Bits burning up

[jimmya]

I had to cut MDF (3 sheets 4'x8' 8 cuts each) and went through 3 band new bits. I have a PR and was running @ .70 feed rate. One time my table was on fire. How can I stop the burning of the bits?

[gerald_d]

Jimmy, we need the following info:
- Diam. of bit
- Rotation speed
- Depth of cut
- Quality of bit (solid carbide or something else?)

[richards]

Jimmy,
Have you downloaded Andrew McClary's Chipload Calculator from the Forum (April 8, 2004)? His calculator will help you find the correct feed speed//RPM combination.

When I plugged in your .70 ips feed rate, 9000 RPM (the lowest possible with my PC 7518) and a 1-flute cutter, I got a chip load of 0.0047. To get a chipload of 0.011, I would have to increase the feed speed to 1.66 ips.

[bjenkins]

To answer your question well, we need the information Gerald indicated. I can tell you from experience that MDF is hard on HSS bits. I won't use anything but carbide. Heat is almost always the result of too low of a chip load-- that translates to too high of cutter RPM or too slow of feed rate or BOTH.

[jimmya]

Gerald

- Diam. of bit 1/4"
- Rotation speed 18000
- Depth of cut 1/4"
- Quality of bit? solid carbide 2 flute
Mike
I didn't know there was a Calculator, I'll see if I can find the post.

Thanks for all the info and help

Jimmy

[gerald_d]

Jimmy, the dimensions/speed look safe against starting fires. I would be suspicious of those bits being true carbide as opposed to something much lesser. I once used the freebie 1/4" bit that came with a new Bosch router and also made smoke within about 5 minutes. Suggest that you try another bit supplier.

[wayne_walker]

Jimmy,

Here is a link to the chip calculator.

http://www.talkshopbot.com/forum/mes...html#POST13778

I was not aware of it either.

Good Luck,

Wayne

[jimmya]

Gerald
Thanks for your reply, I'll try and find another supplier.

Wayne
Thank you for the link, I think it's going to help knowing I'm close to the RPM and feed rates.

Thanks for all your help
Jimmy

[richards]

Jimmy,
At 18,000 RPM, feed speed of .70 ips and a two-flute cutter, you're getting a chip load of about 0.001, or about 8 times smaller than the recommended minimum chip load on the chart that I use. According to the spreadsheet formula that I use, if you have to use a feed speed of 0.7 (42 inches per minute) and a 2-flute cutter, your RPM setting needs to drop to about 2,600. That's probably much slower than you can run your router. An alternative would be to run your router at about 16,000 RPM, use a 1-flute cutter, and set the feed speed to about 2-ips.

My chart lists the minimum chip load for MDF at 0.008 and the average chip load at 0.015. To get that 0.015 chip load, you'll need to slow the router to 13,000 RPM, increase the feed rate to 3 ips, and use a 1-flute bit; however, if your router is the same as mine, at 13,000 RPM it might have enough power to cut to a depth of 1/4-inch per pass.

There are always trade-offs, depending on the equipment that you have. With my alpha, I cut MDF at about 5.5 ips, 19,000 to 21,000 RPM, using a 1-flute 3/8-inch cutter. I can make a clean pass with the depth set at 0.65 inches and then a final clean-up/tabbing pass to the material's full .75 inch depth. If I had a spindle, according to the chip load calculator, I would try 16,000 RPM, 8 ips and a 2-flute cutter. Disregarding ramping, I would get a 0.015 chip. A little trial and error would show whether I needed to modify things to get both the finish and bit life that I desired.

[jimmya]

WoW
Thanks for the info Mike, I'm going to play with the speed rate and RPM's this weekend. I have a PR modified not sure how fast I can run it but I'm going to put it though the mill and see. http://www.talkshopbot.com/forum/mes...tml?1088522060

Jimmy