HK/CETME Mandrel Rental/Purchase

[r.erichsen]

To the mods: sorry if this is the wrong spot, I was looking for increasing exposure for this within the fairly small build community.

As our Inland Empire HK/CETME build party approaches, it's becoming clear that we will probably not have enough time to screw around with hunks of steel to come up with a dimensionally correct mandrel for both CETME and HK receivers (HK being the bigger of the two in width if I recall, but just a smidge) and calculate in the proportion of spring-back from the CRS of the receiver flat (what is it? 1% is what I've heard, but until we try it, unverified). Both will require experimentation and probably the sacrifice of some flats to determine if our numbers are right.

Does anyone have a mandrel to rent or buy, preferably with the external bits to fit into the rails and squeeze the receiver against the mandrel and hopefully squish the rails flat in the process (about 8 of the builders have the older round rail ORF or early GW flats, only 4 bought the flat rail PTR or second gen GW flats).

I'm asking anew here because this sort of tooling seems to change hands frequently and anyone with such a tool might be inclined to make some jing with it.

If you have a mandrel around, please PM me with details regarding rental or out right purchase. Our BP date is Oct 29th, so obviously we need to get something done here pretty quickly.

R

[HKILLER]

member rpmfly has one for rent.

[bladeworks123]

To the mods: sorry if this is the wrong spot, I was looking for increasing exposure for this within the fairly small build community.

As our Inland Empire HK/CETME build party approaches, it's becoming clear that we will probably not have enough time to screw around with hunks of steel to come up with a dimensionally correct mandrel for both CETME and HK receivers (HK being the bigger of the two in width if I recall, but just a smidge) and calculate in the proportion of spring-back from the CRS of the receiver flat (what is it? 1% is what I've heard, but until we try it, unverified). Both will require experimentation and probably the sacrifice of some flats to determine if our numbers are right.

Does anyone have a mandrel to rent or buy, preferably with the external bits to fit into the rails and squeeze the receiver against the mandrel and hopefully squish the rails flat in the process (about 8 of the builders have the older round rail ORF or early GW flats, only 4 bought the flat rail PTR or second gen GW flats).

I'm asking anew here because this sort of tooling seems to change hands frequently and anyone with such a tool might be inclined to make some jing with it.

If you have a mandrel around, please PM me with details regarding rental or out right purchase. Our BP date is Oct 29th, so obviously we need to get something done here pretty quickly.

R

Where would a person find out about the details of the build party? When, where, etc? And how would someone get to attend?

[r.erichsen]

Where would a person find out about the details of the build party? When, where, etc? And how would someone get to attend?

Unfortunately we're all booked up on slots (I think we ended up with 14), scheduled for Oct 29th and it's in the Inland Empire, Southern Cali which from your location probably wouldn't have been ideal anyway. We have a thread in Calguns.net on the topic.

We'll be doing these every six months or so if you happen to be out near San Bernardino for some reason. We're trying to keep the costs down to around $50-60 depending on what our tooling is going to cost to rent. We already have the bending jig and did a couple of test bends on known-quality flats. The owner of the jig reported a 40% failure rate, but with care in initial alignment and during the fold it doesn't look like it should be anywhere near that bad with a quality flat (the GW flat we used had excess material on one "half" that will have to be cut-off and made symmetrical in addition to having round rails to deal with). Neither I nor the main organizer providing the space for the BP are HK experts, that's going to be coming in the form of our local HK builders with dozens (if not close to a hundred) builds under their belts.

I plan to fold two for my purposes and then it will be TIG time for all the extra welding chores my requirements are going to heap on top of the typical weld seams, barrel trunnion and cocking tube. We're spending the next few weekends fabricating tools that will be of benefit, including properly sized heat sinks for the tube and receivers, modifying clamps, taking measurements, etc.

We may have a slot left for "observer only" but I'd have to check with the main organizer first before offering that if you might be out in our neck of the woods.

R

[bladeworks123]

Well, thanks, but I don't have much need to be a participant in a bend party, or to be an observer. I am having enough of those on my own..... But thought I might be able to help if you were close enough. You are right though, it needs to be a little closer than that, I won't be in California for another couple of months. We will be in the LA area in December.

[r.erichsen]

We'd love to have someone with your experience. Maybe in late Spring or early Summer we'll be doing another HK build party. If we plan ahead maybe you can make that one. I'll send you a PM if you'd like, off in the future though that may be.

R

[holescreek]

Is it common to charge for attendance at build parties or is it just a California thing? We have them all over the country on WG and have never heard of such a thing. 40% failure rate on a flat is very bad IMO. You can get that rate using a pipe on a workbench.

