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Fly cutting flat and parallel

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PANAGIOTIS EVRIPIOTIS30/01/2020 12:23:40
19 forum posts

Hello everyone,

I want to ask a question (or a few) about milling something flat and parallel under certain conditions to find ways to overcome some problems.

So I got a manual milling machine trammed and ready to go. I want to fly cut pieces of steel of dimensions of 130mm x 50mm x 5mm so as you can see it is a pretty thing part. I want to reduce the thickness with the fly cutter and as stated above make the two big sides flat and parallel.

I usually clamped the piece on two parallels, milled one side and the flipped it around to mill the other. If I am not mistaken that should theoretically give me what I need but that is not the case.

First question would be this: Is this approach of fly cutting one side and then the opposite correct for making two sides flat and parallel or should I use the theory of squaring up stock and go from side to side to make everything square and parallel. I am not sure as to the importance of squaring up the small sides (perimeter of the stock)

Second question: The stock since it is that thin most of the times come with some warp into it. The problem with that is that when I tighten the piece on the vise and give it a few taps the warp goes away, then I mill and when I unclamp of course the warp comes back. I thought of outsourcing those parts to a machinist with a surface grinder but theoretically the problem should persist since the macgnetic chuck power of such a machine should take out the warp and when removed from it, introduce it back up.

So I would appreciate any info, videos, books, resource or whatever regarding the subject (especially the 2nd one) because I am having a bit of trouble overcoming those problems.

Thanks in advance everyone!

Bazyle30/01/2020 12:36:46
5779 forum posts
216 photos

You need to make some side or edge clamps so that you are not flattening it by clamping. See Harold Hall's website for ideas. Use a straight edge or your parallel and a feeler gauge to measure the warp when not clamped and then check that your clamping has not altered that.
In theory machining both sides is best to even out the change in stresses from the work but If it is mild steel you still have some risk that it will warp in a new way. It may help if you can stress relieve it first by heating - I'm not sure what temperature and for how long would be suitable though.

Robert Atkinson 230/01/2020 12:58:04
902 forum posts
17 photos


I'd suggest you don't do all of one side at a time. Take all of the surface plus a bit off one side, flip it over and take the same off the other side. If possible stress relieve and then take a second cut at the first side, flip and do a final cut.

Robert G8RPI.

PANAGIOTIS EVRIPIOTIS30/01/2020 13:02:41
19 forum posts

Keep em coming guys just to clarify the steel is a stainless hardenable tool steel and it comes already annealed and stress relieved.

I will read your comments more carefully a bit later to make sure I understood the ideas.

Also let me know your thoughts on getting flat and parallel both sides. In theory would it be enough to mill one side flat and then just flip it to do the other side? (Assuming there are no warps)

Edited By PANAGIOTIS EVRIPIOTIS on 30/01/2020 13:08:41

Andrew Johnston30/01/2020 13:11:41
5921 forum posts
663 photos

How flat and how parallel? There's no point in discussing possible methods until we know what is acceptable.


Martin Connelly30/01/2020 14:01:00
1649 forum posts
179 photos

Before I got down to Robert's post I was thinking that I would take off the high spots on both sides with my belt sander. Next point is that clamping a thin plate in a vice will run the risk of distorting it if it is tightly clamped. I think I would clamp on blocks at three points and then put screw jacks under three other points to support it without distortion. Then machine one side the minimum to clean up by moving the clamps around as needed. Turn over and repeat then check the result. At this point you will know if it is flat and what thickness you need to to remove to get to the target.

Beware of flycutters if you want flat. Any slight out of tram will give a surface that is concave. The bigger the diameter the more concavity you will have. It may be better to do multiple passes with a smaller diameter milling cutter to get a flatter finish. It all depends on what you mean by flat and what is needed from the finished part.

Martin C

PANAGIOTIS EVRIPIOTIS30/01/2020 14:55:35
19 forum posts

@ Andrew I have a manual mill and I fly cut on parallels. I am trying to achieve a uniform flatness with an accuracy of +- 0.02mm maximum across the whole piece. I can take that of with some sanding on a surface plate or something later. Regarding flatness and parallelism I want to achieve the best results I can with the setup I explained in the first post.

@ Martin I thought about the belt sander too and it could work to get the warp out by using very light pressure on the sander and then move to the milling machine. I might give it a try.

