Thursday, December 11, 2014

Linear guides

This post will deal with linear guides.

Basically linear guides are the parts that guide the machine movement.  They move in a lateral fashion.

There are many different types, however for this post I will be elaborating on guides that you can buy for a relatively low cost, and guides that can be fabricated locally with a minimum of cost at your local machine shop.  

This post will grow as I add more drawings and explanations to it.  

The first guide is the smooth round rod.  

You want to balance the size of the guide rod (referred to as just "rod") with the mass of the moving parts.  Suffice to say, if if does not move, by that I mean that that axis is moved by another axis, then you can go larger size without paying the weight penalty.

All CNC machines will have at least one axis that is moved on a second axis.  So for example left and right is on the part that goes up and down.  I will leave out the Axis letter designation for now.

You want the guide rod. In this case round rod to be as straight and as smooth as possible.  It can be bought online, or acquired locally in what is called drill rod.  You want the chrome plated stuff if possible.

Now you can use locally acquired stainless round bar stock, however you may have issues with flexing over longer runs and or not being perfectly straight.  This may or may not be an issue depending on circumstances discussed below.

The round bar will ride on either a linear bearing, or a bushing.  Linear bearings will last much longer, but they are more expensive.  They are also usually not available in English measurement sizes that the stainless or drill rod is commonly found in.  So you can get local rod but no linear bearings...

Bushings on the other hand can be fabricated locally on a lathe with a very amount of labor.

Simply they use either Delrin Nylon, or High Density Polyethylene or Brass.  I personally prefer the plastic.  The plastic and brass can be purchased as round stock and drilled out with a drill bit.  

The clearances have to be tight.  Slop or wobble and your tool will wobble, too tight and you will over work the drive mechanism.    

To check local round bar stock, you will need a mirror or piece of glass.  You place the bar flat on the glass and roll it.  Look where the metal meets the glass and see if it matches flat with the glass.

Now for making a milling machine if that bar is bowed by 0.1mm then your overall work over the entire part will vary by 0.1mm.  You need to ask yourself if it is good enough for what you are doing.

For the purpose of making a CNC Plasma table or a wood router, then yes this is good enough.  CNC plasma due to the nature of the plasma jet will vary by a small amount.  Changes in torch height of the bow is located in a up and down axis will have a negligible effect on the machine operation as the auto height control will compensate for this.  

More about CNC plasma and wood routers in later posts.

The next type of linear guide I don't know if it has a name but it uses square bar stock.  

This type of bar stock can be made locally and be made very straight if the machine used is accurate in itself.

What one does is they purchase a length of one inch bar stock.  If you use mild steel then you need larger sizes for longer runs to combat flexing.  This will work for,runs up to around 4 feet. Longer runs can be accomplished using steel tubing as the weight of the steel will cause,it to sag without a load.  

Once the bar stock is acquired, you need to use a surface grinder to true the bar stock.  Make sure they use coolant when grinding.  It is the same machine used to grind engine heads and many other things.  Surface grinders can also be used to make knives which will be discussed in another future post.

The next step is the roller that fits on the bar.  The bar is turned at a 45 degree angle to the roller like this:

The roller is made on a lathe and that part fits inside a bearing.  Personally for low cost I like to use stainless.  It is between the cost of mild steel and tool steel, and does not corrode easily.  A factor when you operate in harsh environments.  Stainless steel will work harden the longer you use it. So it will deform a little and then reach a point where it is very hard like tool steel.  You will lose some tolerances in the time that it is breaking in, but it can be compensated for and allowed.  

The third version does not need a surface grinder to make but it does need a laser cutter.  If you are lasering (or water jetting) parts this part can be lasered at the same time.  What you need to do is laser a rectangular bar out of 4 or 6 mm steel plate or water jet from 8-12mm plate. This serves as the roller guide and you use a similar roller bearing setup.  This method will make great parallel axis but the flat part can be subject to wobble unless it is bolted to a piece of straight stock like c channel, or square tube.  To get it perfect, you use thin metal shims between the guide and the frame and check for straightness with a piece of piano wire stretched tight.

It is more work, but less crap you have to buy from China.  

This is just just 3 of the many ways you can make these parts of your machines. There are other more expensive methods available but for the purposes of this period of instruction, I will only delve into these.

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