Dave, here's a link to the build party thread. I only found it when I started seeing high traffic on a couple of my photos:
http://calguns.net/calgunforum/showthread.php?t=461229

[r.erichsen]

The costs are lower than a lot of local BPs for those who only build AKs. Lunch is served (with accommodation for vegetarians, kosher and the likes within reason) and drinks are provided (all this is part of Roccobro's BP offering, not mine). The costs cover the food and drink, lost, worn or broken tooling, chemicals and other consumables, chemical disposal and in this case any tool rentals (i.e. the HK/CETME mandrel).

Roccobro also has Friday pre-BPs where it's a casual event with 3-4 people usually doing work on their own and using the tools, or asking advise. Unless he does a lot of work he doesn't charge. That's his "fun day."

The alarmingly high failure rates for flat bending are no doubt "operator failure" and that number sounded WAY too high. We have not experienced anything to suggest that's some sort of consequence of the jig, it works just fine if you are paying attention. The folds we did since acquiring it went as well as they could given the flats we tried it on.

The description of how the jig was used was also a bit odd. We worked out how it was supposed to work intuitively and ended up with a flat that was as good as the flat permitted without further remediation of the flat defects that were present prior to folding. The ORF/GW "round rail" flat 1st gen flats had a lot of problems, the worst of which was a lack of symmetry left to right and the rough cuts and less than ideal curves that left a fair gap at the barrel trunnion. All of this can be fixed (its not a bashed fender after all) with time and effort and the right mandrel to clean up the dimensions.

R

[bladeworks123]

I posted a thread somewhere with the problems associated with the lack of symmetry from side to side. That is a definite issue, and unless you have a straightening mandrel or a jig that bends off of the creases in the flat, you will spend a lot of hours getting the flat right. I looked at your posting that Holes referenced above and it looks like you are using a jig that centers on the flat. Do yourself a huge favor and measure and mark a line in the center between the rails. do not count on the flats being centered on the pre-stamped holes in the center unless you are dealing with an Hk flat or one of Tims new flats from Gewehr works. Otherwise your flat will come together fine at the bottom but will be loose on the left side, tight on the ejection port side, and your trunnion will be off center by about 1 to 1.5 mm. Just enough to have alignment issues with your cocking tube and bolt carrier drag after you weld up. I'm going to go out on a limb here and say that 99% of the success or failure of your builds are going to be receiver out of spec issues. But then I am absolutely anal about everything being aligned before I ever start to weld anything up. I also was going to comment on the failure rate you posted but didn't want to come across as being a Johhny Raincloud. I'm more politically correct than Holes....:Smile_Default:. Take your time on the first couple, and evaluate what you have going on. If they aren't close to being straight and symmetrical,,,,,,figure out why and correct the problem before you bend a whole shop full of them.

[holescreek]

I just went back and looked at that thread again. They sure do have a funny way of doing things on that forum, there are at least 5 pages gone since the last time I looked at it that included a pretty detailed critique of the GW flats.

[r.erichsen]

I posted a thread somewhere with the problems associated with the lack of symmetry from side to side. That is a definite issue, and unless you have a straightening mandrel or a jig that bends off of the creases in the flat, you will spend a lot of hours getting the flat right. I looked at your posting that Holes referenced above and it looks like you are using a jig that centers on the flat. Do yourself a huge favor and measure and mark a line in the center between the rails. do not count on the flats being centered on the pre-stamped holes in the center unless you are dealing with an Hk flat or one of Tims new flats from Gewehr works. Otherwise your flat will come together fine at the bottom but will be loose on the left side, tight on the ejection port side, and your trunnion will be off center by about 1 to 1.5 mm. Just enough to have alignment issues with your cocking tube and bolt carrier drag after you weld up.

Good guidance and we'll be applying it even on the new GW flats and the PTR flats that I have been happy with in all respects, dimensionally and in appearance. Most of the builders are showing up with at least one ORF flat that they must have scrounged up that will no doubt give trouble. We will plan accordingly with the measurement and marking you suggested above.


I'm going to go out on a limb here and say that 99% of the success or failure of your builds are going to be receiver out of spec issues. But then I am absolutely anal about everything being aligned before I ever start to weld anything up.

That's how we roll too - the first bend was an eye opener to what to pay attention to. We've since been very critical of the flats to be used and were taking measurements and noting who had the old ORF or first generation GW round rail flats to know which flats might need additional finessing.


I also was going to comment on the failure rate you posted but didn't want to come across as being a Johhny Raincloud.

