New build - base on SX1L or X3?

[wildwestpat]

Hi Dave

I am a few steps ahead of you in the building of a small cnc mill. As others have pointed out rigidity is the goal and makes a useful yard-stick for making your choice once you have decided on the XY&Z travels that you need.

After a lot of searching on the web for information on the various choices I decided that any mill with a round column or a quill was not going to deliver for similar reasons. The problem with achieving rigidity with a single round shaft is going to be difficult if a reasonable axis feed is going to be achieved. The mills that use the single round column and or a quill are designed to make plunging cuts and do not readily support the horizontal pressures induced by milling without locking the quill and or column. Whilst the locking and unlocking of the quill and or column could be automated the control would interrupt any 3D tool path. It is for this reason I ruled out a number of other wise very attractive manual mills. I then found the Redbull machines which are available in the UK from Amerdale and Bigdog in the US. After a lot of debate I bought a CJ12 (CJ9512) which has gib adjustable slide ways for all axis. The standard machine achieves 140 mm y axis 320mm X axis and 210mm Z axis movement.

My goal is to get the three axis fitted with ball screws although the screws fitted are smooth and the backlash is reasonable. I have the drawings done for the parts and have at last accumulated the metal to make all the bits. Screws are from Marchantdice 16mm. The design has taken me some time as I want to avoid cutting into the castings of the frame other than the odd small drill hole. My fear is that the castings may be under stressed and that these stresses could cause the casting to warp after cutting anything other than a small hole. (By way of history normal workshop practice is to rough machine castings and then leaves them for a few days or heat treat them before final finishing to size. Obviously we don’t know if this is carried out in the Oriental factories where most of the machines come from.) Incidentally there are a number of different factories involved in making small mills in the Far East as well as similar ones bearing different badges from the same factory.

Suggest you investigate some of the later offerings from Amerdale as you are in the UK . If you contact them you will I am sure get good advice on the differences.

The main differences between the Sieg X2 and the Redbull CJ12 are in the thickness of the castings. For example the base casting where the flanges extend the whole length of the machine and the Z axis mast where the casting is closed in all faces with the exception of two small apertures on the back face towards the top of the mast.

If you are in the SE England a visit to Amerdale London E11 and possibly to WARCO in Surrey would help clear up a lot of the finer points on rigidity. There is nothing like applying a bit of sideways grunt to the axis to see if they move!

Good luck – once you have decided just go for it. Now I have retired I hope to start cutting metal soon if I can get ahead of the domestic painting chore.

[slashmaster]

I totally understand everything about a round column being a bad thing but you're saying a square column with a quil is bad too? Even if it is locked tight all the way up in the spindle? This is the first time I ever heard of people roughing a part and ultimately getting better results if they leave it a few days before they finish it. Can you tell more about that please?

Originally Posted by wildwestpat Hi Dave I am a few steps ahead of you in the building of a small cnc mill. As others have pointed out rigidity is the goal and makes a useful yard-stick for making your choice once you have decided on the XY&Z travels that you need. After a lot of searching on the web for information on the various choices I decided that any mill with a round column or a quill was not going to deliver for similar reasons. The problem with achieving rigidity with a single round shaft is going to be difficult if a reasonable axis feed is going to be achieved. The mills that use the single round column and or a quill are designed to make plunging cuts and do not readily support the horizontal pressures induced by milling without locking the quill and or column. Whilst the locking and unlocking of the quill and or column could be automated the control would interrupt any 3D tool path. It is for this reason I ruled out a number of other wise very attractive manual mills. I then found the Redbull machines which are available in the UK from Amerdale and Bigdog in the US. After a lot of debate I bought a CJ12 (CJ9512) which has gib adjustable slide ways for all axis. The standard machine achieves 140 mm y axis 320mm X axis and 210mm Z axis movement. My goal is to get the three axis fitted with ball screws although the screws fitted are smooth and the backlash is reasonable. I have the drawings done for the parts and have at last accumulated the metal to make all the bits. Screws are from Marchantdice 16mm. The design has taken me some time as I want to avoid cutting into the castings of the frame other than the odd small drill hole. My fear is that the castings may be under stressed and that these stresses could cause the casting to warp after cutting anything other than a small hole. (By way of history normal workshop practice is to rough machine castings and then leaves them for a few days or heat treat them before final finishing to size. Obviously we don’t know if this is carried out in the Oriental factories where most of the machines come from.) Incidentally there are a number of different factories involved in making small mills in the Far East as well as similar ones bearing different badges from the same factory. Suggest you investigate some of the later offerings from Amerdale as you are in the UK . If you contact them you will I am sure get good advice on the differences. The main differences between the Sieg X2 and the Redbull CJ12 are in the thickness of the castings. For example the base casting where the flanges extend the whole length of the machine and the Z axis mast where the casting is closed in all faces with the exception of two small apertures on the back face towards the top of the mast. If you are in the SE England a visit to Amerdale London E11 and possibly to WARCO in Surrey would help clear up a lot of the finer points on rigidity. There is nothing like applying a bit of sideways grunt to the axis to see if they move! Good luck – once you have decided just go for it. Now I have retired I hope to start cutting metal soon if I can get ahead of the domestic painting chore.

