anybody here have a jig to hold flathead heads on a mill to allow machining they would be willing to share pics of? ive got some ideas but would like to see what works....thanks
I thought by now, a few hours, somebody would have an answer. I did an iron flat six head in a Bridgeport, shimmed the head flat and paralell to the table, put clamping screws thru the spark plug holes. Took .050 off. used a fly cutter. Did a few passes to get to the 050. Frank
here's what i'd likely do...make 3 or 4 standoffs from 1" or larger round steel bar stock, tap one end 1/2-13 to screw onto a stud, this end mounts to the tee nuts on your machines table, or to a subplate. on the other end of the standoff, i'd turn a length to fit snugly into the head stud holes, with a shoulder for the counterbore on the head to rest on. then drill and tap the end of the standoff for a 5/16 or so bolt, countersunk at a very sharp angle, sharper than a flathead screw. then sawcut the part that fits into the head stud holes(the mandrel) into quadrants. now take some appropiate flathead screws, or socketheads may work, and turn a corrosponding angle to match the counterbore in the mandrels. the end result, is you set the head onto the mandrels, shim as necessary because no doubt the counterbores are not equal depths, as you tighten the socket head screws the mandrel expands to tightly grip the bolt holes. this will gove you unobstructed access to the entire surface of the head.
ray thats a great idea. clamping thru the plug holes wont allow for machining for valve clearance so thats out. i was thinking of using knife edge clamps i saw in MSC catalog but your suggestion makes lots of sense. thank you..
Again, I think your biggest challenge isn't how to hold it so much as it is how to reference/index it - what will you use as the reference? The surface of the head? Could be warped, cut at an angle, etc. Use the plug holes? Are they parallel to the bores? Use the bolt holes? Probably the closest thing to square with the block, but they aren't necessarily precision drilled/reamed/bored. In the end, I think it's probably a compromise - use what you think is the best reference for your heads - Holding it down will only be the partial battle. I've seen witness marks where apparently heads were place in some sort of vice-like device that used 4 screws to hole the heads in place...
if i'm thinking of the right clamps, toe clamps with a sliding clamp that exerts force both down and sideways, that would work too, wouldn't require all the other machining "my" way would require. see below too... all in a days work for a machinist! part of getting the part setup is running an indicator across the surface of the head and adjusting the setup to get it flat in relation to the machine. i've use the technique you mentioned as well machining castings, works well and is fairly easy to make the fixturing too. it's about as easy as drilling and tapping a couple of holes in a block of steel and grinding a bolt to a point that digs into the casting. as you mentioned though, it does leave marks on the part being clamped, especially if you use the serrated "grippers" that are available. works great where cosmetics don't matter, not a good choice for heads that are already polished, or the super rare ones just purchased off ebay for 2 grand. that's one of the reasons i suggested the labor intensive, but non-damaging method i did. another reason is repeatability, should it be necessary to reload the head for some reason, it's nice to have it locate in the machine as close to possible as it was before. once there are machined features on a cast part, i like to locate off those machined features because they're generally more consistent than a raw casting.
Yes, but now the head surface - an unknown - is flat in relation to the machine. An unknown plus a known is still an unknown. I see flatheads as kind of unique in this aspect. To my knowledge, there is nothing that is referenced to anything else on the engine (that's repeatable, at least). I've squared things up in mills before (but I'm not machinist by a long shot), but you're squaring the piece to the machine based on already machined surfaces. For something as "unreliable" as a casting that's been machined in an unknown manner, well, I go back to my uknown + known = unknown again. As I ponder this, I think the best thing to set the machine up to would be the bolt holes for level - make the bolt holes parallel to the cutter plunge axis for a mill. At least it would be square to the block at that point (still making the assumption the bolt holes are square & correctly machined). I think the deck of the head is probably the last place I'd index for an unknown head - especially one that's already been machined in its past... I just checked a set of big name, expensive heads and there was so much variation between chambers, it's obvious the heads weren't indexed to anything, really. Of course, some of this is the nature of castings, but these are too far off...
