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Tool for rounding and edge of metal plate

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Martin Dowing26/11/2020 21:54:08
355 forum posts
8 photos

I need to round an edge of metal plate (45HRc) to precision radius of 15mm.

Because plate is wide turning is out of question.

The obvious idea is to use dividing head with tailstock to mount said plate and patiently proceed with inching bit by bit and taking straight cuts with end mill, then smoothing radius so produced with sandpaper etc.

There is also a simpler method:

One can buy corner rounding end mills which are essentially "inverted form of ball mill"

I can envisage 2 ways of using them.

First would be to use them perpendicular to plate, make a cutting pass, invert plate and make yet another pass (there will be witness mark for sure and other issues with geometry calling for careful machine setup.

Second way is to set end face of plate under angle of 45 deg to the axis of mill and proceed with a single cut.

This looks like the best way to proceed but could you comment on it?

Henry Artist27/11/2020 01:06:44
121 forum posts
46 photos

Can you show a diagram (with dimensions) of the part you wish to make? Doesn't have to be anything fancy - quick sketch on the back of an envelope will do.

It would also be handy to know what the metal is and what the part is for.

JasonB27/11/2020 08:07:31
22604 forum posts
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As said a picture would help as I can't see how any size plate could have it's edge rounded by turning without hitting other parts. Your mention of coming from both sides suggests 30mm thickness plate but if holding plate at 45deg you will only be able to cover approx 20mm plate and it won't be a flowing roundover.

Where have you seen that size rounding cutter? it's going to be big with approx a 50mm dia head and maybe 25mm shank and also cost a lot, a 6mm radius carbide one for upto 45HRc costs £96



Edited By JasonB on 27/11/2020 08:17:54

Martin Dowing27/11/2020 09:36:14
355 forum posts
8 photos

Jason have made correct drawing of what I want to make. Sadly I don't have access to autocad/autodesk software to make such a nice drawing.


You can round corners of plate on lathe as long as radius is such that similar plate could be made by milling round barstock. Unfortunately it is not the case in my situation.

These cutters are cheap.

HSS variete of 8mm radius you can get from China for $10 and solid carbide version below $80. Free p&p. Shank is 20mm so ER32 collet of this size will do.

Those for 6mm are twice cheaper with 16mm shank.

So would you go for 45 deg angle and a single pass?

My plate is 6mm thick mage of 6mm steel gauge plate, 45HRc (Toolox 44).

@Henry Artist,

This will be a slider used in quite elaborate spindle of rotary vacuum pump.

Edited By Martin Dowing on 27/11/2020 09:38:46

Sorry to edit your post but the supplier linked does not meet the code of conduct conditions on the forum

Edited By Mike Poole on 27/11/2020 09:50:31

John Haine27/11/2020 10:01:52
4639 forum posts
273 photos

When I had to round the ends of some crank webs I took advantage of the fact that these will be a pivot pin at the centre of the semicircle. So I made a pin of the right diameter to be a good fit in the hole and clamped the bar on to an angle plate so the pin was resting on the edge of the plate. Then milled a series of flats on the bar end with an end mill set at the correct height to give the desired radius with the bar clamped at different angles. Finally smoothed the polygonal surface by filing. From what I remember it was a pretty quick process. Could be done more precisely clamping down to a rotary table with the semicircle axis on the table axis. Nowadays I'd just use the CNC.

Martin Dowing27/11/2020 10:21:08
355 forum posts
8 photos


I have Myford dividing head with overhead bar serving for tailstock so I could do it.

But these cutters are cheap if bought from China.

So why not to try?

Hopper27/11/2020 10:33:31
6217 forum posts
321 photos

How do you propose to cut a 15mm radius with 6mm or 8mm radius cutters? I am confused as to exactly what you want to do.

The usual way of rounding the end of a piece of flat bar as shown in Jason's drawings is to clamp the flat bar to a rotary table in the mill, with any size end mill cutter you like, say 12mm diameter, and use the rotary table to rotate the job so the side of milling cutter cuts the radius on the job. The radius is determined by the distance from the centre pivot point of the rotary table to the edge of milling cutter.

You could do it in the lathe by mounting a small face plate to your dividing head and clamping the flat bar to the face plate. And hold any old decent sized milling cutter in the three jaw chuck. (If the chuck is not bellmouthed it will hold a milling cutter without slipping just fine, in my experience over some years now.

Martin Connelly27/11/2020 12:01:14
2125 forum posts
222 photos

Sounds like a Ø30mm concave horizontal milling cutter would do the job (the right hand example Jason gave is what is wanted I think).

This would be expensive so I would be thinking of making my own with some gauge plate mounted in a fly cutter holder. A Ø30 hole is easy to create, angled cutter to give relief behind the edge, relatively easy heat treatment, sharpening would be a bit harder to get right.

