Slot Drill problems

Whenever I use a slot drill to machine a fork end or slot I usually end up with an oversize irregular shaped slot.

I am using my Myford Super 7 with the cutter held in the 4 jaw and a machine vice on a vertical slide and traversing with the cross slide.

My latest disaster was set up almost perfectly in every direction but to no avail.

I can only think that it is a problem with swarf build up. How to clear the swarf whilst cutting?

what do other readers think?

Make sure there is no slop in your cross slide and the gib is snug. Lock the saddle, then fine cuts with a sharp cutter. Vertical slides are not very rigid set ups.

Peter,

Simon has addressed the matter of irregular shape, but

How accurately can you [do you] set the cutter to run true in your 4-Jaw ?

... Any eccentricity will be doubled in the width of the slot.

MichaelG.

Peter,

As .Simon suggests you do need to nip up all axis gibs to ensure best rigidity. Are you using the same size cutter as the width of slot required ?

It is best to use a smaller diameter cutter so you can then take a finishing pass on each side of the slot to get the finished width.

Perhaps some cutting oil or whatever you use will assist and I use air jet at 20lbs to keep the swarf clear.

You dont mention the size of cutter but perhaps you need more rpm on the spindle to reduce the tooth load with a 2 flute slot drill ?

Emgee

Hi Peter,

I have never had the problem you describe. So either it is wrong milling procedure or a problem with the mill.

One of the most potentially challenging jobs of recent years was to mill the channel section slide bars for my 5"g Terrier Stepney. Despite my worries and misgivings they ended up extremely good much to my surprise, having only a 3rd hand Dore Westbury mill.

For forked ends such as valve gear bits I drill out and hacksaw out most of the material to be removed first. Knuckle joints on coupling rods are rather problematic. Probably best finished with a warding file.

Cheers,

Julian

4 jaw. I know it is probably the best you have, but does it really give enough support to the milling cutter. I know that on occasion, I have put an end mill into the 3 jaw jacobs drill chuck on my Warco mill as I can't be bothered to change to the Autolock. Invariably, the cutter comes loose in the chuck and spoils the work.
BobH

Don't go too deep with each pass, if you try taking too big a cut even a 2-flute cutter will deflect and cut a wider slot.

Also try and have the whole of the fork within the vice jaws, this will stop any tendancy for the two sides to spread out during machining.

Sharp cutter is the other thing to check.

Thanks for all the replies. Been trying to improve my slotting before starting cylinder machining on my Pansy. Don't want to mess up the ports.

My latest attempt ( the one that prompted my question) was a through slot in 3/8in square BMS. I was using a 3/16in slot drill set up with only 0.0005 run out and taking 0.020in cuts and obviously hand feeding the cross slide. When feeding into cutter I could feel a lot of 'juddering' at the cross feed handle. Must recheck everything for slop before having another go.

The resulting slot ended up 0.219in wide which suggests a major problem with my technique. Did forget to lock saddle though.

Edited By Peter Maloney on 04/08/2016 15:56:00

TOGETHER YOU'VE CRACKED IT

Just tried out another slot and I cannot believe the difference -almost perfect.

1) Made sure saddle was locked.

2) Cross-slide gibs were ok.

3) Vertical slide gibs were adjusted.

4) Saddle locked for each cut.

5) Spindle speed increased and juddering at cross feed handle went away and the lathe seemed a lot happier.

THANKS GUYS

End-Mill and Slot-Drill Milling cutters are not intended to cut slots exactly to their marked size, the way to mill an accurate width slot is to rough it out slightly under size and cut to the required width with light finishing cuts.

- Nick

Edited By Nick Hulme on 16/09/2016 09:49:00

I have found it helpful to use a cutter undersize for the slot, and to cut both sides in turn, so that the tendency for the cutter to push everything over to one side is applied equally to both sides. Even if the gibs etc are tight, eliminating any slack, there will be bending of the whole set-up which you cannot remove. It also helps to change the speed if there is any tendency to resonate - which may be what your juddering was about. And finish to size using a light feed, both sides the same as before.

