other. It seemed to work pretty well as I don't have a banana. I also choose to pull the bead in a single line to minimize heat instead of stitching. I looks kind of ugly, but worked out OK.

Pulling in a straight line....I call that a straight drag....as opposed to making "C" shapes or "dropping dimes". But most of your frame is stich welded. Like this -- -- -- -- -- -- --. I think we're mixing up some terms here.

I'm putting a lot of faith in the 3 x 6 surface plate and my homemade power scraper.

That power scraper is interesting! What kind of a tool is on there? Do you need to sharpen it often? How is it working out for you?

Also, I'm the lookout for an autocollimator.

I was thinking, what do you need one of those for....then I looked at your design. Yeah. Crud. I see now. You want to make the mounting surfaces for your rails flat for your two long axis, is that the reason? And you're going to scrape them flat? Am I close?

Alternatively you could make a custom straight edge (because your design has welded the table in the way), and use wax paper or cosmetics to find the high spots.

Will one switch be able to stop movement on the Z axis in both directions? (Up and Down).

No. You can choose to only use one limit switch for the Z to stop it if it hits top of travel. As the dimensions of the bits that you are using and the material height you will be cutting will vary, you can still crash the Z on the downstroke if you mess up your code or setting your Z zero, even if you haven't come to the end of travel of your Z axis. There is nothing stopping you from adding a switch at the bottom of travel as well, if you want to, but by then, you may have already crashed.

Typically you might see, 3 switches for the long axis, 2 for the short, and one for the Z. 3 switches on the long axis is so that you can independently home your gantry on both sides so that it does not get out of skew. This requires 2 switches at one end. You need to figure out what your board can support.

Same switches for homing and limits.

[JakeInCharlotte]

Hi NIC.

I typically weld in a little "c" or cursive "e", that is a stronger weld IMHO but puts a lot of heat in the joint. For most of this project I simply dragged the puddle in a straight line in an attempt to control the warp.

As for the power scraper, it works great! I converted a cheap drill press into a slow speed grinder with diamond honing discs.



Scraping is slow but amazing. I can take off .0002 with each pass.

A couple scraping lessons I have learned:

• Scraping steel make some nasty slivers, and they go everywhere.
• Drill and mill everything before scraping. The new stresses from a single hole can throw you out .010 easily.
• Try to get within .003 before scraping. This can be tough with a loose milling machine. I go from sneaking up to a dimension to blow right past in no time at all.

My plan to align the table rails is to set the surface plate on top of the table and shoot down from there, in combination with some homemade straight edges as you mentioned.

I am completely ignorant about the wiring for a CNC. Maybe I used the word limit when I should have said homing. The basis for my design is an Onsrud router and they used 1 on the Z. The switch is a Honeywell GLC series. I guess my question is, does the software remember which direction the axis was moving when the switch is tripped?

[pippin88]

Hi Pippin and others - Mild Steel will not change shape over time unless some other factor is brought into play (red heat, more machining, considerable harmonic vibration, sand blasting, mechanical overloads ). The welded structures internal stresses change with change in geometry (machining) with a bit of luck and slowly working each side it may come into tolerance. Or it may not. JIC and others, there is no reason to 100% weld the bits together in a machine part application. So stitching it together is a much better strategy to minimise distortion if stress relief is not available. Hard soldering or brazing are much better alternatives if thermal SR is not available. Peter

Good to have an answer.

So machine the weldment, check the geometry is satisfactory, further machining as necessary and then get on with the build.