EbayEnd Mill Quality

Last night I used a 4 flute end mill to cut a slot in cast iron. The end mill was sourced from ebay, it was sold as 14mm and the marking on the shank said 14mm. However, I ended up with a slot that measured 14.25 (ish!)mm . Can anyone think of a good explanation for this (other than a dodgy end mill). It seems quite hard to measure the diameter of the end mill, especially as the shank is narrower than the diameter. I could convince myself it measures 14.25 (ish!) mm.

David

4-flute cutters are not good for cutting slots, try a 2-flute one also known as a "SLOT Drill" for now obvious reasons.

Basically as the lead tooth on a 4 flute takes a cut it will pull the cutter sideways and the following tooth will take material off the side of the slot. With a two flute cutter while one flute is cutting the other is not in contact with metal so won't make the slot wider.

Slop in teh mill could also cause problems.

So if you want an accurate slot use a good sharp 2-flute cutter at full width or a smaller one and take 3 passes, one to remove most of the material down the middle and then a sizing cut down each side

Measure across the "cylindrical" part of the flutes with vernier calipers? It may be you have some play or flexibility in the spindle or machine. Generally if you want to cut a slot with an endmill it's better to use one smaller than the slot and finish the slot to size afterwards. Or use a slot drill. Problem with an endmill is that the edge cutting across the slot creates a transverse force and if that deflects anything then the edge cutting along the slot enlarges it. A 2 flute slot drill avoids that.

Jason - unless you have an asymmetrical or centre-cutting end mill, aren't they rotationally symmetrical? In which case how can there be a lead tooth?

I would be measuring the concentricity of the 4 teeth as well as the diameter (across the teeth at the cutting end), using a DTI.

+1 on what Jason said.

You generally don't use an end mill to cut a precision slot of its own diameter. Use a two flute slot drill instead.

Or to cut a precise 14mm slot, use say a 12mm cutter and cut your slot, then take a finishing cut along each side of the slot to get your 14mm slot.

The problem with a four flute endmill in this application is that three teeth are cutting at once. The lead tooth is cutting the middle of the radius at the front of the slot and pushes the cutter sideways etc.

Jason is quite correct. George Thomas also explains why a 4 flute end mill will cut an oversize slot in his excellent book The Model Engineer's Workshop Manual pages 77 & 78.

I usually buy 3 flute milling cutters these days as they can be used for both end milling and accurate slot cutting.

Eric

My thoughts..

How about comparing the width of grooves with a) a slow cutter traverse and b) a series of 1/4 cutter width plunges along the groove track?

With any traversing cut you have both climb and conventional milling going on so a potential for side forces and cutter oscillation

I'm still at the stage where I'd prefer to destroy cheap cutters rather than expensive one and even with 2-flute mills I get the centre-line of my slot pulled to one side if being aggressive.

A 14mm cutter will usually have the same width shank and I'd expect it to have been ground along it's length before the flutes were ground..unless that second part can raise the flutes then the shank diameter measure should suffice?

Fact of life : It's going to happen.

Take the advise, no need for experimentation.

David Slot drils (2 flute) should be used to cut slots though if a very accurate slot is required then a smaller cutter should be used to rough out the central slot then the table moved to allow a further cut on each side of the slot. ensure each cut is conventional cutting, i.e. not climb milling. It would have been more helpful if the size/ type of mill had been specified and the depth of cut, and if the cutter was normal length or long series,the longer the cutter the more it will wander about.

Thanks everyone. There had to be a rational explanation and another lesson learned! Luckily, my slot size didn’t matter too much and I’ll just have to size the reciprocal component accordingly.

David

pgk pgk, I have not come across any milling cutters with a 14mm shank, surely this is none standard?

I got it from ebay quite a while ago - and the seller has long since disappeared.

Kwil, Standard Carbide cutters usually have a shank Ø that is also the cutter Ø unless the cutter is less than 6mm Ø ie, 4mm Ø cutter on 6mm shank. although I have ones that are 3 mm cutter with a 3mm shank. so with carbide it's likely to be 14 mm on 14mm shank. I think it's different for HSS. The OP's end mill is 14 Ø so it probably on a 12 mm shank. [ if it's HSS ].

'The OP's end mill is 14 Ø so it probably on a 12 mm shank. [ if it's HSS ].' - yes that is the case.

I agree with everyone on here that a two or even three flute(seems to be the magic all rounder but unusually uncommon) for cutting a slot whereas a four flute tends to be better for facing edges or small jobs.

Either that or you might have some slack in one of the slides which needs tightening up, or you have too much vibration or cylindrical run out from the tool, or you may have a slight taper from the position of the vice. It could of course, be none of these things; you may have a rock solid machine with the architecture and neatness of finely chiseled granite and be the envy of all your friends around you.

Michael W

.

I buy (now) most of my milling cutters off ebay from a supplier under the name of cncpoorboy.

Never had fault with delivery speed or the quality of the product.

Nick

Although in fairness and in my case I don't think the fault was with the tool!

Muzzer by lead tooth I mean the one that is at the front of the cut and taking the full tooth loading not a specific tooth out of the four. Seems the rest know what I meant anyway

He's my first choice too, been using him since he worked out of his garage!

Just up the road from me so I try and pop in.

I have been using carbide end mills from bestones2010 on Ebay. 2mm, 3mm & 4mm with 4mm shank and 6mm with 6mm. I have been very happy with performance, cost and delivery

Bruce