Circles 1/8" smaller diameter than designed

[glassdale]

I'm doing probably the simplest task on the CNC ProPlus imaginable and suddenly I'm having problems. I'm trying to cut circles of diameters of 6", 8", 10", etc. up to 22". The last time I did this was last year and it went with few problems. However, this time all of the circles have been cut exactly 1/8" smaller than they should be. I've gone over everything and can't find anything wrong. It seems curious that each circle, no matter what diameter is the same amount too small. I tried to correct this by adding 1/8" to the diameter, but that didn't cure the problem (obviously not a permanent solution, but I'm trying to finish the current job). I had to go to 3/16" larger to get the circles to be the correct size.

Another problem that has suddenly appeared is that when I'm cutting the circles, the machine cuts a notch at the entry/exit point. I've watched closely when the bit enters the stock, and it's not a problem with the wood slipping, but the bit drills down, then moves to the inside of the circle by about 1/8". Then again, when the bit is exiting the cut, it moves back to the notch that was left when the bit entered the wood.

Neither of the problems happened previously, but then I haven't used the machine in several months. I really would appreciate it if anyone has any insights or help they can give me.

I'm attaching both the .crp and .tap files for one of the circles - 6". On my machine I get a circle of 5 7/8" from them.

Thanks in advance for your help.

Dale

[4DThinker]

A couple of things to check:

-What size bit were you using? Not all bits are the diameter that they claim. Most often I find my 1/8" single flute bits making plunge holes smaller than 1/8". My students often confuse 3/16" bits for 1/4" bits as well. Make sure your bit isn't actually a 5/16" bit.
-Are you using the right post processor for your machine?
- Are you sure you haven't accidentally click on the allowance offset option in the profile toolpath settings?
- Are you sure you've picked the right outside/inside/on option? Cutting ON the vector with a 1/8" bit will leave any circle 1/8" smaller than cutting outside the vector with the same bit.


I can't tell from the file provide if you are trying to cut a 6" hole or cut out a 6" circle. The circle measures 6.1875.

[glassdale]

4D,

Thanks for the reply.
-I am using a 1/8" bit, which is actually 1/8" diameter - I measured it.
-The post processor I'm using is the same one that I have always used - CNCShark-USB Arcs (inch).
- The allowance offset option is set to 0.
- I used the inside option, which is what I've used in the past with no problems.

As far as the circle/hole issue, what I'm doing is cutting several circles out of 1 piece of stock. I'll cut a 6" circle, then 8", then 10", etc. Each circle then has an outer rim of 7/8" width due to the 1/8" bit. I only sent the 1 file to simplify things. If I can figure out why it happens to the 6" circle, then it should fix all of the others also. If the circle measures 6.1875", then I must have mistakenly saved that circle under the 6" file name. The 6.1875" is actually 6 3/16", which finally gave me the 6" circle. I've increased all of the diameters by the same 3/16" and they've been coming out correctly.

Thanks again.