Help please

[Rob Davis]

Here's a new one for us and I am almost reluctant to admit this happened but I can't figure out why it did so am needing help.
We were making parts like we have done before and were interpolating holes that are .415 and .300 in diameter using a .250\" comp bit in .75\" MDF. What happened was that the MDF got red hot in three of the 256 holes in a 5x10 sheet, the vacuum was pulling air through the holes and feeding the fire, by the time we figured out what was up we had burned through the waste board, through the table board and now have 3 charred holes all the way through our table of about 5\" in diameter. It was rather exciting in a sick sort of way.

So where did we go wrong? Is the bit dull, even though it is almost new? Could we be rotating the wrong direction? Are we plunging to fast (100)? We don't know why now when we have done this before without difficulty.

So I also need some info on a new waste and table board. We have patched the holes but it is a temporary fix. I need to figure it out because I have to run this order again and I am reluctant to do so until I can explain why. The problem is you don't know you have a fire until you hear the air rushing to the vacuum pump and it starting to smell like smoke in the exhaust of the vacuum. Unfortunately it is already a roaring fire by then.

Any suggestions?

[Nick M Singer]

Sounds catastrophic!!! but also like the spindle is running in reverse

[Bill Rutherford]

Rob,
Personally, I would recommend never interpolating a hole that is full depth with a compression bit. Rolling nest plunges all the way through a hole before beginning the interpolation so during that plunge move there is nowhere for the chips to go. This is what caused your bit to get hot. I would recommend interpolating that hole with an up-spiral bit to allow you to get the chips out.

If you are programming in MasterCam then you could take the hole in two passes (3/8\" at a time) using the depth cuts command. This still is not ideal because you are plunging past the upcut portion of a compression bit but it is better then plunging all the way through at one time.

I am not sure why you only had problems on 3 out of the 256 holes, perhaps you had just finally reached the \"critical\" point in bit heat.

As for the table board we use a piece of 3/4\" Nordboard MDF from our regular board supplier. We don't run wasteboards but as I understand it the wasterboard is just a piece of 1/4\" MDF. I have found that regular MDF works better then UltraLight for the table board.

Just my two cents, I will be interested to hear what other responses you get.

[David Hall]

Rob,
I agree with Bill, you need an upcut spiral for boring without burning. (These bits are less than half the price of a compression bit)

I would also replace the collet that held the bit. It's probably won't hold a bit for much longer and even worse, the heat could have warped it, creating an out of balance situation that can cause premature wear on your spindle. For the $40 for a new collet it's not worth risking your spindle.

Clean the filters on your vacuum pump, they're full of junk now.

Call Thermwood and find out what I missed.

Count your blessings, it's stuff like this that can cause dust collection systems to explode.

Regards,
Dave

[Jody Wilmes]

This could be a result from loading a Left hand-tool instead of the normal Right-hand tool, running the spindle in reverse, dullness, or the tooling type. Tooling types that could cause this are downspirals and 2-flute compression tools as these tools do not allow the material to leave the hole, causing heat. I have used single-flute compression tools successfully without burning, but be careful. As mentioned earlier, thankfully the fire didn't reach the dust collector. Have a good day guys.

[Rob Davis]

Thank you for the replies. We had considered much of these but a real privilege to have the ability to get input from others. I still can't resolve why only 3 holes out of 256 and why now versus no problem before.......It can't be the reverse rotation I don't suspect as it should have done it more?

By way of info, the fire was a smoldering one and not in the dust collector becasue it was getting sucked into the table and vacuum. However, had we not heard air whistling and Estopped, we would maybe have passed the head over burning embers and had a tug of war over which way the sparks were to go. If the dust collector had won, bad news could have been the result. I complemented my operator on doing the right thing and not worrying about a sheet of material we wasted.

If we resolve it forever, I'll post you. Until then, regards,

Rob

[David Hall]

I suspect every hole was charred black when the bit was plunged in. The interpolation then cleaned up the charred MDF and when the bit was extracted most of the smoldering pieces were wisked away to the dust collector. Finally, in a few of the holes, a hot piece stayed/dropped in the hole and started the fires.

[Rob Davis]

Update.... Based on all your responses and your sage advice, we re-evaluated the whole operation we were using to cut these parts. We had always used comp bits because we always used a comp bit. Now we switched to an upcut bit for drilling and interpolating the holes and actually even now use a straight cut for outline tool. We cut faster and while it leaves a small ratty edge, on these parts that is immaterial. Thanks for all the help.

What we actually found out were multiple problems: 1) the bit was dulling faster as we were running more parts than we thought because business picked up (good bad problem) 2) we had always had a small black mark in the holes but never an issue before 3) the dust collection fan had eaten a piece of cardboard and the fan was not turing at maximum rpm and so leaving additional dust in the hole generating heat . Turns out we were living on the edge with this operation for some time and the right situation occurred and we had a near catastrophe! But in re-evaluating the whole process, we have a better solution in every regard!

So thanks for all the advice and many of you were right on target in your thinking. My favor to you would be to learn from our mistake and you check your stuff out too. It is rarely ever one big problem but a colection of littler ones that lead to trouble.

[Rob Davis]

Update.... Based on all your responses and your sage advice, we re-evaluated the whole operation we were using to cut these parts. We had always used comp bits because we always used a comp bit. Now we switched to an upcut bit for drilling and interpolating the holes and actually even now use a straight cut for outline tool. We cut faster and while it leaves a small ratty edge, on these parts that is immaterial. Thanks for all the help.

What we actually found out were multiple problems: 1) the bit was dulling faster as we were running more parts than we thought because business picked up (good bad problem) 2) we had always had a small black mark in the holes but never an issue before 3) the dust collection fan had eaten a piece of cardboard and the fan was not turing at maximum rpm and so leaving additional dust in the hole generating heat . Turns out we were living on the edge with this operation for some time and the right situation occurred and we had a near catastrophe! But in re-evaluating the whole process, we have a better solution in every regard!

So thanks for all the advice and many of you were right on target in your thinking. My favor to you would be to learn from our mistake and you check your stuff out too. It is rarely ever one big problem but a colection of littler ones that lead to trouble.