[mwood3]

By way of history normal workshop practice is to rough machine castings and then leaves them for a few days or heat treat them before final finishing to size.

Ive never hear of this practice either, im interested...

[wildwestpat]

Hi folks

To amplify further on the old fashined and I believe correct way to machine iron castings as this appears to be of interest. In my isolated turret I thought this was common knowledge.

The process of casting involves the poring of molten and in the case of the small mill castings into moulds made from a special sand which is imprinted with a matrix or pattern in the shape of the form to be cast. For the complex shapes as in a lathe bed or mill table there will be at least two parts to the mould. Simple shapes such a blocks of iron only require a simple depression that can be filled from the foundry ladle. Feed channels are added and vents hollowed out in the sand on what is usually the lower half and the upper half lowered into place. Further complications are added if cores are added to reduce the amount of machining - as for example the inside of the gear box on the Sieg mills. The cores are often made from sand and sugar mix that is baked in a mould and supported in the sand mentioed previously. Sounds complicated and is a bit of a black art! The real expertise is in getting the molten iron to fill the resulting cavity without bubbles. This was and I believe still is where the skill of the foundry is evident.

The casting cavity is obviously over dimensioned as the cast iron shrinks on cooling. The cooling process is controled or alowed to take place naturally with these simple castings. This leaves the resulting metal with a hard outer skin which also contains some sand to add to the first cut problems. Old hands use a blunted cutter with a generous rounded lip to remove the first layer. Normally prior to machining the castings would be fettled - the sprue caused by the feed and vent ducts reffered to above removed. The castings should then be left for a time to what the old hands called maturing which can be speeded up by heat cycling and or mechanical means such as hammering.

For precision items such as gear wheels and machine beds the 'matured' casting would then get its first machining. Care would be taken to leave sufficient metal to allow for any resulting twisting caused by the removal of the tensions in the stock that is taken away. On big castings the movement is vissible to the naked eye. A further period was then alowed for the internal stresses to stabilise before the finishing to size - grinding flat etc.

A trawl of the web will bring up some further reading material. I think I have given you enough detail to see the sort of problems taht aris. Obviously having a big delay between machining operations is not compatible with low cost production. Whilst casting techniques have progressed this is still a problem particularly for low cost foundry work. This link is worth a look http://www.sawmillcreek.org/showthread.php?t=19751 as is any text book on machining castings.

Thus with some of the major surgery undertaken to install ball screws may not be a good idea. Correcting any twists that might be induced would require some serious scraping using the three flats method to bring the surface back flat. In the context of the slides on a machine there is the issue of having the mating surfaces aglined as well as flat. Again text books should be consulted on the three flats process as it is not a trade secret just the sort of boring work handed out to the youngsters to do under the watchful eye of the old hands!

Hope this satisfies - good luck.