I try to make it as tight as needed on the vise. I could theoretically measure if there is any distortion easily firstly on a surface plate and then after I clamp it to check if there are any differences. Regarding the flycutter point that you made, I understand that and I am pretty sure that my head is properly trammed although I will check again before the I do it again. Usually the parts that I am trying to flatten have a bow in the middle and so far I have done only a few and haven't noticed if the bow was there in the beginning or it was introduced during milling. I would like to know if it is actually possible due to the heat generated from a depth of cut of 0.05mm cut for example to distort the piece but that is probably a matter for another post. Right now I am trying to make sure that I have a bulletproof set up and troubleshoot after that if the problem persists.

Also I would like to avoid using small endmills and clamps that I will have to move around to save some time from the operation if possible.

@ Bazyle I checked those side clamps and I don't see how I could use them in this situation due to the thickness of my workpiece as well as the fact that I want to face the whole piece and not just part of it. Also I don't know how they will react when force is applied from a cutter to a high spot from the warp.

@Robert sorry mate I didn't quite catch what you mean. Are you suggesting that I should do what I have been doing with the difference of breaking the procedure into 2 flips instead of 1 and to a stress relieve in between?

I have been trying to find something like theoretical information and not specific approaches in the beginning of how machinists deal with slightly twisted or bend parts that have to be square parallel and flat. All with a 2k manual milling machine.

Dave Halford30/01/2020 15:09:39
1274 forum posts
12 photos

Mag chucks aren't strong enough to warp 5mm plate, what you get is poor grip because of the air gap.

I would clamp straight to the milling table bowed side down, then shim the turned up edges so they can't run away from the cutter pressure.

Then check for flat before flipping.

Tony Pratt 130/01/2020 15:33:32
1403 forum posts
6 photos
Posted by Dave Halford on 30/01/2020 15:09:39:

Mag chucks aren't strong enough to warp 5mm plate, what you get is poor grip because of the air gap.

I would clamp straight to the milling table bowed side down, then shim the turned up edges so they can't run away from the cutter pressure.

Then check for flat before flipping.

A decent mag chuck will pull down a 5 mm plate to some degree. Seen it & done it.


PANAGIOTIS EVRIPIOTIS31/01/2020 08:15:27
19 forum posts

@ Dave this is seems like a practical solution that could theoretically lead to the desired results.In order to do that I just need to get bigger pieces of that steel to clamp them on the table.

The bows are very subtle and you can notice them only by placing a parallel on it or checking on a surface plate, so I would guess that a mag chuck as Tony said would take such a warp out but I am just making guesses now.

ChrisB31/01/2020 09:10:07
576 forum posts
192 photos

I would start with a thicker piece of stock ( say 10mm ) and square it off the sides first, the initial thickness of the stock combined with the 50mm width of your workpiece should not result in any warping. Then put the workpiece in the vice clamping the sides and take a face cut, rotate it and face the other side to your 5mm thickness.

PANAGIOTIS EVRIPIOTIS31/01/2020 09:17:00
19 forum posts

@ChrisB Thats an idea but unfortunately not possible because it would double the price of the steel that is already extremely expensive

Wallybox31/01/2020 10:16:11
4 forum posts

You'd have to be very careful if using a machine vice that your not putting a bow into the workpiece. clamp then tap down with a copper mallet and make sure both parallels can't move from under it by giving them a little tug. like has been said already better to clamp directly to the table and to clamp using downward pressure.

Alternatively buy the stuff on ebay! search "ground flat stock gauge plate"

PANAGIOTIS EVRIPIOTIS31/01/2020 11:24:57
19 forum posts

@Wally that is what I always do when clamping something into a vise but that does not counter the problem of a twisted or bowed workpiece of course.

Another thing is to give it to someone to grind it with a surface grinder but if the part is twisted or bowed I don't know how it will react. I cannot buy it ground because it is a very specific tool steel which cannot be found in a ground state, only by the company that produces it and it is about triple the price. If I can't find a way to do it myself then I will resort to that solution but due to the cost of that I need to be sure that I cannot do it in a time effective way.

Howard Lewis31/01/2020 11:26:03
4400 forum posts
4 photos

Two comments

1 ) . Stainless could work harden, despite having been previously annealed.

2 ). 0.02mm ( 20 microns ) is a pretty tight tolerance for milling, you are looking for 0.000787 inches in old units.

To me that calls for surface grinding, not milling.


Tony Pratt 131/01/2020 11:45:00
1403 forum posts
6 photos

Tapping down onto parallels to m/c the first side is exactly what you don't want to do, you have then added a stress which will show itself when you release the vice, the work should just be laid on the parallels but not tapped down & then machined, hopefully when the vice is released the face will remain flat.

