Porting using filing instead of grinding

[WhizzMan]

I've been thinking about analogies how to explain this. You can not control engine RPM in your car without a throttle pedal. It all depends on what gear you are in, if you are on a hill, how heavy the car is and what not, what RPM your engine will run, unless you give feedback by alternating the throttle angle. By constantly modifying throttle angle, you will be able to control the engine, but if you want a steady speed without constant interactive adjustment, you want cruise control. That will make your car go the same speed, regardless of hills, wind and other changing load factors. For Grinding, you don't want your motor running redline RPM, only to stall to 1200 rpm once you disengage the clutch and are running only at half throttle. Try driving your car on only the idle screw, no cruise control and no throttle pedal. That is what you have with electric motor speed controllers that don't have feedback. They are excellent for aquarium pumps, ceiling fans and similar purposes. These see a constant load and only require one adjustment to keep the desired speed. The feedback is what is the cruise control is in your car, a means to keep the grinder running the same speed, regardless how hard you are pressing it against your work piece. DIY store light dimmers and the controller tricky found, are like a manual adjustment screw for your throttle plate, great for a generator, but not practical for vehicles.

[Guy Croft]

Seems a cogent explanation Homme, thanks.

Can we please now get away from controllers and back on the topic.

G

[WhizzMan]

Good idea. Are there any techniques to prevent a grinder from "bouncing"? I find that if you press just a bit too hard or change angle, the grinder will "dig in" and when you try to correct by lowering pressure, it will bounce off. Being patient and keeping pressure low are obvious, but once you've gotten to the point where it's going wrong, are there tricks to avoid the tool skipping away? I feel that a lot of the irregularities are caused by runaway tools.

[Guy Croft]

You must make sure that the tool is never cutting on more than 1/3 max of its circumference at any time. With stones you need to use lubricant, WD40 or 'Duck Oil' spray are ideal. Most burrs need lubricant, the only exceptions I know of are Garryson 'Alucut' types which have very pronounced rake but they are quite aggressive and you have to hold on very tight as they can 'grab' (if the teeth are forced to cut on more than 1/3) and run round the inside of the port leaving deep scarring - a bit like a knurled finish.
The problem of 'grabbing' is worse with torque-feedback electric tools because the motor winds the power on. Heavy duty air tools are a lot safer.

GC

[TomLouwrier]

In milling -or cutting in general- there are 2 ways of moving your tool: either it is moving in the same direction as it is turning, or -logically- the reverse: you are dragging the tool against the direction of cutting.

The first method is used only to make a final pass over your piece to give it a good appearance. You can only cut a very little amount at a time, or the tool wil try to pull itself forward instead of cutting the material, suddenly leading to a rough surface or nasty accidents. This can only be done right if you have very good control over your tool, meaning a mill that has very little play in its feed mechanism and a solid positioning of the tool itself. Your hand and arm do not give that stability.

The second way of cutting means you 'drag' the tool against the direction it would want to take by itself. Because the forces of cutting and feed are opposed there is no chance of the tool advancing on its own account and cutting more than you want it to. This is the usual and correct way of taking off large chunks of material safely and quickly. There is much less danger of the workpiece coming out of the machine clamp to meet its maker (ie: you).
Drawback is that the chips often get between the cutter and the piece, so it is more likely to get a scratched or scarred surface.
Also it is less suitable for cutting very small amounts because the tool's teeth then get no real 'bite' to start the next cut.

In my experience it is far easier to pull a tool then to push it. Same goes for using an angle grinder: you can pull it towards you and hold it down; you can hardly push it away or let it 'pull' itself forward while you control it. It will 'bite' and start jumping around, possibly breaking the disc and/or ending up in your body.

So the main rule is to attack the piece in a way that lets you start the cutting at a point from where you can drag it against the natural direction, taking the play out of the feed (in this case your hand) by pulling it.

Hope that helps.

regards
Tom

[Nobby]

Very usefull information guys - thanks. It never dawned on me that using different tools will make *some* jobs easier (drill vs air tool)

I haven't done that much porting (only tidying up a cast iron manifold for the 20vt) but I have done a fair bit of dress up polishing of engine parts such as coilpack covers, fuel rails, plenums etc... Its certainly a very hard and long process to achieve a good finish and I'm sure cylinder head prep is no better.