minimill fixture plate

[krymis]

I searched and did not get the answers that made sense. I have a part or few parts I want to make on my X2 cnc mill. I am using mach3 to control the machine. I would like to use a fixture system like what i saw on the Industrial hobbies web site.
Question 1. Should I use a fixture plate with subfixtures or just pin each individual part fixture? What are pros and cons? I was leaning to the IH version of doing this.

Question 2. should the plate be flush with the rear of the table or hanging over the rear? Same for the front of the table... If hanging over how much should it hang over?

Question 3. How do you make a Zero point on the fixture?

Question 4. How do you set the fixture offset in the program to allow multiple parts to be machined?

here is the type of part I will be machining...How many do you think I can make at one time?

and an idea of the type of fixture i will use can be seen in the summer 2009 issue of the digital machinist (the taig t-nut fixture).

[HimyKabibble]

Doesn't look to me like a fixture is really needed for that part. Cut them from a strip of material, and leave tabs at either end that you remove after machining. Or, make a very simple fixture consisting of a plate with a series of threaded holes. Clamp the raw stock to the plate, then mill the internal holes. Then insert "plugs" roughly the shape/size of the holes, and bolt them down to the plate. You can then mill the entire outer profile - on the first pass, leave about 0.050" at the bottom to hold everything secure, and take that off with the finishing pass. I do lots of parts that way, many of them much larger than those.

Regards,
Ray L.

[MIKE JEFFERS]

just draw the whole fixture up in your cad program
a straight edge(couple of dowels/pins into tee slot or table edge) and a 10mm dia hole
( 3/8" ish) for your datum
put these in the cad drawing too then just lay out your parts adding any additional
fixturing cam it and off you go
i can't see the pic due to being at work but this will work for most parts

[krymis]

ok can you explain the datum and where to put it? I mean is it arbitrary and can can be put anywhere? Also how do you set the code for fixturing?(G54 and such?)

I figure i can get 3 of these on the table in the work envelope. I have 8.25"x4.5" travel on the machine and the table is 3 11/16x 15 7/8

[MIKE JEFFERS]

are you using a cad program to draw this part ?
if so you will have a point of referance (X0,Y0,Z0)
if as i sugessted you lay out your sub table (jig) in your cad program then
layout your stock (put them on different layers) you can then set your datum (point of referance) anywhere you want usually bottom left (so X,Y are all +)
now when you use your cam program to code it it can be set to mill all 3 of your
parts as one program simples

you could use datum shift to do multiples but if your just starting out mebee not
this time

[krymis]

Thanks for the help. That made sense.

[wildwestpat]

Hi Krymis

Can we assume that the tolerances for the part are better than 0.0001 inch? If so then your best bet is to machine these at one go from bar stock. invert your drawing so that the largest bore is upper most assuming you are using a three axis vertical mill. Surface the top face and proceed to rough mill the outer shape - leave say 0.0005 for final trim after you have end milled the interior. Mill the interior to size but to a greater depth than required determined by the saw blade you have at your disposal. Trim the outer profile as milling the inner may have distorted the outer depending on the cutting pressures used on bringing the inner wall to dimension. Using a circular saw or thin wall cutter remove the finished job from the bar stock using small feed rates. It might be anadvantage to use a sacrificial inner support if you have problems with the surface finish which you have not specified.

May I suggest that you imagine how the part will be machined as you draw as CAD is only part of the designers thought proccess from idea to manufacture. Keep thinking and re thinking tolerances and finishes as every dimension needs both! Sorry to labour this point as it is so fundemental to the design process.

Have fun and stay safe.

Pat