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Dovetail cutter on brass

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Martin Shaw 105/02/2018 21:22:16
48 forum posts
8 photos

Hi folks

I'm manufacturing new gib strips for the lathe slides and I'm using a 60deg HSS cutter on CZ121 brass. The cutter is at 500rpm and the feed rate is I guess about 60mm/min, but the finish is distinctly ridged. I'm obviously getting something wrong, but would anyone care to suggest what.

Regards

Martin

Neil Wyatt05/02/2018 21:41:44
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It could mean the cutter is 'swallowing its own swarf'?

A light 'climb milled' finishing cut, just a couple of thou, might give you the finish you seek. Maybe try a faster rpm or slightly lower feedrate.

Neil

daveb05/02/2018 21:52:31
557 forum posts
10 photos

As Neil said and the cutter needs to be very sharp.

Martin Shaw 105/02/2018 22:10:08
48 forum posts
8 photos

Thanks, it's a brand new cutter so that shouldn't be the issue, I'll try a light climb pass at a higher rev and see what the result is. I'm going to lap the finish anyway but obviously don't want to spend ages polishing off the cutting marks.

Regards

Martin

Joseph Noci 106/02/2018 06:04:38
288 forum posts
623 photos

Martin, what is the cutter nominal diameter and how many cutting teeth?

Joe

FMES06/02/2018 06:27:52
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444 forum posts
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Without knowing your tool diameter, guessing your spindle speed is about right, however I would suggest a feed rate of around 40 mm/min

Regards

Lofty

JasonB06/02/2018 07:18:01
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If it is just a thin gib strip is it adequately supported, could be getting pushed away from the work. Don't think I would bother with a DT cutter for gib strips just hold the strip at the correct angle to do the two narrow edges.

Douglas Johnston06/02/2018 09:16:57
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486 forum posts
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A slightly ridged surface, perhaps smoothed a touch with wet and dry paper on a surface plate, might give a good oil retaining surface for a gib.

Doug

KWIL06/02/2018 09:36:01
2933 forum posts
55 photos

There is one major disadvantage in using Brass as a Gib strip, namely the Gib Screws will mean that wear will occur locally and not along the full length as brass lacks overall regidity.

Yes CZ121 contains lead which acts as a lubricant, but so does the carbon in the cast iron mating faces. Better location can be obtained by the use of Guage Plate as a gib as it is so much more rigid.

Martin Shaw 106/02/2018 17:59:49
48 forum posts
8 photos

Joe 20mm dia 8 teeth

Lofty Thanks, does the info above change your view?

Jason A possibility, but I am starting from a piece 1/2" by 1/4". I dont have an angular vice, so the DT cutter is the less expensive of the choices.

Doug In my involvement with lubrication on full size steam locomotives it is now the generally accepted view that "oil grooves" are undesirable, at least in rotating components as they break up the film between surfaces, Of course a gib strip isn't under the same load, but.

KWIL I've no doubt your right however if it ends up as planned the gib strip will be about 8mm between faces so it will be fairly rigid. I am trying out a mod to the Sieg design so that the void between the slides is as completely filled by the gib strip as possible, ideally to the point that the adjisting screws are solely for snugging up. This is to try and get the gib and slide mating surface to stay fully in contact and will hopefully do something about the currently next to useless design. Taper gibs would be better but there's not a lot of meat. I have done Steve Jordan's mod to the saddle and that has improved things beyond belief. If it all works out I'll do it in steel.

Thanks for all the help

Regards

Martin

John Haine06/02/2018 18:08:37
1895 forum posts
110 photos

Martin, if you use brass strip of the thickness you need you can chamfer the edges by clamping it in the dovetail it will fit in using a length of silver steel and toolmakers clamps in the vee, with the slide clamped upside down on the mill table. Do one edge, then turn it over with a thin parallel packing to get clearance.

Martin Shaw 106/02/2018 18:24:14
48 forum posts
8 photos

John

That is clever, far too much for my little brain. It's also likely more precise than my way so I'll file in the useful to remember drawer, thanks.

Martin

Joseph Noci 106/02/2018 19:44:37
288 forum posts
623 photos

Interesting variations of feeds and speeds put forward by various folk.

From the Franken Catalogue, which gives feed speeds and cutter RPM for almost every conceivable cutter type:

For a 20mm diameter, 8 tooth HSS cutter in brass ( 'chip cuttings' ) and for cast Iron, nodular graphite-

Feed per tooth = 0.014mm/tooth

Feed per cutter rev - 0.112mm/rev

Peripheral cutter speed in m/min - 45m/min = approx 725rpm

Feed at 725rpm = 725 X 0.112mm/rev = approx 80mm/min

I use their tables regularly, and they work for me.

Martin, does your cutter have acceptable runout? If poorly ground and out of true, only part of the full complement of teeth are in contact and then the feed rate would be to high..

Joe

FMES06/02/2018 21:07:13
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Posted by Martin Shaw 1 on 06/02/2018 17:59:49:

Joe 20mm dia 8 teeth

Lofty Thanks, does the info above change your view?

Have a look at **LINK**

nice useful calculator

Regards

Lofty

Edited By *.* on 06/02/2018 21:16:54

Saxalby06/02/2018 21:08:21
143 forum posts
10 photos

I agree with Neil Wyatt, a light cut climb milling with give a good finish

Andrew Johnston06/02/2018 21:39:21
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3728 forum posts
452 photos

I haven't quite grasped why a dovetail cutter is needed to make a gib strip. Notwithstanding my lack of understanding the speeds and feeds mentioned seem awfully slow. I'd be running at 1500rpm and a chip load of 0.05mm/tooth giving a feedrate of 600mm/min.

Andrew

Martin Shaw 106/02/2018 21:43:35
48 forum posts
8 photos

Joe

The cutter is brand new from Arc, and whilst it's probably not the best cutter money can buy, it would I hope be more than adequately made. As to run out on the mill, to be honest I haven't measured it, but I've no reason to think it poor, other work I've used it for seems accurate enough for my limited skills, perhaps I should inform myself.

Lofty

Thanks for the link, that may save me some grief.

Regards

Martin

Martin Shaw 106/02/2018 21:49:18
48 forum posts
8 photos

Andrew

In my naievety I thought it the sensible approach, others have given me an alternative view. You may well be right regarding speed and feed rate, I'm not able to make a judgement, so I made the best possible guess with my limited knowledge, which is why my question is in the beginners section.

As an afterthought my stock is 150mm long which means that a full cut would according to your numbers take 15 seconds, surely much too quick.

Regards

Martin

Edited By Martin Shaw 1 on 06/02/2018 21:52:04

Edited By Martin Shaw 1 on 06/02/2018 21:52:29

norman royds 206/02/2018 22:51:43
31 forum posts

I martin the best approach for beginners is the highest speed and the lowest feed and some times it might easier to hand feed and in time you will learn more about it take time to learn speed& feed & depth of cut and I havnt looked at chart fore years good luck regards norm

JasonB07/02/2018 07:23:32
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Posted by norman royds 2 on 06/02/2018 22:51:43:

I martin the best approach for beginners is the highest speed and the lowest feed ..............

Just what you don't want as you will be rubbing your way through the work.

Andrew, I suspect the DT Cutter is being used to form a rhomboid shape, as I hinted earlier I would prefer to tilt the work to do this on the two narrow edges

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