If the steel is magnetic [able to be held on a magnetic chuck] there are techniques that can be used to eliminate bow & twist on a surface grinder.


Andrew Johnston31/01/2020 11:54:25
5921 forum posts
663 photos
Posted by PANAGIOTIS EVRIPIOTIS on 30/01/2020 14:55:35:

@ Andrew I have a manual mill and I fly cut on parallels. I am trying to achieve a uniform flatness with an accuracy of +- 0.02mm maximum across the whole piece.

Forget parallels, flycutters and machine vices. A tolerance of +/-0.02mm is doable but not simple. First I'd ensure that the material is stress relieved. We don't know what the material is, so It's diffcult to suggest temperatures and times.

I'd the put the material on the milling machine table against a thin sheet as a stop. Then hold the material using side clamps with eccentric screws. This is what I mean by side clamps, the small gold hexagons:

side and face.jpg

If the clamps are too thick use a piece of thinner gauge plate as a filler. Don't tap the material down, just let it sit in it's natural position. Then I'd take a series of shallow cuts with an end mill, say 10mm diameter. It may not look pretty but if the mill is trammed properly it'll be flatter than using a flycutter. Once one side is machined turn the material over, clamp and machine the second side.

When clamping the material you need to be clean room obsessive to ensure there is no dirt, swarf or dust between the material and table. Brush it and then run over with a kitchen towel soaked in acetone or similar. It would be a good idea to stone the table before starting to remove any small burrs.

Of course the above assumes that the top of the table is parallel to the dovetails in both X and Y to better than the tolerance wanted. If not then all bets are off.


PANAGIOTIS EVRIPIOTIS31/01/2020 13:01:01
19 forum posts

@ Howard Lewis It doesn't sound that much to me for such a small piece but on the other hand I am an amateur so I could be completely mistaken. Surface grinding is the ideal way to go but the problem of a bowed or twisted piece remains even for surface grinding.

@Tony So you are suggesting to just lay the workpiece on the parallels and properly tighten the vise? I guess for the first pass the bow should be facing upwards in order for the workpiece to sit straighter on the parallels and avoid making it worse. The steel is magnetic but I have no practical or theoretical knowledge on how to remove bows and twists on a magnetic chuck and it sounds interesting to me. If you have any resources regarding the subject please let me know.

@Andrew I get what you mean and I can imagine that it would work with the assumptions made. I have no experience with the side clamps, do they apply downward pressure at all? If not then I guess that in this situation the bow should be facing up else the workpiece will rock here and there during milling.

Andrew Johnston31/01/2020 13:34:03
5921 forum posts
663 photos
Posted by PANAGIOTIS EVRIPIOTIS on 31/01/2020 13:01:0

I have no experience with the side clamps, do they apply downward pressure at all? If not then I guess that in this situation the bow should be facing up else the workpiece will rock here and there during milling.

In theory there is some resistance to upward movement of the work due to friction. But it would be best to orient the work so that it sits without rocking before clamping.


IanT31/01/2020 13:59:40
1800 forum posts
176 photos

I've not needed to do any work to the same degree of 'flatness' but not having access to a surface grinder, I use either machining or lapping to get my work "flat" (by which I mean "flat" within the limits that I can measure)

It seems that the finished part here is 130 x 50 x 5mm but I don't think I've seen what thickness the material is starting out as. Assuming the twist/distortion is not that much, then it should be possible to lap the part flat on one side before machining (or grinding) the other. If the material is already annealed then hopefully there will not be too much stress present but again I don't know what material thickness needs removing? I'd machine the width/length to dimension before doing any 'depth' work, as these seem less critical and easier to achieve.

As for work holding, another possible approach would be to superglue it to a sub-plate machined in situ - which should make the receiving surface as "flat" as your machine can manage. I've done this with thin parts (a lot thinner than 5mm) which came out good enough for my purposes but I was measuring thous not microns.

If you can afford to have the part surface ground, then that probably sounds the best way to go - but perhaps you could do some preparatory work (e.g. lapping) first to avoid any problems with work holding distortion. The material is 'stainless' (?) so will a 'mag' chuck actually hold it?

Just to repeat - I don't consider myself a 'precision' machinist and my machinery isn't either - but if I need something 'flatter' than I can normally machine it - then I have to lap or scrape it. In this case lapping may get at least one surface flat enough for your needs...just some thoughts which hopefully will help...



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