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Spinning bit (mdf)

858 Views 8 Replies 4 Participants Last post by  ShutterBunny
I'm trying to figure out what caused my bit to spin, and get a bit chipped. I am trying to cut a t-slot to make my pantograph frame a bit more adjustable. I was cutting into MDF wood.

The bit (keyhole) spun so much it wore off the writing that was on the side and was very hot. The cutting side also now has little chips in it. Now, I'm new to plunge routers but it wasn't vibrating, went in fairly smoothly for a first try and didn't sound like it was struggling and didn't burn but when I got to the end of my short practice cut the bit stayed in the wood. - I was supposed to follow it back to the entrance point I assume, but the point is it easily fell out.

Possible issues:
1. The 1/4" bit only slides about half way down the collet. (4 slit style, flares out at the top. There is just the collet and one nut. I'd post a pic but can't yet.)
2. Speed was too high (I had it set to E so not max but close)
3. Not a good enough bit for MDF (Irwin brand)
4. Just didn't tighten the nut enough.
5. Combo of all above?

I got my plunge router used ($20) and the only place I can find it existing on google is on a police auction site lol so no manual. It's a TradeMaster, 1hp with adjustable speed (A-F, 9000-29000rpm).

What to you think caused the bit to spin?

Also, would it be dangerous to try it again with the bit chipped?
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Irwin are cheap bits and it's possible the shaft was slightly under size. Also that is a very cheap router. It wouldn't have a top notch collet in it so it may not have been gripping it that well. You also said something about it only fitting half way into the collet. That isn't enough. At the least it should be about 7/8 all the way in and that only when you desperately need just a little more depth to finish a job. You'll probably get sloppage and may damage the collet if you go less.

Hi Lana and welcome. The keyhole bit is a problematic design. The problem is that it can't get rid of the chips it makes very easily. The best method if you can do it is to make a slot with a straight bit that is equal to the narrow slot the keyhole bit makes and then use the keyhole bit to make the wider channel underneath. This gives those chips an escape route. And yes you are supposed to back it up to the start before you release the plunge in normal use (vertical orientation in your piece). Generally to be able to do that you need to be using some type of guide. If the two step process is too hard then cut a bit and then back up and out and blow the chips out of the hole.
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Hey, Lana; welcome! This subject has come up in the past. One issue is that the chips have nowhere to go as the cut proceeds. A solution to that is to make a cut the whole length with a straight bit, the diameter of the slot portion, ie not the lower keyhole. This removes a lot of the waste prior to making the final keyhole cut. Now the waste has somewhere to go, and a lot of the heat buildup and stress is eliminated, or at least reduced
As to the router itself, where did you buy it from? Is it possible that there was a problem before you took possession?
If you go through the archives here, you'll find lots of helpful info on quickly returning a razor edge to your bits...won't remove chips mind you, but at least the remaining edge will be sharp. Having said that, replacing it is the right way to go.
Since you're up here, N. of 49, you already know that Lee Valley is a great resource...
Lee Valley Tools - Online Catalog
pgs 297 - 307

They carry really decent bits.
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Hi Lana, The two bits approach is the way to cut this kind of slot, and cutting with a one pass type bit in MDF will burn up almost any bit PDQ. MDF is really just sawdust formed with glue into what passes for a sheet of wood. It is weak, hard to glue and finish, and the sawdust will ruin your lungs in a hurry. The one use for it is to make the second layer of a table you want to be flat. But there are other, better ways to make a plywood table top stay flat. There aren't many MDF fans here.

I have some old MDF scraps I keep around to push things through the router. When the bit exits the good piece, it drives into the MDF, which basically eliminates tearout on the back side of the good wood piece. MDF is cheap and expendable for this purpose.

A 1hp router is not particularly powerful so you feed rate has to really slow down. You don't want to be pushing the router forward against resistance, particularly in MDF. Generally bargain priced tools don't hold up very well. For a starter router, give the Bosch 1617 EVSPK kit, which includes both the fixed and plunge base and a half and quarter inch collets. CPO tools often has refurbished units that have a great reputation here. This kit used to go on sale for about $200. The fixed base can be used to mount the router in a table, and there's even a "key" you can get to adjust the height from above the table. There are lots of entries you can search for on how to make a basic router table without spending very much. A table mounted router is far safer than one used freehand.

I think Stick will come along shortly with a batch of pdf files with safety and user information galore. Download and read them is my personal recommendation.

Routers are one of the most useful tools you can have, right next to a decent table saw.
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As to the router itself, where did you buy it from? Is it possible that there was a problem before you took possession?
Yard sale so yes there could be something off about it but the shop seemed clean and well maintained if that means anything.
Thank you all for the info, I've never used a drill to make a channel I'll have to try that. I think I'll leave the MDF for now until I can buy a better bit and/or router.
No, that's not what I meant, Lana. I was talking about a straight bit in your router! If it's a plunge router, even better. (No drilling required)
Set your final depth, locate your start of the channel, and plunge the straight bit down...having preset the maximum depth of the plunge, that's now the bottom of your finished keyhole slot.
Switch to the keyhole bit....make sure the depth is exactly where you want it... then proceed to clean out the channel making it into your keyhole profile. If you've got a dust/chip collection setup that'll make life so much better.
MDF dust is contraindicated for a long life...yours, more than the bit's.
Haha of course! Duh. Thanks Dan. I'll only be making two paths in the MDF so it shouldn't take too much time off me. I will wear full protection when I do. Just have to figure out my collet problem now.
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