Martin C

Andrew Johnston27/11/2020 12:11:22
6577 forum posts
701 photos

Define precision.

I wouldn't call radius cutters a precision tool. Let's ignore the confusion and assume we're talking about a cutter with a radius of 15mm. That's potentially a pretty large width of cut. I doubt a small milling machine will be happy. I don't have any radius cutters that are used vertically. But I picked up a set of radius cutters for the horizontal mill some years back. They work well:


But you need a rigid setup to avoid chatter and a poor finish. I only use them for making parts look pretty. If I needed a precision radius I'd use a ball mill on the CNC mill with a 3D adaptive stepover. In the absence of a CNC mill I'd use a rotary table as mentioned above.

A problem with using the above methods (except CNC) is that it is easy to go a thou or so too far with the cut. This leaves a small ledge or indent on the work which is suprisingly difficult to remove by filing. By the way I can't see sandpaper having much effect on gauge plate. Better to use emery cloth.

The method of rotating the work in a vice and using an endmill to produce a series of flats has never worked for me.


JasonB27/11/2020 13:03:56
22604 forum posts
2643 photos
1 articles

So it sound like the one on the left right of my earlier sketch which if your plate is only 6mm thick won't present a large amount of cutter engagement. If you can mount your plate at 45deg then I would do it that way to avoid having to set up twice and risk things not lining up. You may even get lucky with a router cutter which would allow the work to stay flat though 16mm / 5/8" radius will be more common 

yes now I know what you are after it could be done on a lathe if opposite edges less than 30mm apart.

roundover 3.jpg

Edited By JasonB on 27/11/2020 13:10:03

Edited By JasonB on 27/11/2020 15:33:16

DC31k27/11/2020 14:23:48
662 forum posts
2 photos
Posted by JasonB on 27/11/2020 13:03:56:

roundover 3.jpg


The challenge with the 45 degree method (RHS of sketch) is positioning the cutter correctly so the mid-depth of the plate passes through the radius centre. Easy to do on a picture, some thinking needed for real metal objects.

The LHS will only work if the cutter radius is half the plate thickness. Otherwise the curve produced will always be imbalanced on the plate.

Edited By DC31k on 27/11/2020 14:26:43

JasonB27/11/2020 15:32:56
22604 forum posts
2643 photos
1 articles

LHS has the work flipped over so you cut half way from each side, just risk of getting a step.

Martin Dowing27/11/2020 17:25:28
355 forum posts
8 photos


My stupidity, sorry. 15mm is diameter.

old mart27/11/2020 18:23:07
3728 forum posts
233 photos

Corner rounding milling cutters would do the job, but you would need a solid carbide one with that steel. I have a small solid carbide one which came in a job lot, four flute and about 3mm rad.

peak427/11/2020 18:32:28
1678 forum posts
179 photos
Posted by Martin Dowing on 27/11/2020 17:25:28:


My stupidity, sorry. 15mm is diameter.

Genuine question, does anyone actually market 7.5mm radius cutters?
19/64" is close, but I'd have thought even that would fit in between normal preferred size cutters.


Martin Dowing27/11/2020 18:39:08
355 forum posts
8 photos

@old mart.

Have already ordered solid carbide one of radius 8mm from China. Cheap enough.

Rated up to 50-55 HRc.

Just getting advice how to use it properly.

JasonB27/11/2020 18:40:37
22604 forum posts
2643 photos
1 articles

If you said you wanted an accurate 15mm why have you bought 8mm which will give 16mmcrook

Martin Dowing27/11/2020 18:46:57
355 forum posts
8 photos


Yes, Chinese company which cannot be mentioned here for some reason and which I easily found on Aliexpress is selling very decently looking and also cheap solid carbide radius cutters up to 8mm radius in increments of 0.25mm, so there is a very wide range offer of them.

Martin Dowing27/11/2020 18:58:28
355 forum posts
8 photos

@Jason, I have measured original part which need to be replaced.

Radius there is 7.5mm so we have 15mm diameter, however I have inserted it to the pump to observe how it works with its mating surface.

Base on this I am confident that 8mm radius will also do as good as original and 6mm one should work as well.

However a bit larger one should improve seal, so I went for 8 mm.

Maximum radius there which should still work is in range of 10-12 mm.

Any larger and sharp corner of slider would start scratching bore of pump during rotation.

If you are interested I can post photo of parts of disassembled pump and it should be obvious for most how this pump works (it is rotary vacuum pump with eccentric running spindle working in oil).

Edited By Martin Dowing on 27/11/2020 19:00:05

JasonB27/11/2020 19:01:22
22604 forum posts
2643 photos
1 articles

I'd be interested in a link to the cutter by PM, can understand the shape of the part.

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