Another daft question...

It's been a while since I asked a daft question, so I figure I'm due
I'm making a (small) number of parts on my CNC Taig that need three
holes spotting, then an outline cutting around them. I may need to make
more of these later.
Current plan is to make a jig with the three holes represented by
spigots, and a zero mark.
I'll spot the holes in a number of blanks, then fit the jig to the mill.
Drill the holes in the blanks, then clamp them to the jig using nuts
threaded on to the spigots. Zero the X and Y on the zero mark, the Z on
the blank and then nibble my way through with an end-mill. At the end I
take a full-depth "clean-up" pass removing about half a millimetre
around the outline to final size.
Two questions:
1) Does this sound like a reasonable approach?
2) What's the best way to hit the zero mark with "reasonable" accuracy?
I'm thinking of loosely mounting a piece of silver steel in a collet,
then jogging the table slowly until it drops into a drilled hole in the
jig. Better (cheap!) ideas?
Reply to
Nigel Eaton
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Yes, a lot of operations on CNC machines require special jigs to hold the parts in position.
This is only a "cheap" idea if you have a finger type DTI, but now you've got a CNC machine I'd say it was an essential item for clocking parts square, etc. The standard way to set a CNC machine to a datum point is to mount a DTI in the spindle and drop it into the datum hole. Rotating thespindle by hand jog the part until you get a constant reading all the way round the edge, then the mill is at the centre of the hole. It is a good idea to work in X or Y at a time, taking the reading from side to side and cantering the part. This is actually easier to do than it is to explain, and it gets easier with practice (honest!).
Reply to
Kevin Steele
Sounds like you are doubling up a bit. Try this idea but it is open to modification as you don't state the size of the part , thickness or holes sizes.
Draw your jig plate up and superimpose a blank on it before cutting. Spot the three holes and spot 4 more for say 6mm dowel pins, two at the top and two at the left side of the superimposed blank Also spot two at each end of the jig plate on the horizontal centerline one for a dowel to be inserted to line up with the tee slot and one for a countersunk? clamp screw to hold the jig plate to the bed. Make the 0,0 point at the top left corner.
Remove from the machine and drill all the holes thru and tap the three spigot holes any convenient size.
Refit to the machine using two dowels to line the jig plate up horizontally using two of the centerline dowel holes sticking down into the tee slot. This ensures you always get a repeat position when you refit.
Zero the machine at the top left 0,0 point using an edge finder.
Insert 4 dowel pins to act as stops and push your first part up to the pins and find some way to get a clamp onto the part missing the 3 spigot holes, your call.
Call up the jig plate program and this time delete all the dowel holes and program the three spigot holes in and the finished shape of the blank. resave as the parts name.
First opp is to either drill or circular mill the three holes to size. Don't worry about hitting the jig plate, make it deep enough so you have a decent depth of thread in it. Once the three holes are in, fit screws into the threaded holes to secure the part, remove the 4 dowel pins and clamp, swap tools, if needed and mill the outside. Depending on the part and sizes you may well be able to do the whole job with one tool
By not moving the part you are guarantee complete accuracy and repeatability.
Your part 2 doesn't apply as you only ever need to find the top left corner with an edge finder. It also pays to stamp that corner with a 0,0 datum for future reference. Also stamp the name of the program on the jig and make one extra piece that always stays on the jig so in your advancing years you can spot what the bloody thing was for. Don't ask our scrap man how I found that out
-- Regards,
John Stevenson Nottingham, England.
Reply to
John Stevenson

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