Nasty MDF question

[Michael Simpson Virgina]

I am testing the assembly of my new clock design and ran out of 1/2" MDF. I ran down to my local Home center (HD) and picked up a 2' x 4' sheet.

I brought it home and now I remember why I hate MDF in general. This stuff is like a dang rock. Its nothing like the last batch I got. I cant cut a decent round hole as it causes the 1/8" bit to deflect, even at slower speeds.

Overall I can cut the stuff with enough accuracy for all the other parts of the clock. But since the holes have to support brass tubes nice and snug, I am having problems.

So my fear is that if I generate Gcode and others download the code they may not get consitant holes based on the material. I am tempted to just jave the CNC peck a 1/8" pilot hole and dirll the actual holes with a drill press. That way I know the holes will be consitant no mater what the material.

I guess I could slow the CNC down to a crawl but Im afraid I will burn up the bit. I am takeing a 1/8" cut per pass. Would I be better off keeping the speed up and just reducing the cut depth to 1/16" when I cut the arbor holes?

Keep in mind I am using a two flute 1/8" spiral upcut bit.

[Ken Shea]

Onsrud makes great end mills for specific material including MDF.

http://www.onsrud.com/

[Shane Sura]

Being new I have been using the rule of no more depth than half the diameter. As such if using a 1/8 bit I go for about .07 per pass. I just recently picked up a 1/8 down cut from Niagara and using that have seen no deflection.

[Michael Simpson Virgina]

This MDF is like none I have ever worked with. It bogs down my Festool miter saw when I cut ir. The funny thing is even though its much more dence it does not seem to any stronger or more ridgid than any other MDF.
I will reduce the depth and see how well that works. That said I am leaning toward just cutting a 1/8" pilot and drilling them out with a drill press.

[Shane Sura]

Nothing Wrong with that idea. I am not yet at the point that I feel that everything has to be done on the CNC.

[Gerry Grzadzinski]

Sounds like you might have gotten some high density fiberboard. I've seen it occasionally mixed in with the regular mdf at my HD.
Sounds like you also need a sharper bit. It shouldn't be that much harder to cut, if the bit is sharp. Keep in mind that any type of MDF dulls tools rapidly.

[Steve Ryan]

MDF comes in many densities. Home Desperate buys whatever they get a good deal on. Only MDF I use is the light weight stuff for my spoil board.

[Michael Simpson Virgina]

Well my problem was definatly a worn bit. First off its not .125 in diameter. its more like .123. So thats going to cause some press fit problems. Replacing the bit and taking a shallower cut gave me a perfect hole. I was just making the bit do too much work.

[Mick A Martin]

I tend to cut 1/3 of the bit diameter in depth, it might take longer but less stress on the machine and bit.

Mick

[Michael Simpson Virgina]

Yea, I know what you mean. There seems to be a big thing about how fast you can cut a part. And in the past I have hogeed out a part at high speed. But it comes at a cost. More wear on my bit an Acme screws. And the cuts are not as clean and accurate.

[Gerry Grzadzinski]

And in the past I have hogged out a part at high speed. But it comes at a cost. More wear on my bit................

Actually, cutting faster reduces wear on the bit. Cutting faster results in larger chip loads, which keep the tool cooler. Heat is the enemy of bits, and causes premature wear. Of course, if you increase the rpm when you increase the speed, it doesn't help much. But most people tend to use far too high of rpm's when cutting.

I work on big industrial machines, cutting out parts at 1000 ipm in one pass. A general rule of thumb on these machines, for maximum tool life, is to cut as fast as possible, while still maintaining acceptable cut quality.

[Chris Fournier]

I'm probably telling you something that you already know but there are several different fromulations of MDF and some is "router friendly". I'd talk to a local wholesaler and get the best board for your product and processes.

[Michael Simpson Virgina]

High speed also puts a lot more lateral load on the router bearings and CNCACME nuts or R&P parts many of which are plastic (Derlin) on DIY CNCmachines. This is even true for some of the smaller commercial machines.