Loads of parallels and clamp it through the plug holes! On my head most of the bolt holes are all the same height so it sat on them.
the machinist may have to make some adjustments, but the fact remains, the deck surface is the largest datum to go off of. yes, you have to measure the depths of the combustion chamber etc. in the process of setup and factor in any adjustments necessary, for example if it was milled .030 thinner on one end than the other you'd set it up so there is .030 difference on your indicator from end to end, rather than as close to zero as possible, but you'd still want to use the largest datum you can.
i've always though.....(but never done) if you made 4 threaded rods to thread into the spark plug holes...threads only the depth so they dont interfere with cutting...then run a nut in the slot on the mill table.....use stand off posts in the 4 corners of the head to level it by pushing up on the bolt holes...inturn pulling against the rod in the plug holes take your guess at getting the surface where you think its flat... lots of ways to skin a cat i guess! good luck! Zach
The face of the bolt holes is spot-faced during final machining. Was that perfectly parallel across all 24 holes & all to the same depth? But let's say it's angled from chamber to dome (over the short dimension of the heads) - you can't compare chamber depth to dome depth (particularly if the heads in question have been redomed) to get any sort of accurate datum to base it on. I understand if the chamber on one end is different from the chamber on the other end - that's straight-forward for one axis - it's the short side axis that's tricky, I think. Granted, all of this is a devil's advocate academic exercise - folks machine flatheds every day and honestly, you're not going to do much, if any, harm (particularly on a street engine) if you're off a wee bit - and the head surface probably makes the most sense for most operations. That said, I still don't think there's anything you can absolutely reference off of if you want your deck to be perpendicular to, say, crankshaft centerline (which, the cranks in the flatty are offset anyway, so...)
I know this may sound stupid but after hours of set up on the mill on the first set of "swap meet heads" that I had epoxyed around some heavy pits I found hitting the surface on the big sander did just as well for a low buck street motor. Fast and easy
the thing is, you're thinking the deck surface in relation to the rest of the casting. Which in reality isn't important. the deck surface is perfectly 90 degrees to the bore after you machine it regardless of the the relationship between the deck surface and the rest of the core. It seems to me you are also trying to point out that you can end up with different combustion chamber volumes if you machine the head parallel to the deck height. Any competent machinist that has milled heads before should realize he needs to check the depth of the domes to see if they are in the same plane as the deck. all of this is VERY simple and takes maybe 20 minutes for a good machinist to setup within less than a thousandth. btw, milling the heads isn't the best way to make them nice and flat. best by far is to put them in a blanchard grinder. quick easy and you can do as many as you can fit on the table at once. (prolly 4-5 sets easy) or if you want them within a few helium light bands I can have my uncle throw them on the flat lap. the machining done to these motors is very rudimentary. a warped aluminum head will flatten out and seal when bolted down, as long as it's close
I beg to differ. It is important with cast chambers. With fully machined chambers, I concede your point. This is mostly irrelevant to my point - perhaps I'm not making it well enough. What you say here is 100% true and is important with regards to gasket sealing, etc. However, using just this logic (which I know no machinist would do), you could have a perfectly machined head with nice surfaces true to the block/bore & yet have cock-eyed bolt holes you can't get a bolt through (angle milled heads). (Yes, I'm using the ridiculous to illustrate the absurd! ) Yes, but on heads that have already been machined, or unknown heads, you don't know if the dome is an accurate point of reference - moreso if the heads have been redomed in their past. Again, I'm splitting hairs in an academic, devil's advocate manner - we're talking variances, in most cases, in the thousandths...irrelevant on most engines. The casting variance is much greater...
yes you really are. you are talking about scenarios where it would only matter if you were building a full on blown/race motor. in this case you probably wouldn't risk using an unknown piece of speed equipment that needs to be repaired anyway. even on race engines this is splitting some pretty fine hairs. In a pinch I've run two different heads on my VW midget motors when I punched an exhaust valve at 11pm and had a race the next day at 11am. totally different porting, different size valves and combustion chambers. I couldn't tell the difference and the motor ran very strong.
Yep. Were I build a world record breaking engine, I'd likely machine my own heads from scratch, or at least run fully machined chambers... I'll bow out now & quit stirring pots!
nothing wrong with that . ive had model a heads and such cleaned up and it works ok. it does tend to dig in on the leading edge and round it some and it doesnt allow for other work such as clearancing for valve and pistons
someone who know what they are doing can have better luck with a belt sander than an amateur with a mill. you get all crazy clamping down through spark plugs holes etc. and you can have heads that were flatter BEFORE you tried decking them.
Anyone remember the Lempco #50 "scrub board" head surfacers? They actually would do a good job if you kept the wheel sharp.
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