[airbrush]

Programming for top or bottom of part, advantages disadvantages to each method.
What's your preferences?

I'll mainly be doing wood with my home machine. Top of part I would always need to know my thickness before hand, while bottom of part I could do at a certain thickness with maybe some air cutiing time, but time is not a huge concern since this is just a hobby.

[Jon.N.CNC]

You have almost answered your own question,set your origin at bottom if you are doing contours will reduce the amount of marking of your spoil/sacrificial board however wont guarantee any pockets are the correct depth from front face if the material thickness deviates so its down to the job in hand.

Really its just what side you want the oversize/undersize to be.

For example if you were making a box that was fully rebated, the distance between the rebate depths would need to be correct so as to accept the joining board so the oversize would need to be on the outside of the box or bottom face so you would set origin at the top.

Hope this helps.

[airbrush]

Right, I have a mix of both. Simple contour stuff and some with contour and pocketing. I guess I can probably just do top of part and set some extra depth to cut through. Like you said, sacrificing your spoil board more.
Not a huge concern for now. Will try it and see how things work out for me and adjust down the road.
Thx

[mmoe]

It really comes down to personal preference. If you measure your stock properly, there should be no significant difference that would have any real world advantage or disadvantage. Yes, you may be off one way or the other by a few thousandths of an inch at times, but the discrepancy will be almost exclusively material thickness variance that will cause the same problems no matter which way you tackle the job. If you were to run three sheets of MDF, for example, with the same program to generate a series of the same parts, they will vary simply because the MDF is not perfect and these are not the types of parts where you are going to face mill the material. It's just the nature of the material that it will not be perfect.

I've done it both ways and have come to prefer the origin at the top, but that is due to the fact that my machine is mechanically configured better for that method than for the origin at the bottom since you really get the most accurate measure of the tool position when the stock is on top of the spoilboard when vacuum is involved. With the table open, the vacuum drops and the spoilboard essentially lifts a few thousandths here or there, and in some places much more than others, so you can't know for sure how accurate the measurement of the tool length is for any given point on the table. If you add the stock, it seals the vacuum table better and you get a deeper vacuum that is closer to what will be applied during the machining process, which provides a better measurement. Since the spoilboard is covered, measuring the tool length to the spoilboard is just not possible, so it must be made relative to the top of the stock instead. Either way, I take a digital caliper measurement in several places of the thickness of the stock, then determine a value based on those measurements. The machine you have will probably play the biggest role in determining what process is best for you and you'll develop preferences based on what works best.

[MikeC8]

I've always used the bottom of the part on CNC routers, but it depends on where you need your accuracy.