To be perfectly clear, the "40% failure rate" was not our experience using the jig, but was the reported experience of the previous owner who donated the jig to get an HK build party going. I also strongly suspect he had been using the non-symmetric ORF or GW flats and did not advise measuring or marking as you did - that would have been helpful to know on our first attempt. We have to be kind given his donation, but it's reasonable that had he not had such troubles folding flats we may not have obtained a free $350 tool. We had plans to modify it for better reproducibility with the PTR flat but found we could not confine the group to a known-good receiver flat (the flat rail, improved GW flat wasn't available yet). When we found we were unable to herd the cats given that most already had some old ORF flat already, we figured we'd have to plan accordingly. It's probably best to just treat them all like they have some amount of dimensional anomaly and carefully measure and mark each one before folding them. No center line markings? No time on the jig until you do. Seems a reasonable way to avoid wasted time and receiver rescue attempts.


I'm more politically correct than Holes....:Smile_Default:. Take your time on the first couple, and evaluate what you have going on. If they aren't close to being straight and symmetrical,,,,,,figure out why and correct the problem before you bend a whole shop full of them.

We were extremely slow and careful, if we knew the flats were going to be that awry we'd have take more precautions. The flat slowly closed to within a mere 1/16" of an inch with the tabs for the bottom of the receiver stock block, mag well and front trunnion all appearing to line up. It wasn't until we took it out of the jig, clamped it and tack welded it in with temporary welds that we noticed the lack of symmetry in the original stamping. The only way I think we can fix that first receiver attempt is by breaking the temporary tack welds and trimming off the .053" of excess on the left half of the receiver. We'll probably use a marker and straightedge to indicate the line and use a cutoff wheel down the bottom of the receiver until the excess is removed and then rechecked. When we get a mandrel around here we'll clean up the interior and exterior dimensions, flatten the rails and then recheck prior to moving on to TIG'ng, barrel pressing and the likes.

Prior to the BP I want to fold a couple of correctly spec'ed receivers (with dimensions from this forum) and run a mandrel through them or just make judicious use of my rubber mallet and wooden tent peg to coax the bolt carrier down the receiver. We've also considered using old bolt carriers to make a mandrel - the CETME seems sized about right for as a starting point and is a lot cheaper for our experiment.

We may have some questions for you in a couple of days when we give this another try.

R

[r.erichsen]

There are two threads Holes - one for the jig and thoughts on flats, a survey on current flats from GW and PTR and another thread for the build party. I haven't updating anything in the jig thread in quite awhile. That was the thread where Roccobro posted the first images from when I was in his shop with the GW and the PTR that we compared side by side and then folded one of the two GW flats to determine how the jig worked and how the receiver could be expected to turn out. The instructions left a lot to be desired, which was compounded with the funkiness of the receiver stamping asymmetry.

The second thread for the BP planning didn't have images associated with the GW flat, it was just me complaining about things on the new GW flat that I noted when I opened the package. I did however acknowledge they were a lot better than the first version, but still weren't as attractive as the PTR flat and roughly equal in dimensions and trunnion fit.

R

[bladeworks123]

...The only way I think we can fix that first receiver attempt is by breaking the temporary tack welds and trimming off the .053" of excess on the left half of the receiver. We'll probably use a marker and straightedge to indicate the line and use a cutoff wheel down the bottom of the receiver until the excess is removed and then rechecked. When we get a mandrel around here we'll clean up the interior and exterior dimensions, flatten the rails and then recheck prior to moving on to TIG'ng, barrel pressing and the likes.

Prior to the BP I want to fold a couple of correctly spec'ed receivers (with dimensions from this forum) and run a mandrel through them or just make judicious use of my rubber mallet and wooden tent peg to coax the bolt carrier down the receiver. We've also considered using old bolt carriers to make a mandrel - the CETME seems sized about right for as a starting point and is a lot cheaper for our experiment.

We may have some questions for you in a couple of days when we give this another try.

R

Trimming metal off of one side is not the answer. If you do that, you will find that your mag well and fcg rails will be too narrow. The problem lies in the length around the trunnion. Not much you can do about it except align everything after the bend is done.

[holescreek]

I've been involved in over 20 receiver bends (all ORF stuff) and have never noticed any mismatches or alignment issues. During the bending operation the ejection port side does tend to rise faster but the seams always come together evenly. I use layout die and scribe the centerline for a 3/16" pin hole that guides the flat into the die while pressing, I do use the existing slot in the rear of the flat to guide the back end down.

Maybe someday a GW flat will make it's way into the shop and I can check it out. It's taken two years of builds to use up all the flats I bent back when I first made the jig. I have one more receiver to use and 3 ORF flats left in the box.

[bladeworks123]

I've been involved in over 20 receiver bends (all ORF stuff) and have never noticed any mismatches or alignment issues. During the bending operation the ejection port side does tend to rise faster but the seams always come together evenly. I use layout die and scribe the centerline for a 3/16" pin hole that guides the flat into the die while pressing, I do use the existing slot in the rear of the flat to guide the back end down.

Maybe someday a GW flat will make it's way into the shop and I can check it out. It's taken two years of builds to use up all the flats I bent back when I first made the jig. I have one more receiver to use and 3 ORF flats left in the box.