Cheers, Tim

One thing which has not been mentioned is " climb milling". The way a milling cutter should work is like a wood saw, The material should always be forced into the teeth of the cutter. When cutting a slot using both sides of a cutter, on one side the material is going into the cutter teeth, on the other side, the material is actually travelling in the same direction as the cutter teeth. So to make sure that the teeth cut, the spindle speed has to be increased. Every time a tooth bites, it tries to push the material away down the backlash.

If the initial slot is cut narrow, then a finishing cut down in side in the correct feed direction would be the best you could do.

Frank

I must have been lucky, but in my experience a decent quality 2 flute slot drill will cut a pretty accurate slot. Out of interest I've just measured a blind 5/16" key way cut in one pass in the rear axle for my traction engine; material was EN8. The slot drill was Dormer, can't remember speeds and feeds, but appropriate for HSS tooling in medium carbon steel. A stack of slip gauges equal to 0.312" is a nice snug fit. A stack equal to 0.313" went partly in, but I had to use a jewellers screwdriver to lever it back out again. I suspect the difference is on the same order of magnitude as the accuracy of the slot drill in the first place.

While the leading and trailing teeth on the cutter do see different speeds one would have to be grossly out on spindle speed or feedrate for it to have any material effect. Let's assume we have chosen a spindle speed to give a surface speed of 100 ft/min, ie, HSS tooling in low carbon steel. So if the feedrate is zero, each tooth is moving across the material at 100 ft/min. So what feedrate would people use for a slot drill in low carbon steel, 12 inches/min? If so then one tooth peaks at 101 ft/min and the other at 99 ft/min. In other words 1% and I suspect not significant.

Andrew

Not what we taught at tech college, UK slot drills are made to a tighter tolerance than the USA ones to facilitate cutting slots in passes varying only in depth. They would have laughed at work if you used an undersized one then cut a bit each side. Why make cutter to precision dimensions if you only cut one side.

I believe that the cut one side cones from not locking the opposite axis or not using BS form cutters.

Steve

Key ways in shafts are cut in one pass and have to be a good tight fit to avoid slop and fretting.

Not what we taught at tech college,

Quite right, but I very much doubt you were using the set up as described in the opening post to this thread.

Horses for courses, or make appropriate concessions to the ideal. The cutter, the machine and the actual process methodology need to to be considered in context. Cutters will only cut to their actual size in a good tool holder in a perfectly rigid set up. And, yes, not all cutters are perfect. There will always be a tolerance, no matter what combination of machine, cutter and operator.

If your machine is "Industrial" then I'm sure your cutter will cut close to size with a slow pass, if it isn't then it's not about the differential in cutting speed from side to side, it's about the cutter climbing in the cut and using any available slack in the system to move to one side.

Modern CNC HSM techniques, even on extremely rigid machines are at speeds & feeds which cause cutter deflections and even when using the same cutter almost always involve roughing passes with an allowance for a finishing pass, if it were possible to cut exactly on path at an economical speed then there would be no reason for finishing passes,

- Nick

Yeah, and one can only make a part to tolerance in one pass - industry experience shining through there bud!

Don't just lock the carriage, lock any axis that is not being moved to make the cut. So if using the cross slide to make the cut, lock the vertical slide and the carriage. If your vertical slide does not have lock bolts, drill and tap a couple of holes in the appropriate place. Let the tip of the drill just enter the gib strip when drilling the holes so the noses of the lock bolts can engage the gib strip without pushing it sideways. Or if you are using the vertical slide to take the cut, lock the cross slide as well as the carriage.

+1 on slot drill should machine to size. That is the point of a slot drill. Keyways are cut as one pass wide, as are other slots. If you wanted to muck about with smaller cutters machining one side of the slot at a time, you would be better off to use a four-flute end mill cutter that will remover more metal and be more rigid.

Make sure your cutters are gripped well into the chuck without too much overhang to minimize flex.

My experience has alway been the same as Andrew; a two-flute slot drill has always produced a satisfactory slot or keyway in a single pass (assuming it's in as-new condition). I can't say the same about centre-cutting three-flute slot drills, though. It appears that the alternate cutting of one side then the next affects the accuracy to some degree.

Regards, Steve