[ozstockman]

Hi guys,

I hope anyone can explain me what I am doing wrong as it starts driving me crazy.

Below are two pictures of 140mm high letter cut out of 9mm thick MDF. If you look at the left part of the letter on the first picture you will notice a bad pass trim and the right part of it is clean and smooth. Th second picture is just zoomed version of the first one.

Actually it was only one letter from one job that have some part of the cut side in smooth nice cut. All rest pieces including this letter have this awfull trim all around them. Somewhere it's just a small thin trim that easy to sand off but in most cases it's quite thick. Another weird thing is that a cut left on waste material is perfectly smooth everywhere.

And it's not only that job that hasn't gone well, everything I have cut since I got a CNC Router was almost that bad on cut sides.

I cut the letter with a new 4 flutes 2.5mm carbide end mill. Spindle speed was 12000rpm and feed rate was 5000mm/min. Start depth was 2.5mm, cut depth 7.00mm. And for the mill settings cut pass was set to 2.5mm and stepover to 1.00mm


Thanks in advance.

Cheers,

Mike

[ger21]

If the waste material is good, try cutting in the other direction.

[ozstockman]

I have done some tests and it seems that I am getting the best results with no trims only when the bit climbs. However I have tried to search for "Climb vs Conventional" on the forum and what I've found is opposite to my results. I am more confused now than ever.

[ozstockman]

Thanks Gerry, that's probably why it was so bad. The whole job was cut with conventional direction. I will try climbing to see how it will go.

[ger21]

You may find that different materials need to be cut in different directions.

[ozstockman]

Climbing has worked great. Apart from broken mill at the most end I got all workpieces except one with nice smooth cut.

How do you guys usually handle broken mills in terms of replacing and continuing the job it was broken on. As far as I understand the process a new mill should be installed with Z set to zero. It obviously cannot be done at position it was broken so a spindle should be stopped and raised. Then it should be moved at some place where broken mill can be remove and new one installed relative to z=0.

I tried both stopping the cycle and holding it and turning off spindle. Then I moved the spindle up and close to home position to replace the mill, set Z=0 as it should be. However after I started cycle again from the point of code it was stopped for some reason there was always some offset in X or Z positions. Today it was only small 1mm offset which ruined one letter only but I had it once off by 100mm and it ruined everything that was already cut.

[ozstockman]

I have just read here that MDF is usually cut with conventional direction and a downcut mill. I am using upcut mills. Is this why conventional cutting doesn't work for me while climbing does a very good job?

[ger21]

It would seem to me that your getting flex in your machine. When cutting MDF, there really shouldn't be much of a difference whether climb or conventional cutting. What you're seeing is the tool being in a different position on each pass. Not really a cut quality issue. Cut quality for climb vs conventional would be better determined by making a single pass, and seeing which side of the cut is better. From my experience, the climb side will have a rougher top edge, but the cut edge should be very similar.

[jweinhold]

What ger21 stated maybe true.

2 items to look at is using a larger diameter tool and the other is are you applying tool compensation, or are you machining on tool center? Tool comp will help miniumize offset tool pathes from tool to tool.


jweinhold

[ozstockman]

The replaced tool is absolutely identical and I do apply tool compensation. Is it possible that it happens because I apply some force to tighten a new tool in a collet? I guess it can move my spindle a bit to create some shift in X or/and Y.

If you think that may be the case is there any technic to avoid this problem?
At this point I only cut stuff out and it's always one tool job. So I need to replace the tool untill job's done only if it's broken. However if I am going to do some carving or something more complex there is a big chance I will need to use more than one tool. Then it I guess it will be a big problem for me.

cheers,

Mike

[Big-tex]

What kind of router do you have. Looks to me like you have accuracy issue.

[ozstockman]

What kind of router do you have. Looks to me like you have accuracy issue.

it's shg0609 by excitech Is there any way I can fix it?

[Big-tex]

First of all cut with downspiral tooling in convetional direction (MDF) stepdown= to no more that diam. of tooling. use ramping, calculate rpms and feedrate using chipload chart as per tool manufacturer.

1. lets do some testing- cut circle diam 100mm and check for accuracy if perfect circle (if you have any backlash)
2. do same with 100mmx100mm square

Lets see pictures and observations

P.S looks like you have Chinese machine with following specs.
Precision Machining precision 0.05mm
Repeat location accuracy 0.02mm