That's because you have a clear understanding of the necessity of making sure you identify the true center of the receiver. Unlike many who just eyeball it or don't take the time to measure, mark and do the layout before they start the bend. (That is probably the explanation for the 40% fail rate) That's what I was getting at. Measure and mark the centerline, to be sure your bend is right at the top of the receiver, and so you have equal distance between the rail folds. Then adjust at the bottom if you need to. If you don't you will fight with it later. I've bent several of the ORF flats too and they are pretty close. As long as the jig or the bending process only exerts force on the top portion of the receiver as it wraps around the recoil guide tube, even the out of kilter ones usually never lack more than a 1/16th of an inch of meeting correctly, but regardless of whose flats they are, they all need a little tweeking here and there. Since I found the out of symmetry issue on the trunnion pockets on some, I've started checking measurement as center between the rails on the older flats. The bottom comes out just about a 1/16th of an inch high on one side, and then starts to line up better at the rear of the mag well. But after the bend is done the mandrel will slide through a lot easier because the rails are more parallel to each other. Then with the mandrel, or even an old carrier clamped in place, it is easy to correct the trunnion pocket and mag well, with a couple of well directed whacks with the mother of all rubber mallets. That's why we have mandrels. :084:

[r.erichsen]

That's because you have a clear understanding of the necessity of making sure you identify the true center of the receiver. Unlike many who just eyeball it or don't take the time to measure, mark and do the layout before they start the bend. (That is probably the explanation for the 40% fail rate) That's what I was getting at. Measure and mark the centerline, to be sure your bend is right at the top of the receiver, and so you have equal distance between the rail folds. Then adjust at the bottom if you need to. If you don't you will fight with it later. I've bent several of the ORF flats too and they are pretty close. As long as the jig or the bending process only exerts force on the top portion of the receiver as it wraps around the recoil guide tube, even the out of kilter ones usually never lack more than a 1/16th of an inch of meeting correctly, but regardless of whose flats they are, they all need a little tweeking here and there. Since I found the out of symmetry issue on the trunnion pockets on some, I've started checking measurement as center between the rails on the older flats. The bottom comes out just about a 1/16th of an inch high on one side, and then starts to line up better at the rear of the mag well. But after the bend is done the mandrel will slide through a lot easier because the rails are more parallel to each other. Then with the mandrel, or even an old carrier clamped in place, it is easy to correct the trunnion pocket and mag well, with a couple of well directed whacks with the mother of all rubber mallets. That's why we have mandrels. :084:

Copious notes taken - many thanks guys. I intend to make the second CETME/HK receiver I've ever bent come out better than the first. I was not well informed nor had I done my homework before jumping into the first fold with the scant written documentation that came with the jig.

Another question that nags at me is how important the calculation of the effect of spring back is in the dimensions of the mandrel. Say if the mandrel I hope to rent shows up and is dimensionally identical to a CETME or HK bolt carrier, would that not be relatively "oversize" and therefore not take into account the minor, if important effect of receiver spring-back? Should I see some degree of undersize in the mandrel throughout and some compromise to accomodate both HK and CETME bolt carriers? My expectation is that for the CETME carrier, it will be looser if the mandrel was designed to accommodate (or designed specifically for) the slightly wider HK bolt carrier, correct? Am I over-thinking this aspect of the tooling and the bolt carrier dimension is good enough?

R

[holescreek]

As the one who made the mandrel you are renting I guarantee that it is not the same dimension as your bolt carrier. The HK carrier is not dimensionally different anywhere that matters. I made an instruction sheet that went with every mandrel sold. Follow the guidelines and you'll be alright. I can't emphasize enough to resist squeezing the rails too hard, too fast and keep the press ram over the rail section.

I have run into one series of ORF flats (based on the number stamped on the rear of the flat) that wants to collapse inward when the mandrel is removed. The two "bad" flats were very old, purchased years before ORF went out of business, probably from an earlier run. I believe the difference was they did not have an "S" in the serial number.

[r.erichsen]

As the one who made the mandrel you are renting I guarantee that it is not the same dimension as your bolt carrier. The HK carrier is not dimensionally different anywhere that matters. I made an instruction sheet that went with every mandrel sold. Follow the guidelines and you'll be alright. I can't emphasize enough to resist squeezing the rails too hard, too fast and keep the press ram over the rail section.

I have run into one series of ORF flats (based on the number stamped on the rear of the flat) that wants to collapse inward when the mandrel is removed. The two "bad" flats were very old, purchased years before ORF went out of business, probably from an earlier run. I believe the difference was they did not have an "S" in the serial number.

Good to know. Too bad you don't have another mandrel to put into circulation. I'm concerned we will not get that rental unit in time.

R