Cutter size for little mills

[bobbybeef]

In using a minimill I have used 1/2 inch cutters(2 flute and more) and 1/4 inch both flute types. With variable speed I generally adjust to what seems comfortable.
I wonder what gives me the better finish the little cutter doing twice as many passes or the bigger cutter doing one pass and a finishing cut at the same depth.
Time is not a problem to this old codger but it seems to me that the smaller cutter has the better finish.
I would be grateful if someone with proper training and experience could talk on this and related points.
Thank you,
bobby.

[ERBenoit]

The finish of a finished cut can be attributed to many factors. Condition of machine, cutting tools and workpiece.

Generally, the more flutes you have on an endmill the better of a finish you will end up with. Two flute endmills have the most chip space and are usually used in slotting, pocketing operations where chip removal is a primary concern. Three flute endmills have about the same chip space, but have more "body" to the tool increasing its rigidity. That IMO is a plus, especially with smaller diameter endmills. Four, six or eight flute endmills will give you a better finish as they may be fed at a faster feed rate, however chip space becomes a concern, due to a decreased chip space.

Chip re-cutting will be another cause of a less than ideal finish. When pocketing or slotting, a two flute endmill will have less chip re-cutting than a multi flute endmill for obvious reasons.

When slotting, the endmill is being deflected by cutting forces from one side of the cut against the other. This may leave you with a less than ideal finish. I avoid this by using a smaller endmill than the slot and climb mill each side of the slot with the finish pass.

Climb milling will leave a better finish as there is less chip re-cutting. Any chip re-cutting is done against the un-finished surface.

[bobbybeef]

thank you ERB,
Yes I feel that the smaller diameter cutters with four spirals do a good job.
My worry is that when I take a reasonable cut say 20 thou in mild steel the mill breathes with each advance of the tool into the work. if I go pretty slowly it seems to be a bit better but not that much that I feel happy about it. I am using low revs and cautious advance of the table.
the gibs are tight but not so tight that it is impossible to advance the table .I am using 3MT collets snugged up with the drawbar.
The chips are quite small and come of cleanly where as the chips from the larger cutter !/2 inch pushed up a burr on the work side. maybe it was not as sharp as it should have been.
i have done a lot of lathe work including using a milling attachment but am only now beginning to use a minimill.
grateful for any and all advice.
bobby.

[Michael Az]

Also remember cutting oil. I always use it. Any hardware will have the heavy pipe cutting oil by the gallon for around $8.00 It is messy but I feel necessary. Use wd40 for aluminum. About $10 a gal at home depot. Somethig else when you need a good finish is to leave a little material when you can and finish cut with a new cutter.
Michael

[matador]

If you have the choice,rough cut with the big'un,at slowish speed,then finish with the small cutter,using higher speed.

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Hans

[spope14]

The advantage of small er cutters on a finish cut is not so much finish, which can be excellent, but flatness of a surface.

To note, finish is something of personal taste. We all know a bad finish when we see it, but the "taste" thing is do you like more "tooling pass lines" or less. I used to like less (wide cutter), but now have learned to like more.

Back to flatness. Smaller cutters take a smaller bite of width. If your mill is slightly off tram, use smaller cutters, for the steps between passes is less. If your mill tends to "flex" with larger cutters, use th smaller ones and "stepping" between passes with the mill will be less.

[bobbybeef]

Thank you Michael,
Tried using spray pak cutting oil as that was to hand and it certainly helped a lot. Will try the liquid type fluid from an oil can tonight.
Matador and S Pope,
thank you for very good points. i guess big and small is the way to go.
now for the next question if you can help please jump in. I have one of those sets of 3 flycutters. I have a job that is about midsize and the midsize cutter with its bit protruding about 1/2 inch will just cover it. It cuts OK but I think I detect some spring in the tool. Would a better finish be obtained if the large fly cutter was used ,even if with the cutting bit is retracted it extends well over the work piece. The work piece is work hardened mild steel.
When I say work hardened it was originally a drive block in a reciprocating mower which was part of a grain harvester. Circa 1935. I replaced it in 1980 and thats how it found its way into the scrap box.
Anyway it could be a bit harder than regular BMS.
Regards,
bobby.

[bobbybeef]

Well I used the big flycutter with a HSS cutter properly sharpened on the grinder and it did a fair job. slow and steady was the go. BUT when I changed the HSS cutter for a carbide one we really shifted some swarf. It just went thru that old iron like a hot knife thru butter. The finish reflected the shape of the tool but was even.
There was no heavy vibration and the cut was 25 thou. So far the carbide shows no tendency to break loose from its steel carrier. Its silver soldered on,I think. Could be brazed.It is covered with paint so I will scrape some off and check. should one be stronger than the other or are they
about the same.
Regards to all
bobbybeef.

[Al Messer]

Bobby, which brand of mini-mill do you have?

Al

[bobbybeef]

Yes Al,
I take your point. the milling capacity is listed on the Headstock plate.
for the record it is a MINI VERTICAL MILLING/MACHINE.x 100mm y 228mm z 215mm
speeds 0 to 2500 rpm. Drill/endmill to 13mm Face milling 25mm.
there is no registration number. my lathe has one on the lathe bed. The electical box ans a carbide set of lathe bits have the brand CE on them.
I think it is a Seig. Painted red just for variation.
supplied by Minitech.com.au. Queensland.
both lathe and mill came in stout boxes with every thing tied or bolted down for no movement in transport.
They were both adjusted and checked by Minitech and could have been used right out of the box. But having been to minilathe .com I did some stripping and cleaning anyway. didnt find anything unusual. A very good product for the price and work I do.
In relation to the face milling jobs I have been doing on steel, I reckon I was trying to drive the wee mill a bit hard. much better results with smaller cuts and and better speed. Still have not used high range. Will be working on brass next week so will try high range then.
Regards,
bobbybeef.

[Al Messer]

Thanks for the info and good luck with your machine. BTW, there is a fellow Ozonian that lives in Rockley that posts on the Chaski Lists from time to time.

[bobbybeef]

I have to ask Al,
What is an Ozonian?
Regards,
bobby.

[Al Messer]

Bobby, it is a term of endearmenr taught to me by one of your fellow countrymen. If it is not, I apologize profusely.

Al

[bobbybeef]

Thank you Al,
Ah a Queenslander is the source. That explains a lot. They have a tendency to forget their words and so make up new ones.
We are having the Veterans of 173 Airborne Brigade Sep US Vietnam 65/66 visit us this month for a reunion with the Aust Veterans of 1st Battalion Royal Australian Regiment Battle Group ( It was integrated with 173 under Brig Gen Butch Williamson) All those years ago so many young men not young any more.
There are lots of ties that bind us Cousin.
Regards,
bobbybeef.